Temporal range: Early Cretaceous, 133–130 Ma
Abyssosaurus is known only from the holotype specimen, Museum of Chuvash Natural Historical Society (MChEIO) no. PM/1, a partial postcranial skeleton. The holotype was collected in Poretskii District of Chuvashia, near Mishukovo, dating to the late Hauterivian faunal stage of the Early Cretaceous, about 133-130 million years ago. The specimen is thought to occupy an intermediate position between the Late Jurassic Tatenectes and Kimmerosaurus and the Late Cretaceous Aristonectes and Kaiwhekea. Berezin (2011) considered Abyssosaurus to represent the first reliable record of Aristonectidae in Russia. A large phylogenetic analysis performed by Roger Benson and Patrick Druckenmiller found it to be a derived cryptoclidid closely related to Colymbosaurus.
- Benson, R. B. J.; Druckenmiller, P. S. (2013). "Faunal turnover of marine tetrapods during the Jurassic-Cretaceous transition". Biological Reviews: n/a. doi:10.1111/brv.12038.
- A. Yu. Berezin (2011). "A new plesiosaur of the family Aristonectidae from the early cretaceous of the center of the Russian platform". Paleontological Journal 45 (6): 648–660. doi:10.1134/S0031030111060037.
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