Ctenocystoidea

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: Wuliuan–Sandbian PreꞒ Ꞓ O S D C P T J K Pg N
A specimen of Ctenocystis
Scientific classification
Kingdom:	Animalia
Superphylum:	Deuterostomia
Clade:	Ambulacraria
Phylum:	Echinodermata
Class:	†Ctenocystoidea Robison & Sprinkle, 1969
Type genus
Ctenocystis Robison & Sprinkle, 1969
Genera
†Conollia †Courtessolea †Ctenocystis †Etoctenocystis †Gilcidia †Jugoszovia †Pembrocystis

Description

A selection of early echinoderms, including the ctenocystoid Ctenocystis (middle).

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

Relationships

Courtessolea         Jugoszovia     Conollia         Pembrocystis       Etoctenocystis       Gilcidia     Ctenocystis

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

The following genera of ctenocystoids have been named:[6][10][11]

Genus	Discoverers	Named	Age	Location	Notes
Conollia	Domínguez Alonso	1999	Sandbian	United Kingdom	The geologically youngest genus[3]
Courtessolea	Domínguez Alonso	1999	Wuliuan	France	The most basal genus[2]
Ctenocystis	Robison & Sprinkle	1969	Middle Cambrian	United States	The first genus discovered[5]
Etoctenocystis	Fatka & Kordule	1985	Middle Cambrian	Czechia
Gilcidia	Domínguez Alonso	1999	Middle Cambrian	Australia  Denmark  France
Jugoszovia	Džik & Orłowski	1995	Middle Cambrian	Poland
Pembrocystis	Domínguez Alonso	1999	Middle Cambrian	United Kingdom

Distribution

Ctenocystoids were widespread during the Middle Cambrian, and have been found in the United States, the United Kingdom, Australia, France, Czechia, 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

The 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

1. Rahman, Imran A.; Clausen, Sébastien (2009). "Re‐evaluating the palaeobiology and affinities of the Ctenocystoidea (Echinodermata)". Journal of Systematic Palaeontology. 7 (4): 413–426. doi:10.1017/S1477201909990046.
2. Zamora, Samuel; Rahman, Imran A.; Smith, Andrew B. (2012-06-06). Keith A. Crandall (ed.). "Plated Cambrian bilaterians reveal the earliest stages of echinoderm evolution". PLoS ONE. 7 (6): –38296. doi:10.1371/journal.pone.0038296. ISSN 1932-6203. Retrieved 2020-03-26.
3. 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. ISSN 0567-7920. Retrieved 2020-04-02.
4. Zamora, Samuel; Rahman, Imran A. (2015). Andrew Smith (ed.). "Deciphering the early evolution of echinoderms with Cambrian fossils". Palaeontology. 57 (6): 1105–1119. doi:10.1111/pala.12138. ISSN 0031-0239.
5. Robison, R. A.; Sprinkle, J. (1969). "Ctenocystoidea: New Class of Primitive Echinoderms". Science. 166 (3912): 1512–1514. doi:10.1126/science.166.3912.1512.
6. 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.
7. David, Bruno; Lefebvre, Bertrand; Mooi, Rich; Parsley, Ronald (2000). "Are homalozoans echinoderms? An answer from the extraxial-axial theory". Paleobiology. 26 (4): 529–555. doi:10.1666/0094-8373(2000)026<0529:AHEAAF>2.0.CO;2.
8. Dominguez-Alonso, Patricio; Jefferies, Richard (September 2005). A cladogram for the Deuterostomia based on molecular-biological and fossil evidence (PDF). Symposum of Vertebrate Palaeontology and Comparative Anatomy. London. p. 30.
9. Rahman, Imran A. (2009). "Making sense of carpoids". Geology Today. 25 (1): 34–38. doi:10.1111/j.1365-2451.2009.00703.x.
10. 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.
11. 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.
12. 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. doi:10.1017/S0016756817000152.
13. 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.
14. 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). doi:10.1038/ncomms2391. ISSN 2041-1723.