Diagnosis

See Scholze et al. (Reference Scholze, Golubev, Niedźwiedzki, Schneider and Sennikov2019).

Remarks

As briefly outlined by Poschmann et al. (Reference Poschmann, Hegna, Astrop and Hoffmann2024), the familial-level taxonomy of Pseudestheria and related genera is confusing, and repairing it is beyond the scope of the present work.

Genus Pseudestheria Raymond, Reference Raymond1946

Type species

Pseudestheria brevis Raymond, Reference Raymond1946; by original designation.

Diagnosis

Small to very large valves; oval to round shape; straight to slightly curved dorsal margin; umbo convexly curved, position of the umbo submedial to anterior and inframarginal to supramarginal; larval valve very small to small; variable number of growth lines; pitted (punctate) ornamentation (from Scholze et al., Reference Scholze, Golubev, Niedźwiedzki, Schneider and Sennikov2019; see also Scholze, Reference Scholze2021; Poschmann et al., Reference Poschmann, Hegna, Astrop and Hoffmann2024 for remarks).

Pseudestheria diensti (Gross, Reference Gross1934)

Figures 2, 3.3–3.8, 4, 5.1–5.4, 6.1–6.6

Reference Mauz1933      Paracyclas rugosa Goldfuß; Mauz, p. 275.

*Reference Gross1934     Estheria diensti Gross, p. 309, figs. 1–6, 8, 9.

Reference Raymond1946      Pseudestheria diensti; Raymond, p. 244.

Reference Raymond1946      Palaeolimnadiopsis ? eifelensis Raymond, p. 271.

Reference Raymond1946      Pseudestheria subcircularis Raymond, p. 244.

Reference Novozhilov and Новожилов1953a     Euestheria eifelensis; Novozhilov, p. 948, fig. 2b.

Reference Novozhilov and Новожилов1961      Concherisma eifelense; Novozhilov, p. 86 (in part).

Reference Novozhilov and Новожилов1961      Glyptoasmussia willweratica Novozhilov, p. 62, fig. 25, pl. 15, fig. 5.

Reference Franke and Franke2006      Estheria diensti; Franke, pl. 11, fig. 2.

Reference Becker, Franke and Franke2012      Estheria diensti; Becker and Franke, pl. 2, figs. 7, 8.

Reference Poschmann, Schoenemann and McCoy2016      conchostracan; Poschmann et al., fig. 4b.

Reference Poschmann, Hegna, Astrop and Hoffmann2024       Pseudestheria diensti; Poschmann et al., p. 543, figs. 2a–f, 6b, c, 7e, f, 9g, h.

Reference Poschmann, Hegna, Astrop and Hoffmann2024      Pseudestheria cf. P. diensti; Poschmann et al., p. 545, figs. 3a–c, 6f.

Figure 3. Individual clam shrimp specimens from the Consthum mass layer. (1) Undetermined, relatively small specimen EiB551b_3. (2) Undetermined, relatively small specimen EiB551b_4. (3) Pseudestheria diensti, mid-sized specimen, EiB551b_7. (4) Pseudestheria diensti, mid-sized specimen, EiB551b_5. (5) Pseudestheria diensti, EiB551b_2. (6) Pseudestheria diensti overlying Adelophthalmus sievertsi head shield, EiB551a_5. (7) Pseudestheria diensti, large specimen, EiB551b_10; note protruding right valve at lower margin. (8) Pseudestheria diensti, large specimen; note bivalved preservation. Scale bars = 5 mm.

Reference Poschmann, Hegna, Astrop and Hoffmann2024       (?) Palaeolimnadiopsis ? eifelensis; Poschmann et al., figs. 3d, 6a.

Reference Poschmann, Hegna, Astrop and Hoffmann2024      Palaeolimnadiopsis ? eifelensis; Poschmann et al., p. 556, figs. 4a–c, 6e.

Figure 4. Pseudestheria diensti from Consthum showing striated dorsal regions. (1, 2) Specimen EiB535a and b, part and counterpart, respectively. (3, 4) Details from the dorsal regions in (1) and (2), respectively. (5, 6) Specimen EiB551b_1 and detail from the dorsal region, respectively. (7, 8) Specimen EiB551b_13 in dorsal view and detail, respectively; arrow indicates striation. (1, 2, 5) Scale bars = 5 mm; (3, 4, 6–8) scale bars = 1 mm.

