299-252 million years ago


By the Early Permian, the most primitive therapsids had evolved. Tetraceratops was intermediate between pelycosaurs and therapsids.

Tetraceratops, once classified as a pelycosaur, is now considered the most primitive therapsid which is intermediate between the pelycosaurs and therapsids. It possessed three pairs of horns on its premaxillary, prefrontal, and angular bones. There are a number of characteristics of the teeth and jaws which unite Tetraceratops and all therapsids (the loss of precanine and ectopterygoid teeth; reduction of quadrate, epipterygoid, and interpterygoid vacuity; and the modification of the lower temporal fenestra). There are also a number of traits which unite all therapsids other than Tetraceratops as being more advanced (such as the increased size of the septomaxilla, maxilla, loss of the basipterygoid articulation, equalization of incisor size, and the exclusion of the lacrimal bone from the narial region). Pelycosaurs are first known from the Middle Pennsylvanian, reach their greatest diversity in the Early Permian and become extinct by the Late Permian. Tetraceratops, the oldest and most primitive therapsid, is known from the Early Permian (Laurin, 1990).