The skull of Mesenosaurus romeri, a small varanopseid (Synapsida: Eupelycosauria) from the Upper Permian of the Mezen River basin, northern Russia

Restudy of Mesenosaurus romeri, based on new and previously described cranial materials from the Upper Permian of the Mezen River basin of northern Russia, confirms its assignment to the synapsid eupelycosaurian family Varanopseidae. Comparisons with other members of the family support a pattern of relationship that recognizes two clades: one is composed of Mesenosaurus and Mycterosaurus for which the subfamily designation Myctersaurinae is proposed, and the other includes the remaining wellknown varanopseids Elliotsmithia, Varanops, Varanodon, and Aerosaurus for which the subfamily designation Varanodontinae is proposed. Among the late Paleozoic synapsids, Varanopseidae has the longest fossil record, extending from the end of the Carboniferous to well into the Late Permian, and the widest geographical distribution, including North America, South Africa, and Russia.