Ctenocystoidea is an extinct clade of echinoderms, which lived during the Cambrian and Ordovician periods. Unlike other echinoderms, ctenocystoids had bilateral symmetry, or were only very slightly asymmetrical. They are believed to be one of the earliest-diverging branches of echinoderms, with their bilateral symmetry a trait shared with other deuterostomes. Ctenocystoids were once classified in the taxon Homalozoa, also known as Carpoidea, alongside cinctans, solutes, and stylophorans.[1] Homalozoa is now recognized as a polyphyletic group of echinoderms without radial symmetry. Ctenocystoids were geographically widespread during the Middle Cambrian, with one species surviving into the Late Ordovician.
| Ctenocystoidea Temporal range:
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| Ctenocystis utahensis fossil, National Museum of Natural History | |
Scientific classification 
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Superphylum: | Deuterostomia |
| Clade: | Ambulacraria |
| Phylum: | Echinodermata |
| Class: | †Ctenocystoidea Robison & Sprinkle, 1969 |
| Type genus | |
| Ctenocystis Robison & Sprinkle, 1969
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| Genera | |
Description
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Like other echinoderms, ctenocystoids had a skeleton made of stereom plates. Ctenocystoids had near bilateral symmetry, with some species exhibiting slightly asymmetric plate shapes. Like cinctans and Ctenoimbricata, most ctenocystoids had large marginal plates surrounding the sides of their body, but unlike cinctans and Ctenoimbricata, which had only one row of marginal plates, most ctenocystoids had two rows of marginal plates. Courtessolea had only one row of marginal plates, like cinctans and Ctenoimbricata,[2] whereas Conollia lost the marginal plates entirely.[3] The anus of ctenocystoids was surrounded by a pyramidal periproct as in other echinoderms. It was located at the posterior end, defining a clear anterior-posterior body axis unlike other echinoderms.[4]
All ctenocystoids had a ctenoid apparatus, a comb-like arrangement of movable plates at the anterior end of the animal.[5][3] Beneath the ctenoid apparatus was a mouth, bordered on each side by a food groove.[1]
Classification
editRelationships
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| Phylogenetic relationships within Ctenocystoidea[6] |
Ctenocystoids are likely among the most basal stem-group echinoderms.[1] They have also been interpreted as aberrant blastozoans and as stem-group hemichordates.[7][8] The presence of stereom plates indicates that they most likely belong to the echinoderm total group, rendering a hemichordate affinity unlikely.[9]
Courtessolea was probably the most basal ctenocystoid, given its anatomical similarities to Ctenoimbricata and cinctans.[2] Conollia and Jugoszovia may be closely related to each other, as both have a reduced marginal frame compared to other ctenocystoids.[3]
Genera
editThe following genera of ctenocystoids have been named:[6][10][11]
| Genus | Discoverers | Named | Age | Location | Notes |
|---|---|---|---|---|---|
| Conollia | Domínguez Alonso | 1999 | Sandbian |  United Kingdom
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The geologically youngest genus[3] |
| Courtessolea | Domínguez Alonso | 1999 | Wuliuan |  France
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The most basal genus[2] |
| Ctenocystis | Robison & Sprinkle | 1969 | Middle Cambrian |  United States
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The first genus discovered[5] |
| Etoctenocystis | Fatka & Kordule | 1985 | Middle Cambrian |  Czech Republic
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| Gilcidia | Domínguez Alonso | 1999 | Middle Cambrian |  Australia,  Denmark, France
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| Jugoszovia | Džik & Orłowski | 1995 | Middle Cambrian |  Poland
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| Pembrocystis | Domínguez Alonso | 1999 | Middle Cambrian | United Kingdom
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Distribution
editCtenocystoids were widespread during the Middle Cambrian, and have been found in the United States, the United Kingdom, Australia, France, the Czech Republic, Poland, Spain, and Morocco.[11] The earliest ctenocystoids date to the beginning of Stage 5 of the Cambrian,[12] now known as the Wuliuan age,[13] or possibly slightly earlier, in late Cambrian Stage 4.[14] Most species date to the Wuliuan and Drumian ages of the Cambrian.[3] The geologically youngest ctenocystoid, the only one known from the Ordovician, is Conollia, from the Sandbian of the United Kingdom.[3]
History
editThe class Ctenocystoidea was named in 1969 by Richard A. Robison and James Sprinkle.[5] It originally contained one species, Ctenocystis utahensis. The name comes from the Greek words ktenos, meaning "comb", and kystis, meaning "sac". It was originally assigned to the echinoderm subphylum Homalozoa.
References
edit- ^ a b c Rahman, Imran A.; Clausen, Sébastien (2009). "Re-evaluating the palaeobiology and affinities of the Ctenocystoidea (Echinodermata)". Journal of Systematic Palaeontology. 7 (4): 413–426. Bibcode:2009JSPal...7..413R. doi:10.1017/S1477201909990046. S2CID 129093037.