Reference Poschmann, Hegna, Astrop and Hoffmann2024      Pseudestheria subcircularis; Poschmann et al., p. 549, figs. 4d–f, 6d.

Holotype

The specimen figured by Gross (Reference Gross1934, fig. 5), now in the Museum für

Figure 5. (1–3) Pseudestheria diensti from Consthum, specimens with high H/L ratios. (1) Specimen EiB552b_2. (2) Specimen EiB552b_3. (3) Specimen EiB560a. (4) Pseudestheria diensti from Consthum, specimen with low H/L ratio, EiB559a. (5–8) Palaeolimnadia stevenbeckeri n. sp. from Consthum. (5) Holotype EiB555. (6) Slightly deformed paratype EiB558. (7) Paratype EiB702. (8) Paratype EiB722. White arrows indicate anterodorsal swelling. Scale bars = 1 mm.

Naturkunde Berlin, Germany, number MB.A.0043 (Poschmann et al., Reference Poschmann, Hegna, Astrop and Hoffmann2024).

Description

Clam shrimp individuals attributed to Pseudestheria diensti were found scattered within the most fossiliferous layer. On one (?bedding) plane, the clam shrimp carapaces form laterally discontinuous pavements with a density of up to 30 specimens per 10 cm², which can be extrapolated to around 30.000 specimens per m².

In 33 measured, reasonably well-preserved specimens, the height ranges from (?1.6) 2.3 to 6.2 mm (average 4.1 mm), the length from (?2.4) 3.2 to 9.2 mm (average 5.9 mm), the length/width ratio from 0.55 to 0.89 (average 0.70), and the number of growth lines ranges from about 9 to 21 (Table 1). The specimens share the following morphological traits. Carapace shape elongate-oval to round with a weakly expressed umbo in anterior-median to median-anterior and marginal position, dorsal margin long and almost straight to very slightly curved, larval valve very small and hardly recognizable; microsculpture indistinct (as in the Willwerath specimens; see Poschmann et al., Reference Poschmann, Hegna, Astrop and Hoffmann2024), probably consisting of a dense, fine network of reticulate cells; some specimens among both, those with very high as well as those with very low height/length (H/L) ratios, show a longitudinal, fan-like striation in the region between umbo and posterior end of the dorsal margin. The striations originate from a common point near the umbo and are roughly perpendicular to the distal growth lines.

Table 1. Morphometric data of Pseudestheria diensti from Consthum.

