Lithostratigraphy and geochronology

The largely undeformed stratigraphy of the Nama Basin in southern Namibia was well described by Germs (Reference Germs1972a, Reference Germs1974, Reference Germs1983) and then put into a sequence stratigraphic framework by Saylor et al. (Reference Saylor, Grotzinger and Germs1995, Reference Saylor, Kaufman, Grotzinger and Urban1998 Reference Saylor, Poling and Huff2005), Smith (Reference Smith1999), and Saylor (Reference Saylor2003). Briefly, the Nama Basin is divided into three subbasins by topographic highs that were present during sedimentation. Ediacaran fossils are found only in the two western subbasins, Zaris and Witputs, which are separated by the Osis arch (Fig. 1). Upper Nama Group sediments belonging to the Cambrian Fish River and largely Ediacaran Schwarzrand subgroups extend across the Osis arch; the older Ediacaran Kuibis Subgroup is thicker, more carbonate-rich, and apparently more complete in the north (Fig. 2). The only distinctive Kuibis unit that crosses the arch is the Kliphoek Member of the Dabis Formation, which projects as a tongue into the southern edge of the Zaris subbasin (Fig. 2; Germs, Reference Germs1983, fig. 3; Germs and Gresse, Reference Germs and Gresse1991, fig. 3). Fortunately, the limestone overlying this tongue, which is clearly the Mooifontein Member of the Zaris Formation, preserves the older rising limb of a pronounced positive carbon isotope excursion (OMKYK, Figs. 2, 3; OME of Bowyer et al., Reference Bowyer, Zhuravlev, Wood, Shields, Zhou, Curtis, Poulton, Condon, Yang and Zhu2022) that is well characterized from thick, carbonate-rich sections in the Zebra River area (Figs. 1–3; Grotzinger et al., Reference Grotzinger, Bowring, Saylor and Kaufman1995; Saylor et al., Reference Saylor, Kaufman, Grotzinger and Urban1998; Smith, Reference Smith1999; Wood et al., Reference Wood, Poulton, Prave, Hoffmann and Clarkson2015) and is older than a prominent volcanic ash bed with a U–Pb age of 547.36 ± 0.23 Ma (Grotzinger et al., Reference Grotzinger, Bowring, Saylor and Kaufman1995; Bowring et al., Reference Bowring, Grotzinger, Condon, Ramezani, Newall and Allen2007; Schmitz et al., Reference Schmitz, Singer, Rooney, Gradstein, Ogg, Schmitz and Ogg2020). As the peak of the Omkyk excursion can be followed southward in the Mooifontein Member (Fig. 3), its presence above fossiliferous horizons of the underlying Buchholzbrunn and Kilphoek members in the Witputs subbasin provides a minimum age of about 548 Ma for those assemblages (Fig. 2; Saylor et al., Reference Saylor, Kaufman, Grotzinger and Urban1998).

Figure 2. Lithostratigraphy of the Nama Group and stratigraphic ranges of taxa in this work. The lower parts of the successions of the Zaris and Witputs subbasins differ across the Osis ridge, and the ~2 million-year hiatus (vertical lines) proposed here for the Witputs subbasin is novel. Rock units shown are mainly members of the Kuibis (K) and Schwarzrand (S) subgroups rather than the formations as these are the most commonly used and mappable lithological subdivisions. The recommended nomenclature for the first-order sequence stratigraphy is developed from Saylor et al. (Reference Saylor, Grotzinger and Germs1995, Reference Saylor, Poling and Huff2005), Smith (Reference Smith1999), and Saylor (Reference Saylor2003), and the terms apply to the sequences above each labeled boundary. The stratigraphic ranges shown are limited to genera and larger taxonomic groups to provide a clear overview of the distribution of key elements of the biota; specific details are provided in the systematic paleontology section. “OMKYK” shows the stratigraphic position of the Omkyk positive carbon isotope excursion (Bowyer et al., Reference Bowyer, Zhuravlev, Wood, Shields, Zhou, Curtis, Poulton, Condon, Yang and Zhu2022) in both subbasins (Fig. 3), and the gray triangles within the Feldschuhhorn Member represent pinnacle reefs.

Figure 3. Measured stratigraphic sections from a north–south transect across the Osis ridge at farms Aar, Mooifontein, and Mamba (Fig. 1); rock types are siliciclastics (red/dark gray), carbonates (brick-wall patterns), and siltstones (green/light gray). The canyon section (Aar) incudes almost all of the Dabis Formation from granitic basement to the top of the Mooifontein limestone member; the Amphitheatre section, which transects the famous Pteridinium locality on Aar (UCLA 7307), is the source of the carbon isotope values shown by triangles (Supplemental dataset 1); the thinner and unfossiliferous Mooifontein section, closer to the Osis ridge, provided the carbon isotope values shown as gray filled circles (Supplemental dataset 1). The Mamba section, on the north side of the Osis ridge, has a siliciclastic tongue of Kliphoek Member between the Mara and Mooifontein limestone members (Fig. 2; Germs, Reference Germs1983, fig. 3).); carbon isotope values from this section (black filled circles, Supplemental dataset 1) and those from the far thicker section along the Zebra River (gray triangles; Saylor et al., Reference Saylor, Kaufman, Grotzinger and Urban1998) are correlated by normalizing the thicknesses between the basal unconformities and a distinctive stromatolitic marker bed, visible in both sections. Fossiliferous horizons sampled in this study are shown by their UCLA numbers. The single- and double-headed arrows are current directions, and the rose diagram illustrates the orientation of 10 transported specimens of Pteridinium simplex measured at UCLA 7307 (all measurements corrected for –19° magnetic declination). Lithologic symbols: brick wall patterns = limestones, dark (black) and light (gray); rhomboidal brick patterns = dolomites; red = sandstones and arenites; recessive units, gray or green = mainly siltstones.

A volcanic ash bed near Nooitgedacht (Fig. 1) at the “basal contact” of the siliciclastic Nudaus Formation, the oldest unit of the Schwarzrand Subgroup, has a U–Pb age of 545.27 ± 0.11 Ma (Nelson et al., Reference Nelson, Ramezani, Almond, Darroch, Taylor, Brenner, Furey, Turner and Smith2022), thus implying an approximately 2 million-year hiatus between the Kuibis and Schwarzrand subgroups in the Witputs subbasin, where the Nudaus lies directly upon the Mooifontein (Fig. 2); carbonate and low-energy siliciclastic sedimentation seems to have continued through this hiatus in the north.

Another volcanic ash bed ~10 km southwest of Nooitgedacht in the Nasep Member of the heterogeneous Urusis Formation, which completes the Ediacaran section of the Schwarzrand Subgroup in the Witputs subbasin, has a U–Pb age of 542.65 ± 0.15 Ma (Fig. 2; Nelson et al., Reference Nelson, Ramezani, Almond, Darroch, Taylor, Brenner, Furey, Turner and Smith2022). This constrains the fossiliferous intervals of the Schwarzrand Subgroup to between about 543 and 539 Ma in the Witputs subbasin, but the older Nudaus Formation is fossiliferous north of the Osis arch (UCLA 7320, Fig. 2; Darroch et al., Reference Darroch, Boag, Racicot, Tweedt, Mason, Erwin and Laflamme2016) and perhaps at Gründoorn, about 60 km from Karasburg (Figs. 1, 2; Haughton, Reference Haughton1960; Glaessner, Reference Glaessner1978). The Nama section at Charliesput, ~95 km east of Karasburg, is almost entirely siliciclastic (Germs, Reference Germs1972a, fig. 22; Reference Germs1974, fig. 4). Glaessner (Reference Glaessner1978) followed Haughton (Reference Haughton1960) in attributing the two slabs of sandstone that preserve the type specimens of Archaeichnium haughtoni Glaessner, Reference Glaessner1963 to the Kuibis Formation equivalent, the Nababis Formation, but the stratigraphic range of Archaeichnium elsewhere suggests a younger, Schwarzrand equivalent provenance (Fig. 2).

The best-dated part of the Nama succession spans the Ediacaran–Cambrian boundary (Grotzinger et al., Reference Grotzinger, Bowring, Saylor and Kaufman1995; Linnemann et al., Reference Linnemann, Ovtcharova, Schaltegger, Gärtner and Hautmann2019; Nelson et al., Reference Nelson, Ramezani, Almond, Darroch, Taylor, Brenner, Furey, Turner and Smith2022), and the relevant U–Pb ages for this work are summarized in Figures 2 and 5. The Precambrian–Cambrian boundary has traditionally been placed at a profound erosional surface at the base of the Nomtsas Formation (e.g., Germs, Reference Germs1972a; Grotzinger et al., Reference Grotzinger, Bowring, Saylor and Kaufman1995; Wilson et al., Reference Wilson, Grotzinger, Fischer, Hand and Jensen2012), but recently it has been suggested that the boundary should be lowered into the underlying Spitskop Member of the Urusis Formation (Fig. 2) because trace fossils of the Treptichnus pedum (Seilacher, Reference Seilacher, Schindewolf and Seilacher1955) type have been found within the Spitskop Member (Linnemann et al., Reference Linnemann, Ovtcharova, Schaltegger, Gärtner and Hautmann2019; Bowyer et al., Reference Bowyer, Zhuravlev, Wood, Shields, Zhou, Curtis, Poulton, Condon, Yang and Zhu2022) and treptichnids lower down (Jensen et al., Reference Jensen, Saylor, Gehling and Germs2000; Darroch et al., Reference Darroch, Cribb, Buatois, Germs and Kenchington2021). The significance of these observations remains a matter for debate; here we follow the traditional view on the grounds that bone fide examples of Treptichnus pedum are found abundantly in the Nomtsas Formation (Fig. 25.3, 25.4; Wilson et al., Reference Wilson, Grotzinger, Fischer, Hand and Jensen2012) whereas those present in the Spitskop Member (Fig. 25.1, 25.2; Germs, Reference Germs1972b, pl. 2, fig. 1; Jensen et al., Reference Jensen, Saylor, Gehling and Germs2000; Darroch et al., Reference Darroch, Cribb, Buatois, Germs and Kenchington2021; Turk et al., Reference Turk, Maloney, Laflamme and Darroch2022) are treptichnids but not Treptichnus pedum, a pattern seen elsewhere (Jensen, Reference Jensen2003), including Nevada (Tarhan et al., Reference Tarhan, Myrow, Smith, Nelson and Sadler2020). The persistence of characteristically Ediacaran fossils such as Pteridinium and Swartpuntia to near the top of the Spitskop Member is mirrored by the presence of Ernietta and other Ediacaran taxa immediately beneath the well-characterized Precambrian–Cambrian boundary in Nevada and Sonora, Mexico (Smith et al., Reference Smith, Nelson, Strange, Eyster, Rowland, Schrag and Macdonald2016, Reference Smith, Nelson, O'Connell, Eyster and Lonsdale2022; Hodgin et al., Reference Hodgin, Nelson, Wall, Barron-Diaz, Webb, Schmitz, Fike, Hagadorn and Smith2021; Nelson et al., Reference Nelson, Crowley, Smith, Schwartz, Hodgin and Schmitz2023). Thus, we regard all of the fossils discussed in this work, with the exception of T. pedum from the Nomtsas Formation, to be Ediacaran, not Cambrian, in age.

Biostratigraphy

The regional distribution of the Ediacara fauna and associated calcareous fossils and trace fossils was first documented by Germs (Reference Germs1972a–Reference Germsc, Reference Germs1973). He showed that Ediacaran fossils (Rangea, Pteridinium, Ernietta) are common in the triangular area bounded by Aus (16.25°E, 26.67°S), Helmeringhausen (16.82°E, 25.88°S), and Goageb (17.22°E, 26.75°S) at the siliciclastic to carbonate transition from the Kliphoek to Mooifontein members, since separated out as the Buchholzbrunn Member (Germs and Gresse, Reference Germs and Gresse1991; Germs, Reference Germs1995)—which we use here (Figs. 2, 3)—or as the Aar Member (Hall et al., Reference Hall, Kaufman, Vickers-Rich, Ivantsov and Trusler2013). Germs also found one specimen of Rangea higher in the succession, just above the Mooifontein limestone in the Neiderhagen sandstone Member, Nudaus Formation, at Chamis (17.00°E, 26.05°S), but as the fossil was not in situ, its stratigraphic position may be questionable. A rather different assemblage, interbedded with carbonates, was found at a single site in the base of the Huns limestone, Urusis Formation, Schwarzrand Subgroup at Arimas (17.00°E, 27.70°S). Thus, there seemed to be two principal assemblages of Ediacaran organisms, an older one characterized by Pteridinium and Ernietta and a younger one that lacked both genera but yielded a variety of tubular and trace fossils, plus the new genus Nasepia (Germs, Reference Germs1972a, Reference Germsc, Reference Germs1973). This situation languished until Grotzinger et al. (Reference Grotzinger, Bowring, Saylor and Kaufman1995) showed that Pteridinium and Swartpuntia (Narbonne et al., Reference Narbonne, Saylor and Grotzinger1997) extended almost up to the disconformity that separates the Cambrian Nomtas Formation from older Schwarzrand units. There has been one recent report of an “indeterminate erniettomorph” in the Nomtsas Formation in South Africa (Nelson et al., Reference Nelson, Ramezani, Almond, Darroch, Taylor, Brenner, Furey, Turner and Smith2022), but both the fossil and its stratigraphic level need further assessment.

In the carbonates, Germs (Reference Germs1972a, Reference Germsb) reported Cloudina from the Mara, Mooifontein, and Huns limestone members of the Wiputs subbasin, but all of the described material was from a bioherm, the Driedoornvlagte reef (Grotzinger et al., Reference Grotzinger, Watters and Knoll2000, Reference Grotzinger, Adams and Schröder2005; Adams et al., Reference Adams, Schröder, Grotzinger and McCormick2004; Wood et al., Reference Wood, Poulton, Prave, Hoffmann and Clarkson2015), in the Zaris subbasin (Fig. 1). All occurrences of Cloudina in both subbasins were subsequently summarized by Grant (Reference Grant1990) and Yang et al. (Reference Yang, Warren, Steiner, Smith and Liu2022). Namacalathus is found with Cloudina in the Driedoornvlagte reef (Germs, Reference Germs1972c, pl. 1, fig. 4; Grotzinger et al., Reference Grotzinger, Adams and Schröder2005; Penny et al., Reference Penny, Wood, Curtis, Bowyer, Tostevin and Hoffman2014; Wood et al., Reference Wood, Poulton, Prave, Hoffmann and Clarkson2015) and in the Omkyk Member in the Zebra River section on Donker Gange (UCLA 7319; Grotzinger et al., Reference Grotzinger, Watters and Knoll2000), but apparently without Cloudina in the pinnacle reefs of the Feldschuhhorn Member at Swartkloofberg, although both are present near ash 4 in the Dundas section on Swartpunt (Fig. 5; Wood et al., Reference Wood, Poulton, Prave, Hoffmann and Clarkson2015, fig. 14).

Trace fossils have been important components of Nama Group biostratigraphy since the pioneering studies of Germs (Reference Germs1972a, Reference Germsc) and Crimes and Germs (Reference Crimes and Germs1982). Their taxonomy has been reviewed and revised by Darroch et al. (Reference Darroch, Cribb, Buatois, Germs and Kenchington2021), leading to the elimination of characteristically Phanerozoic genera such as Zoophycos and Diplocraterion. What was left are putative cnidarian resting or dwelling traces (Conichnus, Bergauria) and possible narrow, horizontal burrows (Helminthopsis) in the Kuibis Subgroup and more diverse ichnofossil assemblages in the Schwarzrand Subgroup (Darroch et al., Reference Darroch, Cribb, Buatois, Germs and Kenchington2021, fig. 18b). The only sizable, Cambrian-like traces are Streptichnus nabonnei Jensen and Runnegar, Reference Jensen and Runnegar2005 from the uppermost Spitskop Member (UCLA 7375, Fig. 5) and Parapsammichnites pretzeliformis Buatois et al., Reference Buatois, Almond, Mángano, Jensen and Germs2018 from lower in the Spitskop in the Fish River area; the rest are narrow, subhorizontal burrows or levée-lined trenches (Archaeonassa, Gordia, Helminthoidichnites, Helminthopsis) that are similar to co-occurring tubular body fossils and, when inadequately preserved, may be confused with them. Although Streptichnus is “Cambrian-like” and has been invoked to lower the eon boundary into the Spitskop Member (Linnemann et al., Reference Linnemann, Ovtcharova, Schaltegger, Gärtner and Hautmann2019), its only other known occurrence is in the Ediacaran of China (Xiao et al., Reference Xiao, Chen, Pang, Zhou and Yuan2021; Mitchell et al., Reference Mitchell, Evans, Chen and Xiao2022).

The taxon of greatest interest, first found by Germs (Reference Germs1972a, Reference Germsb), is an ichnospecies that Jensen et al. (Reference Jensen, Saylor, Gehling and Germs2000) referred to Treptichnus, but not T. pedum (Fig. 25.1, 25.2; Darroch et al., Reference Darroch, Cribb, Buatois, Germs and Kenchington2021, fig. 13). Our work reinforces this picture (Fig. 2), but we present clear examples of horizontal burrows (Gordia sp.) in the Kiphoek Member (Fig. 24.11, 24.13, 24.14), provide evidence for bioturbation down to depths of several centimeters in the Nasep and Huns members, and show that Archaeichnium haughtoni (Figs. 20, 21) is a body fossil rather than a trace fossil (Glaessner, Reference Glaessner1963; Turk et al., Reference Turk, Maloney, Laflamme and Darroch2022). We also suggest that structures (Fig. 12) that Darroch et al. (Reference Darroch, Cribb, Buatois, Germs and Kenchington2021) called “guitar strings” are the tool and mold marks of current-transported erniettomorphs—probably Pteridinium—rather than sponge wall fragments.

Chemostratigraphy

Carbonate hand samples, spaced 1 m apart where possible, were collected by MRS and BR from limestone sections measured by them and/or JGG from the Mooifontein Member on Aar, Mamba, Mooifontein, and Twyfel farms; from the Omkyk Member at Swartmodder on Omkyk; from the Urikos, Neiderhagen, and Vingerbreek members on Saurus; and from the Huns Member on Arimas and Swartkloofberg; but samples from only some of those sections were processed because of funding constraints. All isotopic data are tabulated in Supplemental dataset 1 and plotted against stratigraphic heights in Figure 2. Our results confirm previous and subsequent studies (Kaufman et al., Reference Kaufman, Hayes, Knoll and Germs1991; Saylor et al., Reference Saylor, Kaufman, Grotzinger and Urban1998; Smith, Reference Smith1999; Wood et al., Reference Wood, Poulton, Prave, Hoffmann and Clarkson2015). Overall, the record is monotonous apart from the Omkyk excursion, which stands out from background but is not well expressed elsewhere except, perhaps, South China (Bowyer et al., Reference Bowyer, Zhuravlev, Wood, Shields, Zhou, Curtis, Poulton, Condon, Yang and Zhu2022). Whether or not the negative values from the Mara and other lower Kuibis members, which Bowyer et al. (Reference Bowyer, Zhuravlev, Wood, Shields, Zhou, Curtis, Poulton, Condon, Yang and Zhu2022) term the “basal Nama excursion” (BANE), represent a post-Shuram, pre-basal Cambrian excursion (BACE) negative event of intercratonic significance is uncertain, given the paucity of other occurrences (Chai et al, Reference Chai, Wu and Hua2021; Yang et al., Reference Yang, Rooney, Condon, Li, Grazhdankin, Bowyer, Hu, Macdonald and Zhu2021). Conversely, the position of the BACE, which is missing from the Nama succession, remains unresolved. Previously, it was thought to have been eliminated by the basal Nomtsas disconformity, but recent U–Pb ages suggest other possibilities (Linnemann et al., Reference Linnemann, Ovtcharova, Schaltegger, Gärtner and Hautmann2019; Hodgin et al., Reference Hodgin, Nelson, Wall, Barron-Diaz, Webb, Schmitz, Fike, Hagadorn and Smith2021; Bowyer et al., Reference Bowyer, Zhuravlev, Wood, Shields, Zhou, Curtis, Poulton, Condon, Yang and Zhu2022; Topper et al., Reference Topper, Betts, Dorjnamjaa, Li, Li, Altanshagai, Enkhbaatar and Skovsted2022; Nelson et al., Reference Nelson, Crowley, Smith, Schwartz, Hodgin and Schmitz2023). In summary, the positive Omkyk excursion and the lower Kuibis negative intervals are the only features of the carbon isotope record of the Nama Group that are potentially useful for more than regional correlation.

Stratigraphic information

See Figures 2–5, Appendix, and the discussion under Materials and methods.

