Palaeoscolecid

Palaeoscolecida
Temporal range: Upper Early Cambrian–Silurian[1]
Scientific classification
Kingdom: Animalia
Clade: Cycloneuralia
Class: †Palaeoscolecida
Conway Morris & Robinson, 1986
Families, genera and species
(sensu Harvey et al. 2010[2])[3]

See text

The palaeoscolecids are a group of extinct ecdysozoan worms resembling armoured priapulids. They are known from the Lower Cambrian[4] to the late Silurian;[5] they are mainly found as disarticulated sclerites, but are also preserved in many of the Cambrian lagerstätten.[6] They take their name from the typifying genus Palaeoscolex.[5]

Morphology

Palaeoscolecids bear an annulated trunk ornamented with circular patterns of phosphatic tesselating plates; a layered cuticle; and an armoured proboscis.[2] They are long and narrow, and can reach tens of centimetres in length. Their cuticle is annulated, typically in complete rings, but sometimes the rings split or only encircle part of the trunk. Each annulus is essentially identical to its neighbours; the only trunk differentiation is at the anterior and posterior.[2] The anterior is radially symmetrical, typically comprising an introvert, whereas the trunk is bilaterally symmetrical.[2] The posterior hsots the terminal anus and sometimes one or two hooks.[2] There is no one character that unites the palaeoscolecids as a clade (indeed they are likely paraphyletic), and few individual specimens contain all characteristic palaeosolecid traits.[2]

Taxonomic position

Palaeoscolecids are a little tricky to define, and probably represent a paraphyletic grouping. Their most current systematic diagnosis[7] references their annulated worm-like body form, the presence of rows (usually) of phosphatic plates, and a straight gut, with the anus at the end of the animal. The group contains a wide and continuous spectrum of morphological variety, making further division of the group difficult; moreover, non-palaeoscolecid taxa likely evolved from palaeoscolecid-like ancestors, and it is thus difficult to demarcate a single clade that corresponds to the palaeoscolecid concept.[7]

They are considered to belong to the cycloneuralia,[8] although their position within this group is unresolved; they may lie with the priapulids or nematomorpha.[9] They have also been described as a sister-group to the ecdysozoa,[10] although as more characters are described a position closer to the priapulids becomes most probable.[2] A nematomorph affinity appears to be an artefact that results from under-sampling of the priapulid stem group.[2] Their relationship with Archaeopriapulida is unclear, and either group may be paraphyletic to the other.[3]

Other genera include Cricocosmia from the Lower Cambrian Chengjiang biota.[11] Their relationship with the archaeopriapulida is also unclear; it could be that both groups are a paraphyletic assemblage containing the priapulids.[1]

Taxonomy

As palaeoscolecids may represent a grade rather than a clade, drawing up a formal taxonomy proves problematic.[2] What is more, two parallel taxonomies exist: a form taxonomy for sclerites, and a true taxonomy for articulated fossils. The most recent holistic study of priapulids by Harvey et al. (2010) defines a core of palaeoscolecids characterized by a cuticle that is made up of interlocking plates of multiple sizes, and a looser assemblage (palaeoscolecids sensu lato) including other unconfirmed and palaeoscolecid-like forms:

Palaeoscolecids sensu stricto

Articulated macrofossils

Articulated microfossils

(from Orsten-type deposits, preserved in three dimensions)

Palaeoscolecidae From Australia
From China

Palaeoscolecids sensu lato

Other long and narrow Palaeozoic worms that exhibit an invariant body width are commonly referred to the palaeoscolecids, even though they lack the cuticular structure that defines the group; this 'Palaeoscolecid sensu lato' group includes Louisella, Cricocosmia, Tabelliscolex, Tylotites and others.[2] Maotianshania and, by extension, the family Maotianshaniidae, was excluded from the "Palaeoscolecids sensu strictu" by Harvey et al. (2010), but it has been argued that members of this family do exhibit the requisite cuticular structure, if discreetly.[13]

Status impossible to determine from current material

It's possible that Markuelia represents an embryonic Palaeoscolecid.[18]

Linnean taxonomy

  • Order Cricocosmida | Han et al. 2007[19]
    • Defined by the presence of an unarmoured neck between the proboscis and the trunk, and a single pair of posterior hooks.[19]
    • Family Cricocosmiidae
      • Tabelliscolex hexagonus
      • Tabelliscolex maanshanensis
      • Tabelliscolex chengjiangensis
      • Cricocosmia jinningensis
      • Houscolex
    • Maotianshaniidae
      • Maotianshania cylindrica|Sun and Huo, 1987
    • Palaeoscolecidae
      • Wronascolex
        Defined by presence of Hadimopanella sclerites with three to ten nodes.[20]
      • Palaeoscolex
        Defined by presence of Milaculum-type plates, i.e. rectangular with parallel rows of nodes[20]
      • Ashetscolex Muir et al 2014[21]
      • Sanduscolex Muir et al 2014[21]
    • Tylotitidae
      • Tylotites petiolaris|Luo and Hu 1999

