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 in 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 descendents.

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 Latin 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 "grab-bag" taxon for any archosaur that wasn't a crocodilian, a pterosaur, or a dinosaur. Because the cladistic paradigm only recognises monophyletic taxa as natural groups, and because Thecodonts are a paraphyletic group (i.e. they include among their descendents 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.

Traditionally, the Order Thecodontia 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 .  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) has Rauisuchia, Ornithosuchia , and the traditional category incertae sedis ("uncertain standing", literally "seat") instead of Pseudosuchia. This is the last major textbook that still recognises the taxon Thecodontia. In Michael J. Benton 's Vertebrate Paleontology (first ed. 1990, 2nd ed. 1997) the term "Archosauria" is used instead.

External links

• Evolution Wiki: Thecodont
• Five types of tooth implanting in non-mammals, including thecodont type.

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.)