Geologic setting

The Nama Group of southern Namibia records >3,000 m of Ediacaran–Cambrian mixed carbonate–siliciclastic sediments over an area of approximately 125,000 km² (Saylor et al., Reference Saylor, Grotzinger and Germs1995). These successions are thought to represent material derived from the Kalahari Craton and later sourced from the uplift associated with the Damara Orogen, deposited in a foreland basin formed in response to orogenic activity during the assembly of Gondwana (Germs, Reference Germs1983; Stanistreet et al., Reference Stanistreet, Kukla and Henry1991; Saylor et al., Reference Saylor, Grotzinger and Germs1995; Grotzinger and Miller, Reference Grotzinger, Miller and Miller2008).

The Nama Group south of Windhoek is divided into two sub-basins, the Zaris (north) and Witputs (south), which are separated by the Osis Arch, interpreted as a paleotopographic high (Germs, Reference Germs1983; Grotzinger and Miller, Reference Grotzinger, Miller and Miller2008; Fig. 1). The Nama exposures in both sub-basins are subdivided into three subgroups; in ascending stratigraphic order, these are the Kuibis, the Schwarzrand, and the Fish River. Sediments in both basins can broadly be split into two sedimentary successions, a siliciclastic–carbonate succession comprising the Kuibis Subgroup, and a broadly siliciclastic succession comprising the Schwarzrand Subgroup (including the Nomtsas and Vergesig formations) (Germs, Reference Germs1983; Saylor et al., Reference Saylor, Grotzinger and Germs1995; Grotzinger and Miller, Reference Grotzinger, Miller and Miller2008). The Schwarzrand Subgroup encompasses the Ediacaran–Cambrian boundary, with the upper Ediacaran units (Nudaus and Urusis formations) locally incised by the Nomtsas Formation, which contains abundant Treptichnus pedum Seilacher, Reference Seilacher, Schindewolf and Seilacher1955 and is thus identified as Cambrian (Wilson et al., Reference Wilson, Grotzinger, Fischer, Hand, Jensen, Knoll, Abelson, Metz, McLoughlin, Cohen and Tice2012). As such, the Ediacaran–Cambrian boundary is generally thought to lie stratigraphically between the Urusis and the Nomtsas (Narbonne et al., Reference Narbonne, Saylor and Grotzinger1997; Wilson et al., Reference Wilson, Grotzinger, Fischer, Hand, Jensen, Knoll, Abelson, Metz, McLoughlin, Cohen and Tice2012); however, some have placed the boundary further down within the upper Spitskop Member (uppermost Urusis; see Linnemann et al., Reference Linnemann2019). Ash beds dating from strata below the latest-known occurrence of Ediacaran macrofossils and at Farm Swartpunt have yielded ages of 540.095 ± 0.099 Ma and 538.99 ± 0.21 Ma (Linnemann et al., Reference Linnemann2019), which suggests the Ediacaran–Cambrian boundary falls somewhere between 538 and 539 Ma. In the northern Zaris sub-basin, the Urusis Formation is thinner and consists of fluvial to shallow marine sandstone and green shale facies (Saylor, Reference Saylor2003). In the southern Witputs sub-basin, the Urusis Formation grades seaward into a carbonate shelf divided chronologically into the Nasep, Huns, Feldschuhhorn, and Spitskop members (Saylor, Reference Saylor2003; Grotzinger and Miller, Reference Grotzinger, Miller and Miller2008). The Urusis Formation varies in thickness throughout the Witputs sub-basin, reaching a maximum thickness of 1,000 m near the Gariep Belt and thinning to less than 200 m proximal to the Osis Ridge (Saylor, Reference Saylor2003).

Figure 1. (1) Map of the Nama Group in southern Namibia, with the Zaris (ZS) and Witputs (WS) sub-basins indicated (modified from Darroch et al., Reference Darroch, Cribb, Buatois, Germs, Kenchington, Smith, Mocke, O'Neil, Schiffbauer, Maloney, Racicot, Turk, Gibson, Almond, Koester, Boag, Tweedt and Laflamme2020). (2) Generalized stratigraphy for the Witputs sub-basin (adapted from Darroch et al., Reference Darroch, Cribb, Buatois, Germs, Kenchington, Smith, Mocke, O'Neil, Schiffbauer, Maloney, Racicot, Turk, Gibson, Almond, Koester, Boag, Tweedt and Laflamme2020; geochronological dates follow Linnemann et al., Reference Linnemann2019. (3) Higher-resolution map of study area in the Witputs sub-basin. Farm Arimas (star), Canyon Roadhouse (triangle), and Fish River Canyon (FRC) are marked.