- ^ a b c Zamora, Samuel; Rahman, Imran A.; Smith, Andrew B. (2012-06-06). "Plated Cambrian bilaterians reveal the earliest stages of echinoderm evolution". PLOS ONE. 7 (6). Keith A. Crandall (ed.): –38296. Bibcode:2012PLoSO...738296Z. doi:10.1371/journal.pone.0038296. ISSN 1932-6203. PMC 3368939. PMID 22701623.
- ^ a b c d e f Rahman, Imran A.; Stewart, Sarah E.; Zamora, Samuel (2015). "The youngest ctenocystoids from the Upper Ordovician of the United Kingdom and the evolution of the bilateral body plan in echinoderms". Acta Palaeontologica Polonica. 60 (1): 39–48. doi:10.4202/app.00048.2013. hdl:1983/6531d6a6-2b00-43d0-8f2d-0a8d5df836d0. ISSN 0567-7920. Retrieved 2020-04-02.
- ^ Zamora, Samuel; Rahman, Imran A. (2015). "Deciphering the early evolution of echinoderms with Cambrian fossils". Palaeontology. 57 (6). Andrew Smith (ed.): 1105–1119. doi:10.1111/pala.12138. hdl:1983/951cc086-7ed1-469b-98ac-2cc338c4e823. ISSN 0031-0239.
- ^ a b c Robison, R. A.; Sprinkle, J. (1969). "Ctenocystoidea: New Class of Primitive Echinoderms". Science. 166 (3912): 1512–1514. Bibcode:1969Sci...166.1512R. doi:10.1126/science.166.3912.1512. PMID 17742852. S2CID 26730941.
- ^ a b Domínguez Alonso, Patricio (1999). Sistemática, anatomía, estructura y función de Ctenocystoidea (Echinodermata, Carpoidea) del Paleozoico Inferior (PhD). Universidad Complutense de Madrid. ISBN 84-669-2421-3.
- ^ David, Bruno; Lefebvre, Bertrand; Mooi, Rich; Parsley, Ronald (2000). "Are homalozoans echinoderms? An answer from the extraxial-axial theory". Paleobiology. 26 (4): 529–555. Bibcode:2000Pbio...26..529D. doi:10.1666/0094-8373(2000)026<0529:AHEAAF>2.0.CO;2.
- ^ Dominguez-Alonso, Patricio; Jefferies, Richard (September 2005). A cladogram for the Deuterostomia based on molecular-biological and fossil evidence (PDF). Symposium of Vertebrate Palaeontology and Comparative Anatomy. London. p. 30.
- ^ Rahman, Imran A. (2009). "Making sense of carpoids". Geology Today. 25 (1): 34–38. Bibcode:2009GeolT..25...34R. doi:10.1111/j.1365-2451.2009.00703.x.
- ^ Domínguez Alnoso, Patricio; Jefferies, Richard P. S.; Gil Cid, Dolores (2002). "An annotated check-list of genera and species of carpoids". Coloquios de Paleontología. 53: 33–68. ISSN 1132-1660.
- ^ a b Zamora, Samuel; Lefebvre, Bertrand; Javier Álvaro, J.; Clausen, Sébastien; Elicki, Olaf; Fatka, Oldrich; Jell, Peter; Kouchinsky, Artem; Lin, Jih-Pai; Nardin, Elise; Parsley, Ronald; Rozhnov, Sergei; Sprinkle, James; Sumrall, Colin D.; Vizcaïno, Daniel; Smith, Andrew B. (2013). "Cambrian echinoderm diversity and palaeobiogeography". Geological Society, London, Memoirs. 38 (1): 157–171. doi:10.1144/M38.13. S2CID 130481550.
- ^ Lefebvre, Bertrand; Lerosey-Aubril, Rudy (2018). "Laurentian origin of solutan echinoderms: new evidence from the Guzhangian (Cambrian Series 3) Weeks Formation of Utah, USA". Geological Magazine. 155 (5): 1190–1204. Bibcode:2018GeoM..155.1190L. doi:10.1017/S0016756817000152.
- ^ Zhao, Yuanlong; Yuan, Jinliang; Babcock, Loren E.; Guo, Qingjun; Peng, Jin; Yin, Leiming; Yang, Xinglian; Peng, Shanchi; Wang, Chunjiang; Gaines, Robert R.; Esteve, Jorge; Tai, Tongsu; Yang, Ruidong; Wang, Yue; Sun, Haijing; Yang, Yuning (2019). "Global Standard Stratotype-Section and Point (GSSP) for the conterminous base of the Miaolingian Series and Wuliuan Stage (Cambrian) at Balang, Jianhe, Guizhou, China". Episodes. 42 (2): 165–184. doi:10.18814/epiiugs/2019/019013.
- ^ Smith, Andrew B.; Zamora, Samuel; Álvaro, J. Javier (2013). "The oldest echinoderm faunas from Gondwana show that echinoderm body plan diversification was rapid". Nature Communications. 4 (1): 1385. Bibcode:2013NatCo...4.1385S. doi:10.1038/ncomms2391. ISSN 2041-1723. PMID 23340425.