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 to nest with Tseajaia within the Limnoscelidae and within the Lepidosauromorpha, far from the Synapsida, but closer to Eothryis. No previous published studies tested Tetraceratops with Tseajaia, so once again, a too small of an inclusion set nested another taxon by default.
Tetraceratops and Tseajaia were derived from a sister to Romeria? primus, Eothyris and Limnoscelis. Whether or not a lateral temporal fenestra was present or not in the broken chaos of the posterior skull of the specimen, has little to no bearing on the nesting of Tetraceratops. Sister taxa were fenestrated and unfenestrated. The squamosal parts appear to have been present largely within the orbit. Moreover, what was considered a jugal ascending process and postorbital was actually a rotated juga lposterior process and quadratojugal. See below.
Much smaller and distinct from Tseajaia, the skull of Tetraceratops was taller and narrower. The prefrontal expansions had become two of the four "horns". The orbit and postorbital were taller than wide. Otherwise Tetraceratops shared with Tseajaia bumps over the nares, convex ventral margin of maxilla and mandible, premaxillary tusks, caniniform maxillary teeth, jugal with quadratojugal process, supratemporal "horns," maxilla lower than lacrimal and several other traits.
Together Tetraceratops and Tseajaia form a clade as sisters to Limnoscelis within the Limnoscelidae and this clade was a sister to Orobates.
Placing Tseajaia in the matrix of Amson and Laurin (2011) nested it with Tetraceratops as a basal synapsid. Removing Tetraceratops left Tseajaia at the same leaf and node above Haptodus, not as an outgroup. This experiment demonstrated the error of adding Tetraceratops to the Synapsida without first employing a larger umbrella study of reptile relationships.