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Palaeoscolecid

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The palaeoscolecids are a group of extinct ecdysozoan worms resembling armoured priapulids. They are known from the Lower Cambrian[7] to the lower Ludfordian (late Silurian);[8][2] they are mainly found as disarticulated sclerites, but are also preserved in many of the Cambrian lagerstätten.[9] They take their name from the typifying genus Palaeoscolex.[8] Other genera include Cricocosmia from the Lower Cambrian Chengjiang biota.[10] Their taxonomic affinities within Ecdysozoa have been the subject of debate.

Palaeoscolecida
Temporal range: Upper Early Cambrian–Ludfordian[1][2]
The Burgess Shale palaeoscolecid Scathascolex minor. Image from Smith (2015)[5][6]
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Clade: Cycloneuralia
Class: †Palaeoscolecida
Conway Morris & Robinson, 1986
Families, genera and species
(sensu Harvey et al. 2010[3])[4]

See text

Morphology

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Palaeoscolecids bear an annulated trunk ornamented with circular patterns of phosphatic tesselating plates; a layered cuticle; and an armoured proboscis.[3] 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.[3] The anterior is radially symmetrical, typically comprising an introvert, whereas the trunk is bilaterally symmetrical.[3] The posterior hosts the terminal anus and sometimes one or two hooks.[3] 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.[3]

Growth

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Palaeoscolecids can grow by the continuous addition of plates, or by the continued growth of individual plates.[11]

Taxonomic position

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Palaeoscolecids are somewhat challenging to define, and probably represent a paraphyletic grouping. Their most current systematic diagnosis[12] 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.[12]

They are considered by some to belong to the Cycloneuralia,[13] although their position within this group is unresolved; they may lie with the priapulids or Nematomorpha.[14] They have also been described as a sister-group to the Ecdysozoa,[15] although as more characters are described a position closer to the priapulids becomes most probable.[3] This said, their pharynx has the sixfold symmetry that likely characterised the ancestral ecdysozoan, rather than the fivefold symmetry of priapulans.[16] A nematomorph affinity appears to be an artefact that results from under-sampling of the priapulid stem group.[3] Their relationship with Archaeopriapulida is unclear, and either group may be paraphyletic to the other.[4][1] Some authors choose to include paleoscolecids within Priapulida.[17]

Martin R. Smith and Alavya Dhungana suggested in a 2022 publication that palaeoscolecids are a grade including sister taxa to Panarthropoda, highlighting similarities between the dorsal plates of taxa such as cricocosmiids and those of lobopodians such as Microdictyon.[18] This proposal was made in response to a 2021 paper that found in a phylogenetic analysis that paleoscolecids were stem-group priapulids.[19]

Taxonomy

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As palaeoscolecids may represent a grade rather than a clade, drawing up a formal taxonomy proves problematic.[3] 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

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Articulated macrofossils

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Articulated microfossils

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

Palaeoscolecidae From Australia
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From China
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Palaeoscolecids sensu lato

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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.[3] Maotianshania and, by extension, the family Maotianshaniidae, was excluded from the "Palaeoscolecids sensu stricto" by Harvey et al. (2010), but it has been argued that members of this family do exhibit the requisite cuticular structure, if discreetly.[22]

Status impossible to determine from current material

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It is possible that Markuelia represents an embryonic Palaeoscolecid.[29]

Linnean taxonomy

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Order Uncertain
Order Cricocosmida Han et al. 2007[30]
Defined by the presence of an unarmoured neck between the proboscis and the trunk, and a single pair of posterior hooks.[30]
Other Palaeoscolecidae or unassigned
Microfossil material

Genus level taxonomy

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Palaeoscolex

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Palaeoscolex has been abused as a wastebasket 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.[31][38] As such, P. ratcliffei and P. huainanensis should not be included in Palaeoscolex.[31]

Wronascolex

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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.[31]

Guanduscolex, Wudingscolex

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Though these genera have sclerites that resemble Hadimopanella knappologicum, they remain valid genera.[31]

Mafangscolex

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This genus[39] has simple sclerites with a single (small but prominent) node in the middle, so can be separated from Palaeoscolex[31](unless this simplicity is taphonomic). Its introvert has a six-fold symmetry, whereas its proboscis has quincuncially arranged teeth that resemble those of other Cambrian ecdysozoan worms.[16]

Utahscolex

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Originally described from the Spence Shale of Utah, Utahscolex has four transverse rings of plates per annulus, arranged as two 'bands' of double rows of plates separated by a central naked zone. Occasionally, single row bifurcates into two rows (for up to 6 rows per annulus). The plates are circular, and unornamented. Platelets and microplates are absent.

