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. |