Locality information

We used the UCLA locality numbering system (e.g., UCLA 7307) for sites, occasionally at the same place in stratigraphic order, and numbered each piece of rock collected accordingly. Important fossils were then given decimal numbers corresponding to localities (e.g., 7307.1, 7307.2, etc.). Parts of an individual fossil were given the same decimal number, and different fossils on the same slab are identified by letters (e.g., 7307.3A, 7307.3B, etc.). These UCLA numbers for individual fossils were replaced by National Earth Sciences Museum numbers (GSN F) after the collection was repatriated to Namibia, but the UCLA locality numbers (e.g., UCLA 7307) still pertain. Locality details are described in the Appendix. Because this work was carried out before GPS became widely available, particularly in the Southern Hemisphere, the geographic positions of localities were plotted on photocopies of the relevant 1:50,000 scale topographic maps of South West Africa issued by the Surveyor General, Department of Justice, Government of Namibia. These map records were used recently to find the exact locations of the sites on Google Maps and to obtain their decimal longitudes and latitudes (Appendix); Google Maps uses the WGS84 standard.

Names of higher taxa

There is an emerging consensus that the majority of Ediacaran soft-bodied organisms are metazoans, but their phylum and class level assignments remain uncertain. To provide a framework for discussion, we assign taxa to some extinct higher taxa (e.g., class Archaeocyatha) and indicate under remarks and discussion where such plesions probably join the tree of life (e.g., stem Demospongiae).

Type species

Arimasia germsi n. gen. n. sp. from the Huns Member of the Urusis Formation, Schwarzrand Subgroup, Arimas farm, Namibia.

Diagnosis

As for the type species by montypy.

Etymology

Named for Arimas farm, the type locality.

Remarks

Nothing similar to Arimasia has been described from the Neoproterozoic or, so far as we are aware, from the Phanerozoic. It seems to be a one-walled, solitary, and sessile animal, preserved as a composite mold of both inner and outer surfaces, perhaps resembling an unmineralized version of a monocyathid archaeocyath or a vauxiid sponge.

Holotype

GSN F 1960H from the Huns Member of the Urusis Formation, Schwarzrand Subgroup, UCLA 7376, Arimas farm, Namibia.

Diagnosis

Centimeter-scale, porous, rugose, horn-shaped skeletons that appear to have been unmineralized or, perhaps, demineralized.

Description

The holotype (Fig. 6.1, 6.4) is a narrow, conical object, 2 cm long, that has a sealed rounded base and about eight irregular, co-marginal rugae in the lower two-thirds of the structure; the open end of the skeleton was apparently circular but is now flattened by compaction that extends downward toward the rugose part of the cone; the cone surface is evenly granular, giving the impression of a fine mesh, which cannot be fully resolved because of the finite grain size of the matrix; a paratype (Fig. 6.2, 6.3) displays the mesh more clearly; the cells are 200–300 μm apart and appear to be packed like honeycomb; other specimens are more regularly rugose (Fig. 6.6, 6.7) and eight of 10 individuals on one small slab seem to be preferentially oriented, suggesting that the cones may have been tethered to the substrate (Fig. 6.5).

Figure 6. Arimasia germsi n. gen. n. sp., Huns Member, Urusis Formation, UCLA 7326, Arimas farm. (1, 4) Holotype, GSN F 1960H, showing rugose form, the apparently porous nature of the body wall, and other individuals (GSN F 1960A, GSN F 1960B, GSN F 1960C) on the same surface. (2, 3) GSN F 1960C, also showing the porous body wall. (5) A single surface with at least 10 specimens of A. germsi, eight of which (white numerals) are opening upward in this view, and the other two (yellow numerals) are facing downward, GSN F 1954. (6) External mold of one of the largest specimens, GSN F 1958. (7) Three, possibly current-aligned, specimens, GSN F 1955. (1, 2) scale bars = 5 mm; (3) horizontal scale bar = 2 mm; inclined scale bar = 1,000 μm; (4, 6) scale bars = 1 cm; (5, 7) scale bars = 2 cm.

Etymology

Named for Gerard J.B. Germs, in celebration of the fiftieth anniversary of the publication of his groundbreaking, University of Cape Town, Ph.D. dissertation on the stratigraphy and paleontology of the lower Nama Group (Germs, Reference Germs1972a).

Remarks

Antcliffe et al. (Reference Antcliffe, Callow and Brasier2014) reviewed all of the then published reports of the oldest fossil sponges and recommended caution in making such claims. They proposed two selection criteria that should always be met and summarized a passing grade as: “The characters claimed for are useful for detecting sponges in the fossil record and have been reliably shown to be present in the particular candidate fossil.” In their opinion, the oldest fossil sponges are siliceous hexactinellid spicules from the Fortunian of Iran (also China; Chang et al., Reference Chang, Feng, Clausen and Zhang2017) and Archaeocyatha from the Tommotian (Cambrian Stage 2) of Siberia, a conclusion that has not been effectively challenged by subsequent reports of sponge-like fossils of Ediacaran or earlier ages (e.g., Turner, Reference Turner2021). Antcliffe et al. (Reference Antcliffe, Callow and Brasier2014, p. 999) also concluded that “the ancestral archaeocyathan sponges must occur in the [Fortunian] Purella antiqua Zone and would have consisted of small, simple rounded cups, each provided with a single, weakly calcified wall, perforated by simple pores.”

Arimasia germsi appears to pass these two selection criteria by having the sponge characters described in advance by Antcliffe et al. (Reference Antcliffe, Callow and Brasier2014) if our interpretation of the granular texture of the fossil is correct. The cell size of the wall mesh, 200–300 μm (Fig. 6.3), is comparable to the average interpore distance in single-walled Archaeocyatha, such as Archaeolynthus contractus Hill, Reference Hill1965 (~330 μm; Hill, Reference Hill1965, pl. 1, fig. 1), but is larger than the diameter of the pores, which in double-walled Archaeocyatha is commonly ~100 μm or less (Gravestock, Reference Gravestock1984; Antcliffe et al., Reference Antcliffe, Jessop and Daley2019). Thus, Arimasia may be viewed as an unmineralized, apparently single-walled archaeocyath, and perhaps also as a stem group demosponge, if that is where the Archaeocyatha fit into the Porifera (Antcliffe et al., Reference Antcliffe, Callow and Brasier2014). There are also possible similarities to the unmineralized vauxiid sponges, which first appear in the Cambrian Stage 3 Chenjiang biota (Wei et al., Reference Wei, Zhao, Chen, Hou and Cong2021) and are regarded by some as being on the pathway to the keratose demosponges, now thought to be the monophyletic or paraphyletic sister group of the spiculate Heteroscleromorpha (Erpenbeck et al., Reference Erpenbeck, Sutcliffe, Cook, Dietzel, Maldonado, van Soest, Hooper and Wörheide2012; Wörheide et al., Reference Wörheide, Dohrmann, Erpenbeck, Larroux, Maldonado, Voigt, Borchiellinijj and Lavrov2012; Plese et al., Reference Plese, Kenny, Rossi, Cárdenas, Schuster, Taboada, Koutsouveli and Riesgo2021).

Although there is widespread agreement that the Archaeocyatha are hypercalcified aspiculate sponges (Rowland, Reference Rowland2001; Debrenne et al., Reference Debrenne, Zhuravlev and Kruse2012; Antcliffe et al., Reference Antcliffe, Callow and Brasier2014) rather than some kind of calcified alga (Kazmierczak and Kremer, Reference Kaźmierczak and Kremer2022), their position within the poriferan total group remains so uncertain as to be almost ignored (e.g., Botting and Muir, Reference Botting and Muir2018). Arimasia may provide a way forward in that it is demonstrably older than any known spiculate sponge, was apparently unmineralized, and is similar in body form to the organic-walled vauxiids (Rigby, Reference Rigby1980, Reference Rigby1986; Botting et al., Reference Botting, Muir and Lin2013; Luo et al., Reference Luo, Zhao and Zeng2020; Wei et al., Reference Wei, Zhao, Chen, Hou and Cong2021), which Luo et al. (Reference Luo, Yang, Zhuravlev and Reitner2021) have suggested might be demineralized archaeocyaths. Alternatively, Arimasia, Vauxia, and the archaeocyaths may all have been aspiculate stem group sponges, and therefore the vauxiids are not demineralized archaeocyaths (Luo et al., Reference Luo, Yang, Zhuravlev and Reitner2021) but, instead, were unmineralized members of the lineage leading to the aspiculate demosponges. This hypothesis would require the acquisition of siliceous spicules independently in the Hexactinellida and the Demospongiae, a proposal that has been vigorously rejected by nearly all sponge paleobiologists (e.g., Botting and Muir, Reference Botting and Muir2018) but has recently received some molecular support (Aguilar-Camacho et al., Reference Aguilar-Camacho, Doonan and McCormack2019).

Type species

Pteridinium simplex Gürich, Reference Gürich1933 from the Kliphoek Member of the Dabis Formation, Kuibis Subgroup, Aus district, Namibia, by monotypy.

Other species

?Paradoxides carolinaensis St. Jean, Reference St. Jean1973, from the Floyd Church Formation, Albemarle Group, Stanly County, North Carolina, USA; Pteridinium nenoxa Keller in Keller et al., Reference Keller, Menner, Stepanov and Chumakov1974; Inkrylovia lata Fedonkin in Palij et al., Reference Palij, Posti, Fedonkin, Keller and Rozanov1979.

Diagnosis

Frondose organisms, up to at least 0.4 m long, that are made of three equal-sized organic-walled vanes that are set lengthwise about an axis that extends from the curved proximal end to the acute distal growing tip; each vane is constructed from sealed tubular modules that meet alternatively or oppositely at the axis, depending on their order about it, and are concave toward the distal end of the frond; margins of the vanes are defined by smooth, thickened edges against which the modules terminate without narrowing.

Occurrence

Kliphoek, Buchholzbrunn, and Mooifontein members of the Kuibis Subgroup and Nudaus, Nasep, Huns, and Spitskop members of the Schwarzrand Subgroup, Nama Group, Namibia (Fig. 2; Appendix); basal Ediacara Member, Rawnsley Quartzite, South Australia (Glaessner and Wade, Reference Glaessner and Wade1966; Gehling and Droser, Reference Gehling and Droser2013); Floyd Church Formation, Albemarle Group, North Carolina, USA (St. Jean, Reference St. Jean1973; Gibson et al., Reference Gibson, Teeter and Fedonkin1984; McMenamin and Weaver, Reference McMenamin and Weaver2002); Syuzma/Verkhovks member, Ust-Pinega Formation, Onega Peninsula, Russia (>552.85 ± 0.77 Ma; Keller et al., Reference Keller, Menner, Stepanov and Chumakov1974; Fedonkin, Reference Fedonkin1981; Ivantsov and Grazhdankin, Reference Ivantsov and Grazhdankin1997; Grazhdankin, Reference Grazhdankin2004; Ivantsov et al., Reference Ivantsov, Vickers-Rich, Zakrevskava and Hall2019); Shibantan Member, Dengying Formation, China (<550.1 ± 0.06 Ma; Chen et al., Reference Chen, Zhou, Xiao, Wang, Guan, Hua and Yuan2014; Xiao et al., Reference Xiao, Chen, Pang, Zhou and Yuan2021; Yang et al., Reference Yang, Rooney, Condon, Li, Grazhdankin, Bowyer, Hu, Macdonald and Zhu2021); doubtfully, Ukraine (Fedonkin, Reference Fedonkin, Velikanov, Aseeva and Fedonkin1983), Canada (Narbonne and Aitken, Reference Narbonne and Aitken1990), and Iran (Vaziri et al., Reference Vaziri, Majidifard, Darroch and Laflamme2021).

Remarks

It is not clear why Pflug's (Reference Pflug1972) class Erniettomorpha has been widely adopted in preference to his Pteridinomorpha, which has page precedence, but we follow that practice for the probable clade (Dececchi et al., Reference Dececchi, Narbonne, Greentree and Laflamme2017) that includes Pteridinium Gürich, Reference Gürich1933, Ernietta Pflug, Reference Pflug1966, Phyllozoon Jenkins and Gehling, Reference Jenkins and Gehling1978, Swartpuntia Narbonne, Saylor, and Grotzinger, Reference Narbonne, Saylor and Grotzinger1997, and perhaps Ventogyrus Ivantsov and Grazhdankin, Reference Ivantsov and Grazhdankin1997, and Miettia Hofmann and Mountjoy, Reference Hofmann and Mountjoy2010. Inkrylova lata, the type species of Inkrylova Fedonkin in Palij et al., Reference Palij, Posti, Fedonkin, Keller and Rozanov1979, appears to be a junior synonym of Pteridinium nenoxa Keller in Keller et al., Reference Keller, Menner, Stepanov and Chumakov1974. The relationships of these and other species of Pteridinium are discussed under the description of P. simplex. Pteridium Gürich, 1930 (Gürich, Reference Gürich1930b) was a nomen nudum replaced, perhaps unnecessarily, by Pteridinium Gürich, Reference Gürich1933; Onegia Sokolov, Reference Sokolov1976 is a nomen nudum applied to Keller's species nenoxa by Sokolov (Reference Sokolov1976) and Grazhdankin (Reference Grazhdankin2004). As discussed under the genus Ernietta Pflug, Reference Pflug1966, the holotype of the type species, E. plateauensis Pflug, Reference Pflug1966, appears to be a specimen of Pteridinium simplex, so technically Ernietta becomes a subjective junior synonym of Pteridinium. However, we propose that an application be made to the International Commission on Zoological Nomenclature (ICZN) to replace the holotype of E. plateauensis with a neotype, the holotype of Erniograndis sandalix Pflug, Reference Pflug1972, to retain current usage of this well-established name.

Neotype

Gürich's (Reference Gürich1933) specimens were lost during World War II so Richter (Reference Richter1955) nominated a neotype, a sandstone cast of part of a frond with two visible vanes (SMNH XXX 660f), probably from the Kliphoek Member, Kuibis Formation, on Plateau or Aar farm, Aus district, Namibia (Richter, Reference Richter1955, pl. 1, fig. 1a, b; Darroch et al., Reference Darroch, Gibson, Syversen, Rahman, Racicot, Dunn, Gutarra, Schindler, Wehrmann and Laflamme2022, fig. 1a, b).

Diagnosis

A “three-vaned, ribbon-like frond” (Ivantsov et al., Reference Ivantsov, Narbonne, Trusler, Greentree and Vickers-Rich2016, p. 540) in which the modules are less well expressed in the outer halves of the vanes.

Description

Elongate, frondose organisms formed of three equal-sized, undivided vanes that radiate from a common axis and may exceed 0.4 m in length without signs of expansion or tapering in width; vanes are composed of curved to straight tubular modules that are commonly, but not always, more topographically expressed near the axis than the periphery; modules maintain a similar-sized cross section across the vane and terminate abruptly at the distal margins; vanes terminate axially in closed, polyhedral ends that either alternate with those of other modules in a zig-zag fashion or are directly opposed, depending on position around the axis (Fig. 8.8–8.10; Runnegar, Reference Runnegar2022, figs. 9, 10); in two specimens with visibly narrowing vanes, the curvature of the modules is convex in the direction of narrowing and the angle of narrowing is 10° or less (Fig. 11.1, 11.2; Richter, Reference Richter1955, p. 249, pl. 6, fig. 7; Runnegar, Reference Runnegar2022, fig. 9a); whether the tapering of the vanes is unidirectional or bidirectional is unknown; vane margins are commonly obscure but when well preserved are delineated by a narrow differentiated edge (Figs. 9.2–9.4, 10.3) that was stiff enough to imprint other individuals (Fig. 7.3–7.5); vane curvature generally coaxial but inconsistent; two of the vanes are frequently opposite each other at the axis and either lie parallel to bedding or curve quasi-symmetrically to partially embrace the third vane (Fig. 11.6–11.8), thus producing W-shaped cross sections (Fig. 7.5).

Figure 7. Pteridinium simplex Gürich, Reference Gürich1933, Aarhauser sub-member, Kliphoek Member, Dabis Formation, UCLA 7307, Aar farm. (1) A 25 cm thick bed of micaceous quartz sandstone overlain by dislodged joint blocks of the bed, many of which are fossiliferous. (2) Overturned piece of the same bed showing current scoured base. (3) Reassembled pieces of one joint block that contains two subparallel specimens of P. simplex, GSN F 1855A and GSN F 1855B, each composed of three vanes, A1, A2, A3, etc., lying with their axes parallel to bedding. (4) Two parts of the same block, viewed perpendicular to bedding, with the upper edges of vanes A1 and A3 indicated by white and black arrows, respectively. (5) Foreshortened oblique view of the reassembled block showing cross sections of vanes A1 and A3 on the sawn surface. (6) Lateral view of vane B3 with its lower edge parallel to bedding. (7) End piece viewed from the top to show the relative positions of vanes A3, B2, and B3. (8) Sawn edge of the end piece in (7) showing the cross-sectional curvature of vanes A1, A3, and B1. (1) Camera lens cover = 60 mm; (2) brush = 25 cm; (3, 4) scale bar = 5 cm; (5) scale bar = 3 cm but variable scales due to foreshortening; (6–8) scale bar = 3 cm.

Materials

Seven specimens (GSN F 1853–1859) from UCLA 7307; ~20 plaster casts of SMSWA specimens; ~20 specimens, mostly from Plateau and Aar farms in the SMNH collection (Richter, Reference Richter1955); the Pflug (Reference Pflug1970a) collection, plus numerous examples observed in the field at Aar farm and in the “museum” at Plateau farm, including the excavation and casting of the Seilacher slab and other specimens in 1993 (Fig. 10.1, 10.2; Crimes and Fedonkin, Reference Crimes and Fedonkin1996; Seilacher, Reference Seilacher1997, Reference Seilacher2007; McMenamin, Reference McMenamin1998; Grazhdankin and Seilacher, Reference Grazhdankin and Seilacher2002; Ivantsov et al., Reference Ivantsov, Vickers-Rich, Zakrevskava and Hall2019).

Taphonomy

The numerous specimens that have been observed, extracted, and studied from the Amphitheatre site (UCLA 7307) on Aar farm (Figs. 3, 7–9, 10.1–10.4, 11.1–11.3; Richter, Reference Richter1955; Grazhdankin and Seilacher, Reference Grazhdankin and Seilacher2002; Vickers-Rich, Reference Vickers-Rich, Fedonkin, Gehling, Grey, Narbonne and Vickers-Rich2007; Meyer et al., Reference Meyer, Elliott, Wood, Polys and Colbert2014a, Reference Meyer, Elliott, Schiffbauer, Hall, Hoffman, Schneider, Vickers-Rich and Xiaob, Darroch et al., Reference Darroch, Gibson, Syversen, Rahman, Racicot, Dunn, Gutarra, Schindler, Wehrmann and Laflamme2022) are all from a set of quartz sandstone beds that have been named the Aarhauser sandstone submember by Hall et al. (Reference Hall, Kaufman, Vickers-Rich, Ivantsov and Trusler2013) (Fig. 3). The bed that was the source of the Seilacher slab is about 0.4 m thick, has a deeply erosive base (Fig. 7.2), a horizontally bedded to low-angle cross-stratified upper part (Fig. 8.1, 8.2, 8.4), a middle zone with cylindrical specimens of Pteridinium (Fig. 10.4; Crimes and Fedonkin, Reference Crimes and Fedonkin1996, pl. 2c, d), and a thicker, poorly laminated lower part that is richly fossiliferous. P. simplex is widespread in the laminated to upper cross-stratified part, typically as long, straight segments of two-vaned fronds seen in plan view on bed surfaces, as W-shaped intersections on east–west joints, and as upright, stretched single vanes on the faces of north–south joints (Figs. 8.1, 8.2, 9.2–9.4; Darroch et al., Reference Darroch, Gibson, Syversen, Rahman, Racicot, Dunn, Gutarra, Schindler, Wehrmann and Laflamme2022, fig. 5). Almost invariably, the axes of vanes seen on joint faces lie parallel to bedding and are commonly at the bottom of the vanes, even when the whole organism is twisted through 180° about a horizontal axis (Figs. 8.1, 8.2, 8.4, 8.6, 8.7, 9.6, 11.6–11.8). There is no evidence that the upright middle vane, the “chaperone wall” of Grazhdankin and Seilacher (Reference Grazhdankin and Seilacher2002), routinely switches places with one of the other vanes during the 180° folding, as shown in the sketch (Fig. 8.5) from Grazhdankin and Seilacher (Reference Grazhdankin and Seilacher2002), as previously noted by Meyer et al. (Reference Meyer, Elliott, Wood, Polys and Colbert2014a). That would require improbable twisting about two different rotational axes. The other common U-turn is a hairpin bend (Figs. 9.5, 9.7, 9.8, 19.1, 19.2; Richter, Reference Richter1955, pl. 7, fig. 11; Vickers-Rich, Reference Vickers-Rich, Fedonkin, Gehling, Grey, Narbonne and Vickers-Rich2007, fig. 108; Meyer et al., Reference Meyer, Elliott, Wood, Polys and Colbert2014a, figs. 3–6), in which the fold axis is vertical rather than horizontal. One such U-turn, found in situ in 1993 (Fig. 9.5, 9.7, 9.8), was at the upstream end of the hairpin-shaped fossil, as shown by the orientations and of 10 flat-lying specimens measured on the top surface of the same outcrop (Fig. 3). Thus, most if not all of the examples of P. simplex found in the upper, laminated to cross-laminated layers of the Aarhauser sandstone appear to have been transported by northward-flowing high-velocity currents, as suggested previously (Jenkins, Reference Jenkins1985; Elliott et al., Reference Elliott, Vickers-Rich, Trusler and Hall2011; Darroch et al., Reference Darroch, Gibson, Syversen, Rahman, Racicot, Dunn, Gutarra, Schindler, Wehrmann and Laflamme2022).

