Kazakhstanosaurus efimovi Yakupova et Akhmedenov sp. nov. (Ichthyosauria, Undorosauridae), a New Species from the Upper Jurassic Deposits of the Middle Volga Region

The skeleton fragments of a new ichthyosaur species, Kazakhstanosaurus efimovi Yakupova et Akhmedenov sp. nov., from the Middle Volgian substage of the Upper Jurassic of the Middle Volga region (Ulyanovsk region, Gorodishche section) are described. The species is assigned to the genus Kazakhstanosaurus V. Efimov Akhmedenov et Yakupova, 2019 (fam. Undorosauridae), which has been recently described from the same-age deposits of Kazakhstan. The new species is diagnosed by the following characters: the subquadrate shape of the trunk vertebrae; the structure of the shoulder girdle and forelimbs, which have a two-facet distal end of the humerus and a weak articulation of the first finger; an increased size of all parts of the skeleton; the costal facets positioned on the sides of the end of the ribs; the connection of the costal facets with a bone crest. The facets of the ribs are connected by a bony ridge. The discovery of Kazakhstanosaurus efimovi confirms the connection of the Russian Sea during the Late Jurassic with the warm waters of the Tethys Ocean. Probably, the progenitors of undorosaurids descended from the Triassic and Early Jurassic ichthyosaurs that lived in the tropical regions of the eastern part of the Tethys Ocean. In the Late Jurassic, these ichthyosaurs migrated to the territory of modern Kazakhstan and Eastern Europe, settled in the shallow Russian Sea, and gave rise to representatives of the families Undorosauridae and Platypterygius.