Remarks

The morphological traits and their variation observed in the Consthum clam shrimp mass layer correspond largely to the morphology and variation documented in the approximately contemporaneous Willwerath association (Poschmann et al., Reference Poschmann, Hegna, Astrop and Hoffmann2024), with the exception of size variation, which is much greater among the Consthum specimens, suggesting the presence of various growth stages. Clam shrimp from Willwerath were attributed to Pseudestheria diensti (Gross, Reference Gross1934), Pseudestheria subcircularis Raymond, Reference Raymond1946, and Palaeolimnadiopsis ? eifelensis Raymond, Reference Raymond1946 (see Poschmann et al., Reference Poschmann, Hegna, Astrop and Hoffmann2024 for history of research). Poschmann et al. (Reference Poschmann, Hegna, Astrop and Hoffmann2024) recorded a fourth morphotype closely corresponding to Pseudestheria diensti but with a slightly different carapace outline. These authors furthermore stated that the observed differences in these taxa may be attributed to taphonomic/preservational and/or ontogenetic/sexual variation and suspected that all taxa previously described from Willwerath may represent just one species. Pseudestheria subcircularis and Palaeolimnadiopsis ? eifelensis should then be synonymized with Pseudestheria diensti as diagnosed by Scholze et al. (Reference Scholze, Golubev, Niedźwiedzki, Schneider and Sennikov2019). Due to a very limited sample size from Willwerath (about a dozen specimens), Poschmann et al. (Reference Poschmann, Hegna, Astrop and Hoffmann2024) refrained from a formal taxonomic act pending a larger base to substantiate their view. In his emendation of Pseudestheria, Martens (Reference Martens1983) showed, among other characters, a bending of growth lines along the dorsal margin at the contact of the left and right valves. This character state can be recognized only in appropriately three-dimensionally preserved specimens and could not be seen in the specimens from Willwerath (Poschmann et al., Reference Poschmann, Hegna, Astrop and Hoffmann2024). By contrast, some exceptionally preserved specimens from Consthum do show a fine, fan-like, subparallel striation at the dorsal margin (Figs. 4, 5.1, 5.3, 5.4, 6.1–6.4, 6.6), which corresponds to the structure elaborated by Martens (Reference Martens1983, Reference Martens2020). This striation is interpreted to reflect a bending of the growth lines at the dorsalmost edge of each valve and their course along the “inner” dorsal rim (Martens, Reference Martens1983, fig. 7). Thus, the actual hinge of the valves runs along a median depression. The presence of this feature, which in Martens’ (Reference Martens1983, Reference Martens2020) view defines the family Pseudestheriidae Martens, Reference Martens1983, furthermore backs an attribution to Pseudestheria, the most common Early Devonian genus (Liao and Shen, Reference Liao and Shen2022). This feature may also explain some variation in our Pseudestheria samples, which seems unusual at first. We suspect that the particular morphology of the dorsal region in Pseudestheria in concert with various degrees of sediment cover of some parts and with a possible sexual dimorphism may have promoted different outlines of the valve as a result of compression as well as seemingly various degrees of protrusion of the umbo. The result is fossils with low (Figs. 3.8, 4.5, 5.4, 6.4) to high (Figs. 5.1–5.3, 6.5, 6.6) H/L ratios and straight (Figs. 3.6, 5.4, 6.4) to more rounded (Figs. 3.3, 3.4, 5.1–5.3, 6.5, 6.6) outlines of the dorsal margin—in other words, with an elongate “eifelensis preservation” to rounded “subcircular preservation.” In addition, Liao and Shen (Reference Liao and Shen2022) mentioned that among their specimens assigned to Pseudestheria cf. P. diensti, the carapace shape varies from oval and sub-oval to round and considered these different shapes as possibly due to compression deformation or sexual dimorphism. This fan-like feature on the dorsal margin may be distinctive, but it cannot yet be treated as a phylogenetic panacea. Although it may be widespread, we cannot as yet determine exactly how widespread it is among clam shrimp due to the rarity of its preservation. Martens’ (Reference Martens1983, Reference Martens2020) interpretation of the feature implies that we should often see the outer growth lines overprinted on the fan-like lines in compressed fossils with rigid carapaces. While we see this in the specimens illustrated by Tasch (Reference Tasch, Barlow and Burkhammer1975a, note: Tasch’s specimens have the fan-like lines at a significantly higher angle than in the Consthum II specimens), we do not see overprinting in the Consthum specimens. In a re-examination of the type specimen of Palaeolimnadiopsis (T.A.H.), we did not see the fan-like dorsal lines mentioned by Martens (Reference Martens1983). (Note: Raymond, Reference Raymond1946, spelled Palaeolimnadiopsis two different ways, Paleolimnadiopsis [p. 270] and Palaeolimnadiopsis [p. 271, at the introduction of P. carpenteri]. As the first usage (Paleolimnadiopsis) is the only instance of that spelling outside of the table of contents, we regard it as an error; Raymond likely intended Palaeolimnadiopsis as the proper spelling.) Thus, we feel that use of this feature to unite Pseudestheria and Palaeolimnadiopsis is invalid. Palaeolimnadiopsis itself does not seem to be a synonym of Pseudestheria. Palaeolimnadiopsis is represented by a number of distinctive species later in the Paleozoic. We suspect that some of the Devonian species assigned to Palaeolimnadiopsis may need re-evaluation.

