The affinities of the Early Permian synapsid Tetraceratops insignis have been reevaluated several times since the early 20th century, being considered as an eothyridid, a sphenacodontid, or a therapsid. This controversy continues into the 21st 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.

Key words: Therapsida, phylogeny, biogeography, center of origin, range extension, Paleozoic, Pangaea, North America.

Eli Amson [eli.amson@etu.upmc.fr], Michel Laurin [michel.laurin@upmc.fr], CNRS UMR 7207, “Centre de Recherches sur la Paléobiodiversité et les Paléoenvironnements”, Muséum national d'Histoire naturelle, Bâtiment de Géologie, Case Postale 48, 43 rue Buffon, 75231 Paris cedex 05, France.