Study localities and sedimentology

Two localities in the Witputs sub-basin, Canyon Roadhouse and Farm Arimas, expose the Nasep–Huns transition in wide lateral extent (Fig. 1.3). The Canyon Roadhouse exposures (27°31′16.5″S, 17°48′43.4″E) are located approximately 25 km northeast of the Fish River Canyon (FRC) and immediately adjacent to the Gondwana Collection Canyon Roadhouse Lodge. Farm Arimas (27°41′36.1″S, 17°1′50.5″E) is approximately 55 km west of the central portion of the FRC.

At Canyon Roadhouse, the lowest Nasep exposures begin with 0.5 m of recrystallized limestone with mudstone chip inclusions (Fig. 2). This is immediately followed by 1.5 m of coarse-grained sandstone, which bears lithological and textural similarities to Ernietta Pflug, Reference Pflug1966 horizons found at localities such as Farm Hansburg (see Bouougri et al., Reference Bouougri, Porada, Weber and Reitner2011; Gibson et al., Reference Gibson, Rahman, Maloney, Racicot, Mocke, Laflamme and Darroch2019; Maloney et al., Reference Maloney, Boag, Facciol, Gibson, Cribb, Koester, Kenchington, Racicot, Darroch and Laflamme2020); however, these beds have yet to yield any Ediacaran body fossils. These successions fine upward over 2 m into a medium-grained sandstone with small pebble-sized clasts and are capped with a thin layer of fine-grained sand. Another fining-upward interval occurs from 6 to 9 m, recording 20 cm beds of thick, coarse-grained sandstone interbedded with ~30 cm layers of fine-grained sandstone with oscillation ripples (Fig. 3.4). The Nasep–Huns boundary likely falls somewhere in the intermediate ~6 m of nonexposure as the next outcrops appear at ~15 m and consist of >2 m layers of interbedded mudchip-dominated limestone and micaceous fine-grained, silty sandstone, some containing coarser-grained sand. There is no exposure from 27 to 35 m, where ~4 m of weathered black limestone outcrops. After a thin >1 m interval of the same carbonate material with sandy inclusions at 43 m, the section resumes at 50 m with the characteristic weathered Huns carbonate before ceasing at approximately 56 m.

Figure 2. Stratigraphy of the Nasep–Huns exposures at Canyon Roadhouse, with (right) fossil-bearing intervals expanded. ‘sh’ = shale; ‘ss’ = siltstone; ‘fs’ = fine sand; ‘ms’ = medium sand; ‘cong.’ = conglomerate.

Figure 3. Sedimentary structures from Canyon Roadhouse: (1) climbing ripples; (2) mudchips (arrowed); (3) tabular cross bedding; (4) quasi-symmetrical ripples; (5) bioturbated slab exhibiting a number of unidirectional tool marks. Scale bar = 5 cm.

The Nasep–Huns transition at Farm Arimas is more extensive than its counterpart at Canyon Roadhouse, with approximately 96 m of vertical exposure (Fig. 4). The uppermost Nasep constitutes the lower 15 m of the section and is composed of reddish-brown medium-grained sandstone with intermittent oscillation ripples and channelization, as well as meiofaunal traces. The Nasep–Huns boundary is presumed to fall within the successive 10 m of nonexposure as the next exposure (at approximately 25 m) indicates a transition into the characteristic Huns limestone. The next 30 m alternate between <2 m thick exposures of Nasep-type sandstone and more extensive 3–5 m exposures of weathered, sandy, black limestone with ~25 cm thick beds and small-scale laminations. A thrombolite horizon within the bedded limestone appears around 56 m, and the nonthrombolitic sandy limestone continues upward in massive, meter-scale blocks until approximately 68 m. This is succeeded by a 15 m interval of nonexposure, which is capped by approximately 13 m of the massive limestone.

Figure 4. Stratigraphy of the Nasep–Huns exposures at Farm Arimas, with (right) fossil-bearing intervals expanded. Star indicates approximate location of the treptichnid-bearing horizons described by Jensen et al. (Reference Jensen, Saylor, Gehling and Germs2000) and Buatois and Mángano (Reference Buatois, Mángano, Mángano and Buatois2016). ‘sh’ = shale; ‘ss’ = siltstone; ‘fs’ = fine sand; ‘ms’ = medium sand; ‘cong.’ = conglomerate.