References

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  1. ^ a b Wills, M. A. (1 April 1998). "Cambrian and Recent Disparity: the Picture from Priapulids". Paleobiology. 24 (2): 155–286. doi:10.1666/0094-8373(1998)024[0177:CARDTP]2.3.CO;2. JSTOR 2401237. S2CID 88647544.
  2. ^ a b c Howard, Richard J.; Parry, Luke A.; Clatworthy, Innes; D'Souza, Leila; Edgecombe, Gregory D. (May 2024). "Palaeoscolecids from the Ludlow Series of Leintwardine, Herefordshire (UK): the latest occurrence of palaeoscolecids in the fossil record". Papers in Palaeontology. 10 (3). doi:10.1002/spp2.1558. ISSN 2056-2799.
  3. ^ a b c d e f g h i j k Harvey, T. H.; Dong, X.; Donoghue, P. C. (2010). "Are palaeoscolecids ancestral ecdysozoans?". Evolution & Development. 12 (2): 177–200. doi:10.1111/j.1525-142X.2010.00403.x. PMID 20433458. S2CID 16872271.
  4. ^ a b 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. Bibcode:2014JPal...88..371M. doi:10.1666/13-082. S2CID 85627132.
  5. ^ Smith, Martin R. (2015). "A palaeoscolecid worm from the Burgess Shale". Palaeontology. 58 (6): 973–979. Bibcode:2015Palgy..58..973S. doi:10.1111/pala.12210. S2CID 86747103.
  6. ^ Smith, Martin R. (2015). "Lagerstatten". Data from: A palaeoscolecid worm from the Burgess Shale. Dryad Digital Repository. Dryad. doi:10.5061/dryad.cf493. hdl:10255/dryad.92916.
  7. ^ 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. Archived from the original (PDF) on 2011-07-16. Retrieved 2009-06-03.
  8. ^ a b 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.
  9. ^ 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 (1–2): 31–37. Bibcode:2005PPP...220...31Z. doi:10.1016/j.palaeo.2003.03.001.
  10. ^ 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.
  11. ^ Xian, Xiao-Feng; Eriksson, Mats E.; Zhang, Hua-Qiao (2023). "Growth patterns of palaeoscolecid sclerites from the Furongian (Upper Cambrian) Wangcun section, western Hunan, South China". Palaeoworld. doi:10.1016/j.palwor.2023.03.005. S2CID 257601719.
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  13. ^ 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. CiteSeerX 10.1.1.620.4930. doi:10.4202/app.2009.0058. S2CID 54037571.
  14. ^ conference, http://gf.tmsoc.org/Documents/Mikro2009/GFSP15.pdf#page=82
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  16. ^ a b Yang, Jie; Smith, Martin R.; Zhang, Xi-Guang; Yang, Xiao-yu (2020). "Introvert and pharynx of Mafangscolex, a Cambrian palaeoscolecid" (PDF). Geological Magazine. 157 (12): 2044–2050. Bibcode:2020GeoM..157.2044Y. doi:10.1017/S0016756820000308. S2CID 219092881.
  17. ^ Whitaker, Anna F.; Jamison, Paul G.; Schiffbauer, James D.; Kimmig, Julien (December 2020). "Re-description of the Spence Shale palaeoscolecids in light of new morphological features with comments on palaeoscolecid taxonomy and taphonomy". PalZ. 94 (4): 661–674. Bibcode:2020PalZ...94..661W. doi:10.1007/s12542-020-00516-9. ISSN 0031-0220. S2CID 211479504.
  18. ^ Smith, Martin Ross; Dhungana, Alavya (2022). "Discussion on 'Tabelliscolex (Cricocosmiidae: Palaeoscolecidomorpha) from the early Cambrian Chengjiang Biota and the evolution of seriation in Ecdysozoa' by Shi et al. 2021 (JGS, jgs2021-060)". Journal of the Geological Society. 179 (3). Bibcode:2022JGSoc.179..111S. doi:10.1144/jgs2021-111. S2CID 244850491.
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  20. ^ 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. Bibcode:2016AcGlS..90..807Y. doi:10.1111/1755-6724.12724. S2CID 132852259.
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  22. ^ a b c Streng, Michael; Ebbestad, Jan Ove R.; Berg-Madsen, Vivianne (2016). "Cambrian palaeoscolecids (Cycloneuralia) of southern Scandinavia". Papers in Palaeontology. 3: 21–48. doi:10.1002/spp2.1067. S2CID 89406537.
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  24. ^ 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: 86–97. doi:10.1017/S0016756809990082. S2CID 54916222.
  25. ^ Xian, Xiao-Feng; Eriksson, Mats E.; Zhang, Hua-Qiao (2023). "Growth patterns of palaeoscolecid sclerites from the Furongian (Upper Cambrian) Wangcun section, western Hunan, South China". Palaeoworld. doi:10.1016/j.palwor.2023.03.005. S2CID 257601719.
  26. ^ Zhang, X.-G.; Pratt, B. R. (1996). "Early Cambrian palaeoscolecid cuticles from Skaanxi, China". Journal of Paleontology. 70 (2): 275–279. Bibcode:1996JPal...70..275Z. doi:10.1017/s0022336000023350. S2CID 132183034.
  27. ^ a b 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
  28. ^ Xian, Xiao-Feng; Eriksson, Mats E.; Zhang, Hua-Qiao (2023). "Growth patterns of palaeoscolecid sclerites from the Furongian (Upper Cambrian) Wangcun section, western Hunan, South China". Palaeoworld. doi:10.1016/j.palwor.2023.03.005. S2CID 257601719.
  29. ^ 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. Bibcode:2012Palgy..55..613D. doi:10.1111/j.1475-4983.2012.01148.x. S2CID 53485143.
  30. ^ a b 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.{{cite journal}}: CS1 maint: multiple names: authors list (link)
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  32. ^ Whitaker, Anna F.; Jamison, Paul G.; Schiffbauer, James D.; Kimmig, Julien K. (2020). "Re‑description of the Spence Shale palaeoscolecids in light of new morphological features with comments on palaeoscolecid taxonomy and taphonomy". PalZ. 94 (4): 661–674. Bibcode:2020PalZ...94..661W. doi:10.1007/s12542-020-00516-9. S2CID 211479504.
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  38. ^ 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.
  39. ^ Shixue Hu (October 2005). Taphonomy and palaeoecology of the early Cambrian Chengjiang Biota from eastern Yunnan, China. Vol. 7. Berliner Paläobiologische Abhandlungen. pp. 1–197. ISBN 3980853179.
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