Figure 8. Folded specimens of Pteridinium simplex Gürich, Reference Gürich1933, Aarhauser sub-member, Kliphoek Member, Dabis Formation, UCLA 7307, Aar farm. (1) Field photograph of a dislodged but upright 20 cm thick joint block with a specimen of P. simplex folded about a horizontal axis (white dot with circle around it) and with longer upper part of the organism extending downstream. (2) Enlargement of (1) to show details of the limbs. (3) Field photograph of base of block excavated by the Seilacher team in 1993 (Seilacher, Reference Seilacher1997) showing two similarly folded specimens of P. simplex, GSN F 758 and GSN F 576 (arrows indicate positions of horizontal fold axes), which became the basis for the “canoe” model for Pteridinium (Grazhdankin and Seilacher, Reference Grazhdankin and Seilacher2002). (4) Field photograph of another dislodged joint block showing prominent horizontal lamination and one vane of a P. simplex that is tightly folded about a horizontal axis (white dot with circle around it). (5) Part of drawing (Grazhdankin and Seilacher, Reference Grazhdankin and Seilacher2002, fig. 5C, republished with permission) used to explain both the canoe and vane substitution models for the growth of Pteridinium; note that vane substitution requires both twisting through 180° and folding about a horizontal axis. (6) Oblique view of small joint block that has been broken and sawn to reveal details of the kind of folds seen in (1, 2, 4), with the fold axis indicated by the arrows (GSN F 1857). (7) Same specimen as (6), lateral view. (8–10) Weathered fragment (GSN F 1856) that shows how the proximal ends of the three vanes, V1, V2, and V3, interlock; the modules of vanes V1 and V2 are opposite each other, whereas the modules of each alternate with those of V3 (9, 10). (1) Brush = 25 cm; (3, 4) camera lens cover = 60 mm; (6, 7) scale bar = 5 cm; (8–10) scale bar = 1 cm.

Figure 9. Field and other images of a U-shaped specimen of Pteridinium simplex Gürich, Reference Gürich1933, GSN F 1858, that had been exposed by excavation during or before 1993. (1) U-shaped end piece (5, 7, 8) in place and with the trailing vanes, V1 and V2, extending northward in the direction of downstream transport; an extracted piece with part of V1 (2–4) is in the foreground. (2–4) Field images of V1 that show the modules leaning downstream and the linear distal edge of the vane. (5) U-turn with axis (AX) at periphery. (6) Plaster cast of a specimen folded like a taco about a horizontal axis, SMSWA 45730.1 now GSN F 1878. (7, 8) Three parts of the U-bend showing the curvature of the axis and the positions and orientations of V1 and V2 on both sides of the turn (downstream is toward the bottom of the page). (1) Hammer = 33 cm; (3) coin = 23 mm; (4) scale bar = 2 cm; (5–8) scale bar = 3 cm.

A middle zone of closely packed tubular fossils is more problematical but is almost certainly a death association, perhaps created by close packing of enrolled, hairpin-shaped individuals (Fig. 10.4) rather than a population of sealed underground sausage-shaped organisms, as envisaged by Crimes and Fedonkin (Reference Crimes and Fedonkin1996). As the horizon is less accessible, it has not been well studied.

Figure 10. (1–4) Joint blocks of Aarhauser sandstone member, Aar farm (UCLA 7307), that had been split approximately in half parallel to bedding to reveal the lower sides of a large number of specimens of Pteridinium simplex Gürich, Reference Gürich1933 and then reassembled upside down for molding with silicone rubber by the Seilacher team (Seilacher, Reference Seilacher1997). (1) Dolf Seilacher, second from left, with Mark McMenamin, Hans Luginsland, and Peter Seilacher viewing the “Seilacher slab” ready for molding, August 1993. (2) Richly fossiliferous two-thirds of the Seilacher slab (Seilacher, Reference Seilacher1997, Reference Seilacher2007, Reference Seilacher2008), which inspired the “bathtub” or canoe models for Pteridinium living underground. (3) Seven aligned and four closely packed specimens of P. simplex seen in upper left corner of (2); in cross section, those in contact would resemble tubes. (4) Top of one of the joint blocks showing that the third vane (V3) may be underneath a pair of vanes (V1 and V2) exposed on the surface of the bed. (5) An in situ specimen of Pteridinium carolinaensis (St. Jean, Reference St. Jean1973), Spitskop Member, UCLA 7373, Dundas Hill, Swartpunt farm. (2, 3, 5) Camera lens cover = 60 mm; (4) scale bar = 5 cm.

The lower layers are filled with P. simplex preserved in a somewhat different fashion, as can be seen from an extracted block in the Richter collection (Darroch et al., Reference Darroch, Gibson, Syversen, Rahman, Racicot, Dunn, Gutarra, Schindler, Wehrmann and Laflamme2022, fig. 2). The lower part of this block, a similar specimen figured by Glaessner (Reference Glaessner, Robison and Teichert1979b, fig. 11.1b), and the far more extensive Seilacher slab (Fig. 10.1–10.4; Seilacher, Reference Seilacher1997, Reference Seilacher2007; Grazhdankin and Seilacher, Reference Grazhdankin and Seilacher2002) have doubly curved fronds that—to some—resemble inverted bathtubs or boats (Figs. 8.3, 9.2; Grazhdankin and Seilacher, Reference Grazhdankin and Seilacher2002, text-fig. 1). These are the shapes that have given rise to the canoe model for Pteridinium (Pflug, Reference Pflug1970a; Buss and Seilacher, Reference Buss and Seilacher1994; Ivantsov and Grazhdankin, Reference Ivantsov and Grazhdankin1997; Grazhdankin and Seilacher, Reference Grazhdankin and Seilacher2002; Meyer et al., Reference Meyer, Elliott, Schiffbauer, Hall, Hoffman, Schneider, Vickers-Rich and Xiao2014b; Droser et al., Reference Droser, Tarhan and Gehling2017; Darroch et al., Reference Darroch, Gibson, Syversen, Rahman, Racicot, Dunn, Gutarra, Schindler, Wehrmann and Laflamme2022) and to the hypothesis that Pteridinium lived on, in, or wholly within the sediment (Crimes and Fedonkin, Reference Crimes and Fedonkin1996; Seilacher, Reference Seilacher1997, Reference Seilacher2007; Grazhdankin and Seilacher, Reference Grazhdankin and Seilacher2002; Darroch et al., Reference Darroch, Gibson, Syversen, Rahman, Racicot, Dunn, Gutarra, Schindler, Wehrmann and Laflamme2022). However, these lower layers also preserve specimens that are folded like tacos. In these cases, two oppositely directed vanes are bent through 180° about a horizontal axis, and for geometrical reasons, the third vane must follow one of the other two (Fig. 9.6). This cannot be a life orientation, so the fact that this postmortem topology is found among the canoes is evidence that all were transported before burial. As no one has described or illustrated convergence of the three vanes to form the “prow” or “stern” regions of any specimen of P. simplex, the canoe hypothesis is not supported by observation. Thus, we consider all of the material in the Aarhauser sandstone to have been transported by high-energy events. In this context, the experiments in computational fluid dynamics carried out by Darroch et al. (Reference Darroch, Gibson, Syversen, Rahman, Racicot, Dunn, Gutarra, Schindler, Wehrmann and Laflamme2022) may help understand the fact that in the laminated upper part of the Aarhauser sandstone, the horizontal laminae intersect the vertical or steeply inclined vanes with no trace of edge effects (Figs. 8.2, 8.4, 9.2–9.4; Crimes and Fedonkin, Reference Crimes and Fedonkin1996; Elliott et al., Reference Elliott, Vickers-Rich, Trusler and Hall2011; Darroch et al., Reference Darroch, Gibson, Syversen, Rahman, Racicot, Dunn, Gutarra, Schindler, Wehrmann and Laflamme2022). This would be possible if the sediment were moving by laminar rather than turbulent flow when transport is parallel to the vanes, as shown by the calculated streamlines (Darroch et al., Reference Darroch, Gibson, Syversen, Rahman, Racicot, Dunn, Gutarra, Schindler, Wehrmann and Laflamme2022, fig. 10C). In this scenario, the only individuals to be trapped and buried were those that were concave enough to receive and retain sediment; presumably, all of the rest were blown away like discarded plastic shopping bags in the surf (see artwork by John D. Dawson in Monastersky and Mazzatenta, Reference Monastersky and Mazzatenta1998). Additional support for frequent transport may come from widespread bed base rake and bump structures (Fig. 12), which we interpret as tool marks generated by Pteridinium or another erniettomorph.

Remarks

Pteridinium is an uncommon fossil except at Aar and Plateau farms. It is, therefore, difficult to find populations large enough to compare with P. simplex. The next most frequent occurrences are from localities on the Onega Peninsula, Russia, but there the fossils are fragmentary and frequently deformed (Keller et al., Reference Keller, Menner, Stepanov and Chumakov1974; Fedonkin, Reference Fedonkin1981, Reference Fedonkin, Sokolov and Iwanowski1985; Palij et al., Reference Palij, Posti, Fedonkin, Urbanek and Rozanov1983; Ivantsov and Grazhdankin, Reference Ivantsov and Grazhdankin1997; Grazhdankin, Reference Grazhdankin2004). Fortunately, the second species of Pteridinium to be described, P. carolinaensis (St. Jean, Reference St. Jean1973), seems to differ markedly from P. simplex, so we begin by examining the binary choice, simplex or carolinaensis? If all known specimens of Pteridinium can be comfortably referred to one of these two species, then this interim solution may serve until more quantitative information becomes available. If there are numerous intermediates that cannot be so allocated, then perhaps P. simplex should serve as the only known species of Pteridinium for the time being.

In a careful study of 18 specimens of carolinaensis and 25 specimens of simplex using modern morphometric methods, Meyer (Reference Meyer2010) and Meyer and Xiao (Reference Meyer and Xiao2010) were unable to find any statistically significant differences between these two species on the basis of a landmark analysis designed to capture variability in size, vane shape, and module curvature. They therefore suggested that P. carolinaensis is a junior synonym of P. simplex. However, they could not incorporate rarely preserved features of the fossils, such as overall frond size and shape, in their analysis because these features are seen in too few specimens.

Perhaps the most striking feature of the seven specimens of P. carolinaenis that have been illustrated (St. Jean, Reference St. Jean1973; Gibson et al., Reference Gibson, Teeter and Fedonkin1984; McMenamin and Weaver, Reference McMenamin and Weaver2002; Gibson and Teeter, Reference Gibson and Teeter2011) is that six show terminations, even in specimens comparable in size (~20 cm) to many examples of P. simplex. Similar terminations are known from one specimen from the Spitskop Member (UCLA 7373) identified as P. carolinaensis by Narbonne et al. (Reference Narbonne, Saylor and Grotzinger1997) and from one of the ~50 then-known specimens of P. nenoxa Keller in Keller et al.,Reference Keller, Menner, Stepanov and Chumakov1974 (Fedonkin, Reference Fedonkin1981, pl. 5, fig. 2), but not from any of the numerous specimens of P. simplex. Whether this is a demographic difference is difficult to assess because all of the specimens in the Aarhauser sandstone at Aar could, conceivably, be members of a single long-lived cohort. Given the differences in frond size, module curvature, and module expression across the vanes, we continue to treat simplex and carolinaensis as separate species. P. nenoxa shares those characteristics with P. carolinaensis rather than P. simplex (Fedonkin, Reference Fedonkin, Sokolov and Iwanowski1985), as do specimens of Pteridinium from Bed A (UCLA 7373) at Swartpunt farm (Figs. 10.5, 11.4, 11.5; Narbonne et al., Reference Narbonne, Saylor and Grotzinger1997; Darroch et al., Reference Darroch, Gibson, Syversen, Rahman, Racicot, Dunn, Gutarra, Schindler, Wehrmann and Laflamme2022, fig. 7), so we follow others in considering nenoxa to be a junior synonym of carolinaensis (Runnegar and Fedonkin, Reference Runnegar, Fedonkin, Schopf and Klein1992; Narbonne et al., Reference Narbonne, Saylor and Grotzinger1997; McMenamin and Weaver, Reference McMenamin and Weaver2002; Fedonkin et al., Reference Fedonkin, Gehling, Grey, Narbonne and Vickers-Rich2007). Inkrylovia lata Fedonkin in Palij et al., Reference Palij, Posti, Fedonkin, Keller and Rozanov1979 is probably a preservational variant of nenoxa resulting from expansion of the modules parallel to the axis as a result of soft sediment loading.

Figure 11. (1–3, 6–8) Pteridinium simplex Gürich, Reference Gürich1933, Aarhauser sub-member, Kliphoek Member, Dabis Formation, UCLA 7307, Aar farm. (4, 5, 9–13) Other occurrences of Pteridinium in southern Namibia. (1, 2) An unusual specimen of P. simplex that tapers proximally (right), Plateau farm collection, 1996, UCLA 7327.3 (plaster cast). (3) Severely weathered block of horizontally bedded sandstone with a fragment of one wide vane that preserves some of the distal edge, GSN 1859. (4) Vertically oriented vane of Pteridinium carolinaenis (St. Jean, Reference St. Jean1973) for comparison with (3), GSN F 250, Spitskop Member, Urusis Formation, Dundas, Swartpunt farm, southern Namibia, photographed in Kingston, Canada, 1998. (5) Another specimen of P. carolinaensis from the same locality, GSN F 248, showing the distal edge of one vane well, photographed in Kingston, Canada, 1998. (6, 7) Field photographs, taken on Aar farm by Louis Mazzatenta in 1996, of a tightly folded and twisted specimen of P. simplex, with and without a removable piece, GSN F 1854 (8), that preserves vanes V2 and V3. (8) Another view of GSN F 1854 showing a configuration that has the topology implied by Grazhdankin and Seilacher's (Reference Grazhdankin and Seilacher2002) vane substitution hypothesis (Fig. 8.5). (9) Pteridinium cf. P. carolinaensis (St. Jean, Reference St. Jean1973), plaster cast of SMSWA 45731 now GSN F 1905, “Uit Schwarzkalk” (Mooifontein Member, Zaris Formation), Kosis farm, near Helmeringhausen. (10, 12, 13) Pteridinium sp., three specimens, GSN F 1901, GSN F 1899, GSN F 1900, respectively, from UCLA 7320, Neiderhagen Member, Nundas Formation, Kyffhauser farm, that may be preservational variants of P. simplex. (11) Pteridinium sp., UCLA 7315, shale immediately below Mooinfontein Member, Buchholzbrunn Member, Dabis Formation, Namaland district, near Bethanien, showing axis with two vanes partly obscured by overfolding of another vane or individual, GSN F 1891. (1) Camera lens cover = 60 mm; (2) scale bar = 5 cm; (3, 10, 11) scale bars = 5 cm; (4) scale bar = 2 cm; (5) scale bar = 1 cm; (6–8, 12, 13) scale bars = 2 cm; (9) scale bar = 3 cm.

Holotype

Cast of distal end of frond (NCSM 4041; previously UNC 3062) from the Floyd Church Formation, Albemarle Group, Island Creek, Stanly County, North Carolina, USA (St. Jean, Reference St. Jean1973, pl. 3A; Gibson et al., Reference Gibson, Teeter and Fedonkin1984, fig. 6; McMenamin and Weaver, Reference McMenamin and Weaver2002, fig. 2; Weaver and Ganis, Reference Weaver, Ganis, Hibbard and Pollock2013, fig. 4A), by original designation.

Diagnosis

A three-vaned, leaf-like frond in which the modules are well expressed across the whole width of the vanes.

Description

Frondose organisms apparently formed of three equal-sized, undivided vanes that radiate from a common axis; vane shape is approximately aerodynamic, with a pair of opposed vanes of larger specimens subtending an angle of ~20° at the distal end of the frond and rounded at the proximal end; vanes are composed of curved tubular modules that commonly taper in width across the vanes and terminate abruptly at the distal margins; vanes terminate axially in polygonal ends that alternate with those of other modules in a zig-zag fashion, so far as can be determined; when well preserved, vane margins are delineated by a narrow differentiated edge; vane curvature low to negligible, largely for taphonomic reasons; module curvature within vanes is convex toward the presumed proximal end of the frond (Fig. 10.5) and concave toward the opposite end (St. Jean, Reference St. Jean1973; Gibson et al., Reference Gibson, Teeter and Fedonkin1984); consistency of vane curvature throughout suggests that the organism grew in only one direction.

Materials

Five reasonably complete specimens observed in the field at Dundas Hill, Swartpunt farm, three specimens described and illustrated by Narbonne et al. (Reference Narbonne, Saylor and Grotzinger1997), which were examined at Queen's University, and plaster casts of several specimens from White Sea localities kindly supplied by M.A. Fedonkin and the Museum of Paleontology, University of California, Berkeley.

Taphonomy

All specimens of P. carolinaensis from the peri-Gondwanan Carolina terrane are from float (Meyer, Reference Meyer2010) so that only the nature of the sediments that enclosed them is known. The environment of deposition is thought to have been shallow marine adjacent to a volcanic arc, but there is little to indicate whether the fossils were preserved in place or transported before burial. According to Ivantsov and Grazhdankin (Reference Ivantsov and Grazhdankin1997), the White Sea specimens of P. nenoxa are preserved in the Nama manner, and one actually stands vertically in the sediment (Fedonkin, Reference Fedonkin1981, pl. 29, fig. 2, Reference Fedonkin, Sokolov and Iwanowski1985, pl. 11, fig. 1, Reference Fedonkin, Lipps and Signor1992, fig. 26), although it is unclear why. The fossils are mostly biconvex, ovoid when viewed from below, and are preserved in sandstone that filled broad, erosive-based channels, similar to those seen in the Buchholzbrunn Member.

Remarks

If the Peridinium from the Spitskop Member on Swartpunt farm is correctly identified as P. carolinaensis, then one specimen (Fig. 10.5) preserves the proximal end of the frond, as noted by Narbonne et al. (Reference Narbonne, Saylor and Grotzinger1997). The holotype, paratype, Rock Hole Creek, and Gleaning Mission Church specimens preserve the distal end of the fronds (Gibson et al., Reference Gibson, Teeter and Fedonkin1984, fig. 5; McMenamin and Weaver, Reference McMenamin and Weaver2002, fig. 4; Gibson and Teeter, Reference Gibson and Teeter2011), and a small specimen from the Gleaning Mission Church has both (McMenamin and Weaver, Reference McMenamin and Weaver2002, fig. 5). These fossils show that the vanes of P. carolinaensis narrowed toward each end of the frond, but the curvature of the modules and overall shape of the frond remained unidirectional throughout growth. If P. simplex followed the same pattern of growth, then the specimen shown in Figure 11.1, 11.2 represents the proximal part of the frond, not its growing end.

Remarks

Three specimens from the Nudaus Formation on Kyffhauser farm, north of the Osis ridge (Fig. 11.10, 11.12, 11.13), and one from the Buchholzbrunn Member, Namaland, west of Bethanie (Fig. 11.11) are referred to Pteridinium but not easily to either P. simplex or P. carolinaensis because of the narrowness of their modules.

Description

Marks on sandstone bed bases produced by a comb- or rake-shaped tool that had ~30 tines, each capable of producing rounded grooves, narrow channels, or pairs of closely spaced parallel scratches that are typically 2–5 mm apart. In one case, the paired grooves form a chevron-like pattern (Fig. 12.2).

Remarks

Three examples, two from the Arimas (Fig. 12.3, 12.5A) and one from Kyffhauser (Fig. 12.4), are clearly erniettomorph in character. Presumably, they are impressions of parts of bodies or vanes thrust against the underlying eroded surface by the channel-filling sand that accompanied the transported bodies. In other cases, the tools merely raked or lightly scraped the surface. We interpret the rake marks as being due to the axes of Pteridinium fronds, rather than to bodies of Ernietta, because of the need for a wide head to the rake and because these structures extend well beyond the known stratigraphic range of Ernietta (Fig. 2; Darroch et al., Reference Darroch, Cribb, Buatois, Germs and Kenchington2021). Comparable tool marks have been reported from the Ingletonian of Yorkshire (Rayner, Reference Rayner1957), the Ordovician of Ohio (Osgood, Reference Osgood1970), the Silurian of Scotland (Trewin, Reference Trewin1979), and the Ordovician of Estonia (Vinn and Toom, Reference Vinn and Toom2016). Some are attributed to rolling hard fossils, such as crinoids and corals, but Rayner (Reference Rayner1957) and Trewin (Reference Trewin1979) were able to substantiate rake marks produced by graptolite stipes and their thecae, which served as the tines. These rake marks—if correctly interpreted—prove the presence of Pteridinium in the absence of body fossils, show that premortem transport was ubiquitous, and imply that the axes of the fronds were as stiff as graptolite stipes. The properties of the bump marks rule out both a spicular sponge source and a non-biological origin for these structures (Darroch et al., Reference Darroch, Cribb, Buatois, Germs and Kenchington2021). Tool marks attributed to Pteridinium were described by Fedonkin (Reference Fedonkin1976) and Fedonkin in Palij et al. (Reference Palij, Posti, Fedonkin, Urbanek and Rozanov1983) as the trace fossil Suzmites volutatus Fedonkin, Reference Fedonkin1976. Whether this name should be applied to the Namibian structures remains a matter for future investigation.