The interval of nonexposure at Farm Arimas from 47 to 56 m is of particular taphonomic interest. Judging from material found in nearby float, these fine-grained sandstone horizons preserve a variety of microbially induced sedimentary structures as well as abundant gutter casts, which preserve a variety of scour-related and biogenic structures on their lower surfaces (Fig. 5.2; see also Jensen et al., Reference Jensen, Saylor, Gehling and Germs2000; Buatois and Mángano, Reference Buatois, Mángano, Mángano and Buatois2016). The gutter cast horizons (located at ~48 and ~70 m in the section measured at Farm Arimas and at ~20 m in the section measured at Canyon Roadhouse) are observed in the fine-grained siliciclastics of the uppermost Nasep/lowermost Huns. These structures can be indicative of sediment by-pass resulting in small-scale (5–10 cm in width) downslope flows (see Mángano et al., Reference Mángano, Buatois, West and Maples2002) and are a common source of trace fossils across the Nasep–Huns transition. Comparatively high trace-fossil diversity (multiple ichnotaxa indicating a range of tracemaker behaviors) is present on the underside of the gutter casts sourced from fossil horizons at Farm Arimas and Canyon Roadhouse; by contrast, nongutter casts tend to preserve the majority of body fossils in these intervals.

Figure 5. Sedimentary structures from Farm Arimas: (1) oscillation ripples; (2) gutter cast trace assemblage. Dashed lines indicate approximate cast boundaries. White arrows indicate biotic structures; black arrows indicate sole marks; (3) parabolic flutes and (4) linear tool marks, both with white arrows denoting meiofaunal traces; (5) soft-sediment fluidization structures similar to the pseudofossil Aristophycus (see Knaust and Hauschke, Reference Knaust and Hauschke2004). Scale bars = 1 cm.

Microbially induced sedimentary structures

A number of microbially induced sedimentary structures (MISSs) are present across the Nasep–Huns transition, including Kinneyia and Intrites. These structures are thought to form due to sediment deformation under the microbial mats that blanketed large swathes of the seafloor during the late Ediacaran (Bouougri and Porada, Reference Bouougri and Porada2007; although see Pratt, Reference Pratt2021 for an alternative mechanism of Kinneyia formation). First described by Walcott (Reference Walcott1914), Kinneyia (Fig. 6.1, 6.3) is characterized by linear to slightly curved crests separated by highly variable (0.2–2 cm) distances (Porada et al., Reference Porada, Ghergut and Bouougri2008). Crests are approximately 1–2 mm wide, and the transitions into the intra-crest round-bottomed troughs is extremely steep. These structures occur on upper bedding surfaces, primarily in sandstone or shale. Kinneyia structures appear most commonly between the Neoproterozoic and Ordovician, although similar structures have been found as early as the Archean and possibly as late as the Neogene (Hagadorn and Bottjer, Reference Hagadorn and Bottjer1997; Noffke et al., Reference Noffke, Hazen and Nhieko2003; Carmona et al., Reference Carmona, Ponce, Wetzel, Bournod and Cuadrado2012). Proposed mechanisms of formation for Kinneyia center on unstable sediments beneath the original microbial mat, although the causal forces (e.g., shear-induced mat instability, oscillation of microbial aggregates, liquefied substrate) remain debated (Porada et al., Reference Porada, Ghergut and Bouougri2008; Thomas et al., Reference Thomas, Herminghaus, Porada and Goehring2013; Herminghaus et al., Reference Herminghaus, Thomas, Aliaskarisohi, Porada and Goehring2016). By contrast, experimental work by Mariotti et al. (Reference Mariotti, Pruss, Perron and Bosak2014) suggests that Kinneyia-type structures instead indicate the absence of an overlying mat, arguing the characteristic crest–trough form derives from wave-induced movement of microbial agglomerations at an exposed sediment–water interface.

Figure 6. Microbially induced sedimentary structures from the Nasep–Huns transition at Farm Arimas: (1, 3) Kinneyia; (2) wrinkle mat fabric; (4) Intrites. Scale bars = 1 cm.

Originally interpreted as a body fossil, Intrites (Fig. 6.4) was redescribed by Menon et al. (Reference Menon, McIlroy and Brasier2017) as a fine-grained “sediment volcano” or fluid escape structure that forms as a result of small-scale tears in the overlying microbial mats. As material buildup progressed around the ejecta site, cyanobacteria were likely attracted to the potential for increased access to light on the raised structure; thus, the repeated sediment–biofilm–sediment accretion cycle may have produced microstromatolites in the characteristic Intrites torus form (Gerdes et al., Reference Gerdes, Krumbein, Reineck, Krumbein, Paterson and Stal1994).

