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New insights into the Devonian sea spiders of the Hunsrück Slate (Arthropoda: Pycnogonida) - PubMed

  • ️Mon Jan 01 2024

New insights into the Devonian sea spiders of the Hunsrück Slate (Arthropoda: Pycnogonida)

Romain Sabroux et al. PeerJ. 2024.

Abstract

Background: The sea spiders (Pycnogonida Latreille, 1810) of the Hunsrück Slate (Lower Devonian, ~400 million years ago) are iconic in their abundance, exquisite pyritic preservation, and in their distinctive body plan compared to extant sea spiders (Pantopoda Gerstäcker, 1863). Consequently, the Hunsrück sea spiders are important in understanding the deep evolutionary history of Pycnogonida, yet they remain poorly characterised, impacting upon attempts to establish a time-calibrated phylogeny of sea spiders.

Methods: Here, we investigated previously described and new material representing four of the five Hunsrück pycnogonids: Flagellopantopus blocki Poschmann & Dunlop, 2006; Palaeoisopus problematicus Broili, 1928; Palaeopantopus maucheri Broili, 1929; and Pentapantopus vogteli Kühl, Poschmann & Rust, 2013; as well as a few unidentified specimens. Using X-ray microtomography and Reflectance Transformation Imaging, we describe new fossils, provide evidence for newly revealed anatomical features, and interpret these data in comparison to extant species. We also reinterpret the previously published illustration of the (probably lost) holotype of Palaeothea devonica Bergström, Stürmer & Winter, 1980.

Results: We provide the first detailed description of the cephalic appendages of Palaeoisopus problematicus and revise the interpretation of the organisation of its ocular tubercle. Furthermore, we provide new insights into the structure of the legs and the proboscis of Palaeopantopus maucheri, the first description of the body of Flagellopantopus blocki and describe a new specimen of Pentapantopus vogteli, demonstrating that it had eight legs, in contrast to previous interpretations. We argue that, contrary to previous suggestions, Palaeothea devonica probably had a different body plan from extant pantopods. We discuss the ecological traits of the Hunsrück pycnogonids based on their morphological adaptations, and conclude that there is no compelling evidence of Pantopoda in the Devonian. Through comparative interpretation of the legs as well as general morphology, we can divide the Hunsrück pycnogonids into two morphological groups, while Pantopoda constitutes a third morphological group.

Keywords: Appendage; Chelicerata; Devonian; Emsian; Hunsrück Slate; Konservat-Lagerstätte; Pycnogonida; Reflectance transformation imaging; X-ray microtomography.

© 2024 Sabroux et al.

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Conflict of interest statement

The authors declare that they have no competing interests.

Figures

Figure 1
Figure 1. Palaeiosopus problematicus, holotype SNSB-BSPG 1928 VII 11.

(A) Standard view. (B) Specular enhancement (direction of the light is indicated by the hemisphere on the bottom right, see Material S3 for details). (C) Normals visualization. (D, E) Maximum intensity views of X-ray microtomography. (F) Interpretative drawing. ab: abdomen, ce: cephalon, ov: oviger. WL1-4: walking legs 1–4. Scale bar 5 mm.

Figure 2
Figure 2. Palaeoisopus problematicus, body of specimen NHMMZ PWL 1994/133-LS.

(A) Standard view. (B) Specular enhancement (direction of the light is indicated by the hemisphere on the bottom right, see Material S3 for details). (C) Normals visualization. (D) Interpretative drawing. ch: chelifore, lp1-4: lateral processes 1–4, ot: ocular tubercle, pi: palp insertion, WL1-4: walking legs 1–4. Scale bar 5 mm.

Figure 3
Figure 3. Palaeoisopus problematicus, cephalon and proboscis in ventral view.

