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ABSTRACT
Preparation and restudy of the Lower Permian synapsid Tetraceratops insignis demonstrate that it is related more closely to therapsids than to other Permo-Carboniferous synapsids. As the oldest known therapsid and the only known Lower Permian therapsid from North America, Tetraceratops not only fills a large morphological gap between Permo-Carboniferous sphenacodontids and Permian therapsids from Russian and South Africa, but also provides important new insights into the origins of the latter group. Tetraceratops shares with biarmosuchians and other Permo-Triassic therapsids the presence of a broad, concave shelf on the upper margin of the temporal fenestra, a wide tabular, a braincase attached firmly to the cheek, a reduced quadrate, a postero-median flange of the pterygoid, the loss of ectopterygoid teeth, a reduced ventral plate of the epipterygoid excluded from the basicranial articulation, and a shortened interpterygoid vacuity. Tetraceratops possesses several autapomorphies: a long diastema on the maxilla; bony, possibly hornbearing, processes on the premaxilla, prefrontal and angular; and a large orbital contribution of the lacrimal.
