Thecodont

Thecodont ("socket-toothed" reptile), now considered an obsolete term, was formerly used to describe a diverse range of early archosaurs that first appeared in the Latest Permian and flourished until the end of the Triassic period. The group includes the ancestors of dinosaurs (including birds), and ancestors of pterosaurs, and crocodilians, as well as a number of extinct forms that did not give rise to any descendants.

Definition

Thecodonts are defined by certain shared primitive features, such as the suborbital fenestra (an opening on each side of the skull between the eye sockets and the nostrils) and teeth in sockets. The name Thecodont is Greek for "socket-tooth," referring to the fact that thecodont teeth were set in sockets in the jawbones; an archosaurian characteristic that was inherited by the dinosaurs.

They constitute an evolutionary grade of animals, a "wastebasket taxon" for any archosaur that wasn't a crocodilian, a pterosaur, or a dinosaur (i.e. any basal archosaur). Because the cladistic paradigm only recognises monophyletic taxa as natural groups, and because Thecodonts are a paraphyletic group (i.e. they include among their descendants animals that are not thecodonts), the term is no longer used by most paleontologists, although it can still be found in older (and even fairly recent) books.

Taxonomic History

Traditionally, the order Thecodontia Owen, 1859 was divided into four suborders, the Proterosuchia (early primitive forms, another paraphyletic assemblage), Phytosauria (large crocodile-like semi-aquatic animals), the Aetosauria (armoured herbivores), and the Pseudosuchia (see e.g. Alfred Sherwood Romer's "Vertebrate Paleontology" and Edwin H. Colbert's "Evolution of the Vertebrates"). Of these, only Phytosaurs and Aetosaurs constitute monophyletic groups, and the term Pseudosuchia was simply a catch-all term for any species that didn't fit in one of the other three sub-orders.

Robert Carroll in his book "Vertebrate Paleontology and Evolution" (1988) replaces Pseudosuchia with Rauisuchia, Ornithosuchia, and the traditional category "incertae sedis" (of uncertain placement), while retaining the other three suborders. This is the last major textbook that still recognises the taxon Thecodontia, as it uses a traditional Linnaean based taxonomy.

Brian Gardiner (1982) attempted to define Thecodontia within a cladistic framework, thus giving the old name to a new concept. All recent cladistic studies (e.g. Jacques Gauthier 1986) have confirmed that the traditional Thecodontia is indeed a paraphyletic taxon, the members of which are not united by any shared derived characteristics. As the association of the name with the outdated concept proved to be very strong, it is now considered a historical term only, and its current usage has been abandoned.

All current vertebrate paleontology textbooks, (e.g. Michael Benton's "Vertebrate Palaeontology" (first ed. 1990, 2nd ed. 1997), follow the cladistic approach, and so the name Archosauria is used instead. This includes both the Thecodonts and all their descendants.

ee also

*Paleorrota

External links

* [http://wiki.cotch.net/wiki.phtml?title=Thecodont Evolution Wiki: Thecodont]
* [http://www.palaeos.com/Vertebrates/Bones/Teeth/ToothImplantation.html Five types of tooth implanting in non-mammals, including thecodont type.]
* [http://www.royalsul.com.br/paleo/galeria.asp Dinosaurs of Rio grande do Sul.]

References

* Benton, M. J. 1997, "Vertebrate Paleontology", Blackwell Science Ltd
* Carroll, R. L. 1988, "Vertebrate Paleontology and Evolution", W. H. Freeman and Co. New York
* Colbert, E H. 1969, "Evolution of the Vertebrates", John Wiley & Sons Inc (2nd ed.)
* Gardiner, BG (1982). Tetrapod classification. "Zool. J. Linn. Soc. London" 74: 207-232.
* Gauthier, J., 1986. Saurischian monophyly and the origin of birds. In: K. Padian, ed. The Origin of Birds and the Evolution of Flight. Memoirs California Academy of Sciences 8. pp. 1–55
* Sereno, P. C. 2005. [http://www.taxonsearch.org/Archive/stem-archosauria-1.0.php Stem Archosauria—TaxonSearch] [version 1.0, 7 November 2005]


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Look at other dictionaries:

  • Thecodont — The co*dont, a. [Gr. ? a case + ?, ?, a tooth.] 1. (Anat.) Having the teeth inserted in sockets in the alveoli of the jaws. [1913 Webster] 2. (Paleon.) Of or pertaining to the thecodonts. [1913 Webster] …   The Collaborative International Dictionary of English

  • Thecodont — The co*dont, n. (Paleon.) One of the Thecodontia. [1913 Webster] …   The Collaborative International Dictionary of English

  • thecodont — [thē′kə dänt΄] n. any of an order (Thecodontia) of reptiles of the Permian and Triassic periods, believed to be ancestors of the dinosaurs and crocodilians …   English World dictionary

  • thecodont — I. adjective Etymology: International Scientific Vocabulary thec (from New Latin theca) + odont Date: 1840 having the teeth inserted in sockets II. noun Date: 1840 any of an order (Thecodontia) of Triassic diapsid thecodont reptiles that were… …   New Collegiate Dictionary

  • thecodont — noun presumably in the common ancestral line to dinosaurs and crocodiles and birds • Syn: ↑thecodont reptile • Hypernyms: ↑archosaur, ↑archosaurian, ↑archosaurian reptile • Member Holonyms: ↑Thecodontia, ↑ …   Useful english dictionary

  • thecodont — /thee keuh dont /, n. 1. any of various reptiles of the extinct order Thecodontia, occurring in the late Permian to late Triassic periods and characterized by teeth set in sockets. adj. 2. having the teeth set in sockets. 3. belonging to or… …   Universalium

  • thecodont — Having the teeth inserted in alveoli. [G. theke, box, + odous (odont ), tooth] * * * the·co·dont (theґko dont) [theca + Gr. odous tooth] having the teeth inserted in sockets or alveoli …   Medical dictionary

  • thecodont — [ θi:kə(ʊ)dɒnt] noun Palaeontology a Triassic fossil reptile of a group ancestral to dinosaurs and other archosaurs. Origin C19: from mod. L. Thecodontia, from Gk thēkē case + odous, odont tooth (because the teeth were fixed in sockets in the… …   English new terms dictionary

  • thecodont — the·co·dont …   English syllables

  • thecodont reptile — noun presumably in the common ancestral line to dinosaurs and crocodiles and birds • Syn: ↑thecodont • Hypernyms: ↑archosaur, ↑archosaurian, ↑archosaurian reptile • Member Holonyms: ↑Thecodontia, ↑ …   Useful english dictionary


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