Thecodontia

Thecodontia (meaning "socket-teeth"), now considered an obsolete taxonomic grouping, was formerly used to describe a diverse "order" of early archosaurian reptiles that first appeared in the latest Permian period and flourished until the end of the Triassic 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 of dinosaurs, pterosaurs, and crocodilians, as well as a number of extinct forms that did not give rise to any descendants. The term "thecodont" 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 the antorbital 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. While the taxon "Thecodontia" is obsolete, the term "thecodont" remains in use as an anatomical description of teeth in bony sockets; in addition to species formerly in this group (such as crocodiles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 (any basal archosaur). Because the cladistic paradigm only recognises monophyletic taxa as natural groups, and because thecodonts are a paraphyletic group (they include among their descendants animals that are not thecodonts), the term is no longer used as a formal name by most paleontologists, but it can still be found in older (and even fairly recent) books as a convenient shorthand for the basal archosaurs.

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