Tetraceratops insignis (Matthew 1908) Early Permian, 9 cm skull length was considered an eothyridid (Matthew 1908), a sphenacodontid (Conrad and Sidor 2001), or the basalmost therapsid (Laurin and Reisz 1996, Amson and Laurin 2011). This study found Tetraceratops nesting between Saurorictus and Limnoscelis, far from the Synapsida. No previous published studies tested Tetraceratops with these taxa. Much smaller and distinct from Limnoscelis, the skull of Tetraceratops was taller and narrower, but with a broad palate. The orbit and postorbital were taller than wide. Martensius is a sister taxon with fewer derived traits.

Amson E and Laurin M 2011. On the affinities of Tetraceratops insignis, an Early Permian synapsid. Acta Palaeontologica Polonica 56(2):301-312. online pdf

Conrad J and Sidor CA 2001. Re−evaluation of Tetraceratops insignis (Synapsida: Sphenacodontia). Journal of Vertebrate Paleontology 21: 42A.

Matthew WD 1908. A four-horned pelycosaurian from the Permian of Texas.
Bulletin of the American Museum of Natural History 24:183-185.

Laurin M and Reisz RR. 1996. The osteology and relationships of Tetraceratops insignis, the oldest known therapsid. Journal of Vertebrate Paleontology 16:95-102. doi:10.1080/02724634.1996.10011287.

Sidor CA and Hopson JA 1998. "Ghost lineages and “mammalness”: Assessing the temporal pattern of character acquisition in the Synapsida". Paleobiology 24: 254–273.

wiki/Tetraceratops