The short face and dental structures represent an extreme type of development of predaceous habits far off from those in a pelycosaur ancestor and entirely distinct from the milder dental differentiations of ophiacodontids. The skull measure of "Eothyris parkeyi" is an extremely smaller than for a pelycosaur. "Eothyris parkeyi" is represented by a complete skull, but the postcranial skeleton is unknown. Therefore, there is little foundation to diagnose the extended family. Also, the skull of "Eothyris parkeyi" is described according to that form. Whether many of the striking primitive features were repeated in other genera is unknown.