Specimens IGPB-HS207 (A, E, I, M), IGPB-HS660 (B, F, J, N), SNSB-BSPG 1932 I 67 (C, G, K, O) and NHMMZ PWL 1997/44-45-LS (D, H, L, P). (A–D) Standard view. (E–H) Specular enhancement (direction of the light is indicated by the hemisphere on the bottom right, see Material S3 for details). (I–L) Normals visualization. (M–P) Interpretative drawings. ch: chelifore, pa: palp, pb: proboscis, ov: oviger. Scale bars 5 mm.

Figure 4
Figure 4. Palaeoisopus problematicus, lateral view of the body where the proboscis is potentially visible.

Specimens IGPB-HS457 (A, C, E) and NHMMZ PWL 1995/35-LS (B, D, F). (A, B) Standard view. (C, D) Normals visualization. (E, F) Interpretative drawings. ch: chelifore, pa: palp, pb: proboscis, ov: oviger, te: telson, WL1-4: walking legs 1–4. Scale bars 5 mm.

Figure 5
Figure 5. Palaeoisopus problematicus, cephalon and first trunk segment.

Specimens NHMMZ PWL 1994/133-LS (A, D, G, J), NHMMZ PWL 2008/141-LS (B, E, H, K) and IGPB-AR-340 (C, F, I, L). (A–C) Standard view. (D-E Specular enhancement (direction of the light is indicated by the hemisphere on the bottom right, see Material S3 for details). (D–I) Normals visualization. (J–L) Interpretative drawings; features of the ocular tubercle discussed in the text are coloured (compare also with Fig. 6), putative lateral sense organs are coloured in orange. ch: chelifores, pa: palp, WL1: walking leg 1. Scale bars 5 mm.

Figure 6
Figure 6. Diagram of the organisation of the ocular tubercle ornamenation of Palaeoisopus problematicus as well as a few representatives of the modern Rhynchothorax.

Colours indicate putative homologies and correspond with fig. 5. In the case of the posteriormost dorsal tubercle of R. coralensis, there is no clear evidence of ornamentation, but Staples (2019) represents a low elevation of the tergite around this position which could correspond. Asterisks (*) indicate the tubercles that were interpreted as eyes by Bergström, Stürmer & Winter (1980). lo: lateral sense organ (coloured in orange). Rhynchothorax ocular tubercles diagrams are based on the illustrations of Arnaud (1974), Child (1979, and Staples (2019).

Figure 7
Figure 7. Palaeoisopus problematicus, trunk and cephalon of the specimen IGPB-HS206.

(A) Standard view. (B) Specular enhancement (direction of the light is indicated by the hemisphere on the bottom right, see Material S3 for details). (C) Normals visualization. (D) Interpretative drawing. Black arrowheads point out the dorsal ornamentation of the trunk segments. ch: chelifore, ot: ocular tubercle, pa: palp, WL1-4: walking legs 1–4. Scale bar 5 mm.

Figure 8
Figure 8. Palaeoisopus problematicus, abdomen.

Specimens MB-A-46 (A–D, dorsal view), NHMMZ PWL 1998/122-LS (E–H, dorsal view), IGPB-HS1039 (I–L, lateral view), IGPB-HS660 (M-P, ventral view). (A, E, I, M) Standard view. (B, F, J, N) Specular enhancement (direction of the light is indicated by the hemisphere on the bottom right, see Material S3 for details). (C, G, K, O) Normals visualization. (D, H, L, P) Interpretative drawings, black arrows point out some of the setae along the abdominal lateral margins, white arrows point out the arthrodial membranes of the abdomen. an: anus, vs.: pair of ventral setae. Scale bars 5 mm.

Figure 9
Figure 9. Palaeoisopus problematicus, evidence for the mobility range of the abdomen (in lateral view).

