A gigantic pliosaurid from the Cenomanian (Upper Cretaceous) of the Volga Region, Russia

Abstract

Pliosaurid plesiosaurs were among the largest predators of Mesozoic marine ecosystems attaining giant sizes up to 10–11 m in length, near a quarter of which could be constituted by a massive head. The known Late Cretaceous pliosaurids are comparatively smaller than the stratigraphically older taxa and lack a suite of macropredaceous features, demonstrating a somewhat decline of the group members towards their extinction in the Turonian. However, the early Late Cretaceous (Cenomanian) record of pliosaurids is scarce and mostly represented by isolated teeth, thus hampering the recognition of diversity and disparity of pliosaurids in this time interval. Here we describe a very large isolated cervical centrum (19 cm in maximum diameter) belonging to a brachauchenine pliosaurid. The comparison with other pliosaurids indicates that the centrum belonged to one of the largest known pliosaurids, comparable in size to Kronosaurus queenslandicus, ‘Kronosaurus’ boyacensis and Sachicasaurus vitae from the Aptian–Albian of Australia and Barremian–Aptian of Colombia. This finding indicates that gigantic pliosaurids were also present in the Cenomanian. Thus, the previously inferred decrease in size of pliosaurids in the Late Cretaceous is likely a result of sampling bias, and, if present, it happened not earlier than in the late Cenomanian–Turonian, right before the extinction of the group.