New Data on the Evolution and Ontogeny of Karpinskiosauridae (Tetrapoda, Seymouriamorpha). Part 1. Cranial Morphology of Volgerpeton exspectatus gen. et sp. nov.

A new member of the family Karpinskiosauridae (Tetrapoda, Seymouriamorpha), Volgerpeton exspectatus gen. et sp. nov., is described based on the material from the Monastyrsky Ovrag-D locality, which corresponds to the uppermost Urzhumian of the boundary stratotype of the Severodvinian Regional Stage in the Monastery Ravine sequence of Tetyushi District, Republic of Tatarstan, Russia. The holotype of the new species, a skull of 18 mm long, has the majority of skull roof bones preserved, and belonged to a larval individual. The deep otic notches, absence of the interpremaxillary fenestra, extension of the squamosal into the marginal area of the parietal shield on the dorsal side of the skull, shape and orientation of the anterior edge of this bone, presence of a long anterolateral process in the nasal, a curved shape of the lateral edge of the supratemporal, a quadrangular shape of the postorbital portion of the jugal, a long-rectangular outline of the palatine, the specific pattern of dentition of the lateral elements of the palate, and presence of a long crista praequadrata of the quadratojugal are features that distinguish the genus Volgerpeton from Karpinskiosaurus. In terms of the cranial morphology, the genus Volgerpeton occupies an intermediate position between the known members of Discosauriscidae and Karpinskiosauridae, being similar to the former in the morphology of the otic and cheek regions of the skull, as well as in the shape of premaxillaria, and to the latter in the pattern of palatal dentition and the shape and enlarged size of the tabularia, which together confirms the assumption of the phylogenetic continuity of both the families. The finding of basal karpinskiosaurids in the upper Urzhumian agrees with the stratigraphic data, in particular, to the recent discovery of discosauriscids at the Kluchiki locality (Perm Region, Russia), which is Ufimian–Kazanian in age.