    Genus level taxonomy

    Palaeoscolex

    Palaeoscolex has been abused as a wastebucket taxon for palaeoscolecid macrofossils. The most recent proposal is that Palaeoscolex should only include taxa with Milaculum-type sclerites, as in the type species P. piscatorum.[20][25] As such, P. ratcliffei and P. huainanensis should not be included in Palaeoscolex.[20]

    Wronascolex

    Originally described from Siberia, Wronascolex should now be considered to include all taxa with Hadimopanella sclerites that have 3–10 nodes in a single circle, perhaps including Yunnanoscolex.[20]

    Guanduscolex, Wudingscolex

    Though these genera have sclerites that resemble Hadimopanella knappologicum, they remain valid genera.[20]

    Mafangscolex

    This genus[26] has simple sclerites with a single (small but prominent) node in the middle, so can be separated from Palaeoscolex[20](unless this simplicity is taphonomic).

    References

    1. 1 2 Wills, M. A. (1 April 1998). "Cambrian and Recent Disparity: the Picture from Priapulids". Paleobiology. 24 (2): 155–286. JSTOR 2401237. doi:10.2307/2401237 (inactive 2017-01-30).
    2. 1 2 3 4 5 6 7 8 9 10 11 Harvey, T. H.; Dong, X.; Donoghue, P. C. (2010). "Are palaeoscolecids ancestral ecdysozoans?". Evolution & Development. 12 (2): 177–200. PMID 20433458. doi:10.1111/j.1525-142X.2010.00403.x.
    3. 1 2 Ma, X.; Aldridge, R. J.; Siveter, D. J.; Siveter, D. J.; Hou, X.; Edgecombe, G. D. (March 2014). "A New Exceptionally Preserved Cambrian Priapulid from the Chengjiang Lagerstätte". Journal of Paleontology. 88 (2): 371–384. doi:10.1666/13-082.
    4. ↑ Andrey Y. Ivantsov & Ryszard Wrona (2004). "Articulated palaeoscolecid sclerite arrays from the Lower Cambrian of eastern Siberia" (PDF). Acta Geologica Polonica. 54 (1): 1–22.
    5. 1 2 Xianguang Hou; Richard Aldridge; Jan Bergström; David Siveter; Derek Siveter (2004). The Cambrian fossils of Chengjiang, China: the flowering of early animal life. Wiley-Blackwell. p. 233. ISBN 978-1-4051-0673-3.
    6. ↑ Zhu, M.; Babcock, L.; Steiner, M. (2 May 2005). "Fossilization modes in the Chengjiang Lagerstätte (Cambrian of China): testing the roles of organic preservation and diagenetic alteration in exceptional preservation". Palaeogeography Palaeoclimatology Palaeoecology. 220: 31–37. doi:10.1016/j.palaeo.2003.03.001.
    7. 1 2 3 Smith, Martin R. (2015). "A palaeoscolecid worm from the Burgess Shale". Palaeontology. 58 (6): 973–979. doi:10.1111/pala.12210.
    8. ↑ Conway Morris, S.; Peel, J. S. (2010). "New palaeoscolecidan worms from the Lower Cambrian: Sirius Passet, Latham Shale, and Kinzers Shale". Acta Palaeontologica Polonica. 55 (1): 141–156. doi:10.4202/app.2009.0058.
    9. ↑ conference, http://gf.tmsoc.org/Documents/Mikro2009/GFSP15.pdf#page=82
    10. ↑ Peel, J. S. (March 2010). "A Corset-Like Fossil from the Cambrian Sirius Passet Lagerstätte of North Greenland and Its Implications for Cycloneuralian Evolution". Journal of Paleontology. 84 (2): 332–340. doi:10.1666/09-102R.1.
    11. ↑ Jian Han; Jianni Liu; Zhifei Zhang; Xinglian Zhang; Degan Shu (2007). "Trunk ornament on the palaeoscolecid worms Cricocosmia and Tabelliscolex from the Early Cambrian Chengjiang deposits of China" (PDF). Acta Palaeontologica Polonica. 52 (2): 423–431.
    12. ↑ Yuning, Yang; Xingliang, Zhang (2016). "Distinctive Scleritome with Marginal Tubercles of a New Palaeoscolecid Worm from the Shipai Fauna (Cambrian Epoch 2) at Three Gorges, South China". Acta Geologica Sinica - English Edition. 90 (3): 807. doi:10.1111/1755-6724.12724.