It is a fortunate circumstance that shortly after the revision of Rhenish Lower Devonian clam shrimp by Poschmann et al. (Reference Poschmann, Hegna, Astrop and Hoffmann2024), the Consthum clam shrimp mass layer was discovered in the summer of 2023. With the additional evidence at hand, we can now substantiate that both the Willwerath association and the Consthum mass layer association comprise (mainly) clam shrimp that share important morphological characters and differ mainly in size, the (corresponding) number of recognizable growth lines, and H/L ratios (elongate oval to round). The observed variation can be attributed to intraspecific variability (ontogeny and possibly sexual dimorphism/ecophenotypic variability) and preservation (tectonic and compressional deformation), and perhaps sediment still covering deeply buried parts (for examples of preservational/intraspecific variation in clam shrimp, see e.g., Kozur, Reference Kozur1983; Martens, Reference Martens1983, Reference Martens2020; Kozur and Weems, Reference Kozur and Weems2005, Reference Kozur, Weems and Lucas2010; Gosny, Reference Gosny2010; Astrop et al., Reference Astrop, Park, Brown and Weeks2012; Gallego et al., Reference Gallego, Monferran, Astrop and Zacarías2013; Scholze et al., Reference Scholze, Golubev, Niedźwiedzki, Sennikov, Schneider and Silantiev2015, Reference Scholze, Hamad, Schneider, Golubev, Sennikov, Voigt and Uhl2017, Reference Scholze, Golubev, Niedźwiedzki, Schneider and Sennikov2019; Geyer and Kelber, Reference Geyer and Kelber2018; Hethke et al., Reference Hethke, Fürsich, Morton and Jiang2018, Reference Hethke, Weeks, Schöttle and Rogers2021; Sell, Reference Sell2018; Hethke and Weeks, Reference Hethke and Weeks2020). We found only simple correlations of length to height of the valves or number of growth lines and valve length, while the number of growth lines remains generally the same in (similarly sized) elongate and rounded specimens. Furthermore, the feature of a striated dorsal margin has been documented in both elongate and rounded specimens (Figs. 5, 6). It is thus impossible to draw a clear demarcation between these forms.

Figure 6. Line drawings of clam shrimp from Consthum. (1–6) Variation in Pseudestheria diensti. (1) Specimen EiB535a. (2) Specimen EiB551a_5. (3) Specimen EiB551b_1. (4) Specimen EiB559b. (5) Specimen EiB551b_7. (6) Specimen EiB552b_2. (7, 8) Palaeolimnadia stevenbeckeri n. sp. (7) Holotype EiB555. (8) Paratype EiB558. (1–6) Scale bars = 5 mm; (7, 8) scale bars = 1 mm.

In our view, it is warranted to formally synonymize Pseudestheria subcircularis with Pseudestheria diensti as already suspected by Poschmann et al. (Reference Poschmann, Hegna, Astrop and Hoffmann2024). Present evidence suggests that Palaeolimnadiopsis ? eifelensis is a preservational variant of Pseudestheria diensti (Poschmann et al., Reference Poschmann, Hegna, Astrop and Hoffmann2024) as well (although, as outlined in the preceding, this does not seem to apply more broadly to other species assigned to Palaeolimnadiopsis). Approximately contemporaneous clam shrimp from the Lower Devonian of Belgium and France (Maillieux, Reference Maillieux1939; Defretin, Reference Defretin1950) and originally designated Estheria (Euestheria) stockmansi Maillieux, Reference Maillieux1939 need re-examination and revision and will not be considered herein (see discussion in Poschmann et al., Reference Poschmann, Hegna, Astrop and Hoffmann2024).

Family Paleolimnadiidae Tasch, Reference Tasch1956 sensu Zhang et al., Reference Zhang, Chen and Shen1976

Diagnosis

Carapace with a large larval shell and few growth lines, smooth to faintly reticulate ornamentation. Limnadiform and cycladiform carapace shapes (translation from Zhang et al., Reference Zhang, Chen and Shen1976; see Astrop and Hegna, Reference Astrop and Hegna2015).

Remarks

Astrop and Hegna (Reference Astrop and Hegna2015) observed that past conceptions of the family (i.e., Zhang et al., Reference Zhang, Chen and Shen1976) implied that it was paraphyletic, and Sun and Cheng (Reference Sun and Cheng2022) hypothesized that the family might be polyphyletic due to the variance in ornamentation patterns. Usage of Paleolimnadiidae herein is not meant to imply monophyly. Indeed, the general diagnosis seems to suggest that paleolimnadiids are perhaps plesiomorphic vertexioids with large larval shells. The large larval shell is likely due to the initiation of carapace molt retention happening later in ontogeny.

Genus Palaeolimnadia Raymond, Reference Raymond1946

Type species

Estheria wianamattensis Mitchell, Reference Mitchell1927; by original designation.

Palaeolimnadia stevenbeckeri new species

Figures 5.5–5.8, 6.7, 6.8

Holotype

Figures 5.5, 6.7; specimen Eib555 (left valve, part only).

Diagnosis

A species of Palaeolimnadia with large (up to 3.6 mm long), oval carapace with straight to slightly concave and short dorsal margin, and with a slight bump in anterodorsal position; up to 13 (usually 6–10) growth lines; umbo in submedial and marginal position; larval valve moderately large.

