Abyssosaurus

Abyssosaurus ("bottomless lizard") is an extinct genus of cryptoclidid plesiosaur known from the Early Cretaceous of Chuvash Republic, western Russia. The type specimen, consisting of a fairly complete postcranial skeleton and parts of a skull, was discovered around the left tributary of the Sura River by Vasily V. Mita; it was found alongside the remains of a possible fetus, though this has not been described. Subsequently, the specimen entered the collection of the Museum of Chuvash Natural Historical Society. It was described in 2011 by Alexander Yu Berezin, who named its type species, A. nataliae, after his wife and colleague, Nataliya Berezina. While Berezin initially believed that Abyssosaurus was a basal aristonectid, part of a lineage whose later members evolved into giant filter-feeders, later studies have concluded that it was instead related to Colymbosaurus.

Abyssosaurus was aberrant compared to other cryptoclidids. It was a large animal, with a total length of , of which half was its neck. Its skull was unusually short and triangular in comparison to that of other plesiosaurs, and its eye sockets were very large. Parts of its overall skeleton retain juvenile traits, a condition known as paedomorphosis which is observed in various deep-sea vertebrates. Parts of the gastralia (abdominal ribs) and flippers were very robust, an example of pachyostosis which would have made it less buoyant. The hind flippers were considerably longer than the front flippers, which would have allowed it to float diagonally above the seabed. It has been suggested that Abyssosaurus spent much of its life in the bathypelagic zone, where it would have hunted small invertebrates such as cephalopods and crustaceans.