Cutleria wilmarthi (Lewis and Vaughn 1965; Early Permian, USNM 22099). Laurin (1994) redescribed this small synapsid as a sister to the Sphenacodontidae + Therapsida at a time when therapsids were thought to descend from sphenacodontids. Here Cutleria nests as the most basal therapsid between Ophiacodon + Haptodus on the pelycosaur side and Stenocybus + Hipposaurus on the therapsid side. The postcrania is shown below. The reptile family tree is here. See below for skulls in phylogenetic order.

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.

Laurin, M 1994. Re-evaluation of Cutleria wilmarthi, an Early Permian synapsid from Colorado. Journal of Vertebrate Paleontology 14(1): 134-138.

Lewis GE and Vaughn PP 1965. Early Permian Vertebrates from the Cutler Formation of the Placerville Area Colorado. United States Geological Survey Professional Papers 503-C:1-50.

wiki/Cutleria