Cranial anatomy of Ennatosaurus tecton (Synapsida: Caseidae) from the Middle Permian of Russia, and the evolutionary relationships of Caseidae

The cranial anatomy of the Russian caseid Ennatosaurus tecton is described in detail. Three autapomorphies distinguish E. tecton from other caseids: a broad anterior ramus of the jugal, a large contribution of the frontal to the dorsal orbital margin, and a relatively narrow parasphenoid body. Phylogenetic analysis of Caseidae yields a single most parsimonious tree. The obtained topology posits Ennatosaurus tecton as the sister taxon to the monophyletic clade of the North American caseids Angelosaurus dolani and Cotylorhynchus romeri. Oromycter dolesorum from the Richards Spur locality becomes the most basal member of Caseidae, replacing Casea broilii. In addition, the genus Casea is paraphyletic, whereby C. rutena forms the sister taxon to the clade containing E. tecton, C. romeri , and A. dolani. The current topology reveals the pattern of dental complexity in terms of the number of apical cuspules does not contain a phylogenetic signal and convergently acquired tooth morphologies occur between some caseid taxa.