Figure 12. Tool and impact marks presumably left by erniettomorphs on the bases of flat, undulating, and incised sandstone beds, Huns (2, 3, 5, 7, 8, 10, 11) and Feldschuhhorn (1, 6) members, Urusis Formation and Neiderhagen Member, Nudaus Formation (4) on Arimas (UCLA 7309, 7326), Swartkloofberg (UCLA 7323) and Kyffhauser (UCLA 7320) farms, plus a vane of Nasepia altae Germs, 1972 from Arimas farm (9). (1) Broad comb-like toolmark, field photograph, Swartkloofberg, 1996. (2) Unique bilaterally symmetrical chevron-shaped tool mark, GSN F 1933. (3) Field photograph of an impact cast attributable to Pteridinium. (4) Lower surface and cross section of a shovel-shaped gutter cast, found by D.E. Erwin in 1995, showing Pteridinium-like impact mark on one side (arrow and insert), GSN F 1948. (5) Lower surface of large slab, left in field, showing a Pteridinium-like impact cast (arrow A and insert) and obscure impressions of several co-aligned specimens of Archaeichnium (arrow B), field photograph, 1996. (6) Hand specimen from same site as (1) showing similar comb marks, GSN F 1936. (7) Base of thin sandstone with evenly spaced bifid comb marks, GSN F 1924. (8) Another thin sandstone base with several sets of comb marks, one of which resembles the evenly spaced, bifid scratches of (7), field photograph, 1996. (9) Probable vane of Nasepia altae Germs, 1972 from the type locality but preserved in sandstone rather than limestone conglomerate (Fig. 14.4), GSN F 1909. (10) Third example of evenly spaced bifid comb marks, GSN F 1926. (11) Deep gouge mark on base of sandstone bed, which may or may not have been produced by a biological agent, GSN F 1932. (1) Comb approximately 3 cm wide; (2) scale bar = 1 cm; (3) coin = 25 mm; (4, 6, 7, 9–11) scale bars = 2 cm; (5) camera lens cap = 60 mm; (8) coin = 24 mm.

Type species

Swartpuntia germsi Narbonne, Saylor, and Grotzinger, Reference Narbonne, Saylor and Grotzinger1997 from the Spitskop Member of the Urusis Formation, Schwarzrand Subgroup, Swartpunt farm, Witputs district, Namibia, by original designation and monotypy.

Other species

None.

Holotype

Incomplete frond displaying parts of three vanes, one of which appears to have both upper and lower surfaces preserved, GSN F 238-H, from fossil bed B, Spitskop Member, Urusis Formation, Schwarzrand Subgroup, Dundas Hill, Swartpunt farm, southern Namibia.

Description

Cardioid to heart-shaped frond (Fig. 13.1), formed from at least three equal-sized vanes (Fig. 13.3) that are attached to a voluminous, knobbly axial structure (Fig. 14.6) so that the 50–100 narrow, tubular modules forming the distal parts of the vanes meet the axial structure at an angle of about 45°; those closer to the proximal end meet it nearly perpendicularly or even obtusely, and there is no evidence for a stem or stalk (Fig. 14.1, 14.8); confusion about the number of vanes arises because some appear to have been filled with fine sediment (Fig. 13.3; Narbonne et al., Reference Narbonne, Saylor and Grotzinger1997, figs. 6, 7); in other cases, upper and lower surfaces of the vanes may be superimposed by composite molding so that the spacing of the modules may be halved.

Figure 13. Swartpuntia germsi Narbonne, Saylor, and Grotzinger, Reference Narbonne, Saylor and Grotzinger1997 from beds A, UCLA 7373 (2) and B, UCLA 7374 (1, 3, 4) of Narbonne et al. (Reference Narbonne, Saylor and Grotzinger1997), Spitskop Member, Urusis Formation, Dundas Hill, Swartpunt farm. (1) Paratype, GSN F 423, showing the cardioid shape of the vanes, no evidence of a stem, and preservation of the vane surfaces on at least three levels, photographed in Kingston, Canada, in 1998. (2) GSN F 1886, upper surface of bed and partly overlapped by a specimen of Pteridinium carolinaensis (St. Jean, Reference St. Jean1973). (3) Topotype, GSN F 1887, showing preservation of three (V1–V3) or possibly four vanes if VI* is not just the other surface of vane V1. (4) Paratype, GSN F 245, part and counterpart, showing no sign of a stem but clear evidence for three vanes, as illustrated by Narbonne et al. (Reference Narbonne, Saylor and Grotzinger1997, fig. 9.2), photographed in Kingston, Canada, in 1998. (1) Scale bar = 3 cm and loonie = 26.5 mm; (2) scale bar = 5 cm; (3, 4) scale bars = 3 cm.

Figure 14. Swartpuntia germsi Narbonne, Saylor, and Grotzinger, Reference Narbonne, Saylor and Grotzinger1997 from bed B, UCLA 7374 (1, 2, 6–8) Spitskop Member, Urusis Formation, Dundas Hill, Swartpunt farm, and UCLA 7376, top of Huns Member, Urusis Formation, Swartkloofberg farm (3), plus a paratype of Nasepia altae Germs, 1972 (4) and the “Arimas lycopod” (5), both from UCLA 7326, Huns Member, Urusis Formation, Arimas farm (5). (1, 2, 8) Three views of a three-dimensionally preserved specimen of S. germsi, GSN F 1888, which exposes the proximal parts of the frond folded through about 90° and displaying no evidence for a stem; arrows in (8) mark the edges of one vane; (2) is flipped horizontally to serve as a mirror image of (1). (3) GSN F 1890, stratigraphically oldest known specimen of Swartpuntia, found by M.L. Droser in 1996, preserved in silt-sized carbonate, with axis presumably embedded in the counterpart. (4) Paratype of Nasepia altae Germs, 1972 ISAM K1086, showing distal edge of one vane embedded in a limy matrix that includes rounded limestone clasts (arrow), photographed in Cape Town, South Africa in 1993. (5) The “Arimas lycopod” (enlarged in insert), GSN F 1910A, found by JGG in 1996, may be the decorticated axis judging from circumstantial evidence; the organization of its diagonal arrays of “leaf scars,” analogous to those seen in lycopods, resembles that of the axial nodes of Swartpuntia, which are arranged in a similar fashion (6, 7, arrows). (6, 7) Topotype GSN F 1889 and paratype GSN F 247 (after Narbonne et al., Reference Narbonne, Saylor and Grotzinger1997, fig. 10, republished with permission), of S. germsi that have well-preserved axial nodes. Scale bars = 2 cm except black bar in insert of (5) = 1 cm.

Materials

Four specimens from UCLA 7374 (GSN F 1886–1889) and one from UCLA 7376 (GSN F 1890) on Swartpunt and Swartkloofberg farms, respectively.

Remarks

Swartpuntia was originally reconstructed as a vertically oriented, three-vaned frond supported by a stout cylindrical stem that was attached to an unseen holdfast in the sediment (Narbonne et al., Reference Narbonne, Saylor and Grotzinger1997, fig. 11; Narbonne, Reference Narbonne1998, fig. 1). Only the holotype was thought to show evidence for a distinct stem, but the feature interpreted as the stem (Narbonne et al., Reference Narbonne, Saylor and Grotzinger1997, figs. 6, 7) may well be just an elevated section of the matrix. Hoyal Cuthill (Reference Hoyal Cuthill2022, p. 1211) wrote: “It is notable . . . that the stalk originally described in Swartpuntia (Narbonne et al. Reference Narbonne, Saylor and Grotzinger1997) is not clearly visible even in the classic Namibian material.” We attempted to investigate this problem by collecting an in situ specimen preserved in relatively unweathered carbonate. Preparation of the proximal end revealed that opposing vanes are folded through ~90° across the axis of the putative stem and that the edge of one of the vanes can be followed to the axis (Fig. 14.1, 14.2, 14.8). There is no sign of a continuation of a voluminous axial structure beyond the margins of the frond.

The nature of the axial structure is not well understood, but it appears to have a surface formed of similarly sized, equally spaced, rounded projections that perhaps are arranged like the scales of a pineapple or the Fibonacci spirals of a pinecone (Fig. 14.6, 14.7). JGG found a possibly comparable axis at UCLA 7326 (Arimas), which became known as the “Arimas lycopod” (Fig. 14.5) because of its similarity to the bark of Paleozoic lycopods such as Leptophloeum. It was found at the same level as another float specimen (Fig. 12.9) with a fragment of a vane of Nasepia altae Germs, 1972 (Germs, Reference Germs1972a, Reference Germs1973), the only other example recovered subsequently from the type locality. Thus, the Arimas lycopod may be the decorticated axial structure of Nasepia judging from their co-occurrence and previously recognized similarities between the vanes of Swartpuntia and Nasepia (Fig. 14; Grotzinger et al., Reference Grotzinger, Bowring, Saylor and Kaufman1995; Narbonne et al., Reference Narbonne, Saylor and Grotzinger1997). However, the syntypes of Nasepia are preserved in a carbonate conglomerate (Fig. 14.4), whereas the Arimas lycopod and the new Nasepia vane are both in blocks of sandstone, one of which also contains a poorly preserved specimen of Archaeichnium (Fig. 21.5). The only other penecontemporaneous fossil worthy of comparison with the Arimas lycopod seems to be Gibbavasis kushkii Vaziri, Majidifard, and Laflamme, Reference Vaziri, Majidifard and Laflamme2018 (Vaziri et al., Reference Vaziri, Majidifard and Laflamme2018, Reference Vaziri, Majidifard, Darroch and Laflamme2021) from the Ediacaran of Iran, but the similarities, although striking, are almost certainly superficial.

Swartpuntia has been positively or tentatively identified from the earliest Cambrian of South Australia (Jensen et al., Reference Jensen, Gehling and Droser1998), the latest Ediacaran of Nevada and California (Hagadorn and Waggoner, Reference Hagadorn and Waggoner2000; Hagadorn et al., Reference Hagadorn, Fedo and Waggoner2000), the early Cambrian of California (Hagadorn et al., Reference Hagadorn, Fedo and Waggoner2000), the Ediacaran of North Carolina (Weaver et al., Reference Weaver, McMenamin and Tacker2006), and the Spitskop Member just over the international border in South Africa (Nelson et al., Reference Nelson, Ramezani, Almond, Darroch, Taylor, Brenner, Furey, Turner and Smith2022). Although the South African specimens are only parts of single vanes, their morphology and geographic and temporal proximity give confidence to the identifications. The same is not true for other reports, which should be treated skeptically on a case-by-case basis. The one American specimen that shows more than a fragment of a corrugated surface is LACNMH 12793 (Hagadorn and Waggoner, Reference Hagadorn and Waggoner2000, fig. 4.1, 4.2), which has two characters that may support an assignment to Swartpuntia: fine, dihedral linear striations that may be impressions of modules and an apparently knobbly axial structure. However, neither character is particularly convincing when compared with material from the type locality (Figs. 13, 14). The remainder of the referred specimens, including a sizeable surface from the Cambrian Poleta Formation of California (Hagadorn et al., Reference Hagadorn, Fedo and Waggoner2000, fig. 3.2) may be pieces of erniettomorphs, but in the Cambrian at least, there are many other possibilities (e.g., MacGabhann et al., Reference MacGabhann, Schiffbauer, Hagadorn, Van Roy, Lynch, Morrison and Murray2019; but see Hoyal Cuthill, Reference Hoyal Cuthill2022). However, the Swartpuntia-like fossils discovered by Jensen et al. (Reference Jensen, Gehling and Droser1998) deserve further investigation; there is more than one specimen, which is an important first step, and they display bilateral symmetry, discrete margins, and subdivisions reminiscent of erniettomorphs. They also occur very close to the local base of the Cambrian, which may be equivalent in age to the Ediacaran–Cambrian boundary zone in both Namibia (Linnemann et al., Reference Linnemann, Ovtcharova, Schaltegger, Gärtner and Hautmann2019) and South Africa (Nelson et al., Reference Nelson, Ramezani, Almond, Darroch, Taylor, Brenner, Furey, Turner and Smith2022).

Swartpuntia has been reported with Vendoconularia Ivantsov and Fedonkin, Reference Ivantsov and Fedonkin2002, Ventogyrus Ivantsov and Grazhdankin, Reference Ivantsov and Grazhdankin1997, and Calyptrina Sokolov, Reference Sokolov1965 from the Onega Peninsula of the White Sea area, northern Russia (Ivantsov and Fedonkin, Reference Ivantsov and Fedonkin2002). According to Serezhnikova (Reference Serezhnikova2014), this association is approximately 550 million years old, which would make this the oldest record of the genus. However, until this material has been figured and described, affinity with the Nama occurrences cannot be evaluated. A frond-shaped fossil from northern Norway has been tentatively compared with Swartpuntia (Meinhold et al., Reference Meinhold, Willbold, Karius, Jensen, Agić, Ebbestad, Palacios, Högström, Høyberget and Taylor2022, fig. 4b). This too would represent an occurrence older than those of the Nama Group, but in view of the revised morphology of Swartpuntia presented here, this comparison now is unlikely.

In summary, Swartpuntia is a Pteridinium-like frond that probably had only three equal-sized vanes set about a knobbly, voluminous, axial structure, and it lacked any kind of stem or stalk. Its modules and vanes resemble those of the smaller frond, Nasepia altae, so the discovery of a lycopod-like fossil, similar to the axial structure of Swartpuntia, with Nasepia at its type locality provides circumstantial evidence for a close relationship between the two genera. Swartpuntia/Nasepia is known with certainty only from Namibia and nearby South Africa, but one fragmentary specimen from California may belong to Swartpuntia. All other identifications are based on specimens that are too fragmentary or too little studied to warrant confident assignment to the genus or even to the Erniettomorpha.

Type species

Ernietta plateauensis Pflug, Reference Pflug1966 from the Buchholzbrunn Member of the Dabis Formation, Kuibis Subgroup, Aar farm, Aus district, Namibia, by original designation and monotypy.

Other species

Numerous other generic and specific names, as well as two new orders, four families, and five subfamilies, were proposed by Pflug (Reference Pflug1972) for material on Aar farm that is essentially topotypic. In preparing the Precambrian section of the Introduction volume of the Treatise on Invertebrate Paleontology, Glaessner (Reference Glaessner, Robison and Teichert1979b) attempted to rationalize Pflug's excessive splitting by recognizing only two subfamilies and five genera: Ernietta, Erniofossa, Ernionorma (Erniettinae), plus Erniobeta and Erniograndis (Erniobetinae). Soon after, Richard Jenkins effectively overruled this assessment with the statement: “One of us (Jenkins) has examined Pflug's material and considers that all the specimens he refers to as the ‘Erniettomorpha’ belong to a single genus and species, Ernietta plateauensis Pflug” (Jenkins et al., Reference Jenkins, Plummer and Moriarty1981, p. 71). That interpretation has become the status quo. However, in his summary of the genus Ernietta, Glaessner (Reference Glaessner, Robison and Teichert1979b, p. A101) wrote as follows: “Body compressed at base into U-shape; ribs strongly developed, separated by zig-zag median line; resembling a folded petaloid of Pteridinium.”

When Pflug (Reference Pflug1966) first described E. plateauensis, he thought he had the dorsal carapace of a soft-shelled worm or isopod-like arthropod but noted that the zig-zag dorsal suture was more like a structure found in Pteridinium than any animal except, perhaps Dickinsonia. One distinctive feature was a triangular mark at the topographical pole of the holotype, which he designated segment z (Fig. 19.1). This, he thought, was matched by segment 0 on the opposite side of the axis, and the segments were numbered away from these structures on both sides of the body. If segment z is a real feature of the anatomy, it is not seen in any other known specimens of Ernietta. It is, however, seen occasionally in U-shaped specimens of P. simplex (Fig. 19.2; Pflug, Reference Pflug1972, pl. 34, fig. 1), and it appears to be a tear of the seam between two modules on one side of the organism. Thus, Glaessner's diagnosis of Ernietta was more perceptive than he realized because the holotype of E. plateauensis (Fig. 19.1) is probably the tip of a tightly folded, U-shaped specimen of P. simplex.

One argument against this interpretation is that the discovery site “C” is described as “slate between Kuibis quartzite and black limestone in the lower part of the Nama system” (Pflug, Reference Pflug1966, p. 22), which clearly places it within the Buchholzbrunn Member (Fig. 3). This is the level where Ernietta abounds and Pteridinium is rarely seen (Fig. 2; Elliott et al., Reference Elliott, Trusler, Narbonne, Vickers-Rich, Morton, Hall, Hoffmann and Schneider2016). However, together with Bob Brain, Mark McMenamin, and Friedrich Pflüger, an attempt was made to recollect Pflug's locality C in 1993. Mark McMenamin found the only fossil, a vertical vane of Pteridinium (Fig. 19.7; McMenamin, Reference McMenamin1998), at about the same stratigraphic level and geographic position as the holotype. So far as we know, Pflug's site C has not been resampled since that time; it is ~1.5 km east of the eastern edge of the geological maps of Plateau and Aar farms in Hall et al. (Reference Hall, Kaufman, Vickers-Rich, Ivantsov and Trusler2013) and Elliott et al. (Reference Elliott, Trusler, Narbonne, Vickers-Rich, Morton, Hall, Hoffmann and Schneider2016).

If the holotype of E. plateauensis is a small specimen of P. simplex, as seems likely, then Ernietta becomes a junior subjective synonym of Pteridinium. That is an undesirable outcome, given the long history of the use of Ernietta for a well-understood generic concept (Elliott et al., Reference Elliott, Trusler, Narbonne, Vickers-Rich, Morton, Hall, Hoffmann and Schneider2016; Ivantsov et al., Reference Ivantsov, Narbonne, Trusler, Greentree and Vickers-Rich2016). However, the holotype of E. plateauensis is already an unsatisfactory standard because, as Pflug (Reference Pflug1972, p. 139) noted: “From the collection area around point C [Pflug, Reference Pflug1966, fig. 1b] come the [holotype and paratype] specimens of the genus Ernietta and the specimens numbered 393, 399 of Erniotaxis. Almost all other pieces, with the exception of an Erniobeta colony, were found in collection area E, F.” (~2 km south of the southern edge of the maps of Plateau and Aar and farms in Hall et al. [Reference Hall, Kaufman, Vickers-Rich, Ivantsov and Trusler2013] and Elliott et al. [Reference Elliott, Trusler, Narbonne, Vickers-Rich, Morton, Hall, Hoffmann and Schneider2016]). As discussed in the following, Erniotaxis is an unusual juvenile form of Ernietta, and a “colony of Erniobeta” could mean many things. Thus, it is very difficult to obtain a population of individuals of E. plateauensis on the basis of the topotypic principle that all similar specimens from a bed or set of beds at one place are likely to be conspecific. Without such a sample to assess intraspecific variability, application of the name plateauensis is difficult. For both of these reasons, we recommend that the holotype of another of Pflug's species, Erniograndis sandalix Pflug, Reference Pflug1972, be designated the neotype of E. plateauensis. This recommendation will need the approval of the ICZN before it can take effect; in the meantime, community input is invited. There are precedents for this type of action to preserve useful names.

If there are to be other valid species of Ernietta, then Namalia villiersiensis Germs, Reference Germs1968 has priority over all of Pflug's species except plateauensis. Again, the holotype of N. villiersiensis (Germs, Reference Germs1968, fig. 1, Reference Germs1972a, pl. 23, fig. 1) is not ideal in terms of preservation and the availability of topotypic material, but even worse, it may be missing (it could not be found at the ISAM in 1993). The type locality, Buchholzbrunn, has yielded the juvenile specimens of Ernietta shown in Figure 16, but they are preserved in a very different fashion from the holotype of Namalia villiersiensis. A better comparison is with the sandstone cast of a fossil—similar to those commonly attributed to Namalia villiersiensis or Kuibisia glabra Hahn and Pflug, 1985 (Hahn and Pflug, Reference Hahn and Pflug1985a)—from the Aarhauser sandstone at Aar (Fig. 19.8, 19.9). This specimen was uncovered by the Seilacher team during their excavation in 1993. Thus, Namalia villiersiensis may be the senior synonym of Kuibisia glabra, and both may or may not be conspecific with Ernietta plateauensis (Jenkins et al., Reference Jenkins, Plummer and Moriarty1981; Runnegar and Fedonkin, Reference Runnegar, Fedonkin, Schopf and Klein1992; Vickers-Rich, Reference Vickers-Rich, Fedonkin, Gehling, Grey, Narbonne and Vickers-Rich2007; Ivantsov et al., Reference Ivantsov, Narbonne, Trusler, Greentree and Vickers-Rich2016; but see Grazhdankin and Seilacher, Reference Grazhdankin and Seilacher2002). The apparent differences between N. villiersiensis/K. glabra and neotypic Ernietta plateauensis may be due to preservation in coarse sandstone instead of siltstone.

