Earlier we talked about the role of Ophiacodon at the base of the Therapsida. Several years later that hypothesis is still in direct contrast to traditional thinking (Laurin and Reisz 1996 and other refs below) that holds sphenacodonts and Tetraceratops were basal to Therapsida.
Here (Figs. 1, 2) we’ll add the tall-skull holotype of Varanosaurus, which nests as an outgroup to Ophiacodon, a genus known from several species of increasing size. Cutlerlia nests at the base of the Therapsida, basal to Biarmosuchus and tiny Ictidorhinus (wonder if it is a juvenile lacking fangs?). Cutleria is also close to the other branch of the Therapsida, the Anomodontia (dromasaurs, dicynodonts and their kin.)
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.