New polycotylid plesiosaur skeletons from the Upper Cretaceous of the Southern Urals provide additional diagnostic features of Polycotylus sopozkoi and demonstrate its variation

Since the discovery of the holotype of Polycotylus sopozkoi in the Southern Urals, several skeletons referable to Polycotylus were collected from the upper Santonian – lower Campanian bone bed at its type locality (Izhberda quarry, Orenburg Region, Russia). Here we describe these new specimens, which together with the holotype constitute an ontogenetic series from small juveniles with the propodial length of 27 cm to a large adult individual with the propodial length exceeding 47 cm and traces of senile pathologies of limb elements. The new specimens significantly expand the knowledge of the morphology and variation of the postcranial skeleton in Polycotylus. They also reveal additional diagnostic features of Polycotylus sopozkoi (28 cervical vertebrae, one of the highest counts for polycotylines, and strongly tapered anterodistal processes of cervical ribs), and shared traits with P. latipinnis (divergent anteromedial processes and pronounced anterior shelf of the coracoid), which were previously unnoticed in the known specimens and supplement the generic diagnosis. The observed individual variation in Polycotylus sopozkoi has implications for phylogenetic and ontogenetic studies of plesiosaurs, and for assessment of the taxonomic value of some characters in polycotylids.