Occurrence

Rinnen Quarry west of Consthum, fossil-bearing layer “Consthum II,” at the northeastern part of quarry (coordinates 49.97799, 6.03504). Lower Devonian, lower Emsian Klerf Formation (Schuttbourg Member of the Our Formation sensu Dejonghe et al., Reference Dejonghe, Colbach and Goemaere2017).

Description

Carapace large (L = 3.5–3.6 mm; H = 2.4–2.5 mm), shape oval (H/L = 0.69), with a slight swelling or bump in anterodorsal position (white arrow in Fig. 5.5, 5.6); dorsal margin straight to slightly concave and short (length of the dorsal margin (l) = 2.1–2.3 mm, l/L = 0.60–0.64); umbo in submedial and marginal position; larval valve small to large (height of the larval valve (h)/H = 0.44–0.5, about 0.25 in EiB702); up to 10 growth lines (about 13 in EiB702); ornamentation indistinct, possibly faintly reticulate.

Etymology

For Steven Becker (formerly Carrière Rinnen, Consthum), without whose generous help the Consthum excavations would not have been possible.

Additional material

Paratypes Eib558 (slightly deformed left valve, part only), EiB702 (right valve, part and counterpart), and EiB722 (right valve, part and counterpart).

Remarks

The second species present at Consthum is rare and found scattered in the sediment; it has not been recorded from the Pseudestheria mass occurrence. It has a relatively small carapace with a comparatively large, unornamented larval valve and straight dorsal margin and is therefore assigned to Palaeolimnadia. It shows a conspicuous small swelling or bump in anterodorsal position of the valve, which is probably not preservational but a genuine character of this species. In contrast to Bulbilimnadiidae Kozur and Weems, Reference Kozur and Weems2005, this swelling is situated anterior to, and not on, the larval valve. One specimen (EiB702; Fig. 5.7) has a higher number of growth lines and a smaller larval valve than other specimens assigned to this species. As it perfectly agrees in all other features with comparable specimens, it is assigned here to the same species. Whether this variation is due to preservation or may have some taxonomic significance can be reliably assessed only when more material is available. The new species is easily distinguishable from other Ardenno–Rhenish Early Devonian taxa, including the Belgian and French fossils (cf. Maillieux, Reference Maillieux1939; Defretin, Reference Defretin1950; Poschmann et al., Reference Poschmann, Hegna, Astrop and Hoffmann2024). It is comparable (slightly larger) in size to the type species P. wianamattensis (Mitchell, Reference Mitchell1927) but shows a straighter or even slightly concave dorsal margin. Considering that the occurrence of fossil spinicaudatan clam shrimp species is assumed to have at least some stratigraphical significance, we refrain from a comparison of our Early Devonian species with the Permian to Mesozoic Palaeolimnadia-like species, especially given the vast number of species, which includes many potential synonyms (e.g., Tasch, Reference Tasch1975b, Reference Tasch1987; Zhang et al., Reference Zhang, Chen and Shen1976; Kozur and Weems, Reference Kozur, Weems and Lucas2010; Gallego et al., Reference Gallego, Monferran, Stigall, Zacarias, Hegna, Jiménez, Bittencourt, Li and Barrios Calathaki2020, and literature therein). Palaeolimnadia is distributed mainly from the Late Permian to Jurassic, but Devonian records are rare. Palaeolimnadia subquadrata Novozhilov, Reference Novozhilov and Новожилов1953b from the Devonian of Kazakhstan is a much larger (6.4 mm long) and more rounded (H/L ratio 0.83) species. ?Palaeolimnadia sp. from the Emsian of Hunnan Province, China, is a larger species with a somewhat smaller (estimated) H/L ratio (0.60 versus 0.69) (Liao and Shen, Reference Liao and Shen2022). Palaeolimnadia atava Liu in Liu and Gao, Reference Liu and Gao1985 is comparable in size (up to 3.7 mm long, 2.3 mm high), whereas P. changyangensis Liu in Liu and Gao, Reference Liu and Gao1985 and P. luoyanshanensis Liu in Liu and Gao, Reference Liu and Gao1985 are smaller species. These three species from the Late Devonian of Hubei Province, China, all differ in lacking an anterodorsal carapace swelling and the associated tendency for a slightly concave dorsal margin.