Trace-fossil diversity of the Nasep–Huns transition

The first account of ichnofossils in the Nama was published by Germs (Reference Germs1972), who documented five types of vermiform traces in the Nasep (although Archaeichium Glaessner, Reference Glaessner1963 has since been revised to represent a body fossil; see Glaessner, Reference Glaessner1978). Geyer and Uchman, (Reference Geyer, Uchman, Geyer and Landing1995) reported a number of ichnogenera from the Nasep, including Torrowangea rosei Webby, Reference Webby1970, cf. Trichophycus pedum, two ichnospecies of Palaeophycus Hall, Reference Hall1847, and three ichnospecies of Planolites Nicholson, Reference Nicholson1873 (including cf. P. montanus Richter, Reference Richter1927), in addition to Skolithos Haldeman, Reference Haldeman1840, Brooksella Walcott, Reference Walcott1896, Curvolithus Fritsch, Reference Fritsch1908, and Didymaulichnus Young, Reference Young1972 noted in earlier accounts by Germs (Reference Germs1983) and Crimes and Germs (Reference Crimes and Germs1982). Geyer and Uchman (Reference Geyer, Uchman, Geyer and Landing1995) also described two morphotypes of Skolithos from the Nasep and Huns, a trace historically associated with the Cambrian onward (Mángano and Buatois, Reference Mángano and Buatois2014). If accurate, this would indicate vertical burrowing was occurring far earlier than previously thought, although we note Jensen (Reference Jensen2003) questioned these identifications, instead suggesting they more likely represent body fossils with some part of the organism rooted in the sediment. Jensen et al. (Reference Jensen, Saylor, Gehling and Germs2000) focused exclusively on trace fossils from the Nasep–Huns transition, noting the presence of treptichnids, “small trace fossils,” as well as the body fossils Nasepia Germs, Reference Germs1973, Archaeichnium, and “annulate tubes.”

The primary trace fossils found within Nasep and Huns exposures at Farm Arimas and Canyon Roadhouse represent simple, horizontal vermiform pascichnia or fodinichnia, although there are a number of morphologically distinct forms present under this greater classification (Archaeonassa Fenton and Fenton, Reference Fenton and Fenton1937, Gordia Emmons, Reference Emmons1844, Helminthoidichnites Fitch, Reference Fitch1850, and Helminthopsis Heer, Reference Heer1877; see Fig. 7.1, 7.3, 7.5, 7.6). Also present to a lesser extent are actively filled horizontal forms (Torrowangea; see Fig. 7.4), horizontal burrows with vertical components (treptichnids; see Fig. 7.2), and irregular meiofaunal networks (Fig. 8). These ichnogenera will be described in greater detail in the following.

Figure 7. Nasep–Huns ichnofauna: (1) Archaeonassa from Canyon Roadhouse; (2) treptichnids from Farm Arimas, with arrows denoting individual segments; (3) Helminthoidichnites from Canyon Roadhouse; (4) Torrowangea from Canyon Roadhouse, with irregular constrictions indicated by arrows; (5) Helminthopsis from Canyon Roadhouse; (6) Gordia from Canyon Roadhouse. Filled scale bars = 1 cm; hollow scale bar = 2.5 mm.

Figure 8. Meiofaunal traces from the upper Nasep at Farm Arimas: (1) individual exhibiting movement in and out of the sediment, with dotted line providing trajectory visualization; (2, 3) meiofaunal networks exhibiting both branching (white arrows) and overcrossing (black arrows); (4) trace network; (5) individual specimen dipping below surface and reemerging. (1, 3, 5) Hollow scale bars = 5 mm; (2, 4) filled scale bars = 1 cm.

Archaeonassa are described as convex furrows separated by a midline groove, 1–7 mm in width and often preserved in positive epirelief (Fig 7.1; also see Darroch et al., Reference Darroch, Cribb, Buatois, Germs, Kenchington, Smith, Mocke, O'Neil, Schiffbauer, Maloney, Racicot, Turk, Gibson, Almond, Koester, Boag, Tweedt and Laflamme2020, fig. 9g). Traces are commonly straight to mildly sinuous and are thought to represent pascichnia attributable to a range of invertebrate taxa (Yochelson and Fedonkin, Reference Yochelson and Fedonkin1997). In their original description, Fenton and Fenton (Reference Fenton and Fenton1937) suggested a gastropod origin for Archaeonassa (although this was contested by Yochelson and Fedonkin, Reference Yochelson and Fedonkin1997); Buckman (Reference Buckman1994) later posited potential attribution to echinoderms or arthropods. However, neoichnological work by Matz et al. (Reference Matz, Frank, Marshall, Widder and Johnsen2008) found similar bilobate traces could plausibly be left by nonbilaterian giant protists such as Gromia. Archaeonassa found in late Ediacaran strata from Ukraine suggest a degree of undulation in and out of sediment underneath microbial mats, as well as general movement perpendicular to the paleoshoreline and parallel to tides, potentially indicating very early taxis (Uchman and Martyshyn, Reference Uchman and Martyshyn2020). Archaeonassa is present at both Canyon Roadhouse and Farm Arimas despite being noticeably rarer than at Farm Haruchas (basal Vingerbreek Member, Nudaus Formation, lower Schwarzrand Subgroup; see Bouougri and Porada, Reference Bouougri and Porada2007).

