Cincta

Cincta is an extinct class of echinoderms that lived only in the Middle Cambrian epoch.[1] Homostelea is a junior synonym.[2] The classification of cinctans is controversial, but they are probably part of the echinoderm stem group.[1][3]

Cincta Temporal range: Miaolingian PreꞒ Ꞓ O S D C P T J K Pg N
Trochocystites bohemicus from the Czech Republic with a length of 15 millimetres (0.59 in)
Scientific classification
Kingdom:	Animalia
Superphylum:	Deuterostomia
Clade:	Ambulacraria
Phylum:	Echinodermata
Class:	†Cincta Jaekel, 1918
Families
†Trochocystitidae †Gyrocystidae †Sucocystidae

Cinctans were sessile, asymmetrical animals with a skeleton made of stereom plates and a racquet-shaped body composed of a theca and stele. They may have had a lifestyle similar to modern tunicates, filter-feeding by pumping water through gill slits in their pharynx.

Description

Cinctans were asymmetrical animals, though some species were nearly bilaterally symmetrical.[1] Like all echinoderms, cinctans have a skeleton made of plates of stereom. The body of cinctans was divided into two sections, the main body, called the theca, and a posterior appendage called a stele. The overall shape of cinctans has been compared to a tennis racquet.[4] The theca of cinctans was surrounded on its margins by a frame of large stereom plates called a cinctus, and the dorsal and ventral surfaces were covered in a tessellated arrangement of small plates. The stele was essentially an extension of the cinctus, rather than a discrete appendage, and would have been fairly stiff side-to-side but possibly more flexible up and down.[4] The stele was not a holdfast, but may have served to stabilize the animal.[5] In most cinctans, the overall shape of the theca was only mildly asymmetrical, but in the unusual genus Lignanicystis the theca was highly asymmetrical, convergent on the stylophorans in some respects.[6] The species Graciacystis could reach a theca length of 14.5 millimetres (0.57 in).[7]

The theca of cinctans contained three major openings. The mouth was located on the right side of the anterior end, in the cinctus, and was associated with marginal grooves. All species had a left marginal groove, but the right marginal groove was sometimes absent and always shorter than the left one. The anus was located near the anterior end, on the right side, indicating that the gut was U-shaped. It was surrounded by a cone of plates equivalent to the periproct of modern echinoderms.[5] The largest opening, called the porta, was located at the anterior end and was covered by an operculum. It was likely an atrial opening like that of tunicates.

The asymmetry of the marginal groove likely indicates that cinctans had a water vascular system comprising two hydrocoels, with the left hydrocoel larger than the right.[1] In species with only a left marginal groove, the right hydrocoel may have been absent as in modern echinoderms.

Biology

Cinctans have been hypothesized to be epibenthic suspension feeders, with a lifestyle similar to modern tunicates.[1] They likely rested on the seafloor parallel to the current, with their mouth and porta oriented downstream.[3] They likely fed by actively pumping water through their pharynx.

Cinctans grew largely by the expansion of the plates.[7] A small number of plates were added to the stele, and somewhat more to the dorsal and ventral surfaces, during growth, but the number of plates making up the cinctus remained constant throughout growth. Because of the asymmetric anatomy of cinctans, they likely underwent torsion when metamorphosing from a larva into an adult.[1]

Classification

Chordata     Hemichordata   Pterobranchia     Enteropneusta     Echinodermata   Ctenocystoidea       Cincta       Soluta       Helicoplacus       Camptostroma       Stromatocystites       Kinzercystis       Gogia       Akadocrinus       Pleurocystites     Ubaghsicystis         Macrocystella     Aethocrinus

Phylogenetic position of Cincta within Deuterostomia[8][9]

The classification of cinctans, like that of other carpoid echinoderms, is contentious. They have been hypothesized to be blastozoans, stem-group hemichordates, and stem-group echinoderms.[1]

Phylogenetic analyses have found cinctans to be stem-group echinoderms, intermediate between ctenocystoids and solutans.[8]

Asturicystis   Trochocystitidae   Trochocystites     Trochocystoides         Sotocinctus       Graciacystis       Protocinctus   Gyrocystidae   Progyrocystis     Gyrocystis     Sucocystidae   Ludwigicinctus       Undatacinctus       Sucocystis       Lignanicystis     Elliptocinctus

Phylogenetic relationships within Cincta [10][7]

The internal phylogenetic relationships within cinctans have been difficult to study, due in part to their highly specialized anatomy making it difficult to determine their ancestral anatomy.[10] However, three families are recognized, the Trochocystidae, Gyrocystidae, and Sucocystidae, as well as some basal genera which do not fit into any of the families.