Returning to Glaessner's (Reference Glaessner, Robison and Teichert1979b) revision of Pflug's taxa, do his generic categories help break up Ernietta into distinct morphotypes? His subfamily Erniobetinae comprised two genera, Erniobeta and Erniograndis. A swift survey of Pflug's material may be obtained from the reproductions of his 13 Palaeontographica plates by Vickers-Rich (Reference Vickers-Rich, Fedonkin, Gehling, Grey, Narbonne and Vickers-Rich2007). Excluding plate 34, which deals mainly with E. plateauensis, 10 of the plates are devoted to specimens that generally resemble the shapes shown in Figure 19.3–19.6. The remaining two plates, 38 and 39, illustrate the Erniobetinae—Erniobeta and Erniograndis—which are bulky, internal molds of large specimens such as the proposed neotype for plateauensis (Fig. 15.3; Pflug, Reference Pflug1972, pl. 38, 1, 2, 4; Vickers-Rich, Reference Vickers-Rich, Fedonkin, Gehling, Grey, Narbonne and Vickers-Rich2007, fig. 124; Elliott et al., Reference Elliott, Trusler, Narbonne, Vickers-Rich, Morton, Hall, Hoffmann and Schneider2016, fig. 3). Thus, the Erniobetinae sensu Glaessner (Reference Glaessner, Robison and Teichert1979b) may serve as a population concept for E. plateauensis if the proposed neotype is eventually adopted.

Figure 15. Ernietta plateauensis Pflug, Reference Pflug1966, Buchholzbrunn Member, Dabis Formation, Plateau farm (2, 4, 7) and approximately the same stratigraphic level, UCLA 7378, Twyfel farm (1, 5, 6, 8–10). (1) Classic “sock in a rock” preservation, found in place and photographed in the field, both specimens numbered GSN F 1876. (2) Colorized version of one of five sketches based on plaster cast, YPM 204 508, of specimen in the “museum” at Plateau farm after Seilacher et al. (Reference Seilacher, Grazhdankin and Legouta2003, fig. 11), copyright 2003, the Palaeontological Society of Japan, republished with permission. (3) GSN F 389, holotype of Erniograndis sandalix Pflug, Reference Pflug1972, and proposed neotype for E. plateauensis, photographed in Lich, Germany, 1993, GSN F 389. (4) Duplicate of the plaster cast used as the model for (2), UCLA 7327.2, gifted by the Seilacher team, was used to count the 70+ modules (Fig. 18.6) after tracing the between-module seams with a soft pencil. (5, 6) Two excavated specimens, GSN F 1863 and GSN F 1864, that share rhomboidal distal cross sections (corners indicated by arrows in (5)), viewed from above. (7) Photograph taken in 1993 of the specimen used to make the casts used for (2) and (4). (8–10) Four other excavated specimens, GSN F 1865, GSN F 1866, GSN F 1867, GSN F 1868, respectively, that show the typically pointed shape of the toe and, in the smaller specimens, evidence for growth interruptions. (1) Scale bar = 5 cm; (2, 4, 7) scale bar = 3 cm; (3) scale bar = 3 cm; (5, 6, 8–10) scale bars = 2 cm.

Pflug's plates 31 and 32 may best summarize the second morphotype, which Glaessner included in his subfamily Erniettinae; whether this morphotype may be specifically distinct from plateauensis is discussed in the following under the species description. Finally, plate 37, which shows specimens Pflug referred to Erniotaxis, is very different from all the others. Erniotaxis was one of five of Pflug's generic names that Glaessner (Reference Glaessner, Robison and Teichert1979b, p. A102) dismissed as “unrecognizable.” Our discovery of this morphology on Twyfel farm, where it is associated with larger and more normal specimens (Fig. 17), allows us to show that Erniotaxis is a young growth stage that is allometrically different from larger individuals. The modified generic name “erniotaxid” may therefore serve as informal shorthand for this juvenile morphotype. Finally, we agree with Elliott et al. (Reference Elliott, Trusler, Narbonne, Vickers-Rich, Morton, Hall, Hoffmann and Schneider2016) that Erniocarpus sermo Pflug, Reference Pflug1972 is not a specimen of Ernietta and suggest that while Erniocarpus orbiformis Pflug, Reference Pflug1972 may be, Erniocentrus centriformis Pflug, Reference Pflug1972 is certainly not.

Diagnosis

Sack-shaped, organic-walled bodies, oval or stadium shaped in cross section and U to V shaped in lateral profile, formed of tubular modules that meet in a zig-zag suture at the base, are attached to the outer wall, generated an inner wall by packing together during growth, and terminate distally in either stubby lobes or conical tips.

Occurrence

Kliphoek and Buchholzbrunn members, Dabis Formation, Kuibis Subgroup, Nama Group, Namibia (Fig. 2; Appendix); Lower Member, Wood Canyon Formation, Nevada, USA (Horodyski et al., Reference Horodyski, Gehling, Jensen and Runnegar1994; Smith et al., Reference Smith, Nelson, Tweedt, Zeng and Workman2017, Reference Smith, Nelson, O'Connell, Eyster and Lonsdale2022; Runnegar, Reference Runnegar2022).

Holotype

Tip of U-shaped internal mold (GSN F 429; previously Pflug no. 227) from the Buchholzbrunn Member, Aar farm, Aus district, Namibia by original designation (Fig. 19.1; Pflug, Reference Pflug1966, pl. 1, figs. 1–3, 1972, pl. 34, fig. 4).

Proposed neotype

Nearly complete internal mold (GSN F 389; previously Pflug no. 192) from the Buchholzbrunn Member, Aar farm, Aus district, Namibia (Fig. 15.3; Pflug, Reference Pflug1972, pl. 38, figs. 1, 2, 4; Vickers-Rich, Reference Vickers-Rich, Fedonkin, Gehling, Grey, Narbonne and Vickers-Rich2007, fig. 124; Elliott et al., Reference Elliott, Trusler, Narbonne, Vickers-Rich, Morton, Hall, Hoffmann and Schneider2016, fig. 3; Maloney et al., Reference Maloney, Boag, Facciol, Gibson, Cribb, Koester, Kenchington, Racicot, Darroch and Laflamme2020, fig. 3A).

Description

Body sack-like, composed of as many as 70 tubular modules arranged side by side around the circumference and joined proximally (ventrally) in a zig-zag seam; body cross section is elliptical to stadium shaped; basal profile at right angles to the zig-zag seam is U shaped or rounded V shaped, and parallel to the seam it is U shaped; upper part of body usually truncated postmortally, with the upper parts frequently assuming the cross-sectional shape of a four-pointed star (Fig. 15.5) that is aligned with the symmetry axes; modules are approximately constant in width for any growth stage until they approach the distal (dorsal) margin, where they start to separate from each other and taper toward pointed ends (Smith et al., Reference Smith, Nelson, Tweedt, Zeng and Workman2017, fig. 3d; J.G. Hall et al., Reference Hall, Smith, Tamura, Fakra and Bosak2020, fig. 1b; Runnegar, Reference Runnegar2022, fig. 3a; possibly Narbonne, Reference Narbonne2005, fig. 4b); modules are circular in cross section in the tapering tips, square to rectangular in cross section throughout much of the upper part of the body, and triangular to D shaped near the base, where they are in contact with each other only at the outer wall (Fig. 17.6); rare internal molds (Fig. 15.7) show that the sides of adjacent modules approached each other during growth and then coalesced about one-third of the way to the top of the body.

Materials

Numerous specimens (GSN F 1860–1877, 1880, 1881) from UCLA 7317 and UCLA 7378 on Buchholzbrunn and Twyfel farms and rare specimens from UCLA 7312, UCLA 7313, UCLA 7314, UCLA 7315, and UCLA 7381, all west of the road from Bethanie to Helmeringhausen (Fig. 1).

Ontogeny

Two sizeable blocks of sandstone, each part of a sand-cast gutter fill, were found on the floor of a small road metal quarry in the Buchholzbrunn Member on Buchholzbrunn by SJ and BR in 1995 (UCLA 7317; Fig. 16). The lower surfaces of these blocks preserve numerous small specimens of Ernietta that were transported with the sand and settled first, presumably because they behaved hydrodynamically like pebbles. The fossils are not well preserved, but they do display the modules well enough for them to be counted and compared with specimen size (Fig. 18.6). The smallest identifiable specimen, ~4 mm in diameter (E in Fig. 16.4, 16.5, 16.7), appears to have four modules (Fig. 16.7), although only three are visible in the gutter cast. An even smaller object to the upper left of it (Fig. 16.7) is preserved in the same fashion and may be an ~1 mm larval stage with only one module, such as the tiny White Sea specimens of Dickinsonia costata Sprigg, Reference Sprigg1947 illustrated by Ivantsov and Zakrevskava (Reference Ivantsov and Zakrevskava2022, pl. 1, figs. 1, 2). The subsequent growth of E. plateauensis is summarized in Figure 18.6, which is a plot of countable module number versus body size (length + width/2). The largest individual measured was a plaster cast of a specimen in the Plateau “museum” collection (Fig. 15.4, UCLA 7327.2, YPM 204 508; Seilacher et al., Reference Seilacher, Grazhdankin and Legouta2003, fig. 11, bottom row; Seilacher, Reference Seilacher2007, fig. 1) that has ~70 modules. Thus, body size is a reasonable predictor of module number (Fig. 18.6), although Ivantsov et al. (Reference Ivantsov, Narbonne, Trusler, Greentree and Vickers-Rich2016) found a fairly constant number of modules (~26) in a cohort of similar-sized individuals preserved in a gutter cast. Presumably, modules are added at one or both ends of the zig-zag seam (Ivantsov et al., Reference Ivantsov, Narbonne, Trusler, Greentree and Vickers-Rich2016), but it has not been possible to identify the most recently added modules in those areas because of inadequate preservation. At the base of the body, the modules terminate in separate, rounded ends (Fig. 15.7), so it is not obvious how new modules are generated; they may even be intercalated around the body during growth, as might be recorded by impressions of vertical seams on internal molds (Fig. 15.2, 15.4, 15.7; Jenkins et al., Reference Jenkins, Plummer and Moriarty1981, fig. 5B). However, these structures are better attributed to changes in module width and shape during growth, as discussed in the following.

Figure 16. Juvenile specimens of Ernietta plateauensis Pflug, Reference Pflug1966 preserved on the bases of two sizeable pieces of a gutter cast, found on the floor of a small road metal quarry, Buchholzbrunn Member, Dabis Formation, UCLA 7317, Buchholzbrunn farm, near Goageb. (1) Whole block, GSN F 1860. (2) Part of second block, GSN F 1861; arrows indicate directions of current flow. (3, 6) Enlargements of GSN F 1860 with individuals used for module counts (Fig. 18.6) indicated by letters. (4, 5, 7) Enlargements of parts of GSN F 1861 with smallest identifiable individual labeled E and three of its four modules indicated by L, M, and R (other arrows point to the ends of the modules of a larger individual); the bump above E in (7) may be the base of a tiny one-module postlarva (Fig. 18.6). (1, 2) Scale bar = 5 cm; (3, 4, 6) scale bars = 2 cm; (5) scale bar = 1 cm; (7) scale bar = 5 mm.

One of Pflug's (Reference Pflug1972) taxa, Erniotaxis segmentrix, is a puzzling set of small objects that Glaessner (Reference Glaessner, Robison and Teichert1979b) dismissed as “unrecognizable” and Elliott et al. (Reference Elliott, Trusler, Narbonne, Vickers-Rich, Morton, Hall, Hoffmann and Schneider2016, p. 1024) thought were “part of the midline of a fragmentary Pteridinium fossil.” Our discovery of two similar small specimens on Twyfel farm (Fig. 17.1–17.4) validates Pflug's recognition of the importance of his find, and the two small collections reveal the same features of the internal anatomy of young examples of E. plateauensis (Fig. 16.1–16.5). The striking feature of both is the depth of the walls of the modules and their concave lateral surfaces. In these specimens, the inner edges of the modules are blade-like and close to the axis of the body (Fig. 17.2). The concavity of their lateral walls suggests that there may have been open space between the modules and that they were in contact only at the outer wall. This morphology is seen more clearly in a larger but still youthful specimen from the same site (Fig. 17.6), which is best understood as an inverted fragment of the lower part of the specimen shown in Figure 17.10 (in both examples, the modules taper distally). In these specimens, the modules were filled with carbonate following burial, so the whole structure is preserved in three dimensions and could in one case be separated from the internal mold (Fig. 17.6). The two halves of this specimen show clearly that, at an early growth stage, the modules were D shaped in cross section and in contact with each other only at the seams of the outer wall, best seen in the external mold (Fig. 17.6, left) and modeled in Figure 18.7, 18.8. Larger, more mature individuals have modules that are wholly in contact laterally, resulting in square to rectangular cross sections (Fig. 15.4; Pflug, Reference Pflug1972, pl. 38, fig. 3; Jenkins et al., Reference Jenkins, Plummer and Moriarty1981, fig. 6C; Elliott et al., Reference Elliott, Trusler, Narbonne, Vickers-Rich, Morton, Hall, Hoffmann and Schneider2016, fig. 4.2), except toward their growing terminations, where the modules separated and assumed a hydrostatic (circular) cross section (Runnegar, Reference Runnegar2022, fig. 3a). There are also some contentious components of the growth of Ernietta: (1) whether there were two or more layers of modules in the body wall; (2) the nature of the growing terminations of the body; (3) the significance of waist-like constrictions that are seen in many specimens, including the proposed neotype; (4) whether sand was incorporated into the body during life. These matters are reviewed in the following under remarks.

Figure 17. Juvenile and small specimens of Ernietta plateauensis Pflug, Reference Pflug1966, Buchholzbrunn Member, Dabis Formation, UCLA 7378, Twyfel farm (1–4, 6, 7, 9, 10); UCLA 7308, Aar farm (5); and UCLA 7313, Klipdrif farm (8). (1–4) Unusual, globose specimens preserved in carbonate that resemble the Erniotaxis segmentrix morph (5) in having extraordinarily wide walls between adjacent modules (2) and highly curved outer walls, GSN F 1869 (1–3) and GSN F 1870 (4). (5) Holotype of Erniotaxis segmentrix Pflug, Reference Pflug1972, no. 396 now GSN F 449, photographed in Lich, Germany, in 1993. (6) Part and counterpart of a specimen with carbonate-filled modules that are convex in both outward and inward directions and are in lateral contact only at the outer surface (arrows); GSN F 1874. (7, 9) Excavated block shown in original orientation with two visible specimens of E. plateauensis, one of which is removable and is shown in inverted orientation in (9). (8) A deformed specimen found with other individuals in a small channel ~3 m below the first limestone of the Mooifontein Member, GSN F 1956. (10) Rare example of preservation of the outer surface of the organism as a result of carbonate-filled modules, GSN F 1872. Scale bars = 1 cm.

Figure 18. Growth of Ernietta. (1–5, 7, 8) Super3D models. (6) Scatter plot of size versus number of modules in Ernietta plateauensis Pflug, Reference Pflug1966 (filled circles); the holotype of E. plateauensis, thought to be a deformed specimen of Pteridinium simplex, is represented by the filled square. (1) Perspective view of two identical copies showing how the modules interdigitate along the proximal seam. (2–5) Orthographic views of the base of the model tilted about X by 30° (2) and 20° (3, 4) showing the progressive deconstruction of the model, which is based on rectangular modular cross sections found in mature individuals of Ernietta from Nevada. (7, 8) The basal part of the external layer of the model and three modules of the kinds seen in immature individuals from Namibia (Fig. 17.6), where the cross sections are D-shaped and end proximally in wedge-shaped terminations (arrow), reminiscent of the youthful modules of the erniotaxid morphotype (Fig. 17.2); it is assumed that the D-shaped modules merge distally into mature box-shaped ones.

Taphonomy

An evocative metaphor for the preservation of a mature individual of Ernietta is not a “rock in a sock” (Seilacher, Reference Seilacher1992) but rather a “sock in a rock” (Knoll, Reference Knoll2003, p. 166; Fig. 15.1). Is this morphology the result of mass flow transport and burial or a life orientation? A recent consensus is the latter based on in situ specimens from localities west of the Aar homestead (Elliott et al., Reference Elliott, Trusler, Narbonne, Vickers-Rich, Morton, Hall, Hoffmann and Schneider2016) and sites on Weigkrup and Hansburg farms (Bouougri et al., Reference Bouougri, Porada, Weber and Reitner2011; Maloney et al., Reference Maloney, Boag, Facciol, Gibson, Cribb, Koester, Kenchington, Racicot, Darroch and Laflamme2020), which are close to our localities UCLA 7378 and UCLA 7379 on Twyfel and Weigkrup (Fig. 1). Perhaps the best evidence for this interpretation is the fact that nearly all specimens are oriented with their zig-zag seams down and occur in clusters that are thought to have developed in depressions in the seafloor. However, some of these group occurrences appear to be secondarily transported (Ivantsov et al., Reference Ivantsov, Narbonne, Trusler, Greentree and Vickers-Rich2016). At Twyfel, we also found that rare specimens in the same bed as the upright clusters are inverted, a configuration that is difficult to explain in an in situ community. Given the high probability that transported specimens partly filled with sand would aggregate seam downward in depressions, the life orientation of any of these clusters is questionable.

The carbonate infilling of the modules of the specimen shown in Figure 17.6 is unusual and previously unreported from Namibia. As the specimen is unique, no preparation of it was undertaken, so the identification of the fill, based on microscopic examination of fractured surfaces, is tentative. Another specimen found with it (Fig. 17.10) seems to be preserved in the same way and could be examined with computed tomography in the future. A possibly similar style of preservation has been reported by Ivantsov (Reference Ivantsov2018) from the Ediacaran of Siberia.

Remarks

Jenkins et al. (Reference Jenkins, Plummer and Moriarty1981, fig. 6) published a reconstruction of Ernietta based on Pflug's material, which Jenkins had examined in Giessen with Pflug's assistance. Two key observations, based on an unfigured syntype of Erniograndis sandalix (Pflug no. 182), were the presence, near the base, of a small piece of sediment that had filled the interior of a second outer palisade of modules and an “enigmatic ‘budding’ suture” that encircled the upper part of the internal mold and is also present in the holotype (Fig. 15.3; Pflug, Reference Pflug1972, pl. 38, figs. 1, 2, 4; Vickers-Rich, Reference Vickers-Rich, Fedonkin, Gehling, Grey, Narbonne and Vickers-Rich2007, fig. 124; Elliott et al., Reference Elliott, Trusler, Narbonne, Vickers-Rich, Morton, Hall, Hoffmann and Schneider2016, fig. 3; Maloney et al., Reference Maloney, Boag, Facciol, Gibson, Cribb, Koester, Kenchington, Racicot, Darroch and Laflamme2020, fig. 3A) and several other specimens from Aar (e.g., Fig. 15.2). Jenkins also sketched three cross sections of Ernietta on the basis of sawn specimens, including a paratype of E. sandalix (Pflug, Reference Pflug1972, pl. 39, fig. 1; Jenkins et al., Reference Jenkins, Plummer and Moriarty1981, fig. 5B), which is shown as displaying a voluminous inner layer of modules crossed by rare septa and a tiny, attached fragment of a second layer of modules. Pflug's (Reference Pflug1972, pl. 39, fig. 1) figure of the paratype shows a thick layer of white sediment near the base of the organism, comparable to that illustrated by Ivantsov et al. (Reference Ivantsov, Narbonne, Trusler, Greentree and Vickers-Rich2016, fig. 6E, F) in a similar sawn section, and two or three linear features that could be the intersections of septa. However, their convexity is in the opposite direction to the septa shown in Jenkins's sketch, and the thickness of the adhering second layer of modules is negligible. Furthermore, the other two cross sections sketched by Jenkins each have only one layer of modules. Thus, evidence for a second layer of modules is limited to a few fragments adhering to the bases of syntypic specimens of E. sandalix (e.g., Elliott et al., Reference Elliott, Trusler, Narbonne, Vickers-Rich, Morton, Hall, Hoffmann and Schneider2016, fig. 4.2) and an image of the polished sawn surface of GSN F-1243 from the Teapot locality on Aar (Elliott et al., Reference Elliott, Trusler, Narbonne, Vickers-Rich, Morton, Hall, Hoffmann and Schneider2016, fig. 5.3), which show two layers of honeycomb-like cells. Given that the orientation of the polished surface with respect to the fossil is not clear, that the section may intersect part of the ventral zig-zag seam, and that some mineralized cracks may mimic mineralized organic walls, it also does not provide strong support for the dual- or multiple-wall hypothesis that forms the basis for the remarkably similar reconstructions of Jenkins et al. (Reference Jenkins, Plummer and Moriarty1981, fig. 6A) and Ivantsov et al. (Reference Ivantsov, Narbonne, Trusler, Greentree and Vickers-Rich2016, fig. 7). Because there is no evidence for more than a single wall in the great majority of specimens of Ernietta (Fig. 17.6; Pflug, Reference Pflug1972; Hall et al., Reference Hall, Droser, Clites and Gehling2020; Runnegar, Reference Runnegar2022) or Namalia (Grazhdankin and Seilacher, Reference Grazhdankin and Seilacher2002), caution is recommended until a complete individual or population of individuals showing evidence for more than one palisade wall becomes available. We tentatively attribute those few specimens that have some evidence of more than one layer near their bases to abnormal development, regeneration after injury, or some unidentified taphonomic process.