Gordia (Fig. 7.6; also see Darroch et al., Reference Darroch, Cribb, Buatois, Germs, Kenchington, Smith, Mocke, O'Neil, Schiffbauer, Maloney, Racicot, Turk, Gibson, Almond, Koester, Boag, Tweedt and Laflamme2020, fig. 9d) is composed of nonbranching traces displaying a high degree of self-crossing, a criterion used to differentiate it from the morphologically similar Helminthoidichnites and Helminthopsis (Getty et al., Reference Getty, Sproule, Stimson and Lyons2017). Gordia is most frequently interpreted as unspecialized infaunal pascichnia of annelids or priapulids (Buatois et al., Reference Buatois, Mángano, Maples and Lanier1998; Wang et al., Reference Wang, Lin, Zhao and Orr2009). Occasional abrupt terminal segments (“pustules”) suggest a movement of a proboscid frontal organ into the vertically adjacent sediment, which could indicate priapulid affinities (Wang et al., Reference Wang, Lin, Zhao and Orr2009). Gordia is most typically found in gutter casts from the uppermost Nasep/lowermost Huns but is scarce outside of these structures. A number of the basal Huns specimens record the aforementioned pustules, indicating the tracemakers possessed a degree of vertical movement capabilities.

Helminthoidichnites represents nonmeandering, straight to curved horizontal traces 1–10 mm wide, most typically interpreted as pascichnia and attributed to nematomorphs (Buatois et al., Reference Buatois, Mángano, Maples and Lanier1998; Schlirf et al., Reference Schlirf, Uchman and Kümmel2001; Fig. 7.3). Specimens are generally preserved in negative epirelief or positive hyporelief (Darroch et al., Reference Darroch, Cribb, Buatois, Germs, Kenchington, Smith, Mocke, O'Neil, Schiffbauer, Maloney, Racicot, Turk, Gibson, Almond, Koester, Boag, Tweedt and Laflamme2020). Individuals of Helminthoidichnites exhibit overcrossing between individuals, and some degree of “looping” is observed; however, computer simulations have demonstrated these loops exhibit greater randomness than do those of Gordia (Hofmann, Reference Hofmann1990). Helminthoidichnites is abundant across the Nasep–Huns transition and is most often found on the underside of gutter casts in concert with other vermiform traces.

In contrast to Helminthoidichnites, Helminthopsis (Fig. 7.5; also see Darroch et al., Reference Darroch, Cribb, Buatois, Germs, Kenchington, Smith, Mocke, O'Neil, Schiffbauer, Maloney, Racicot, Turk, Gibson, Almond, Koester, Boag, Tweedt and Laflamme2020, fig. 7b) are nonlooping horizontal trails 1–10 mm wide interpreted as unspecialized fodinichnia and most often preserved in negative epirelief or positive hyporelief (Hofmann and Patel, Reference Hofmann and Patel1989). Traces are winding to meandering and do not touch or self-cross (Fillion and Pickerill, Reference Fillion and Pickerill1990). Individuals are typically preserved in negative epirelief. The presence of marginal ridges suggests the trails were formed by displacement of sediment by the tracemakers and that these structures remained open for a time post passage (Jensen et al., Reference Jensen, Droser, Gehling, Xiao and Kaufman2006). Helminthopsis, together with Helminthoidichnites, represent the most abundant traces found at Canyon Roadhouse and Farm Arimas. As with many of the vermiform traces present at these sites, this ichnotaxon is largely confined to preservation within the gutter cast communities from ~70 m at Farm Arimas and ~20 m at Canyon Roadhouse.

Torrowangea is the sole example of actively filled burrows from the Nasep–Huns (Fig. 7.4; also see Darroch et al., Reference Darroch, Cribb, Buatois, Germs, Kenchington, Smith, Mocke, O'Neil, Schiffbauer, Maloney, Racicot, Turk, Gibson, Almond, Koester, Boag, Tweedt and Laflamme2020, fig. 9e). This ichnogenus is composed of transversely annulated, meandering to sinuous horizontal burrows characterized by intermittent constrictions thought to indicate a degree of peristaltic motion (Narbonne and Aitken, Reference Narbonne and Aitken1990; Kim and Pickerill, Reference Kim and Pickerill2003; Jensen et al., Reference Jensen, Droser, Gehling, Xiao and Kaufman2006). Specimens are always preserved in positive epirelief, and backfill has the same sedimentary composition as the surrounding matrix. Torrowangea is most often interpreted as a vermiform deposit feeder burrowing below the sediment–water interface (Buatois and Mángano, Reference Buatois, Mángano, Mángano and Buatois2016); however, some studies demonstrate the formation of structurally similar traces by benthic foraminiferal pseudopoda (Severin et al., Reference Severin, Culver and Blanpied1982; Kitazato, Reference Kitazato1988). Torrowangea is present at both Farm Arimas and Canyon Roadhouse and typically presents as 3–5 mm wide burrow structures interweaving to form a greater tangled structure. Individual burrows are meandering and characterized by sporadic constrictions, which provide for differentiation from the visually similar Archaeichnium. Torrowangea is found at the gutter cast horizons, although it is not typically associated with the vermiform trace-dominated gutter cast communities. Instead, it typically forms monospecific occurrences.