Specimens NHMMZ PWL 1994/56-LS (A–C), NHMMZ PWL 1995/35-LS specimens 2 (D–F) and 3 (G–I), and IGPB-HS694 (J–L). (A, D, G, J) Standard view. (B, E, H, K) Normals visualization. (C, F, I, L) Interpretative drawings (in I, the abdomen was coloured in yellow for better visibility, and the WL1 of another specimen is coloured in green), black arrows point out some of the setae along the abdominal lateral margins. an: anus. Scale bars 5 mm.

Figure 10
Figure 10. Palaeoisopus problematicus, specimen IGPB-HS582.

(A) General view of the fossil. (B–D) Zoom on the cephalon region: (B) Standard view. (C) Normals visualization. (D) Interpretative drawing. Margins and lines (see text) are highlighted with different colours according to their position. ch: chelifore, pa: palp, ov: oviger. Scale bars: A: 20 mm, D: 5 mm (B, C same scale as D).

Figure 11
Figure 11. Palaeoisopus problematicus, chelifores.

Specimens NHMMZ PWL 1996/18-LS (A, D, G, J), NHMMZ PWL 2008/141-LS (B, E, H, K) and SNSB-BSPG 1932 I 67 (C, F, I, L). (A–C) Standard view. (D–F) Specular enhancement (direction of the light is indicated by the hemisphere on the bottom right, see Material S3 for details). (G–I) Normals visualization. (J–L) Interpretative drawings. Black arrowheads in J indicate some of the potential setae insertions. Margins and lines (see text) are highlighted with different colours according to their position. Scale bars 5 mm.

Figure 12
Figure 12. Palaeoisopus problematicus, palps.

Specimens NHMMZ PWL 1996/18-LS (right palp: A, H, O, V; left palp: B, I, P, W), NHMMZ PWL2008/141-LS (left palp: C, J, Q, X), NHMMZ PWL 1995/35-LS specimens 1 (right palp: D, K, R, Y) and 2 (right palp: E, L, S, Z), NHMMZ PWL 1997/44-45-LS (right palp: F, M, T, AA), SNSB-BSPG 1932 I 63 (left palp: G, N, U, AB). (A–G) Standard view. (H-N) Specular enhancement (direction of the light is indicated by the hemisphere on the bottom right, bottom left or top right; see Material S3 for details). (O–U) Normals visualization. (V–AB) Interpretative drawings. Scale bars 5 mm.

Figure 13
Figure 13. Palaeoisopus problematicus, ovigers.

Specimens NHMMZ PWL 1995/35-LS specimen 3 (unidentified oviger: A, F, K, P), SNSB-BSPG 1932 I 63 (unidentified oviger: B, G, L, Q), NHMMZ PWL 1994/56-LS (unidentified oviger: C, H, M, R) and NHMMZ PWL 1994/133-LS (left oviger: D, I, N, S; right oviger: E, J, O, T). (A–E) Standard view. (F–J) Specular enhancement (direction of the light is indicated by the hemisphere on the bottom right or bottom left; see Material S3 for details). (K–O) Normals visualization. (P–T) Interpretative drawings. Scale bars 5 mm.

Figure 14
Figure 14. Diagram of the interpreted structure of the cephalic appendages of Palaeoiospus problematicus.

(A) Chelifore. (B) Palp. (C) Oviger. (D) Colour code for putative podomeres homology in palps and ovigers, following the nomenclature of Sabroux et al. (2023) and Siveter et al. (2023) (modified from Sabroux et al., 2023 and Siveter et al., 2023).

Figure 15
Figure 15. Palaeoisopus problematicus, first walking pair of legs (WL1).

Specimens NHMMZ PWL 2008/141-LS (left WL1, lateral view: A–D), IGPB-HS636 (left WL1, lateral view: E–H), terminal podomeres of IGPB-HS582 (left WL1, lateral view: I–K; note the ventral spines), IGPB-AR-340 (left WL1, dorsal view: L–N), MB-A-46 (left WL1, dorsal view: O–Q). (A, E, I, L, O) Standard view. (B, F) Specular enhancement (direction of the light is indicated by the hemisphere on the bottom right or bottom left; see Material S3 for details). (C, G, J, M, P) Normals visualization. (D, H, K, N, Q) Interpretative drawings. cx1-3: coxa 1-3, cl: claw, fe: femur, pt: patella, pr: propodus, ta: tarsus, ti: tibia. Scale bars 5 mm.