This suggestion may also apply to Jenkins's encircling suture, which interrupts the modules so severely that they may be significantly narrower above it and not in register with the modules below it (Fig. 15.2, 15.3; Jenkins et al., Reference Jenkins, Plummer and Moriarty1981, fig. 5B), details not captured in the reconstructions (Jenkins et al., Reference Jenkins, Plummer and Moriarty1981, fig. 6A; Ivantsov et al., Reference Ivantsov, Narbonne, Trusler, Greentree and Vickers-Rich2016, fig. 7). Perhaps an explanation for these sutures is that they also represent a response to traumatic injury, such as truncation of the top of the organism by storm surge, followed by subsequent regrowth. This may explain why the suture is lower down in specimens from Twyfel (Fig. 15.9) than in those from Aar (Fig. 15.3).

The nature of the growing ends of the modules is another feature of the two reconstructions that deserves reassessment. In Jenkins's reconstruction, the modules terminate in lappet-like edges, which border a pair of wide lips formed from three concentric palisades of modules (Jenkins et al., Reference Jenkins, Plummer and Moriarty1981, fig. 6A). Ivantsov et al. (Reference Ivantsov, Narbonne, Trusler, Greentree and Vickers-Rich2016, fig. 7) show two concentric rows of similar lappets that flare outward, away from the symmetry axis. These reconstructions contrast with the ones by Monastersky and Mazzatenta (Reference Monastersky and Mazzatenta1998), based on undescribed specimens of Ernietta from Nevada (Horodyski et al., Reference Horodyski, Gehling, Jensen and Runnegar1994), which have the modules terminating in narrowly tapering cones with pointed ends (Runnegar, Reference Runnegar2022, fig. 3a). The holotype of Kubisia glabra has lappet-like terminations (Ivantsov et al., Reference Ivantsov, Narbonne, Trusler, Greentree and Vickers-Rich2016, fig. 8D), adding support for the Jenkins–Ivantsov reconstruction, but another specimen of Ernietta from Namibia seems to have distal terminations like the Nevada examples (Narbonne, Reference Narbonne2005, fig. 4b; Smith et al., Reference Smith, Nelson, Tweedt, Zeng and Workman2017, fig. 3d; Hall et al., Reference Hall, Droser, Clites and Gehling2020, fig. 1b; Runnegar, Reference Runnegar2022, fig. 3a). Does this mean that Ernietta could withdraw and collapse its tentaculate terminations like anemones exposed at low tide, or that there are two distinct, co-existing morphotypes? These questions deserve further investigation.

As mentioned previously, Glaessner's (Reference Glaessner, Robison and Teichert1979b) Erniettinae, best exemplified by the specimen shown in Figure 19.4, 19.5, may prove to be another distinct morphotype/species of Ernietta. If so, Pflug's Ernionorma abyssoides may be the name to use, which is why we re-illustrate an epoxy cast of the holotype (Fig. 19.3; GSN F 485; Pflug no. 280). This morphotype is commonly found as basal pieces formed of numerous closely spaced modules (Fig. 19.5, white dots) but may also have highly variable module widths (Fig. 19.6). A morphometric analysis of populations tied to the type localities for the holotypes of E. sandalix and E. abyssoides will be needed to answer this question.

Figure 19. Ernietta plateauensis Pflug, Reference Pflug1966, Buchholzbrunn Member, Dabis Formation, UCLA 7308, Aar farm (1, 3) and approximately the same stratigraphic level, UCLA 7379, Wegkruip farm, plus Pteridinium simplex Pflug and Namalia villiersiensis Germs, UCLA 7307, Aarhauser sub-member, Kliphoek Member, Dabis Formation, Aar farm (2, 7–9). (1) Plaster cast of holotype of Ernietta plateauensis Pflug, Reference Pflug1966, no. 227 now GSN F 429, probably a deformed and torn specimen of P. simplex (2) that should be replaced by a neotype such as the holotype of Erniotaxis segmentrix Pflug, Reference Pflug1972 (Fig. 15.3). (2) Plaster cast of a deformed and torn specimen of P. simplex, SMSWA 45370.2 now GSN F 1879, that shows a similar triangular lesion to the one in the holotype of Ernietta plateauensis, which Pflug (Reference Pflug1972) termed an “apicostomatous aperture”; however, note that there are three vanes (V1–V3) preserved in this specimen. (3) Underneath view of epoxy cast of the holotype of Ernionorma abyssoides Pflug, Reference Pflug1972, no. 280 now GSN F 485, donated by H.D. Pflug, for comparison with specimens from Wegkruip farm (4–6); (4, 5) Underneath and lateral views of a weathered but otherwise well-preserved internal mold with the number of visible modules indicated by white dots, GSN F 1880. (6) Four similar-sized specimens to illustrate variations in module size and number, GSN F 1881, GSN F 1882, GSN F 1883, GSN F 1884, respectively. (7) Fragment of one vane of a specimen of P. simplex, embedded in a horizontally bedded sandstone, found by M.A.S. McMenamin at or near the type locality of E. plateauensis, field photograph, 1993, GSN F 2209 (McMenamin, Reference McMenamin1998, p. 85, fig. 5.3). (8, 9) Two views of a specimen resembling the holotype of Namalia villiersiensis that was excavated by the Seilacher team at Aar farm, field photographs, 1993, GSN F 612 (Grazhdankin and Seilacher, Reference Grazhdankin and Seilacher2002, text-fig. 9F–H). (1, 3–6) Scale bars = 1 cm; (2, 7–9) scale bars = 2 cm; (7) coin = 23 mm.

A fourth area of concern for understanding the biology and taphonomy of Ernietta is the long-standing questions as to whether they were epibenthic or endobenthic, whether they received sediment passively in their body cavities during life or actively incorporated sediment into their body tissues to enable them to remain upright if disturbed. There is no doubt that many are preserved with their modules filled with sediment, which is often coarser and better sorted than the matrix that surrounds them (Pflug, Reference Pflug1972; Ivantsov et al., Reference Ivantsov, Narbonne, Trusler, Greentree and Vickers-Rich2016; Hall et al., Reference Hall, Droser, Clites and Gehling2020). This is particularly noticeable in Nevada, where the modules of corrugated bodies or their degraded bag-like remnants are full of clean quartz sand, quite unlike the deep-water, silty matrix in which they are interred (Hall et al., Reference Hall, Droser, Clites and Gehling2020). This suggests that only those bodies that were torn and filled with coarse grains during high-velocity transport were preserved. However, Ivantsov et al. (Reference Ivantsov, Narbonne, Trusler, Greentree and Vickers-Rich2016) opted for a division of function along the length of the modules, with active incorporation of sand in their basal parts, a fluid-filled hydrostatic function for their middle parts, and an aerobic/osmotic function for their distal parts. Evidence for sediment incorporation came from longitudinal thin and polished sections, which showed a wide zone of clean quartz sand between the outer and inner walls and sequential fill of less coarse sediment within the body cavity (Ivantsov et al., Reference Ivantsov, Narbonne, Trusler, Greentree and Vickers-Rich2016, fig. 6E, F), similar to the sawn section illustrated by Pflug (Reference Pflug1972, pl. 39, fig. 1; Jenkins et al., Reference Jenkins, Plummer and Moriarty1981, fig. 6B). However, the problem with these cross sections compared with internal molds of the E. sandalix type is that there is the paucity of partitions attributable to septa—even allowing for the small angles between sections and septa—and the distance between the inner and outer walls is proportionally large compared with the module depths recorded by internal molds. These discrepancies raise the possibility that, although the inner and outer walls were largely intact, the walls and septa had been sufficiently breached to allow coarse suspended grains to enter the wall cavity during transport. Thus, the Namibian and Nevadan specimens may have been preserved under similar conditions. The highly structured nature of the filling of the body cavity of a Namibian specimen (Ivantsov et al., Reference Ivantsov, Narbonne, Trusler, Greentree and Vickers-Rich2016, fig. 6B, C) may provide a reason to doubt this interpretation, but it may also be explicable by waning storm surge sedimentation if the bodies had sufficient mechanical strength to remain open during burial.

Type species

Archaeichnium haughtoni Glaessner, Reference Glaessner1963 from the Kuibis? or Schwarzrand Subgroup, Karasburg district, Namibia, by monotypy.

Other species

None. Two species of Archaeichnium erected on Paleozoic material, Archaeichnium kunmingensis Luo in Luo et al. (Reference Luo, Tao and Gao1994), from the lower Cambrian of Yunnan, China, and Archaeichnium(?) xizangensis Yang in Yang et al. (Reference Yang, Song and Liang1983), from the Upper Carboniferous of Tibet, are trace fossils with longitudinal striations.

Diagnosis

Narrow conical organism with about 10–12 longitudinal ridges that may be the edges of, and internally septate, pleated body wall, possibly enclosed in similarly corrugated unmineralized epitheca.

Holotype

One of several specimens probably preserved in convex hyporelief on a small slab of sandstone, ISAM K4812, ostensibly from the Nababis Formation, Kuibis Subgroup, on Gründoorn farm, near the Ham River, ~60 km east of Karasburg, southern Namibia (Fig. 20.1; Haughton, Reference Haughton1960, pl. 4, 5; Glaessner, Reference Glaessner1963, pl. 2, fig. 1, Reference Glaessner1978, fig. 1), but possibly from the younger Schwarzrand Subgroup in the same section (near 19.295356°E, 28.094153°S); by original designation and monotypy.

Figure 20. Archaeichnium haughtoni Glaessner, Reference Glaessner1963, Nakop Member, Nababis Formation, Gründorn farm (57) (1) and Huns Member, Urusis Formation, UCLA 7309, Arimas farm (2–8). (1) Holotype of A. haughtoni, ISAM K4812, photographed in Cape Town, South Africa, in 1993. (2) Sandstone slab with two specimens, GSN F 1904A (3, 4, 6, 8) and GSN F 1904B (5), that reveal much of the anatomy of the form. (3, 4, 6, 8) Four views of GSN F 1904A taken with different lighting and equipment to show the nature of the body wall and its construction. (5) End piece showing likely origin of growth. (7) A co-occurring external mold that is longitudinally fluted and may represent a cast of the cuticle or tube of Archaeichnium. (1) Coin = 19 mm; (1, 2) scale bars = 2 cm; (3–5) scale bar = 1 cm; (6–8) scale bars = 5 mm.

Description

Longitudinally ridged, tubular fossils that taper gradually or rapidly to a closed end (Figs. 20.5, 21.1), which may have been fixed to the substrate (Fig. 21.1, 21.4); the greatest diameter is typically ~5 mm, and the tubes may exceed several cm in length; the number of longitudinal ridges is ~10–12 assuming that external molds, which have ~3.5 ridges (Fig. 21.6), represent about a third of the circumference; perimortem kinks (Fig. 21.2) and twists (Fig. 20.1) in the tubes suggest that they were unmineralized and flexible; a remarkable specimen (Fig. 20.3, 20.4, 20.6, 20.8) shows either an impression of the side of one ridge or the side view of a flange that either extended outward from the ridge crest or is an internal extension from the body wall; the structure has evenly spaced, radially oriented ridges that are about 0.5 mm apart and of similar length that may have provided structural support or had some other function (see Remarks); rare external molds suggest that, in life, the ridges were narrow and stiff and the intervening wall segments were concave so that the cross section resembled a concave or parabolic star with 10–12 points (Fig. 21.6).

Figure 21. Archaeichnium haughtoni Glaessner, Reference Glaessner1963, Huns Member, Urusis Formation, UCLA 7309, Arimas farm (1), UCLA 7325, Holoog River (2), and Neiderhagen Member, Nudaus Formation, Kyffhauser farm (3–6). (1) Four longitudinally striated individuals with pointed terminations (arrows), presumed to be the origins of growth, on the base of a 3 cm thick sandstone bed with “old elephant skin” texture, GSN F 1906. (2) Two specimens from the Holoog River, one of which is severely kinked (insert), GSN F 1962 and GSN F 1975, respectively. (3) Superb bed base, GSN F 1939, found by D.E. Erwin in 1995, with at least eight tethered and current-oriented individuals, six facing right and two facing left, with the three best-preserved ones indicated by arrows and shown in (4). (4) Three panels enlarged from (3) to show left-facing (top, GSN F 1939A) and right-facing individuals (middle, GSN F 1939B, bottom GSN F 1939C). (5) External mold, GSN F 1949. (6) An external mold, photographed in the field and then discarded, figured as a pseudofossil by Buatois and Mángano (Reference Buatois and Mángano2016, fig. 2.7c) that clearly shows the pleated nature of the body wall; image kindly provided by Luis Buatois, rotated through –90° so that it appears in positive rather than negative relief. (1) Scale bar = 2 cm; (2, 2 insert, 3, 5, 6) scale bars = 1 cm; (4) scale bar = 5 mm.

Materials

Six pieces of sandstone from Arimas (UCLA 7309), two from the Holoog River (UCLA 7325), and one from Kyffhauser (UCLA 7320), most having more than one specimen of Archaeichnium, plus an image of a specimen (Fig. 21.6) figured by Buatois and Mángano (Reference Buatois and Mángano2016, fig. 2.7c), generously provided by Luis Buatois.

Remarks

The holotype and other specimens on the same surface are tubular fossils that are longitudinally ridged and may or may not taper to pointed ends. Haughton (Reference Haughton1960) thought the pointed ends were closed and possibly attached to the seafloor; Glaessner (Reference Glaessner1963, Reference Glaessner1978) rejected Haughton's comparisons to archaeocyathids and thought that the tapering of the tubes was caused by their trajectory out of the bedding plane. We are confident that the tubes tapered to closed ends because they overlay surfaces sealed by microbial mats before being buried by event sands and do not leave those surfaces (Fig. 21.1, 21.2).

Glaessner (Reference Glaessner1978) thought Archaeichnium was some kind of agglutinated sand worm tube, and Turk et al. (Reference Turk, Maloney, Laflamme and Darroch2021, Reference Turk, Maloney, Laflamme and Darroch2022) have compared it to priapulid burrows, but it is clear from the specimen discovered by JGG at Arimas (Fig. 20.6) that Archaeichnium must be a body fossil, not a trace fossil. Presumably, the ladder-like feature preserved in this specimen is part of a tubular and perhaps pleated body wall. As it associated with one of the longitudinal ridges, it may be one of numerous similar structural elements, each comprising part of one of the ridges. If the external molds shown in Figure 20.7, 20.8 do represent casts of the exterior of Archaeichnium, on the basis of their co-occurrence, then all of the wall complexity would presumably lie inside this unmineralized epitheca. Thus, the ladder-like feature (Fig. 20.6) may have extended inward from the ridge crest in the form of a longitudinal septum. An associated external mold (Fig. 20.7) is another similar-sized, tubular object, but it also has ~8 longitudinal corrugations and regularly spaced commissural flanges. It may represent an external mold of a piece of the epitheca of Archaeichnium such that each longitudinal corrugation would house a projecting flange.

We also tentatively assign several specimens on the base of the slab from Kyffhauser (UCLA 7320; Fig. 21.3, 21.4) to Archaeichnium, although they may represent a completely different organism. However, they are conical, taper to a pointed and apparently attached end, are longitudinally ridged, and are comparable to Archaeichnium in size but not in length. Apart from their length/width ratios, they are similar to other specimens of Archaeichnium (e.g., Fig. 21.5). Whether the Kyffhauser specimens are assigned to Archaeichnium makes little difference to the biological interpretation of the fossil. In that context and starting from first principles, Archaeichnium appears to have had radial symmetry and a stiff but flexible body wall and probably lived attached to the substrate. Some of the Kyffhauser specimens somewhat resemble the Cambrian demosponge Takakkawia Walcott, Reference Walcott1920 (Rigby, Reference Rigby1986; Botting Reference Botting2012), but the lengths of the longer ones and the flexibility of the walls effectively rule out a poriferan affinity. Perhaps a more plausible possibility is some connection with those “Precambrian macroorganisms” (Protechiuridae; Ivantsov and Fedonkin, Reference Ivantsov and Fedonkin2002; Ivantsov et al., Reference Ivantsov, Vickers-Rich, Zakrevskava and Hall2019) that possess unmineralized conical thecae. Although most are vastly different (e.g., Protechiurus edmondsi Glaessner, Reference Glaessner1979a), there are intriguing similarities to some (e.g., Vendoconularia triradiata Ivantsov and Fedonkin, Reference Ivantsov and Fedonkin2002) in the pleating of the walls, the hint of duodeciradial symmetry, and the possibility of longitudinal flanges. Ivantsov et al. (Reference Ivantsov, Vickers-Rich, Zakrevskava and Hall2019) pointed out some similarities of the protechurids to conulariids and anabaritids and suggested that all three groups might be basal scyphozoans, so a cnidarian affinity for Archaeichnium is one potentially viable possibility. The reconstruction of the putative Ediacaran anthozoan Auroralumina attenboroughii (Dunn et al., Reference Dunn, Kenchington, Parry, Clark, Kendall and Wilby2022) is also similar in some respects to Archaeichnium, most notably in its longitudinally pleated cup and stem. A third, and perhaps even better, cnidarian comparison is with the Cambrian Stage 4 “tubicolous enigma” Gangtoucunia aspera Luo and Hu in Luo et al., Reference Luo, Hu, Chen, Zhang and Tao1999, which is thought to be a sessile, tube-dwelling stem or early crown medusozoan (G. Zhang et al., Reference Zhang, Parry, Vinther and Ma2022). Some specimens of Gangtoucunia aspera have 16–19 mesenteric septa that appear to extend along the length of the body. The tube is phosphatic and densely annulated like some Schwarzrand Subgroup tubes (Fig. 23), so it is possible that our speculative association of a radially fluted tube (Fig. 20.7) with the longitudinally ridged and possibly septate body of Archaeichnium may prove to be correct.

Another, and more likely, possibility is a relationship to the Burgess Shale and Chenjiang mackenziids Mackenzia Walcott, Reference Walcott1911 (Conway Morris, Reference Conway Morris1993) and Paramackenzia Zhao et al. Reference Zhao, Vinther, Li, Wei, Hou and Cong2021. Each has an elongate, sausage-shaped body with 10–20 longitudinal elements that are thought to be radial septa, either of the cnidarian type (Conway Morris, Reference Conway Morris1993) or the kind found between the modules of Ernietta (Zhao et al., Reference Zhao, Vinther, Li, Wei, Hou and Cong2021). However, in Paramackenzia, each septum houses shallowly inclined tubular structures that are ~1–2 mm long, ~1 mm apart, and ~0.25 mm in diameter, which are comparable in organization and dimensions to the ladder-like feature of Archaeichnium (Fig. 20.6). The tubular structures are thought to represent pore canals that were used to pump water into the body cavity of an Ernietta-like organism (Zhao et al., Reference Zhao, Vinther, Li, Wei, Hou and Cong2021), and some are filled with sediment, giving them the kind of topographic relief seen in the ladder-like feature of Archaeichnium. Although Zhao et al. (Reference Zhao, Vinther, Li, Wei, Hou and Cong2021) advanced a strong case for Ernietta-like modular construction of Paramackenzia, the presence of an inner body wall is still debatable. Conway Morris's (Reference Conway Morris1993, p. 610) description of the body wall of Mackenzia is closer to our concept of Archaeichnium (see Fig. 21.6): “It is conjectured that in life the circumference of the body was not simple but thrown into relatively deep folds and intervening ridges, the expression of which is now seen in the elevated lines and displaced margins. Further support for this comes from the distal end of some specimens which have a lobate appearance.” Thus, it is possible that even the mackenziids are total group cnidarians, although the inferred pore–canal system of Paramackenzia has no counterpart in the Cnidaria. Nevertheless, for all of these reasons, we tentatively refer Archaeichnium to the Cnidaria while acknowledging that new discoveries are needed to further explore that possibility.

Remarks

It is well known that the terminal part of the Ediacaran is replete with tubular fossils preserved in different ways: organic films, which may or may not be phosphatic or phosphatized; composite molds in siliciclastic sediments; calcareous skeletons, frequently originally aragonitic and recrystallized; and siliceous replicas of originally calcareous skeletons. Although the nature of the inhabitants of most of these tubes remains uncertain, the time immediately before the “Cambrian explosion” has become known as “Wormworld” (Schiffbauer et al., Reference Schiffbauer, Huntley, O'Neil, Darroch, Laflamme and Cai2016; Darroch et al., Reference Darroch, Smith, Laflamme and Erwin2018; Chai et al., Reference Chai, Wu and Hua2021) or—less restrictively—as “tube world” (Budd and Jackson, Reference Budd and Jackson2015). The Nama biota is characteristic in that the dominant fossils through much of the succession are vendotaeniids (Cohen et al., Reference Cohen, Bradley, Knoll, Grotzinger and Jensen2009), mineralized tubes of Cloudina (Grant, Reference Grant1990; Yang et al., Reference Yang, Warren, Steiner, Smith and Liu2022), composite molds of the bodies of Archaeichnium, and a variety of smooth or annulated tubes typically filled or cast in positive or negative hyporelief by sandy event beds (Figs. 20–23).

