[4] Vertebral morphometric data also support, though ambiguously, a rather terrestrial lifestyle for Ophiacodon,[5] which could reach a length of 3.6 m (12 ft).

The earliest ophiacodontids resembled varanids in body proportions, while others were larger with elongated skulls and massive shoulder girdles, probably to provide muscle attachment to support the weight of the large head.

Traditionally, Archaeothyris, Ophiacodon, Varanosaurus and the briefly described Baldwinonus, Clepsydrops, Echinerpeton, Stereophallodon and Stereorhachis are included in the Ophiacodontidae.

Below is a cladogram modified from the analysis of Benson (2012),[7] after the exclusion of Echinerpeton:[7] Tseajaia campi Limnoscelis paludis Captorhinus spp.

Protorothyris archeri Caseasauria Ianthodon schultzei Edaphosauridae Sphenacodontia Archaeothyris florensis Varanosaurus acutirostris Ophiacodon spp.