Craniology of the Plesiosaur Abyssosaurus nataliae Berezin (Sauropterygia, Plesiosauria) from the Lower Cretaceous of the Central Russian Platform

Cranial bones of the plesiosaurus Abyssosaurus nataliae Berezin, 2011 from the Upper Hauterivian (Lower Cretaceous) of Chuvashia are described. It differs from other known Cryptoclididae in the evident pedomorphic characters in the skull structure. The skull is strongly hypertrophic; the visceral part is shortened relative to its large occipital region; the orbits are enlarged, the lower jaw is narrow. The jugal is well developed, positioned vertically, and borders most of the posterior orbital wall. This skull structure is evidence of deepwater mode of life. The species diagnosis is revised.