Martensius bromackerensis (Berman et al. 2020; Early Permian, 50cm; MNG 13814 adult holotype; MNG 14230 juvenile and smallest specimen) was originally considered a caseasaur, nesting between Eocasea and Casea. Here Martensius is a sister to Tetraceratops, a taxon omitted by Berman et al. 2020.

Four specimens were found together, from a juvenile to an adult. A small skull is apparent here, as a trait found in most caseasaurs. Martensius indicates that tetraceratopsians were caseasaur mimics. This is the first postcranial material known for a tetraceratopsian.

Berman et al. did not test Tetraceratops, Milleretta, or any casesaur sisters, like Feeserpeton, Australothyris, Acleistorhinus, Eunotosaurusor any other lepidosauromorphs in their phylogenetic analysis. Instead they followed tradition by assuming casesaurs were synapsids.

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

Berman DS, Maddin HC, Henrici AC Sumida SS, Scott D and Reisz R 2020. New primitive caseid (Synapsida, Caseasauria) from the Early Permian of Germany. Annals of the Carnegie Museum 86(1):43–75.

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.

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