Revision of the plesiosaur Polycotylus sopozkoi from the Southern Urals (Russia) confirms the wide distribution of Polycotylus in the Late Cretaceous of the Northern Hemisphere

Abstract

Polycotylids are among the most common plesiosaurians of the Late Cretaceous, however, in Eurasia their findings are rare and fragmentary. In 2016, a partial polycotylid skeleton from the Upper Cretaceous of the Izhberda quarry in the Southern Urals region was described by Efimov et al. as a new species, Polycotylus sopozkoi. Here we revise this holotype specimen and show that many features initially proposed to distinguish the species are the result of misinterpretations. However, P. sopozkoi is indeed referable to Polycotylus and is highly similar to its type species, P. latipinnis. Although only one distinctive trait of the species noted by Efimov et al., the protruding basioccipital tubera with deep carotid canals on their anterodorsal surface, is confirmed here, new observations revealed additional features that allow us to substantiate the validity of P. sopozkoi. The presence of Polycotylus in the Upper Cretaceous of North America and Eastern Europe highlights a wide distribution of some plesiosaurian genera and suggests caution in assumptions of ‘endemic’ plesiosaurian taxa in particular regions of the world.