Treptichnids are an informal grouping of serial vertically penetrating burrows; in the Nasep–Huns, these are represented by discrete ovoid probes typically ~1–3 mm long that follow a curvilinear to semicircular pathway and are characteristically preserved in positive hyporelief (Fig 7.2; also see Darroch et al., Reference Darroch, Cribb, Buatois, Germs, Kenchington, Smith, Mocke, O'Neil, Schiffbauer, Maloney, Racicot, Turk, Gibson, Almond, Koester, Boag, Tweedt and Laflamme2020, fig. 13a, b). The first report of these traces from the Nasep (Germs, Reference Germs1972) details parallel-ridged trails ~3 mm wide that terminate and reappear at regular intervals. Jensen et al. (Reference Jensen, Saylor, Gehling and Germs2000) later identified these traces as Treptichnus isp., noting their morphological similarities to Treptichnus pedum, albeit at a far smaller scale and with greater unidirectional probe tendencies. This suggests the treptichnids (determined by Jensen et al., Reference Jensen, Saylor, Gehling and Germs2000 to be from the Huns rather than the Nasep originally documented by Germs) likely represent more “advanced” bilaterian behavior below the Cambrian boundary (Jensen et al., Reference Jensen, Saylor, Gehling and Germs2000). Treptichnid specimens are found in both the uppermost Nasep and basal Huns at Canyon Roadhouse and Farm Arimas; at both sites, these traces are preserved as part of the larger gutter cast ichnofossil assemblages but are much smaller relative to the other traces.

Meiofaunal networks from this interval are high-density assemblages composed of millimeter-scale horizontal trace fossils 0.3–0.5 mm wide and exhibiting a high degree of overcrossing (Fig. 8). First described by Germs (Reference Germs1972) as “thread-like trails” from the Nasep of Farm Arimas, these traces are often found in conjunction with treptichnids and other vermiform burrowers. Individual traces over- or undercross but do not pass through each other. Some specimens exhibit slight vertical movement in and out of the horizontal plane similar in character to Yichnus levis (Xiao et al., Reference Xiao, Chen, Zhou and Yuan2019) from the terminal Ediacaran Shibantan member of China, although at a much smaller scale. Traces of similar appearance to those figured by Germs (Reference Germs1972) are found in the upper Nasep and lower Huns at both Farm Arimas and Canyon Roadhouse, where they are most commonly preserved within the greater gutter cast trace assemblage and exhibit occasional instances of dichotomous branching (Fig. 8.2, 8.3). Further inspection of these specimens suggests morphological differences between these traces (“Form A” as described by Darroch et al., Reference Darroch, Cribb, Buatois, Germs, Kenchington, Smith, Mocke, O'Neil, Schiffbauer, Maloney, Racicot, Turk, Gibson, Almond, Koester, Boag, Tweedt and Laflamme2020) and those found in the terminal Ediacaran Spitskop Member (“Form B” from Darroch et al., Reference Darroch, Cribb, Buatois, Germs, Kenchington, Smith, Mocke, O'Neil, Schiffbauer, Maloney, Racicot, Turk, Gibson, Almond, Koester, Boag, Tweedt and Laflamme2020, frequently displaying right-angle branching and regular movement above and below the sediment surface), indicating the possibility of two distinct meiofaunal behaviors within the Urusis Formation.

Of note from Arimas is an indeterminate trace fossil (GSN F1643; Fig. 9) composed of meandering 1–3 cm (length) tubes that intertwine to create a larger, braided form. Tubes are 2–3 mm wide, with longitudinal striations ~0.4 mm apart present toward the distal ends. A number of specimens display a shift from the grooved pattern into faint transverse annulations in the medial portion (see Fig. 9.4, 9.6). In multiple instances, tubes appear to dip below the sediment surface and reemerge consistent with the direction of motion, suggesting a degree of bioturbative activity (see Fig. 9.5, 9.6). Structures terminate with minimal evidence of tapering, and some appear to exhibit Gordia-type probing “nubs.” A number of individual tubes are capped by a slightly wider, rounded structure, producing a bulbous effect. This trace material is present on a single slab composed of multiple individuals from Farm Arimas. The horizon lies in the upper portions of Huns exposure (a few meters above the gutter cast/trace horizon at ~70 m) and are likely also the source of Corumbella body fossils described in the next section.

