''Opetiosaurus'' is an extinct genus of Late Cretaceous marine or semiaquatic lizard classified as part of the family Aigialosauridae within the Mosasauroidea. Exclusively found in deposits of

Cenomanian The Cenomanian is, in the ICS' geological timescale, the oldest or earliest age of the Late Cretaceous Epoch or the lowest stage of the Upper Cretaceous Series. An age is a unit of geochronology; it is a unit of time; the stage is a unit in the s ...

age near Hvar, Croatia, the genus contain one valid species, ''O. bucchichi''. It was a small reptile measuring long. The genus was diagnosed as congenetic with '' Aigialosaurus'' by Dutchak and Caldwell (2009),Dutchak, Alex R.; and Caldwell, Michael W. (2009
"A redescription of ''Aigialosaurus'' (= ''Opetiosaurus'') ''bucchichi'' (Kornhuber, 1901) (Squamata: Aigialosauridae) with comments on mosasauroid systematics".
''Journal of Vertebrate Paleontology'' 29 (2): 437-452. being re-diagnosed as "''Aigialosaurus bucchichi''" and only separated by some minor anatomical differences between specimens of the two species. Madzia & Cau (2017)Madzia, D., Cau, A. (2017)
Inferring "weak spots" in phylogenetic trees: application to mosasauroid nomenclature
PeerJ 5:e3782 show that the two species are not necessarily more closely related to each other than either is to more derived mosasauroids, suggesting not only that ''Opetiosaurus'' is a valid genus, but also calling the validity of the Aigialosauridae as a monophyletic group into question. Aigialosaurus bucchichi

References

Mosasaurs Mosasaurs of Europe {{paleo-lizard-stub