Figure 16
Figure 16. Palaeoisopus problematicus, second to fourth walking pairs of legs (WL2-4).

Specimens IGPB-HS1039 (A–D), IGPB-HS582 (specimen 2, right WL: E–H), MB.A.46 (right WL: IL) and IGPB-HS582 (specimen 1, right WL: M–P). (A, E, I, M) Standard view. (B, F, J, N) Specular enhancement (direction of the light is indicated by the hemisphere on the bottom left, see Material S3 for details). (C, G, K, O) Normals visualization. (D, H, L, P) Interpretative drawings. WL2-4: walking legs 2–4. Scale bars 5 mm.

Figure 17
Figure 17. Juvenile instars of Palaeoisopus problematicus (see also following illustration).

Specimens IGPB-HS942 specimen 1 (A, C, E, G) and NHMMZ PWL 2003/272-LS specimen 1 (B, D, F, H). (A, B) Standard view. (C, D) Specular enhancement (direction of the light is indicated by the hemisphere on the bottom right, see Material S3 for details). (E, F) Normals visualization. (G, H) Interpretative drawings. ab: abdomen, ch: chelifore, ov: oviger, pa: palp, WL1-4: walking legs 1–4. Scale bars 5 mm.

Figure 18
Figure 18. Juvenile instars of Palaeoisopus problematicus.

Specimens NHMMZ PWL 2000/46 (A–D), NHMMZ PWL 1986/3 (E–H) and NHMMZ PWL 1994/54-LS specimen 1 (I–L). (A, E, I) Standard view. (B, F, J) Specular enhancement (direction of the light is indicated by the hemisphere on the bottom left, see Material S3 for details). (C, G, K) Normals visualization. (D, H, L) Interpretative drawings. ab: abdomen, ch: chelifore, ov: oviger, pb: proboscis, WL1-4: walking legs 1–4. Scale bars 5 mm.

Figure 19
Figure 19. Palaeoisopus problematicus reconstruction based on observed material, in dorsal view.

The number of podomeres of the chelifore scapes is speculative (see related section).

Figure 20
Figure 20. Palaeopantopus maucheri, holotype SNSB-BSPG 1929 V 3.

Dorsal (A–D) and ventral (E, F) views of the body. (A, E) Standard view. (B) Specular enhancement (direction of the light is indicated by the hemisphere on the bottom right, see Material S3 for details). (C, F) Normals visualization. (D) Interpretative drawing based of the fossil’s surface. (G) Maximum intensity views of X-ray microtomography. (H) Interpretative drawing of structure revealed with X-ray, overlapped with surface structure (in grey). Pictures (E and F) were mirrored to align with (A–D, G and H). ab: abdomen, ch: chelifore, pa: palp, pb: proboscis, ov: oviger, WL1-4: walking leg 1–4. Scale bar: 2 mm.

Figure 21
Figure 21. Palaeopantopus maucheri, specimen SNSB-BSPG 1930 I 501, posterior view of the body.

(A) Standard view. (B) Specular enhancement (direction of the light is indicated by the hemisphere on the bottom right, see Material S3 for details). (C) Normals visualization. (D) Interpretative drawing based of the fossil’s surface. ab: abdomen, pa: palp, ov: oviger, WL1-4: walking leg 1–4. Scale bar: 5 mm.

Figure 22
Figure 22. Palaeopantopus maucheri, specimen MB-A-45, ventral view of the body.