Figure 9. Indeterminate trace fossils from Farm Arimas (GSN F1643): (1–3) original images; (4–6) annotated material. White dotted lines indicate individual overcrossing probe structures, including indications of subsurface movement. Colored arrows denote dichotomous surface patterns, with blue showing posterior transverse annulation and red marking anterior longitudinal striations. Black arrows indicate abrupt probing “nubs” similar to those of Gordia. Scale bars = 1 cm.

These striated traces are similar to the material described by Glaessner (Reference Glaessner1963) as Archaeichnium haughtoni. While originally classified as a trace, Glaessner (Reference Glaessner1978) later revised his assessment of Archaeichnium to that of a body fossil, although he noted definitive assignment to either one of these categories is difficult. Traces exhibiting similar flexible collapse have been described from the Cambrian of Sweden (see Jensen, Reference Jensen1997, fig. 49). In addition, both the individual tubes and the greater braided mass of the Huns material bear noticeable similarities to priapulid trace material described from the lower Cambrian of Sweden by Kesidis et al. (Reference Kesidis, Slater, Jensen and Budd2019).

The longitudinal striations are consistent (both in approximate size and placement) with sensory papillae ridges (scalids) present on the proboscides of extant scalidophoran priapulids such as Priapulus caudatus Lamarck, Reference Lamarck1816 (see Hammond, Reference Hammond1970a, Reference Hammondb, fig. 1). These grooves remain equidistant throughout the anterior portion of the probes. However, as this material is preserved in positive semirelief, assessment of the full number of striations is difficult.

The materials recovered by our group also exhibit two distinct surface patterns that support scalidophoran affinities: a striated, bulbous distal portion that transitions into a transversely annulated proximal section. Modern priapulids move through sediment by evagination of the frontal introvert (see Calloway, Reference Calloway1975, fig. 1), followed by peristaltic contractions that shorten the annulated trunk. As the peristaltic wave reaches the posterior praesoma, the introvert retracts and the body shifts into the now-vacated anterior space (see Elder and Hunter, Reference Elder and Hunter1980, table 1 and fig. 1). Neoichnological experiments by Kesidis et al. (Reference Kesidis, Slater, Jensen and Budd2019) demonstrated that this method of locomotion results in dual-patterned burrows strikingly similar to the specimens described here. In addition, while the burrows of larger priapulids such as P. caudatus tend to close immediately post passage, smaller species (e.g., Halicryptus spinulosus von Siebold, Reference von Siebold1849) can leave smaller open structures behind, which likely provide greater preservation potential as sediment is infilled (Powilleit et al., Reference Powilleit, Kitlar and Graf1994); however, the presence of microbial mats may affect this taphonomic paradigm (see Yallop et al., Reference Yallop, De Winder, Paterson and Stal1994). This preservational tendency toward smaller burrows is consistent with the specimens collected for this study, which are smaller in both length and width than the exceptionally preserved material described by Kesidis et al. (Reference Kesidis, Slater, Jensen and Budd2019).

The horizontality and slight penetrative behavior of the traces are also consistent with some accounts of priapulid burrowing behavior investigated under laboratory settings. Experiments by Vannier et al. (Reference Vannier, Calandra, Gaillard and Zylińska2010) demonstrated that when constrained to a single horizontal plane, P. caudatus will produce burrow traces morphologically similar to the material described here (see Vannier et al., Reference Vannier, Calandra, Gaillard and Zylińska2010, fig. 1D). However, their imposed vertical restrictions are largely artificial and do not reflect the natural environment of priapulids, which also display tendencies to burrow vertically. As such, our interpretations should be viewed cautiously.

Body fossils

While this study is focused primarily on the ichnofauna of the Nasep–Huns transition, there are also body fossils preserved in the sections that provide important supplementary evidence as to the temporal placement of the units and the suite of ecologies present.