Distribution

All cinctan fossils are from the Middle Cambrian. The earliest cinctan is Protocinctus, which dates to Cambrian Stage 5,[1] now known as the Wuliuan. Their diversity peaked during the Drumian. The youngest cinctans are in the genera Undatacinctus and Sucocystis.[11] Cinctans died out just prior to the beginning of the Furongian epoch, during the Guzhangian age.[12][11] The decline of cinctans was associated with a marine regression, and cinctans may have struggled to migrate as sea levels changed or run out of habitat due to the regression and cooling.[12]

Cinctan fossils are found in the former continents of Siberia, Avalonia, and western Gondwana.[10] They have been found in Czechia, France, Germany, Italy, Morocco, Russia, Spain, and the United Kingdom.[11]

History

The first cinctan named, Trochocystites, was described in 1887.[10] Cinctans were originally considered to be cystoids, but in the early 20th century they were recognized as a distinct group. Because Cincta was originally described as an order, they were later assigned to their own subclass Homostelea, but Homostelea is now considered a junior synonym of Cincta.[13][2]

Under the calcichordate interpretation of carpoids, cinctans were initially interpreted as basal stem-group echinoderms.[4] Later, however, they, as well as ctenocystoids, were interpreted as possible stem-group hemichordates under the calcichordate interpretation.[14]

References

1. Rahman, Imran A.; Zamora, Samuel (2009). "The oldest cinctan carpoid (stem-group Echinodermata), and the evolution of the water vascular system". Zoological Journal of the Linnean Soceity. 157: 420–432.
2. Lefebvre, Bertrand; Derstler, Kraig; Sumrall, Colin D. (2012). "A reinterpretation of the solutan Plasiacystis mobilis (Echinodermata) from the Middle Ordovician of Bohemia". Zoosymposia. 7: 287–306.
3. Rahman, Imran A.; Zamora, Samuel; Falkingham, Peter L.; Phillips, Jeremy C. (2015). "Cambrian cinctan echinoderms shed light on feeding in the ancestral deuterostome". Proceedings of the Royal Society B. doi:10.1098/rspb.2015.1964.
4. Jefferies, R. P. S. (1990). "The solute Dendrocystites scoticus from the Upper Ordovician of Scotland and the ancestry of chordates and echinoderms". Palaeontology. 33 (3): 631–679.
5. Smith, Andrew B. (2005). "The pre-radial history of echinoderms". Geological Journal. 40 (3): 255–280. doi:10.1002/gj.1018.
6. Zamora, Samuel; Smith, Andrew B. (2008). "A new Middle Cambrian stem-group echinoderm from Spain: Palaeobiological implications of a highly asymmetric cinctan". Acta Palaeontologica Polonica. 53 (2): 207–220. doi:10.4202/app.2008.0204. ISSN 0567-7920.
7. Zamora, Samuel; Rahman, Imran A.; Smith, Andrew B. (2013). "The ontogeny of cinctans (stem-group Echinodermata) as revealed by a new genus, Graciacystis, from the middle Cambrian of Spain". Palaeontology. 56 (2): 399–410. doi:10.1111/j.1475-4983.2012.01207.x. ISSN 0031-0239.
8. Timothy P. Topper; Junfeng Guo; Sébastien Clausen; Christian B. Skovsted; Zhifei Zhang (2019). "A stem group echinoderm from the basal Cambrian of China and the origins of Ambulacraria". Nature Communications. 10 (1): Article number 1366. Bibcode:2019NatCo..10.1366T. doi:10.1038/s41467-019-09059-3. PMC 6433856. PMID 30911013.
9. Samuel Zamora; David F. Wright; Rich Mooi; Bertrand Lefebvre; Thomas E. Guensburg; Przemysław Gorzelak; Bruno David; Colin D. Sumrall; Selina R. Cole; Aaron W. Hunter; James Sprinkle; Jeffrey R. Thompson; Timothy A. M. Ewin; Oldřich Fatka; Elise Nardin; Mike Reich; Martina Nohejlová; Imran A. Rahman (2020). "Re-evaluating the phylogenetic position of the enigmatic early Cambrian deuterostome Yanjiahella". Nature Communications. 11 (1): Article number 1286. doi:10.1038/s41467-020-14920-x. PMID 32152310.
10. Smith, Andrew B.; Zamora, Samuel (2009). "Rooting phylogenies of problematic fossil taxa; a case study using cinctans (stem-group echinoderms)". Palaeontology. 52 (4): 803–821. doi:10.1111/j.1475-4983.2009.00880.x.
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. Zamora, Samuel; Álvaro, J. Javier (2010). "Testing for a decline in diversity prior to extinction: Languedocian (latest mid-Cambrian) distribution of cinctans (Echinodermata) in the Iberian Chains, NE Spain". Palaeontology. 53 (6): 1349–1368. doi:10.1111/j.1475-4983.2010.01004.x. ISSN 0031-0239.
13. Caster, Kenneth E. (1983). "A new Silurian carpoid echinoderm from Tasmania and a revision of the Allanicytidiidae". Alcheringa: An Australasian Journal of Palaeontology. 7 (4): 321–335. doi:10.1080/03115518308619615.
14. 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.