Remarks

Similarities in the collar-in-collar construction of the tubes of Saarina hagadorni (Selly et al., Reference Selly, Schiffbauer, Jacquet, Smith and Nelson2020) and Cloudina hartmannae Germs, 1972 (Germs, Reference Germs1972b) (Yang et al., Reference Yang, Steiner, Schiffbauer, Selly, Wu, Zhang and Liu2020, Reference Yang, Warren, Steiner, Smith and Liu2022) have given rise to the term “cloudinomorph” as a group name. If these similarities are thought to be due to relatedness rather than convergent similarity, then the family and group names Saarinidae, Sabelliditidae, and Sabelliditida (Sokolov, Reference Sokolov1965) should take precedence over Cloudinidae Hahn and Pflug, 1985 (Hahn and Pflug, Reference Hahn and Pflug1985b) and cloudinomorph.

Siboglinid annelid worms abound in Russian Arctic waters because of the widespread availability of methane from seafloor clathrates and cold seeps (Karaseva et al., Reference Karaseva, Rimskaya-Korsakova, Smirnov, Udalov, Mokievsky, Gantsevich and Malakhov2022). Discovery of this biodiversity (Ivanov, Reference Ivanov1954, Reference Ivanov1963) led to temporary acceptance of the phylum Pogonophora for the gutless siboglinids (Pleijel et al., Reference Pleijel, Dahlgren and Rouse2009) and presumably to Sokolov's (Reference Sokolov1965, Reference Sokolov1967) hypothesis that his Ediacaran genera Calyptrina, Paleolina, and Saarina—as well as Sabellidites Yanishevsky, Reference Yanishevsky1926—are Precambrian examples of the phylum. A recent in-depth study of the tubes of Sabellidites has supported Sokolov's hypothesis (Moczydłowska et al., Reference Moczydłowska, Westall and Foucher2014), but the discovery by Schiffbauer et al. (Reference Schiffbauer, Selly, Jacquet, Merz, Nelson, Strange, Cai and Smith2020) of a one-way gut in a cloudinomorph—which may be either Saarina or the related genus Costatubus (Selly et al., Reference Selly, Schiffbauer, Jacquet, Smith and Nelson2020)—would reinforce molecular evidence that the Siboglinidae are a significantly younger, highly derived clade of the Annelida (Hilário et al., Reference Hilário, Capa, Dahlgren, Halanych, Little, Thornhill, Verna and Glover2011; Vrijenhoek, Reference Vrijenhoek2013; Georgieva et al., Reference Georgieva, Little, Watson, Sephton, Ball and Glover2019, Reference Georgieva, Little, Maslennikov, Glover, Ayupova and Herrington2021; Capa and Hutchings, Reference Capa and Hutchings2021). However, Eoalvinellodes annulatus (Little et al., Reference Little, Maslennikov, Morris and Gubanov1999) is a pyritized annulated worm tube from a Silurian fossil hydrothermal vent site in Russia (Georgieva et al., Reference Georgieva, Little, Watson, Sephton, Ball and Glover2019), which may imply that it had a chemosymbiotic lifestyle. Some other tube-dwelling polychaetes that inhabit vents and seeps obtain nutrients from bacterial symbionts in their respiratory crowns (Goffredi et al., Reference Goffredi, Tilic, Mullin, Dawson and Keller2020), a less-derived mode of chemosymbiosis that may have been in operation lower in the annelid tree. Thus, a non-siboglinid annelid affinity for the sabelliditid tubes remains a prime possibility and is compatible with the existence of a non-siboglinid, tube-dwelling polychaete (Dannychaeta) in a Cambrian Stage 3 fauna in China (Chen et al., Reference Chen, Parry, Vinther, Zhai, Hou and Ma2020).

Landing et al. (Reference Landing, Geyer, Jirkov and Schiaparelli2021) have also argued for extending the stratigraphic range of siboglinid and sabellid polychaetes into the Ediacaran on the basis of their reinterpretation of the early Cambrian stem gastropod Pelagiella exigua (Resser and Howell, Reference Resser and Howell1938), which preserves two fan-shaped arrays of chitinous chaetae (Thomas et al., Reference Thomas, Runnegar and Matt2020). Their new sabellid genus Pseudopelagiella is based on P. exigua but is considered characteristic of species such as Pelagiella subangulata (Tate, Reference Tate1892), which have triangular apertures (e.g., Mghazli et al., Reference Mghazli, Lazreq, Geyer, Landing, Boumehdi and Youbi2023) and an inner shell layer made from foliated aragonite (Runnegar in Bengtson et al., Reference Bengtson, Morris, Cooper, Jell and and Runnegar1990, fig. 169B). The presence of an identical microstructure in Aldanella attleborensis (Shaler and Foerste, Reference Shaler and Foerste1888; Qiang et al., Reference Qiang, Guo, Li, Song, Peng, Sun, Han and Zhang2023), which Landing et al. (Reference Landing, Geyer, Jirkov and Schiaparelli2021) accept as a stem gastropod, and in the stem lineage bivalves Fordilla troyensis (Barrande, Reference Barrande1881) and Pojetaia runnegari (Jell, Reference Jell1980; Runnegar and Pojeta, Reference Runnegar and Pojeta1992; Vendrasco et al., Reference Vendrasco, Checa and Kouchinsky2011), makes the probability that species referred to “Pseudopelagiella” are annelids rather than mollusks vanishingly small.

Bobrovskiy et al. (Reference Bobrovskiy, Nagovitsyn, Hope, Luzhnaya and Brocks2022) have recently redescribed one of Sokolov's sabelliditid species, Calyptrina striata Sokolov, Reference Sokolov1967, and have extracted biomarker molecules from an organically preserved specimen of its tube. The proportion of cholestane, the diagenetically derived end product of cholesterol, was ~9% in Calyptrina, a little less than the background average for the Lyamtsa locality (~11%; Bobrovskiy et al., Reference Bobrovskiy, Hope, Golubkova and Brocks2020) and thus very different from the much higher average amount (~50%) found in large specimens of Dickinsonia from the same site (Bobrovskiy et al., Reference Bobrovskiy, Hope, Ivantsov, Nettersheim, Hallmann and Brocks2018). Furthermore, the ratio of two cholestane isomers (5ß/5α) was extraordinarily high (~4) in Dickinsonia but similar to that expected from diagenesis (~0.65) in Calyptrina. Conversely, other lighter and heavier steranes from Calyptrina have unusually low 5ß/5α ratios (~0.2) compared with Dickinsonia, where the average value (~0.7) is indistinguishable from the diagenetic expectation. Bobrovskiy et al. (Reference Bobrovskiy, Nagovitsyn, Hope, Luzhnaya and Brocks2022) used these and other data to conclude that almost all of the steranes in Dickinsonia were derived from cholesterol in body tissue that had been decomposed by anaerobic bacteria rather than from dietary cholesterol in a one-way gut, the usual source of 5ß-cholestane in younger paleobiological, archaeological, and forensic contexts (Runnegar, Reference Runnegar2022). Calyptrina, they suggested, had lost its tissue cholesterol by not being decomposed by anaerobes, and the low 5ß/5α ratios of its other steranes was due to the processing of dietary sterols derived from algal food sources by aerobic bacteria. This complex argument depends on many questionable assumptions, including a comparison with Kimberella (Glaessner and Wade, Reference Glaessner and Wade1966), which they assumed to be a bilaterian with an alimentary canal. If that assumption is incorrect, then the case for a gut in Calyptrina, based on biomarkers, is even weaker. Given that 88% of the total steranes detected in Calyptrina come from green algae and that the clay underlying Calyptrina is similar to bulk rock extracts from the White Sea area (Bobrovskiy et al., Reference Bobrovskiy, Hope, Golubkova and Brocks2020, table S1, Reference Bobrovskiy, Nagovitsyn, Hope, Luzhnaya and Brocks2022, table S1), perhaps the simplest explanation is that the part of the tube analyzed was not inhabited at the time of fossilization.

Type species

Calyptrina partita Sokolov, Reference Sokolov1965 by original designation.

Other species

Calyptrina striata Sokolov, Reference Sokolov1967 from the Syuzma beds of the Ust-Pinega Formation (>552.85 ± 0.77 Ma) in a borehole at Obozerskaya (40.31°E, 63.45°N), ~200 km south of Arkhangelsk, Russia (Sokolov, Reference Sokolov1967; Stankovskiy et al., Reference Stankovskiy, Verichov, Grib and Dobeyko1983; Xiao et al., Reference Xiao, Yuan, Steiner and Knoll2002; Ivantsov et al., Reference Ivantsov, Vickers-Rich, Zakrevskava and Hall2019; Bobrovskiy et al., Reference Bobrovskiy, Nagovitsyn, Hope, Luzhnaya and Brocks2022).

Description

Sand-filled tubes, ~5 mm in diameter and up to 10+ cm long, that were probably originally circular in cross section, now slightly flattened, are in places ornamented with regularly spaced well-separated circumferential ridges, which presumably strengthened the tube wall.

Material

Eleven sandstone gutter casts, each containing several to many individual tubes, from Kyffhauser (UCLA 7320) and single specimens, possibly of the same form, on bed bases from Arimas (UCLA 7309) and the Holoog River (UCLA 7325).

Remarks

It is not known whether the corrugated parts of these tubes represent a different stage of growth from the smooth and seemingly thicker parts or are due to differences in preservation. However, there is little doubt that these corrugated tubes are neither body fossils nor trace fossils but rather the secreted dwelling structures of a worm-shaped animal. Rare specimens from the Holoog River (Fig. 22.5) and Arimas (Fig. 22.6) are tentatively included in this form taxon.

Figure 22. Coarsely and regularly annulated tubes, cf. Calyptrina striata Sokolov, Reference Sokolov1967 (2–6), smooth tubes (1, 7), and two important specimens of Archaeichnium haughtoni Glaessner, Reference Glaessner1963 (8, 9) from the Neiderhagen Member, Nudaus Formation, UCLA 7320, Kyffhauser farm (1–4), the Huns Member, Urusis Formation, UCLA 7325, Holoog River (5, 7), and UCLA 7309, Arimas farm (5, 8, 9). (1) Bed base with sandstone casts of numerous small, short, conical tubes plus one wider, coarsely annulated, kinked tube (arrow), GSN F 1941. (2) Base of gutter with sandstone cast of one coarsely annulated tube, GSN F 1944. (3) Top of tube-filled gutter cast, found by D.H. Erwin in 1995, with one annulated tube indicated by the arrow, GSN F 1943. (4) Top, end, and base of small section of a gutter cast with one enclosed coarsely annulated tube indicated by the arrow, GSN F 1945. (5) Cast of irregular annulated tube on bed base, Holoog River, GSN F 1973. (6) A somewhat similar structure, Arimas, GSN F 1934. (7) Small sandstone slab with casts, many presumably current-aligned smooth tubes, GSN F 1976. (8) Recognizable specimen of Archaeichnium haughtoni that is on the same surface as the “Arimas lycopod” (Fig. 14.5), thus demonstrating co-occurrence of these two taxa, GSN F 1910. (9) Quartz filling of Archaeichnium haughtoni that gives some information about its cross-sectional shape before burial and compaction, GSN F 1919. (1, 3, 4, 7) Scale bars = 2 cm; (2, 5, 6, 8, 9) scale bars = 1 cm.

A variety of sparsely annulated tubular fossils have been assigned to Calyptrina striata, which was based on a single compressed pyritized specimen (Sokolov, Reference Sokolov1967; Bobrovskiy et al., Reference Bobrovskiy, Nagovitsyn, Hope, Luzhnaya and Brocks2022, fig. S4H, I). The redescription of the species from numerous White Sea examples preserved in different ways (Bobrovskiy et al., Reference Bobrovskiy, Nagovitsyn, Hope, Luzhnaya and Brocks2022) revealed that the apertural part of the tube had regularly spaced wall thickenings that were robust enough to leave deep grooves in external molds and are clearly visible in mineralized compressions (Bobrovskiy et al., Reference Bobrovskiy, Nagovitsyn, Hope, Luzhnaya and Brocks2022, fig. S4D, H, I).

Between the thickened annulations, the wall has fine longitudinal costae that would not be visible in our material because of the grain size of the sandstone gutter casts. Bobrovsky et al. (Reference Bobrovskiy, Nagovitsyn, Hope, Luzhnaya and Brocks2022) also provided excellent evidence that the apertural end of the tube projected above the seafloor during life and that the longer, buried portion of the tube ran horizontally and changed character gradationally along its length to become finely and regularly annulated or finely and irregularly annulated, features we found in tubular fossils from other localities (Fig. 23.1–23.3, 23.6, 23.9). However, as there is no direct evidence for a biological connection between these variously ornamented tubes, we describe the finely annulated ones separately. Annulated structures from Ediacara identified as the meniscuate trace fossil Taenidium cf. T. serpentinum (Heer, Reference Heer1877) by Jenkins (Reference Jenkins1995) are superficially similar to C. striata but are consistently short and banana shaped (Reid et al., Reference Reid, García-Bellido, Payne, Runnegar and Gehling2017) and probably not circular in cross section.

Figure 23. Various annulated tubes, cf. Sinotubulites baimatuoensis Chen, Chen, and Qian, Reference Chen, Chen and Qian1981 (1–3, 6, 9) and cf. Sekwitubulus annulatus Carbone et al. (4, 5, 7, 8), Feldschuhhorn Member, Urusis Formation, UCLA 7377, Swartkloofberg farm (1, 2), Urikos Member, Zaris Formation, UCLA 7383 and UCLA 7384C, Zaris farm and Zaris Pass (3, 5, 6), and Huns Member, Urusis Formation, UCLA 7309, Arimas farm (4, 7, 8) and UCLA 7325, Holoog River (9). (1, 2) Two views of a kinked tube on a presumed lower fine-grained carbonate bed surface, GSN F 1935. (3) Two finely annulated tubes on the lower surface of a carbonate slab that has OES texture, GSN F 1982. (4) Small piece of crisply annulated tube, GSN F 1918. (5) Another crisply annulated tube, GSN F 1984. (6) Bed base casts of finely annulated tubes (seen in positive relief in insert), GSN F 1966. (7, 8) Narrow annulated tube, seen in bed base context in (8), GSN F 1953. (9) Section of finely annulated tube, bed base, GSN F 1971. (1, 3, 5, 6 insert, 7, 9) Scale bars = 1 cm; (2, 4) scale bars = 5 mm; (6) scale bar = 2 cm; (8) scale bar = 5 cm.

Type species

Sinotubulites baimatuoensis Chen, Chen, and Qian, Reference Chen, Chen and Qian1981, from the Shibantan Member of the Dengying Formation, Shibantan of Yichang City, China, by original designation.

Remarks

We tentatively refer some regularly annulated tubular fossils to this species, which is based on rather poorly preserved type material and has been identified from Ediacaran deposits in many parts of the world (Yang et al., Reference Yang, Warren, Steiner, Smith and Liu2022). As currently diagnosed, the species is a form taxon that is sufficiently broadly defined to accommodate the Namibian material for the time being. There are also some similarities to Wutubus annularis (Chen et al., Reference Chen, Zhou, Xiao, Wang, Guan, Hua and Yuan2014) as discussed in the following.

Description

External molds preserved in calcareous siltstone and sandstone of a an originally cylindrical finely annulated tube, having either dispersed irregular narrow ridges (Fig. 23.1, 23.2) or tightly packed narrow annulations (Fig. 23.3, 23.6, 23.9); length of tube at least 140 mm, diameter 2–6 mm; longest known tube appears to taper from ~3.5 to ~2 mm; severely kinked specimen (Fig. 23.1) demonstrates that the tube wall was unmineralized and flexible.

Material

One large, kinked specimen from Swartkloofberg (UCLA 7377), three slabs with five specimens from Zaris (UCLA 7383), five slabs with one or more specimens from Zaris Pass (UCLA 7384C), and eight slabs from the Holoog River (UCLA 7325), only some of which may be tentatively included in this category.

Remarks

There are similarities to some specimens of Calyptrina striata (e.g., Bobrovskiy et al., Reference Bobrovskiy, Nagovitsyn, Hope, Luzhnaya and Brocks2022, figs S2E, S4C), but in the absence of examples with widely spaced coarse rugae, membership of that species is unlikely. Sinotubulites baimatuoensis has fewer distinctive features, being simply an irregularly to regularly annulated tube, but we tentatively refer these Namibian tubes to that form species. Wutubus annularis (Chen et al., Reference Chen, Zhou, Xiao, Wang, Guan, Hua and Yuan2014) is also similar, but some specimens taper rapidly to a closed apex that is presumed to be the site of attachment to the substrate. Our material tapers far more slowly (Fig. 23.6), and there is no evidence for a closed end. Annulatubus flexuosus (Carbone et al., Reference Carbone, Narbonne, Macdonald and Boag2015) is more regularly annulated and fits better with the Sekwitubus–Corumbella–Shaanxilithes morphotype according to the analysis by Dunn et al. (Reference Dunn, Kenchington, Parry, Clark, Kendall and Wilby2022).

Description

Cylindrical tubes, 2–3 mm in diameter, ornamented with regularly spaced angular ridges that are not obviously collar shaped, are not tapered, and do not appear to have been mineralized.

Material

About half a dozen short pieces of tubular fossils found as external molds on the bases of slabs from Arimas (UCLA 7309), the Holoog River (UCLA 7325), and Zaris (UCLA 7383).

Remarks

There are many similarly ornamented tubular structures in the Ediacaran, and the morphology persists to the present, as exemplified by the living terebellid annelid Glyphanostomum pallescens (Georgieva et al., Reference Georgieva, Little, Watson, Sephton, Ball and Glover2019). Sekwitubulus annulatus is a comparable, incompletely known regularly annulated tubular fossil from the Blueflower Formation, northwest Canada, described by Carbone et al. (Reference Carbone, Narbonne, Macdonald and Boag2015), who compare it with previously described Ediacaran genera.

Remarks

We illustrate but do not describe specimens of Aspidella sp. and Beltanelliformis brunsae Menner in Keller et al., Reference Keller, Menner, Stepanov and Chumakov1974 from Aar (UCLA 7307) and Palaeopascichnus sp. from the Namaland (UCLA 7315) for the sake of completeness. One locality has yielded three specimens of Pseudorhizostomites (Sprigg, Reference Sprigg1949; Fig. 24.6), best interpreted as the removal trace of a frond (Tarhan et al., Reference Tarhan, Droser, Gehling and Dzaugis2015). We also show two examples of scratch circles (Osgood, Reference Osgood1970; Jensen et al., Reference Jensen, Högström, Almond, Taylor, Meinhold, Høybergt, Ebbestad, Agić and Palacois2018), one of which is on a bed base and has a conical plug at its center (Fig. 24.7) that resembles structures described (Darroch et al., Reference Darroch, Cribb, Buatois, Germs and Kenchington2021) as the conical burrows Conichnus (Männil, Reference Männil and Hecker1966) and Bergaueria (Prantl, Reference Prantl1946) and presumably was the entrance to the home of the producer. Other blister-like structures on bed bases (Fig. 24.5) are best interpreted as incipient syneresis cracks.

Figure 24. Miscellaneous body fossils and trace fossils. (1) Aspidella sp., Aarhauser sub-member, Kliphoek Member, Dabis Formation, UCLA 7307, Aar farm, GSN F 1894. (2) Aspidella terranovica Billings, Reference Billings1872, Fermeuse Formation, St. John's Group, Ferryland, Avalon Peninsula, Newfoundland, UCLA 7335.1, for comparison with (1). (3) Beltanelliformis brunsae Menner in Keller et al., Reference Keller, Menner, Stepanov and Chumakov1974, characteristic closely packed aggregate, top of Kliphoek Member, Dabis Formation, UCLA 7311, Kliphoek farm. (4) Palaeopascichnus sp., base of very thin sandstone, Buchholzbrunn Member, Dabis Formation, UCLA 7315, Namaland district, near Bethanien, GSN F 1892. (5) Lens-shaped blisters, possibly sandstone casts of syneresis cracks, base of bed, same locality as (4). (6, 9) radially grooved disks reminiscent of Pseudorhizostomites Sprigg, Reference Sprigg1949, bed base and counterpart cast of bed base, same locality as (4), GSN F 1893 and GSN F 1895, respectively. (7) Sandstone cast of scratch circle and funnel-shaped hole made by rotating tethered object, Huns Member, Urusis Formation, UCLA 7309, Arimas farm, GSN F 1912. (8) Concentric scratch circles on ripple-marked bed top, same locality as (7), GSN F 1917. (10) Archaeonassa isp., positive relief, Urusis Formation, UCLA 7325, Holoog River, GSN F 1979. (11) Gordia isp. in positive hyporelief, thin sandstone bed, Urikos? Member, Zaris Formation, UCLA 7384A, Zaris Pass, GSN F 1970. (12) Helminthopsis isp., sinuous channel, either a trace or a body fossil, on a rippled bed top, same locality as (7), GSN F 1915. (13, 14) Gordia isp., two small slabs from the same bed with possibly the oldest known trace fossils from the Nama Group, Kliphoek Member, Dabis Formation, UCLA 7378, Twyfel farm, GSN F 1920 and GSN F 1921, respectively; the trace fossils occur with syneresis cracks, e.g., left of center in (13). (1–9, 11, 12) Scale bars = 2 cm; (10, 13, 14) scale bars = 1 cm.

