On the affinities of Tetraceratops insignis, an Early Permian synapsid

The affinities of the Early Permian synapsid Tetraceratops insignis have been reevaluated several times since the early 20(th) century, being considered as an eothyridid, a sphenacodontid, or a therapsid. This controversy continues into the 21(st) century, with recently raised doubts about the interpretation of Tetraceratops as the oldest known therapsid, a hypothesis supported by the only redescription of this fossil in the second half of the 20th century. Our study examines the arguments proposed to refute therapsid affinities, and concludes that Tetraceratops indeed is the sister-group of all other known therapsids. The most recently published phylogenetic data matrix that includes Tetraceratops fails to confirm its therapsid affinities. However, adding seven characters to that matrix leads to the conclusion that Tetraceratops is the basal-most and oldest therapsid. The recent suggestion of a Laurasian origin of therapsids appears poorly supported; too few data are available on the distribution of Permian synapsids to settle this question.