Ophiacodontidae

{{Automatic taxobox }}
 * edit_link =
 * color={[Taxobox_colour|Animalia}}
 * fossil_range = Late Carboniferous to Early Permian
 * image = Ophiacodon retroversus.jpg
 * image_width = 250px
 * image_caption = Mounted skeleton of Ophiacodon retroversus in the American Museum of Natural History
 * authority = Nopsca, 1923
 * subdivision_ranks = Genera
 * display_children = 1

Ophiacodontidae is an extinct family of early synapsids from the Carboniferous and Permian. Archaeothyris, and Clepsydrops were among the earliest ophiacodontids, appearing in the Late Carboniferous. Ophiacodontids are among the most basal synapsids, an offshoot of the lineage which includes therapsids and their descendants, the mammals. The group became extinct by the Middle Permian, replaced by anomodonts, theriodonts, and the diapsid reptiles.

Description
Some ophiacodonts were semi-aquatic, and few were fully aquatic, but some were fully terrestrial like Archaeothyris. Several ophiacodontids resembled small lizards, while others were larger with elongated skulls and massive shoulder girdles, probably to provide muscle attachment to support the weight of the large head.

Phylogeny
Traditionally, Archaeothyris, Ophiacodon, Varanosaurus and the briefly described Baldwinonus, Clepsydrops, Echinerpeton, Stereophallodon and Stereorhachis are included in the Ophiacodontidae. Protoclepsydrops was also regarded as ophiacodontid, however there is debate as to whether or not it was a synapsid. Echinerpeton and Sterophallodon were included for the first time in a phylogenetic analysis by Benson (in press). Echinerpeton was found to be a wildcard taxon due to its small amount of known materials. It occupies three possible positions, falling either as the most basal synapsid, as the sister taxon of Caseasauria + more derived taxa, or as an ophiacodontid more derived than Archaeothyris. Below is a cladogram modified from the analysis of Benson (in press), after the exclusion of Echinerpeton: