Thecodontia (meaning 'socket-teeth'), now considered an obsolete taxonomic grouping, was formerly used to describe a diverse "order" of earlyarchosaurianreptiles that first appeared in the latestPermian period and flourished until the end of theTriassic period. All of them were built somewhat like crocodiles but with shorter skulls, more erect pose and usually somewhat lighter. The group includes the ancestors ofdinosaurs,pterosaurs, andcrocodilians, as well as a number of extinct forms that did not give rise to any descendants. The termthecodont is still used as an anatomical description of the tooth morphology seen in these species and others.

Thecodonts are characterized by certain shared primitive features, such as theantorbital fenestra (an opening on each side of the skull between the eye sockets and the nostrils) and teeth in sockets. The namethecodont 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. While the taxon Thecodontia is obsolete, the termthecodont remains in use as an anatomical description of teeth in bony sockets; in addition to species formerly in this group (such as crocodiles and dinosaurs), mammals also possess thecodont dentition, which evolved independently.

They constitute an evolutionary grade of animals, a "wastebasket taxon" for any archosaur other than a crocodilian, a pterosaur, or a dinosaur (anybasal archosaur). Because thecladistic paradigm only recognisesmonophyletic taxa as natural groups, and because thecodonts are aparaphyletic group (they include among their descendants animals that are not thecodonts), the term is no longer used as a formal name by mostpaleontologists, but it can still be found in older (and even fairly recent) books as a convenient shorthand for the basal archosaurs.

Traditionally, the order ThecodontiaOwen, 1859 was divided into four suborders, theProterosuchia (early primitive forms, another paraphyletic assemblage),Phytosauria (large crocodile-like semi-aquatic animals), theAetosauria (armoured herbivores), and thePseudosuchia, awastebasket taxon intended to be paraphyletic to all later archosaurs (see e.g.,Alfred Sherwood Romer'sVertebrate Paleontology andEdwin H. Colbert'sEvolution of the Vertebrates). Of these, only phytosaurs and aetosaurs constitute monophyletic groups, and the termPseudosuchia was simply a catch-all term for any species that didn't fit in one of the other three sub-orders. Pseudosuchia as used in recent literature is astem-based taxon that includescrocodile-linearchosaurs, or all archosaurs (includingcrocodilians) that are more closely related to crocodilians thanbirds.

Robert Carroll, in his bookVertebrate Paleontology and Evolution (1988), replaces Pseudosuchia withRauisuchia, Ornithosuchia (containingOrnithosuchidae and non-dinosaur, non-pterosaurOrnithodira), and the traditional categoryincertae sedis (of uncertain placement), while retaining the other three suborders. This is the last major textbook that still recognizes the taxon Thecodontia, as it uses a traditionalLinnaean based taxonomy derived from that of Romer.

In his 1982 thesis on amniote phylogeny,Brian Gardiner attempted to define Thecodontia within a cladistic framework, containing crocodilians and haemothermata (mammals and birds), thus giving the old name to a new concept. All more recent cladistic studies (e.g.,Jacques Gauthier 1986) have confirmed that Thecodontia as construed by Gardiner is a polyphyletic taxon, the members of which are not united by anyshared 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.

• 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.Stem Archosauria—TaxonSearch [version 1.0, 7 November 2005]

• Evolution Wiki: Thecodont
• Five types of tooth implanting in non-mammals, including thecodont type at theWayback Machine (archived November 26, 2010)
• Dinosaurs of Rio grande do Sul.