Palaeoscolecid Explained

The palaeoscolecids are a group of extinct ecdysozoan worms resembling armoured priapulids. They are known from the Lower Cambrian[1] to the lower Ludfordian (late Silurian);[2] they are mainly found as disarticulated sclerites, but are also preserved in many of the Cambrian lagerstätten.[3] They take their name from the typifying genus Palaeoscolex.[4] Other genera include Cricocosmia from the Lower Cambrian Chengjiang biota.[5] Their taxonomic affinities within Ecdysozoa have been the subject of debate.

Morphology

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

Growth

Palaeoscolecids can grow by the continuous addition of plates, or by the continued growth of individual plates.[6]

Taxonomic position

Palaeoscolecids are somewhat challenging 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.

They are considered by some 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.[11] This said, their pharynx has the sixfold symmetry that likely characterised the ancestral ecdysozoan, rather than the fivefold symmetry of priapulans.[12] A nematomorph affinity appears to be an artefact that results from under-sampling of the priapulid stem group. Their relationship with Archaeopriapulida is unclear, and either group may be paraphyletic to the other. Some authors choose to include paleoscolecids within Priapulida.[13]

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.[14] This proposal was made in response to a 2021 paper that found in a phylogenetic analysis that paleoscolecids were stem-group priapulids.[15]

Taxonomy

As palaeoscolecids may represent a grade rather than a clade, drawing up a formal taxonomy proves problematic. 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.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.[18]

Status impossible to determine from current material

It is possible that Markuelia represents an embryonic Palaeoscolecid.[25]

Linnean taxonomy

Order Uncertain

Defined by the presence of smooth, folded and sclerite-bearing cuticular regions

Order Cricocosmida Han et al. 2007[26]
  • Defined by the presence of an unarmoured neck between the proboscis and the trunk, and a single pair of posterior hooks.

    Defined by presence of Hadimopanella sclerites with three to ten nodes.[27]

    Defined by presence of Milaculum-type plates, i.e. rectangular with parallel rows of nodes

    Other Palaeoscolecidae or unassigned
  • Microfossil material

    • Genus level taxonomy

    Palaeoscolex

    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.[30] As such, P. ratcliffei and P. huainanensis should not be included in Palaeoscolex.

    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.

    Guanduscolex, Wudingscolex

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

    Mafangscolex

    This genus[31] has simple sclerites with a single (small but prominent) node in the middle, so can be separated from Palaeoscolex(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.[12]

    Utahscolex

    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.