(A) Standard view. (B) Specular enhancement (direction of the light is indicated by the hemisphere on the bottom right, see Material S3 for details). (C) Normals visualization. (D) Interpretative drawing based of the fossil’s surface. (E) Maximum intensity views of X-ray microtomography. (F) Interpretative drawing of structure revealed with X-ray, overlapped with surface structure (in grey). ab: abdomen, ch: chelifore, pa: palp, pb: proboscis, ov: oviger, WL1-4: walking leg 1–4. Scale bar: 5 mm.

Figure 23
Figure 23. Palaeopantopus maucheri, legs.

Holotype SNSB-BSPG 1929 right WL1 (A–C), right WL2 (D–F), right WL3 (G–I), right WL4 (J–L), specimens MB-A-45 left WL4 (M–O) and SNSB-BSPG 1930 I 501 left WL3 (P–R). (A, D, G, J, M, P) Standard view. (B, E, H, K, N, Q) Specular enhancement (direction of the light is indicated by the hemisphere on the bottom right, see Material S3 for details). (C, F, I, L, O, R) Normals visualization. (S) Interpretative drawings of all legs, put to the same scale. All scale bars 5 mm, B and C same as A; E, F same as D; H, I same as G; K, L same as J; N, O same as M; Q, R same as P.

Figure 24
Figure 24. Palaeopantopus maucheri reconstruction based on observed material, in dorsal view.

The structure and number of podomeres in the cephalic appendages, as well as the terminal podomeres of the walking legs, are conjectural.

Figure 25
Figure 25. Flagellopantopus blocki, holotype NHMMZ PWL 2004/5024-LS, ventral view.

(A) Standard view. (B) Normals visualization, white arrowheads indicate the immobile joint. (C) Interpretative drawing based of the fossil’s surface. (D, E) Maximum intensity views of Xray microtomography. (F) Interpretative drawing of structure revealed with X-ray, overlapped with surface structure (in grey). ab: abdomen, ch: chelifore, fl: flagellum,ot: ocular tubercle, ov: oviger, pa: palp, pb: proboscis; se: seta, WL1-4: walking leg 1–4. Scale bar: 5 mm.

Figure 26
Figure 26. Flagellopantopus blocki, ovigers of holotype NHMMZ PWL 2004/5024-LS.

Right (A, C, E, G) and left (B, D, F, H) ovigers. (A, B) Standard view. (C, D) Normals visualization. (E, F) Maximum intensity views of X-ray microtomography. (G, H) Interpretative drawing, tentative podomere-per-podomere homology hypothesis indicated by colour codes (refer to Figs. 14 and 36). Scale bars 2 mm.

Figure 27
Figure 27. Flagellopantopus blocki, reconstruction based on observed material, in dorsal view.

The proximal portion of the proboscis, and the structure of the chelifores and the palps are conjectural.

Figure 28
Figure 28. Pentapantopus vogteli holotype and paratype NHMMZ PWL 2010/5-LS.

(A) Standard view. (B) Normals visualization. (C) Maximum intensity views of X-ray microtomography. (D) Interpretative drawing (holotype in yellow, paratype in green). ch: chelifore, ov: oviger, pa: palp, pb: proboscis, WL1-4: walking leg 1–4. Scale bar: 5 mm.

Figure 29
Figure 29. Pentapantopus vogteli, NHMMZ PWL 2010/5-LS, specimen 5.

(A, B) Whole specimen. (C) Close-up on the body (arrow: coxal rings, white arrowhead: abdomen, black arrowhead: palpal claw?). (D) Close-up on WL3 (arrow: terminal claw). (E) Close-up on WL2 (arrow: terminal claw, arrowhead: molariform indentations of the ventral side of WL). (A, C, D, E) Maximum intensity views of X-ray microtomography. (B) Interpretative drawing. ab: abdomen, ch: chelifore, ov: oviger, pa: palp, pb: proboscis, WL1-4: walking leg 1–4. Scale bars: 5 mm (A, B), 2 mm (C), 1 mm (D, E).

