Updated January 11, 2020
with better data in Spindler 2020.
Updated June 14, 2021
with a new tracing of Martensius, a new sister for Tetraceratops.
The phylogenetic position
of the Early Permian Tetraceratops has been controversial. Originally Matthew (1908) considered it an eothyridid. Others considered it 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 within the Diadectomorpha and within the new Lepidosauromorpha. No one else had ever tested such a relationship. The outgroup was Orobates, a taxon that might provide some clues to the post-crania of Tetraceratops, which is otherwise unknown.
Tetraceratops was half the length of Orobates, if the skull proportions are any clue. Tetraceratops also has a distinct and derived skull structure with a highly curved lower jaw, a reduced naris, and fewer large teeth. The famous bony protuberances of the skull are likely intraspecific traits associated with secondary sexual characters and dominance. These can also be seen in similar patterns in Tseajaia. Tetraceratops was a likely herbivore, as were its closest kin. We don’t know what the tail of Tseajaia looked like, but it was likely rather gracile, as in Orobates. It would be interesting to see the shape of the dentary in Tseajaia, but every image I’ve seen has the jaws tightly closed, but the ventral margin is highly curved.
It still seems odd that anyone would think such an odd specimen, like Tetraceratops, was a basal form of such a large clade, like the therapsids, since it resembles none of them. This may represent just another case of having a blind eye to all possibilities because a large reptile tree was not employed. Here it is represents a derived herbivore with sprawling limbs leaving no descendants.
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.