Thelodonti

Thelodonti
Temporal range: Sandbian[1]–Late Devonian,[2] 458–359 Ma
Among the flat-bodied forms are Lanarkia (top left), provided with long, spine-shaped scales, and Loganellia (top right and middle). Other thelodonts, such as Furcacauda from the Devonian of Canada (bottom) are deep-bodied, with lateral gill openings and a very large, forked tail.[3]
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Subphylum: Vertebrata
Infraphylum: Agnatha
Superclass: Thelodontomorphi
Jackel 1911
Class: Thelodonti
Jaekel, 1911
Orders

Thelodonti (from Greek: "nipple teeth")[4] is a class of extinct Palaeozoic jawless fishes with distinctive scales instead of large plates of armor.

There is much debate over whether the group represents a monophyletic grouping, or disparate stem groups to the major lines of jawless and jawed fish.

Shielia taiti

Thelodonts are united in possession of "thelodont scales". This defining character is not necessarily a result of shared ancestry, as it may have been evolved independently by different groups. Thus the thelodonts are generally thought to represent a polyphyletic group,[5] although there is no firm agreement on this point. On the basis that they are monophyletic, they are reconstructed as being ancestrally marine and invading freshwater on multiple occasions.[6]

"Thelodonts" were morphologically very similar, and probably closely related, to fish of the classes Heterostraci and Anaspida, differing mainly in their covering of distinctive, small, spiny scales. These scales were easily dispersed after death; their small size and resilience makes them the most common vertebrate fossil of their time.[7][8]

The fish lived in both freshwater and marine environments, first appearing during the Ordovician, and perishing during the Frasnian–Famennian extinction event of the Late Devonian. Traditionally they were considered predominantly deposit-feeding bottom dwellers, but more recent studies have showed they occupied various ecological roles in various parts of the water column, much like modern bony fishes and sharks. In particular, a large variety of species preferred reef ecosystems, and it has been suggested that this preference was the reason for the development of their unique scales, protecting against abrasion and allowing for the development of more flexible bodies than other jawless fish, which had inflexible armors and were restricted to open habitats.[9]

  1. ^ Sansom, Robert S.; Randle, Emma; Donoghue, Philip C. J. (7 February 2015). "Discriminating signal from noise in the fossil record of early vertebrates reveals cryptic evolutionary history". Proceedings of the Royal Society B. 282 (1800): 20142245. doi:10.1098/rspb.2014.2245. PMC 4298210. PMID 25520359.
  2. ^ Turner, S.; R. S. Dring (1981). "Late Devonian thelodonts (Agnatha) from the Gneudna Formation, Carnarvon Basin, Western Australia". Alcheringa. 5: 39–48. doi:10.1080/03115518108565432.
  3. ^ Janvier, Philippe (1997) Thelodonti Archived 14 March 2013 at the Wayback Machine The Tree of Life Web Project.
  4. ^ Maisey, John G., Craig Chesek, and David Miller. Discovering fossil fishes. New York: Holt, 1996.
  5. ^ Sarjeant, William Antony S.; L. B. Halstead (1995). Vertebrate fossils and the evolution of scientific concepts: writings in tribute to Beverly Halstead. ISBN 978-2-88124-996-9. Archived from the original on 16 February 2017. Retrieved 26 September 2016.
  6. ^ Ferrón, Humberto G; Martínez-Pérez, Carlos; Turner, Susan; Manzanares, Esther; Botella, Héctor (2018). "Patterns of ecological diversification in thelodonts". Palaeontology. 61 (2): 303–315. doi:10.1111/pala.12347. hdl:10550/85568.
  7. ^ Turner, S. (1999). "Early Silurian to Early Devonian thelodont assemblages and their possible ecological significance". In A. J. Boucot; J. Lawson (eds.). Palaeocommunities, International Geological Correlation Programme 53, Project Ecostratigraphy, Final Report. Cambridge University Press. pp. 42–78.
  8. ^ The early and mid Silurian. See Kazlev, M.A., White, T (6 March 2001). "Thelodonti". Palaeos.com. Archived from the original on 28 October 2007. Retrieved 30 October 2007.{{cite web}}: CS1 maint: multiple names: authors list (link)
  9. ^ Ferrón Humberto G., Botella Héctor (2017). "Squamation and ecology of thelodonts". PLOS ONE. 12 (2): e0172781. Bibcode:2017PLoSO..1272781F. doi:10.1371/journal.pone.0172781. PMC 5328365. PMID 28241029.