Corumbella Hahn et al., Reference Hahn, Hahn, Leonardos, Pflug and Walde1982 is described as an annulated tubular structure of length up to ~80 mm and diameter up to 25 mm extending from a basal aboral region (often found extending into the substrate) into an oral region (Babcock et al., Reference Babcock, Grunow, Sadowski and Leslie2005; Pacheco et al., Reference Pacheco, Galante, Rodrigues, Leme, Bidola, Hagadorn, Stockmar, Herzen, Rudnitzki, Pfeiffer and Marques2015). Tubes possess fourfold radial symmetry and suggest a high degree of flexibility. Hagadorn and Waggoner (Reference Hagadorn and Waggoner2000) described a second form of the genus from the Great Basin of the western United States, termed Corumbella new species A, which exhibits a helical twist and lacks the secondary branched polypar of the specimens originally described by Hahn et al. (Reference Hahn, Hahn, Leonardos, Pflug and Walde1982). However, Babcock et al. (Reference Babcock, Grunow, Sadowski and Leslie2005) noted the presence of a similar helical twist in C. werneri Hahn et al., Reference Hahn, Hahn, Leonardos, Pflug and Walde1982 specimens from Brazil, and thus suggested it may in fact bear no taxonomic significance. While members of the genus are most often interpreted as scyphozoan cnidarians, recent work by Walde et al. (Reference Walde, Weber, Erdtmann and Steiner2019) has suggested Corumbella might instead represent calcareous sinotubulitids.

The Corumbella specimens (GSN F1644; Fig. 10) recovered by our group from the Huns represent the first-known occurrence of the genus from Namibia. These individuals are preserved on a single upper-Huns (~73 m) slab from Farm Arimas, with some exhibiting a degree of lateral flexibility consistent with corumbellids from other Ediacaran sites (see, e.g., Hagadorn and Waggoner, Reference Hagadorn and Waggoner2000, figs. 5.4–5.5; Pacheco et al., Reference Pacheco, Galante, Rodrigues, Leme, Bidola, Hagadorn, Stockmar, Herzen, Rudnitzki, Pfeiffer and Marques2015, fig. 4; Vaziri et al., Reference Vaziri, Majidifard, Darroch and Laflamme2021, fig. 2). Septa are clearly visible and form ~0.5 mm rings with defined midlines; neither the oral nor aboral regions appear preserved for any individual. As such, assignment to either C. werneri or a separate species is difficult; the lack of clearly defined torsion in the Huns specimens would suggest affinity with C. werneri, but the limited extent of the preserved individuals prevents assessment as to the presence/absence of the second polypar.

Figure 10. Two Huns Corumbella specimens from Farm Arimas (GSN F1644): (1) individual with multiple faces preserved; (2) specimen with single face preserved exhibiting degree of flexibility. Labels included reference to Pacheco et al. (Reference Pacheco, Galante, Rodrigues, Leme, Bidola, Hagadorn, Stockmar, Herzen, Rudnitzki, Pfeiffer and Marques2015). F = face; LE = lateral edge; ML = midline; R = ring. Scale bars = 1 cm.

A suite of tubular body fossils 1–5 mm in diameter displaying a high degree of morphological variability is found throughout the study area (Fig. 11). Specimens similar to this type have historically been assigned to Archaeichnium; however, this singular taxonomic designation likely obscures the breadth of tubular morphological diversity present across the Nasep–Huns.

Figure 11. Tubular body fossils from the Nasep–Huns: (1, 2, 4) examples from Farm Arimas; (3) example from Canyon Roadhouse. Note preservation of tapered ends (1, 3) and defined annulations (1, 2, 4). Scale bars 1 = cm.

Unlike the possible priapulid trace material, these tubular body fossils do not possess longitudinal striations and are instead characterized by transverse annulations spaced approximately 1 mm apart (Hagadorn and Waggoner, Reference Hagadorn and Waggoner2000). These structures are thought to be flexible due to lack of clean breaks, and the body orientations suggest current alignment. Tentative interpretations suggest similarities to other late Ediacaran tubular metazoans (cf. Droser and Gehling, Reference Droser and Gehling2008; Cai et al., Reference Cai, Schiffbauer, Hua and Xiao2011; Cortijo et al., Reference Cortijo, Cai, Hua, Schiffbauer and Xiao2015; Smith et al., Reference Smith, Nelson, Strange, Eyster, Rowland, Schrag and Macdonald2016; Schiffbauer et al., Reference Schiffbauer, Selly, Jacquet, Merz, Nelson, Strange, Cai and Smith2020). A number of individuals possess conical terminal tapers, and diagnoses are reliant on the presence of these tapered ends. As such, due to the number of annulated and tubular metazoan taxa associated with the Nama Group, identification can be more difficult when the conical tapers are absent. Other tubular taxa present across this interval include “kinked-funnel” structures and stacked cone-in-cone forms that bear similarities to the “cloudinomorph” form grouping described by Selly et al. (Reference Selly2020). Further systematic work is needed to address the breadth of tubular morphological disparity across the Nasep–Huns transition.