Skeletal morphology, phylogenetic relationships and stratigraphic range ofEosimops newtoniBroom, 1921, a pylaecephalid dicynodont (Therapsida, Anomodontia) from the Middle Permian of South Africa

Robert Broom described Eosimops newtoni in 1921 based on a skull collected in Tapinocephalus Assemblage Zone strata in South Africa, and the species has been largely overlooked since that time. Here we present several new specimens of E. newtoni, including a nearly complete skeleton, that allow a much more thorough description of its morphology. Eosimops newtoni is diagnosed by two autapomorphies, a postorbital bar that is anteroposteriorly expanded in lateral view and a posterior median palatal ridge that forms a flattened, Y-shaped platform surrounding the anterior median palatal ridges. Other characters include a single median nasal boss; parietals widely exposed on the skull roof; presence of lateral anterior palatal ridges; anterior edge of caniniform process set off from the palatal rim forming a notch; a small number of ‘postcanine' teeth on the maxilla; a large, ventrally directed transverse flange on the anterior pterygoid ramus; a robust, tall, blocky crista oesophagea; a shovel-shaped jaw symphysis; dentary tables absent; posterior dentary sulcus present and bounded medially by a dorsally convex blade; at least one ‘postcanine' tooth present on the dentary blade; three sacral vertebrae; cleithrum present; dorsal margin of the coracoid foramen formed by the scapula; proximal and distal ends of the humerus offset by approximately 90°; ectepicondylar foramen present; and trochlea and capitellum distinct. Comparison with Brachyprosopus broomi and Koupia koupensis shows that neither species is likely to be a junior synonym of E. newtoni. Phylogenetic analysis reconstructs E. newtoni as the most basal member of Pylaecephalidae. We also reassess the homologies and distribution of the dentary table and posterior dentary sulcus in dicynodonts in light of the jaw morphology of E. newtoni. The stratigraphic range of Eosimops newtoni begins in the Tapinocephalus Assemblage Zone, 1565 metres above the Ecca–Beaufort contact, and extends into the Pristerognathus Assemblage Zone.