Figure 30
Figure 30. Pentanpantopus vogteli, reconstruction based on observed material, in dorsal view.

The articulation of the abdomen is conjectural, inspired by the morphologically similar Haliestes dasos (Siveter et al., 2004, 2023); so is the presence of a claw at the tip of the palps, the structure of the palps and ovigers. The structure of the chelifores and the number of coxal rings are also speculative. The structure of the cephalon relies on the specimen NHMMZ PWL 2007/29-LS, which is possibly a representative of Pentapantopus vogteli.

Figure 31
Figure 31. Specimen NHMMZ PWL 2007/29-LS, dorsal view.

(A) Association with ophiuroid (Bundenbachia beneckei Stürtz, 1886). (B–E) Focus on the specimen. (A and B) Standard view. (C) Specular enhancement (direction of the light is indicated by the hemisphere on the bottom right, see Material S3 for details). (D) Normals visualization. (E) Interpretative drawing. ab: abdomen, ch: chelifore, ot: ocular tubercle, ov: oviger, pa: palp, WL1-4: walking leg 1–4. Scale bar: 10 mm (A), 2 mm (E). B, C, D same scale as E.

Figure 32
Figure 32. Specimen IGPB-HS437, ventral view.

(A) Standard view. (B) Normals visualization. (C) Maximum intensity views of X-ray microtomography. (D) Interpretative drawing. ab: abdomen, ce: cephalon, WL1-4: walking leg 1–4. Scale bar: 5 mm.

Figure 33
Figure 33. Specimen IGPB-HS437, ventral view of the body.

(A) Standard view. (B) Specular enhancement (direction of the light is indicated by the hemisphere on the bottom right, see Material S3 for details). (C) Normals visualization. (D–F) Maximum intensity views of X-ray microtomography. (G) Interpretative drawing based of the fossil’s surface. (H) Interpretative drawing of structure revealed with Xray, overlapped with surface structure (in grey). ch: chelifore, ot: ocular tubercle, ov: oviger, WL1-4: walking leg 1–4. Scale bar: 5 mm.

Figure 34
Figure 34. Undetermined pycnogonids.

NHMMZ PWL 2010/5-LS (specimen 3: A–D: specimen 4: E–H) and IGPB-HS942 specimen 2 (I–L). (A, E, I) Standard view. (B, F, J) Specular enhancement (direction of the light is indicated by the hemisphere on the bottom right, see Material S3 for details). (C, G, K) Normals visualization. (D, H, L) Interpretative drawing. Scale bars 5 mm.

Figure 35
Figure 35. Interpretative drawing of the holotype of Palaeothea devonica.

Drawing based on the X-ray images provided by Bergström, Stürmer & Winter (1980). Scale bar 1 mm.

Figure 36
Figure 36. Walking legs of sea spiders and hypothetical homologies among podomeres.

(A) Reconstruction and hypothesised homology (see text and Table 2 for explanations). (B) Colour code, following the nomenclature of Sabroux et al. (2023) and Siveter et al. (2023). Figure modified from Sabroux et al. (2023) and Siveter et al. (2023).

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Grants and funding

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 101023218 to RS; from the Leverhulme Trust Research Fellowship (RF-2022-167) to PCJD; from a University of Bristol University Research Fellowship (URF) to DP; and from the Natural Environment Research Council (grant NE/T000813/1), the Alexander von Humboldt Stiftung, and the Leverhulme Trust Research Project Grant (RPG-2023-234) to RJG. DP and PCJD were conjointly funded by the Natural Environment Research Council (grant NE/P013678/1) as part of the Biosphere Evolution, Transitions and Resilience programme co-funded by the Natural Science Foundation of China; by the John Templeton Foundation (62220; the opinions expressed in this publication are those of the authors and do not necessarily reflect the views of the John Templeton Foundation), and by a Gordon and Betty Moore Foundation (GBMF9741) grant. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

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