Mitrates are an extinct group of echinoderms, which are grouped with the cornutes and the basal Ceratocystis to form the Stylophora. Mitrates were central to the now-disproven[2] calcichordate hypothesis of chordate origins, but are now seen as either pre-radial-symmetry stem-group echinoderms,[3] or as a derived group either within the blastozoans[4] or near to the origin of crinoids.[5]
Morphology
The organisms were a few millimetres long.[6] Like the echinoderms, they are covered in armour plates, each of which comprises a single crystal of calcite. This is one of the features they share with the latter group, along with a water vascular system, only discovered in 2019.[7] However, they do not display the familiar fivefold symmetry that more recent echinoderms possess, instead being close to (but not fully) bilaterally symmetrical.[6][8]
Their heads had two sides; one, flat, was covered with large “pavement-like”[6] plates, the other, convex, bore smaller plates.[6] Their tails were long and segmented, resembling the stalk of a crinoid or the arm of a brittlestar.[6] At the opposite end was a hole which may have been mouth or anus – or both.[6]
They also bear features reminiscent of pharyngeal slits,[9] a character lost in other echinoderms but present in hemichordates.[6] This caused R. P. S. Jefferies to hold them as the ancestor of all chordates, a theory that has since been disproven.[2]
Behaviour
The mitrate Rhenocystis latipedunculata has been found with associated trace fossils.[10][11] Their interpretation requires an understanding of how the animal was oriented in life; it’s not agreed whether the convex side of the head was up or down, or indeed whether the “tail” was at the front or back of the organism.[6] The trace fossils suggest that they pulled themselves through the mud with their “tail”, and were flat-side up.[6]
Taxa
The following taxa are recognized as being included in the Mitrata as of a 2002 review of the order:[12]
- Adoketocarpus Ruta & Jell, 1999
- Allanicytidium Caster & Gill, 1967
- Anatifopsis Barrande, 1872
- Anomalocystites Hall, 1859
- Aspidocarpus Ubaghs, 1979
- Ateleocystites Billings, 1858
- Australocystis Caster, 1954
- Balanocystites Barrande, 1887
- Barrandeocarpus Ubaghs, 1979
- Bokkeveldia Ruta & Theron, 1997
- Chauvelia Cripps, 1990
- Chinianocarpos Ubaghs, 1961
- Dalejocystis Prokop, 1963
- Diamphidiocystis Kolata & Guensburg, 1979
- Enoploura Wetherby, 1879
- Eumitrocystella Beisswenger, 1994
- Guichenocarpos Chauvel, 1981
- Jaekelocarpus Kolata, Frest & Mapes, 1991
- Kierocystis Parsley, 1991
- Kopficystis Parsley, 1991
- Lagynocystis Jaekel, 1918
- Milonicystis Chauvel, 1986
- Mitrocystella Jaekel, 1901
- Mitrocystites Barrande, 1887
- Mongolocarpos Rozhnov, 1990
- Notocarpos Philip, 1981
- Occultocystis Haude, 1995
- Ovocarpus Ubaghs, 1994
- Paranacystis Caster, 1954
- Peltocystis Thoral, 1935
- Placocystella Rennie, 1936
- Placocystites de Koninck, 1869
- Protocytidium Ruta & Jell, 1999
- Pseudovictoriacystis Ruta & Jell, 1999
- Rhenocystis Dehm, 1932
- Spermacystis Ubaghs, 1967
- Tasmanicytidium Caster, 1983
- Victoriacystis Gill & Caster, 1960
- Vizcainocarpus Ruta, 1997
- Willmanocystis Kolata & Jollie, 1982
- Yachalicystis Haude, 1995
References
- ^ Mitrata in the Paleobiology Database
- ^ a b Boisset, Thomas; Lefebvre, Bertrand; Mooi, Rich; Kroh, Andreas; Winkler, Viola; Adrien, Jérôme; Martin, Markus J. (2024). “Insights into stylophoran anatomy and taphonomy based on an exceptionally preserved mitrate from the Lorraine Group (Upper Ordovician) of New York, USA”. Cah. Biol. Mar. 65: 511–516. doi:10.21411/CBM.A.33CFD1AB.
- ^ Rahman, Imran A.; Zamora, Samuel (July 2024). “Origin and early evolution of echinoderms”. Annual Review of Earth and Planetary Sciences. 52 (1): 295–320. Bibcode:2024AREPS..52..295R. doi:10.1146/annurev-earth-031621-113343. hdl:10141/623070.
- ^ Mooi, Rich; Lefebvre, Bertrand; Guensburg, Thomas E.; Nohejovlá, Martina; Dupichaud, Christophe (December 2024). “Approaches to understanding echinoderm origins. Part 2: Questioning conceptual models”. Cahiers de Biologie Marine. 65: 463–490. doi:10.21411/CBM.A.604F2876.
- ^ Guensburg, Thomas E.; Sprinkle, James; Mooi, Rich; Lefebvre, Bertrand; David, Bruno; Roux, Michel; Derstler, Kraig (2020). “Athenacrinus n. gen. and other early echinoderm taxa inform crinoid origin and arm evolution”. Journal of Paleontology. 94 (2): 311–333. doi:10.1017/jpa.2019.87.
- ^ a b c d e f g h i Gee, Henry (October 2000). “Mitrates on the move”. Nature. 407 (6806): 849–851. doi:10.1038/35038193. ISSN 0028-0836.
- ^ Lefebvre, Bertrand; Guensburg, Thomas E.; Martin, Emmanuel L. O.; Mooi, Rich; Nardin, Elise; Nohejlová, Martina; Saleh, Farid; Kouraïss, Khaoula; El Hariri, Khadija (2019-02-01). “Exceptionally preserved soft parts in fossils from the Lower Ordovician of Morocco clarify stylophoran affinities within basal deuterostomes”. Geobios. 52: 27–36. Bibcode:2019Geobi..52…27L. doi:10.1016/j.geobios.2018.11.001. ISSN 0016-6995.
- ^ “Palaeos Metazoa: Deuterostomia: Stylophora”. Retrieved 2023-03-04.
- ^ Jefferies, R. P. S. (1986). The Ancestry of the Vertebrates. British Museum (Natural History).
- ^ Sutcliffe, Owen E.; Südkamp, Wouter H.; Jefferies, Richard P. S. (March 2000). “Ichnological evidence on the behaviour of mitrates: two trails associated with the Devonian mitrate Rhenocystis”. Lethaia. 33 (1): 1–12. doi:10.1080/00241160050150267. ISSN 0024-1164.
- ^ Rahman, Imran A.; Jefferies, Richard P. S.; SüDkamp, Wouter H.; Smith, Ru D. A. (January 2009). “Ichnological insights into mitrate palaeobiology”. Palaeontology. 52 (1): 127–138. doi:10.1111/j.1475-4983.2008.00838.x. ISSN 0031-0239.
- ^ Domínguez Alonso, Patricio; Jefferies, Richard P. S.; Gil Cid, María Dolores (December 2002). “An annotated check-list of genera and species of carpoids”. Coloquios de Palcontologia. 53: 33–68. ISSN 1132-1660.