Dimetrodon was one of the earliest relatives of mammals. The large "sail" on its back may have been used for temperature regulation, to attract mates, or to frighten off other animals.
Dimetrodon is a member of a group called synapsids. Behind the eye socket in its skull is the synapsid opening. Its function is uncertain, but it may have been a passage for jaw muscles that helped Dimetrodon and other synapsids chew.
Hall of Primitive Mammals