    Notes and References

    1. Andrey Y. Ivantsov . Ryszard Wrona . amp . 2004 . Articulated palaeoscolecid sclerite arrays from the Lower Cambrian of eastern Siberia . . 54 . 1 . 1–22 . 2009-06-03 . https://web.archive.org/web/20110716061730/http://www.geo.uw.edu.pl/agp/table/pdf/54-1/wrona.pdf . 2011-07-16 . dead .
    2. 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 . en . 10 . 3 . 10.1002/spp2.1558 . 2056-2799. free .
    3. M.. L.. M. . Fossilization modes in the Chengjiang Lagerstätte (Cambrian of China): testing the roles of organic preservation and diagenetic alteration in exceptional preservation. Zhu . Palaeogeography, Palaeoclimatology, Palaeoecology . 220 . 1–2. 31–37 . 2 May 2005 . 10.1016/j.palaeo.2003.03.001. Babcock. Steiner. 2005PPP...220...31Z.
    4. Book: The Cambrian fossils of Chengjiang, China: the flowering of early animal life . Wiley-Blackwell . 978-1-4051-0673-3. 233 . 2004 . Xianguang Hou . Richard Aldridge . Jan Bergström . David Siveter . Derek Siveter . David J. Siveter.
    5. Trunk ornament on the palaeoscolecid worms Cricocosmia and Tabelliscolex from the Early Cambrian Chengjiang deposits of China . Jian Han . Jianni Liu . Zhifei Zhang . Xinglian Zhang . Degan Shu . 2007 . . 52 . 2 . 423–431.
    6. 10.1016/j.palwor.2023.03.005. Growth patterns of palaeoscolecid sclerites from the Furongian (Upper Cambrian) Wangcun section, western Hunan, South China. 2023. Xian. Xiao-Feng. Eriksson. Mats E.. Zhang. Hua-Qiao. Palaeoworld. 257601719.
    7. 10.1111/pala.12210. A palaeoscolecid worm from the Burgess Shale. Palaeontology. 58. 6. 973–979. 2015. Smith. Martin R.. 86747103 . free. 2015Palgy..58..973S .
    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 . 10.4202/app.2009.0058. 10.1.1.620.4930 . 54037571.
    9. conference, http://gf.tmsoc.org/Documents/Mikro2009/GFSP15.pdf#page=82
    10. Peel . J. S.. 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. March 2010. 10.1666/09-102R.1. 2010JPal...84..332P. 86256781.
    11. Harvey . T. H.. Dong . X.. Donoghue . P. C.. Are palaeoscolecids ancestral ecdysozoans?. Evolution & Development. 12. 2. 177–200. 2010. 20433458. 10.1111/j.1525-142X.2010.00403.x. 16872271.
    12. 10.1017/S0016756820000308. Introvert and pharynx of Mafangscolex, a Cambrian palaeoscolecid. 2020. Yang. Jie. Smith. Martin R.. Zhang. Xi-Guang. Yang. Xiao-yu. Geological Magazine. 157. 12. 2044–2050. 2020GeoM..157.2044Y. 219092881.
    13. 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 . en . 94 . 4 . 661–674 . 10.1007/s12542-020-00516-9 . 2020PalZ...94..661W . 211479504 . 0031-0220.
    14. 10.1144/jgs2021-111. 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). 2022. Smith. Martin Ross. Dhungana. Alavya. Journal of the Geological Society. 179. 3. 2022JGSoc.179..111S. 244850491. free.
    15. Shi . Xiaomei . Howard . Richard J. . Edgecombe . Gregory D. . Hou . Xianguang . Ma . Xiaoya . March 2022 . Tabelliscolex (Cricocosmiidae: Palaeoscolecidomorpha) from the early Cambrian Chengjiang Biota and the evolution of seriation in Ecdysozoa . Journal of the Geological Society . en . 179 . 2 . jgs2021–060 . 10.1144/jgs2021-060 . 0016-7649. free . 2022JGSoc.179...60S .
    16. 10.1111/1755-6724.12724. 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. 2016. Yuning. Yang. Xingliang. Zhang. 2016AcGlS..90..807Y . 132852259 .
    17. 10.4202/app.00875.2021. First palaeoscolecid from the Cambrian (Miaolingian, Drumian) Marjum Formation of western Utah. 2021. Leibach. Wade. Lerosey-Aubril. Rudy. Whitaker. Anna. Schiffbauer. James. Kimmig. Julien. Acta Palaeontologica Polonica. 66. 239731912. free.
    18. 10.1002/spp2.1067. Cambrian palaeoscolecids (Cycloneuralia) of southern Scandinavia. Papers in Palaeontology. 3. 21–48. 2016. Streng. Michael. Ebbestad. Jan Ove R.. Berg-Madsen. Vivianne. 89406537 .
    19. Müller . K. J. . Hinz-Schallreuter . I. . 1993 . Palaeoscolecid worms from the Middle Cambrian of Australia . Palaeontology . 36 . 549–592 .
    20. 10.1017/S0016756809990082. Palaeoscolecid scleritome fragments with Hadimopanella plates from the early Cambrian of South Australia. Geological Magazine. 147. 86–97. 2009. Topper . T. P. . Brock . G. A. . Skovsted . C. B. . Paterson . J. R. . 54916222.
    21. 10.1016/j.palwor.2023.03.005. Growth patterns of palaeoscolecid sclerites from the Furongian (Upper Cambrian) Wangcun section, western Hunan, South China. 2023. Xian. Xiao-Feng. Eriksson. Mats E.. Zhang. Hua-Qiao. Palaeoworld. 257601719.
    22. Zhang . X.-G. . Pratt . B. R. . 1996 . Early Cambrian palaeoscolecid cuticles from Skaanxi, China . Journal of Paleontology . 70 . 2 . 275–279 . 10.1017/s0022336000023350 . 1996JPal...70..275Z . 132183034 .
    23. 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
    24. 10.1016/j.palwor.2023.03.005. Growth patterns of palaeoscolecid sclerites from the Furongian (Upper Cambrian) Wangcun section, western Hunan, South China. 2023. Xian. Xiao-Feng. Eriksson. Mats E.. Zhang. Hua-Qiao. Palaeoworld. 257601719.
    25. Duan . B. . Dong . X. -P. . Donoghue . P. C. J. . 10.1111/j.1475-4983.2012.01148.x . New palaeoscolecid worms from the Furongian (upper Cambrian) of Hunan, South China: Is Markuelia an embryonic palaeoscolecid? . Palaeontology . 55 . 3 . 613–622 . 2012 . 2012Palgy..55..613D . 53485143 .
    26. Trunk ornament on the palaeoscolecid worms Cricocosmia and Tabelliscolex from the Early Cambrian Chengjiang deposits of China. Han, J., Liu, J., Zhang, Z., Zhang, X., and Shu, D.. 2007. Acta Palaeontologica Polonica . 52. 2. 423–431.
    27. García-Bellido . D. C. . Paterson . J. R. . Edgecombe . G. D. . 10.1016/j.gr.2012.12.002 . Cambrian palaeoscolecids (Cycloneuralia) from Gondwana and reappraisal of species assigned to Palaeoscolex . Gondwana Research . 24 . 2 . 780–795 . 2013 . 2013GondR..24..780G .
    28. 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 . 10.1007/s12542-020-00516-9 . 2020PalZ...94..661W . 211479504 .
    29. 10.1016/j.palwor.2013.06.003. Palaeoscolecidan worms and a possible nematode from the Early Ordovician of South China. Palaeoworld. 23. 15–24. 2014. Muir. Lucy A.. Ng. Tin-Wai. Li. Xiang-Feng. Zhang. Yuan-Dong. Lin. Jih-Pai.
    30. 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 . 10.1111/j.1096-3642.1997.tb00136.x . free .
    31. Book: Shixue Hu. October 2005. Taphonomy and palaeoecology of the early Cambrian Chengjiang Biota from eastern Yunnan, China. Berliner Paläobiologische Abhandlungen. 7. 1–197. 3980853179.

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