    13. 1 2 3 Streng, Michael; Ebbestad, Jan Ove R.; Berg-Madsen, Vivianne (2016). "Cambrian palaeoscolecids (Cycloneuralia) of southern Scandinavia". Papers in Palaeontology. doi:10.1002/spp2.1067.
    14. 1 2 3 4 5 6 7 8 9 10 11 MÜLLER, K. J. and HINZ-SCHALLREUTER, I. 1993. Palaeoscolecid worms from the Middle Cambrian of Australia. Palaeontology, 36, 549–592. 549-592.pdf
    15. ↑ Topper, T. P.; Brock, G. A.; Skovsted, C. B.; Paterson, J. R. (2009). "Palaeoscolecid scleritome fragments with Hadimopanella plates from the early Cambrian of South Australia". Geological Magazine. 147: 86. doi:10.1017/S0016756809990082.
    16. ↑ ZHANG, X.-G. and PRATT, B. R. 1996. Early Cambrian palaeoscolecid cuticles from Skaanxi, China. Journal of Paleontology, 70, 275–279.
    17. 1 2 Duan B, Dong X. 2013. Furongian (Late Cambrian) palaeoscolecid cuticles from Hunan Province, South China: the growth impact on the worm cuticle. Acta Scientiarum Naturalium Universitatis Pekinensis 49: 591–602. https://www.researchgate.net/profile/Xi_Ping_Dong/publication/264275844_Furongian_%28_Late_Cambrian_%29_Palaeoscolecid_Cuticles_from_Hunan_Province__South_China_the_Growth_Impact_on_Worm_Cuticle/links/53d894d70cf2631430c3250c.pdf
    18. ↑ Duan, B.; Dong, X. -P.; Donoghue, P. C. J. (2012). "New palaeoscolecid worms from the Furongian (upper Cambrian) of Hunan, South China: Is Markuelia an embryonic palaeoscolecid?". Palaeontology. 55 (3): 613–622. doi:10.1111/j.1475-4983.2012.01148.x.
    19. 1 2 Han, J., Liu, J., Zhang, Z., Zhang, X., and Shu, D. (2007). "Trunk ornament on the palaeoscolecid worms Cricocosmia and Tabelliscolex from the Early Cambrian Chengjiang deposits of China" (PDF). Acta Palaeontologica Polonica. 52 (2): 423–431.
    20. 1 2 3 4 5 6 7 García-Bellido, D. C.; Paterson, J. R.; Edgecombe, G. D. (2013). "Cambrian palaeoscolecids (Cycloneuralia) from Gondwana and reappraisal of species assigned to Palaeoscolex". Gondwana Research. 24 (2): 780–795. doi:10.1016/j.gr.2012.12.002.
    21. 1 2 Muir, Lucy A.; Ng, Tin-Wai; Li, Xiang-Feng; Zhang, Yuan-Dong; Lin, Jih-Pai (2014). "Palaeoscolecidan worms and a possible nematode from the Early Ordovician of South China". Palaeoworld. 23: 15–24. doi:10.1016/j.palwor.2013.06.003.
    22. ↑ Huang, D.; Chen, J.; Zhu, M.; Zhao, F. (2014). "The burrow dwelling behavior and locomotion of palaeoscolecidian worms: New fossil evidence from the Cambrian Chengjiang fauna". Palaeogeography, Palaeoclimatology, Palaeoecology. 398: 154–164. doi:10.1016/j.palaeo.2013.11.004.
    23. 1 2 3 Hu, S.; Steiner, M.; Zhu, M.; Luo, H.; Forchielli, A.; Keupp, H.; Zhao, F.; Liu, Q. (2012). "A new priapulid assemblage from the early Cambrian Guanshan fossil Lagerstätte of SW China". Bulletin of Geosciences: 93–106. doi:10.3140/bull.geosci.1238.
    24. ↑ "An Ordovician Occurrence of Utahphospha Müller & Miller". Journal of Paleontology. 55: 395–400. JSTOR 1304225. doi:10.2307/1304225 (inactive 2017-01-30).
    25. ↑ Conway, Morris S. (1997). "The cuticular structure of the 495-Myr-old type species of the fossil worm ~Palaeoscolex~, ~P. piscatorum~ (?Priapulida)". Zoological Journal of the Linnean Society. 119: 69–82. doi:10.1111/j.1096-3642.1997.tb00136.x.
    26. ↑ HU, S. 2005. Taphonomy and palaeoecology of the early Cambrian Chengjiang Biota from eastern Yunnan, China. Berliner Paläobiologische Abhandlungen, 7, 1–197.
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