THE OSTEOLOGY AND RELATIONSHIPS OF TETRACERATOPS INSIGNIS, THE OLDESTKNOWN THERAPSID

Preparation and restudy of the Lower Permian synapsid Tetraceratops in signis 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, Tetrace ratops not only fills a large morphological gap between Permo-Carbonif erous sphenacodontids and Permian therapsids from Russian and South Af rica, but also provides important new insights into the origins of the latter group. Tetraceratops shares with biarmosuchians and other Perm o-Triassic therapsids the presence of a broad, concave shelf on the up per margin of the temporal fenestra, a wide tabular, a braincase attac hed firmly to the cheek, a reduced quadrate, a postero-median flange o f the pterygoid, the loss of ectopterygoid teeth, a reduced ventral pl ate of the epipterygoid excluded from the basicranial articulation, an d a shortened interpterygoid vacuity. Tetraceratops possesses several autapomorphies: a long diastema on the maxilla; bony, possibly hornbea ring, processes on the premaxilla, prefrontal and angular; and a large orbital contribution of the lacrimal.