a photograph, of the skull of Hipposaurus (Figs. 1, 2; Haughton 1929; WB123), moved this basal therapsid to the base of the Anomodontia (Fig. 3), one node down (more primitive) in the therapsid skull tree (TST, 67 taxa, Fig. 3) and the same in the large reptile tree (LRT, 1440 taxa).
Broom 1932 considered Hipposaurus a gorgonopsian in the family ‘Ictidorhinidae’. Ictidorhinus nests between Hipposaurus and the gorgonopsians in the TST (Fig. 3). So, for 1932, and prior to the advent of software-driven cladograms, that was a good assessment.
Hipposaurus boonstrai (Haughton 1929, skull length 21cm, length 1.2m) Capitanian, Mddle Permian ~260 mya is a basal therapsid. Derived from a sister to Cutleria, Hipposaurus phylogenetically preceded the Anomodontia and all higher predatory synapsids, including mammals.
Much larger than its ancestors,
Hipposaurus kept the small round skull of its ancestors, but had longer, more slender limbs capable of a more erect pose and probably a faster gait.
Earlier we looked at Dimetropus tracks that were originally attributed to a high-walking Dimetrodon, but fits Hipposaurus those tracks much better. The basalmost therapsid, Cutleria (Lewis and Vaughn 1965; Early Permian) is another possibility known from fewer bones.
The relatively larger size of the WB 123 specimen skull of Hipposaurus
indicates it was a later, larger variety of the as yet unknown last common ancestor of anomodonts and kynodonts (Fig. 4).
Boonstra LD 1952. Die Gorgonospier-geslag Hipposaurus en die familie Ictidorhinidae: Tydskr. Wet. Kuns 12:142-149.
Broom R 1932. The mammal-like reptiles of South Africa and the origin of mammals. Witherby, London, 376 pp.
Haughton SH 1929. On some new therapsid genera: Annals of the South African Museum 28(1):55-78.
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.