Remarks

The Ediacaran ichnofossil record of Namibia has recently been reviewed by Darroch et al. (Reference Darroch, Cribb, Buatois, Germs and Kenchington2021) and Turk et al. (Reference Turk, Maloney, Laflamme and Darroch2022). Our contribution is focused on bilaterian traces from the Kliphoek Member (Fig. 24.11–24.13) and evidence for a substantial infauna of small animals during Schwarzrand time (Figs 25.5, 26.1–26.8). We also discuss the evidence for the first occurrence of treptichnids in the Nama succession and briefly review ichnological arguments for and against treating the upper part of the Spitskop Member as earliest Cambrian.

Figure 25. Ediacaran and Cambrian trace fossils. (1) Treptichnus isp? and Helminthopsis isp., GSN F 1937, base of thin sandstone slab with continuous and intermittent traces preserved in convex hyporelief, both possibly made by the same organism, and comparable to the traces shown in (2), ~160 m above base of Huns Member, Urusis Formation, UCLA 7371, Arimas farm. (2) Treptichnus isp. and numerous microburrows, Ariichnus vagus n. igen. n. isp., all preserved in convex hyporelief, ISAM K4366, Huns Member, Urusis Formation, Arimas farm, found by G.J.B. Germs before 1972, photographed in Cape Town, South Africa, in 1993. (3, 4) Treptichnus pedum (Seilacher, Reference Seilacher, Schindewolf and Seilacher1955), lower bed surfaces, Nomtsas Formation, UCLA 7324, Sonntagsbrunn farm, GSN F 1951 and GSN F 1952, respectively. (5) Subhorizontal burrows excavated and filled by sediment filling a gutter as evidenced by breaks in the continuity of the borrows (arrow), GSN F 1923, found by A.J. Kaufman in 1995, Nasep Member, Urusis Formation, UCLA 7322, Swartkloofberg farm. (6) Gordia isp., looping traces on the top surface of a rippled slab, GSN F 1925, Huns Member, Urusis Formation, UCLA 7326, Arimas Farm. (1–4) Scale bars = 1 cm; (5, 6) scale bars = 2 cm.

Figure 26. Gutter casts and microburrows of Ariichnus vagus n. isp., Buchholzbrunn Member, Dabis Formation, UCLA 7314, Namaland district (3) and Huns Member, Urusis Formation, UCLA 7326, Arimas farm (1, 2, 4–8). (1, 2) GSN F 1911, sandstone cast of large gutter, viewed from side and bottom, with microburrow traces on the shallower parts of the cast. (3) Cross section of a sandstone-filled gutter cast, embedded in a thin sandstone event bed, and comparable to samples found as float elsewhere. (4) GSN F 1929, flat base of an event bed that cast erosional intersections with many microburrows. (5) Upper and lower surfaces of a channel cast topped by hummocky stratification (rectangle shows location of the holotype; arrow indicates ripple crest), GSN F 1931; lower surface enlarged in (6). (6) GSN F 1931, enlargement of lower surface of channel (5, 6) showing numerous casts of microburrows; insert is an enlargement of the holotype, which is on another part of the same surface (5). (7, 8) GSN F 1927, oblique and cross-sectional views of a well-formed channel that has cast microburrows above the level of the white arrows and below the level of black arrows, a stratigraphic interval of ~3 cm. (1, 2, 5, 7, 8) scale bars = 5 cm; (3) camera lens cap = 60 mm; (4, 6) scale bars = 1 cm; (insert in 6) scale bar = 1 mm.

Type ichnospecies

Archaeonassa fossulata Fenton and Fenton, Reference Fenton and Fenton1937 from the Cambrian Series 3, Stage 5, Mt. Whyte Formation, Alberta, Canada (Fenton and Fenton, Reference Fenton and Fenton1937; Yochelson and Fedonkin, Reference Yochelson and Fedonkin1997), by monotypy.

Remarks

A single specimen, GSN F 1979, from the base of the Huns Member at Holoog River (UCLA 7325) shows the characteristic U-shaped end of this groove and ridged trace; better examples were illustrated by Turk et al. (Reference Turk, Maloney, Laflamme and Darroch2022, fig. 7.1) from their Canyon Roadhouse site. Archaeonassa Fenton and Fenton, Reference Fenton and Fenton1937 ranges from the late Ediacaran to the present (Jensen, Reference Jensen2003; Uchman and Martyshyn, Reference Uchman and Martyshyn2020).

Type ichnospecies

Ariichnus vagus n. igen. n. isp. from the Huns Limestone Member, Urusis Formation, Arimas farm, Southern Namibia.

Diagnosis

As for the type species by montypy.

Etymology

Contraction of Arimas (farm) and ichnos, Greek for footprint or track.

Holotype

Burrow shown in insert of Figure 26.6, on slab GSN F 1931, from the Huns Limestone Member, Urusis Formation, Arimas farm, Southern Namibia.

Diagnosis

Narrow, subhorizontal, dichotomously branching burrows that follow irregular paths and occasionally cross over each other.

Description

Narrow, subhorizontal cylindrical burrows, ~0.3 mm in diameter, exhibiting occasional Y-shaped junctions that multiply the total number of terminations away from the burrow entrance; in the holotype, adjacent branches are irregular in the forward direction and cross each other without intersecting (see also Darroch et al., Reference Darroch, Cribb, Buatois, Germs and Kenchington2021, fig. 9c, right arrow; Turk et al., Reference Turk, Maloney, Laflamme and Darroch2022, fig. 8.3); numerous circular cross sections presumably represent vertical segments that connect the subhorizontal tunnels into a complex three-dimensional network.

Etymology

Latin, vagus (roving, wandering), in reference to the paths of the primary branches of the burrows.

Material

Six sandstone gutter casts, GSN F 1924–1931, from the same locality, each with numerous examples of microburrows intersected and cast by the sandstone channel fills plus the specimen collected by Germs at Arimas.

Remarks

The three-dimensional geometry of these microburrow systems is difficult to reconstruct from the curved two-dimensional surfaces on which they are observed. It does, however, appear that they formed systems with true branching and are not merely the result of coincidental interference. The surface expression resembles planar sections of Chondrites burrow systems seen in thin and polished sections and core slices (Ekdale and Bromley, Reference Ekdale and Bromley1982; Bromley and Ekdale, Reference Bromley and Ekdale1984; Baucon et al., Reference Baucon, Bednarz, Dufour, Felletti and Malgesini2020). The diameter of the burrows is smaller than that of most ichnospecies of Chondrites but overlaps with Chondrites intricatus (Brongniart, Reference Brongniart1828), in which the strings are smaller than 1 mm (Fu, Reference Fu1991); Ekdale and Bromley (Reference Ekdale and Bromley1982) and Uchman (Reference Uchman1999) also recorded occurrences with a burrow diameter as narrow as 0.2–0.3 mm. The wandering pathways their primary branches seem to follow (Fig. 26.6; Darroch et al., Reference Darroch, Cribb, Buatois, Germs and Kenchington2021, fig. 9c; Turk et al., Reference Turk, Maloney, Laflamme and Darroch2022, fig. 8.3) distinguish Ariichnus vagus from all previously described ichnospecies of Chondrites.

Reconstruction of the three-dimensional geometry is problematic because of the small size and style of preservation, but it appears to be less complex than is typical for Chondrites, so attribution of the new ichnospecies to Chondrites was not desirable. It should be noted that this would have been the first Ediacaran record of Chondrites, earlier reports of this age having been rejected (Jensen and Runnegar, Reference Jensen and Runnegar2005; L. Zhang et al., Reference Zhang, Buatois and Mángano2022). The ichnogenus is rare also in the Cambrian, without a single accepted Terreneuvian or Series 2 occurrence (Mángano and Buatois, Reference Mángano and Buatois2014; but see Baucon et al., 2022 for possible exception). Chondrites from the Teltawongee Group of New South Wales, Australia (Webby, Reference Webby1984), sometimes cited as Terreneuvian (e.g., L. Zhang et al., Reference Zhang, Buatois and Mángano2022), is in strata of poorly constrained age. Webby (Reference Webby1984) considered the occurrence to be no older than early or middle Cambrian, on the basis of the trace fossils, and they remain the main criteria for the maximum depositional age of the group, with minimum depositional ages obtained from cross-cutting volcanics dated at 505 and 515 Ma (Johnson et al., Reference Johnson, Phillios and Allen2016).

An alternative ichnogeneric assignment of the Nama material could have been to Pilichnus, an ichnogenus that Uchman (Reference Uchman1999) erected as part of the Chondrites group of branched structures. The Cretaceous type material has straight to winding strings 0.15–0.35 mm wide, with dichotomous branches. Subsequent reports have extended this ichnogenus to the Terreneuvian (e.g., Buatois and Mángano, Reference Buatois and Mángano2012). The latest Ediacaran Pilichnus from the Tamengo Formation, Brasil (Adôrno, Reference Adôrno2019) has larger dimensions and rare branching. A possible difference of the Nama material from Pilichnus is that the orientation of Pilichnus is mainly along a horizontal plane, although comparison of this ichnogenus with modern traces has been made with more vertically oriented traces (Hertweck et al., Reference Hertweck, Wehrmann and Liebezeit2007).

At the type locality, the microburrows are preserved on the sides and bases of sandstone gutter casts such as the one from the Buchholzbrunn Member seen in outcrop (Fig. 26.3). The Arimas gutter casts were float samples, but the level from which they came is well constrained (Fig. 4; Turk et al., Reference Turk, Maloney, Laflamme and Darroch2022). The microburrows are confined to certain parts of the channel walls and are not found below or above the presumed bioturbated interval that is ~3 cm deep and lies 1–2 cm below the surface (Fig. 26.7, 26.8). This distribution eliminates all four of the hypotheses proposed by Turk et al. (Reference Turk, Maloney, Laflamme and Darroch2022) for their formation: opportunistic colonization of exposed sediment in gutters, selective preservation in gutters of a ubiquitous infauna, lithologic contrast between gutter-filling sandstone and guttered substrate, and reborrowing of the channel surface by small animals carried with the eroding fluid. However, they also said that “small bilaterian traces … might be more widespread in these intervals than is currently recognized,” as we suggest. As the tops of the channel sands are rippled (Fig. 26.5), the original height of the seafloor was probably lower than the tops of the channel-filling sand (Fig. 26.1, 26.3, 26.5, 26.8). Nevertheless, it is clear that the microburrowed interval began at least 1 cm below the sediment–water interface and continued downward for ~3 cm. This suggests that the tracemakers were exploiting a particular part of the redox gradient and, like Chondrites, may have been adapted to dysaerobic conditions (Bromley and Ekdale, Reference Bromley and Ekdale1984; Savrda and Bottjer, Reference Savrda and Bottjer1987).

Parry et al. (Reference Parry, Boggiani, Condon, Garwood and Leme2017) described a dense occurrence of “meiofaunal ichnofossils” from the terminal Ediacaran of Brazil and identified them as Multina minima Uchman (Reference Uchman2001), an Eocene species of Multina Orlowski in Orlowski and Zylinska, Reference Orlowski and Zylinska1996 from the late Cambrian of the Holy Cross Mountains, Poland (Orlowski and Zylinska, Reference Orlowski and Zylinska1996). However, both of these ichnospecies of Multina are meshes, not networks, and even M. minima has a much larger tunnel diameter than the Brazilian meiofaunal burrows. There is a better size comparison with the Namibian structures, but although the Brazilian traces are described as having “rare dichotomous branches” (Parry et al., Reference Parry, Boggiani, Condon, Garwood and Leme2017, p. 1456; Adôrno, Reference Adôrno2019), the tomographic reconstructions show little evidence for branching. On the basis of the small diameter of their narrowest burrows, Parry et al. (Reference Parry, Boggiani, Condon, Garwood and Leme2017) were able to exclude most bilaterian phyla as possible tracemakers and opted for a nematode-like worm that lacked the ability to move by peristalsis. We think that is unlikely for the Namibian microburrows because of the irregularity of the trajectories of the tunnels. Nematodes move by bending their bodies in a sinusoidal fashion and either make sinusoidal traces (Balinski and Sun, Reference Balinski and Sun2015) or generate “meioturbation” rather than well-defined burrows (Schieber and Wilson, Reference Schieber and Wilson2021). Thus, it seems more likely that the Namibian microburrows were produced by small animals using hydrostatic processes. The significance of a sizeable bioturbated zone, well below the sediment–water interface and apparently decoupled from the widely assumed Ediacaran microbial mat communities, is explored under Discussion. Another kind of subterranean trace fossil community is indicated by Planolites-like burrows intersected by a gutter cast in the top of the Nasep Member on Swartkloofberg (UCLA 7322; Fig. 25.5). Thus, it seems that by the close of the Ediacaran, some bioturbation had moved well below the level of microbial mats. Although probably morphologically distinct, the Nama microburrows compare to, and predate, Fortunian material of Olenichnus irregularis Fedonkin in Sokolov and Iwanowskii (Reference Sokolov and Iwanowskii1985) from Siberia that Marusin and Kuper (Reference Marusin and Kuper2020) interpreted as complex three-dimensional endobenthic tunnel systems made by bilaterians.

Trace fossils are rarely preserved in pot and gutter casts and, if present, are thought to be post-depositional (Myrow, Reference Myrow1992; Jensen, Reference Jensen1997; Mángano et al., Reference Mángano, Buatois, West and Maples2002). However, the Planolites-like burrows from the Nasep Member are interrupted by the gutter cast wall (arrow, Fig. 25.5), and the microburrows from Arimas were clearly exposed by the erosive action that created the gutters. By contrast, the body fossils, which are occasionally preserved in gutter casts or on the bases of channels (Figs. 12.3–12.5, 16, 21.3, 21.4, 22.4), are thought to have been transported by the eroding events and are, like the tool marks, synchronous with them.

Type ichnospecies

Gordia marina Emmons (Reference Emmons1844) from an Ordovician “fine flagging stone” (calcareous turbidite) of the Giddings Brook slice, Taconic Allochthon (Landing, Reference Landing2012), at “Mr. M‘Arthur's quarry,” Jackson, New York, by monotypy.

Material

Three small slabs from Twyfel, GSN F 1920–1922, one specimen from Arimas, GSN F 1925, and other examples observed in the field.

Remarks

Gordia is one of four Ediacaran ichnogenera/behaviors identified in simple horizonal trails (Buatois and Mángano, Reference Buatois and Mángano2016) and is characterized by common self-crossings. The examples we illustrate are typical, but those from the Kliphoek Member on Twyfel (Fig. 24.11, 24.13, 24.14) are older than previous records from Namibia and extend this kind of behavior downward from the Schwarzrand Subgroup (Darroch et al., Reference Darroch, Cribb, Buatois, Germs and Kenchington2021, fig. 18b) into the Kuibis Subgroup. There has been some difference of opinion about the stratigraphic position of the fossiliferous intervals on Twyfel, Weigkrup, Hansburg, and Zuurberg farms (Bouougri et al., Reference Bouougri, Porada, Weber and Reitner2011; Maloney et al., Reference Maloney, Boag, Facciol, Gibson, Cribb, Koester, Kenchington, Racicot, Darroch and Laflamme2020), but we agree with Maloney et al. (Reference Maloney, Boag, Facciol, Gibson, Cribb, Koester, Kenchington, Racicot, Darroch and Laflamme2020) in identifying these horizons as either upper Kliphoek Member or Bucholzbrunn Member rather than Kanies Member. Thus, these specimens of Gordia isp. and some associated finer trails are the oldest known trace fossils from Namibia. A looped trace from the Huns Member on Arimas (Fig. 25. 6) may also be referred to Gordia isp., but it is clearly different in detail from the Twyfel examples.

Type ichnospecies

Helminthopsis magna Heer, Reference Heer1877 by subsequent designation of Ulrich (Reference Ulrich1904, p. 144) or Helminthopsis abeli Książkiewicz, Reference Książkiewicz1977 (Han and Pickerill, Reference Han and Pickerill1995) or Helminthopsis hieroglyphica Wetzel and Bromley, Reference Wetzel and Bromley1996.

Remarks

Wetzel and Bromley (Reference Wetzel and Bromley1996) proposed Helminthopsis hieroglyphica to retain Heer's generic name, which was based on material that should be referred to other ichnogenera, so H. hieroglyphica Wetzel and Bromley, Reference Wetzel and Bromley1996 has gained acceptance as the replacement type species (e.g., Šamánek et al., Reference Šamánek, Vallon, Mikuláš and Vachek2022).

Material

Only two possible examples of many similar structures are illustrated. The long continuous trace in Figure 25.1 is best characterized as the form genus Helminthopsis but may, in fact, have been generated by the producer of Treptichnus isp? (Fig. 25.1, arrows).

Remarks

According to Buatois and Mángano (Reference Buatois and Mángano2016, p. 41) “Helminthopsis displays a tendency to meander,” so we refer simple cylindrical traces with this property to Helminthopsis. However, as noted by many others, it may be difficult to distinguish such ichnofossils from tubular body fossils. The specimen shown in Figure 25.1 is fairly clearly a trace fossil and may, in fact, be a variety of Treptichnus, which occurs next to it on the same slab. The one shown in Figure 24.12 is more ambiguous, and there are many more poorly preserved traces like this in the Schwarzrand Subgroup that could be either trace or body fossils.

Type ichnospecies

Streptichnus narbonnei Jensen and Runnegar, Reference Jensen and Runnegar2005 from UCLA 7375, uppermost Spitskop Member, Urusis Formation, Swartpunt farm, southern Namibia, by original designation and monotypy.

Holotype

One of two adjoining slabs, GSN F 626 (Jensen and Runnegar, Reference Jensen and Runnegar2005, fig. 2b), from the Spitskop Member, Urusis Formation, UCLA 7375, 13 m below the summit of Dundas Hill, Swartpunt farm, southwestern Namibia.

Remarks

The complexity of the Streptichnus burrow system is comparable to that of Treptichnus pedum, so Linnemann et al. (Reference Linnemann, Ovtcharova, Schaltegger, Gärtner and Hautmann2019) advocated lowering the Ediacaran–Cambrian boundary to just below the level of Streptichnus in the Dundas Hill section on Swartpunt (Fig. 5). However, the subsequent discovery of Streptichnus in the Shibantan Lagerstätte of South China (Xiao et al., Reference Xiao, Chen, Pang, Zhou and Yuan2021; Mitchell et al., Reference Mitchell, Evans, Chen and Xiao2022) confirms that it is associated with typical Ediacaran taxa.

Type ichnospecies

Treptichnus bifurcus Miller, Reference Miller1889 from the Carboniferous, Gzhelian, Mansfield Formation of Indiana (Maples and Archer, Reference Maples and Archer1987), by original designation and monotypy.

Holotype

Specimen figured by Seilacher (Reference Seilacher, Schindewolf and Seilacher1955, p. 387, fig. 4a) from the early Cambrian Neobulus shale (Khussak Formation), Salt Range, Pakistan (Buatois, Reference Buatois2018).

Material

Three specimens from the Nomtsas Formation on Sonntagsbrunn, GSN F 1950–1952.

Remarks

See Wilson et al. (Reference Wilson, Grotzinger, Fischer, Hand and Jensen2012) for a full description of this species and its occurrence in Namibia.

Materials

A single slab from Arimas, ISAM K4366, described originally by Germs (Reference Germs1972a, Reference Germsb) and another similar-sized slab collected by us (Fig. 25.2) from Arimas (UCLA 7371).

Remarks

Germs's discovery slab was not in place, so the stratigraphic level from which it was derived is uncertain. Germs (Reference Germs1972b, fig. 2) showed the horizon as being immediately beneath the base of the Huns Limestone Member as he then defined it, which would probably place it above the level of UCLA 7326 (Fig. 4). Jensen et al. (Reference Jensen, Saylor, Gehling and Germs2000) placed Germs's specimen lower down in the Arimas section on the basis of field observations of similar traces by JGG in 1993, and Turk et al. (Reference Turk, Maloney, Laflamme and Darroch2022, fig. 4) indicated a comparable level, just below their “gutter cast horizon” (= UCLA 7326). Buatois and Mángano (Reference Buatois and Mángano2016, fig. 4) suggested that treptichnids occur even lower down, well below the first prominent limestone bed, in the vicinity of UCLA 7309 (Fig. 4). The only sample in our collection we can confidently place in the stratigraphic section is the one shown in Figure 25.1, which was removed from outcrop by SJ in 1996. The similar slab collected by Germs could easily have moved downslope from that level. Thus, UCLA 7371 (Fig. 4) is the oldest certain occurrence of Treptichnus in Namibia.

We refer these traces to Treptichnus because of the great range of preservational variants found in some examples of the type species, including structures that closely resemble Germs's “discontinuous trails with three ridges” (Getty et al., Reference Getty, McCarthy, Hsieh and Bush2016, fig. 4.1, 4.2). In any case, Treptichnus is a form taxon, which could have been produced by different kinds of animals in the Ediacaran and the Cambrian, so our use of the generic name does not necessarily imply biological continuity across the eon boundary.