''Opetiosaurus'' is an extinct genus of

Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''creta'', the ...

marine or semiaquatic lizard classified as part of the family

Aigialosauridae Aigialosauridae (from Greek, ''aigialos'' = "seashore" and ''sauros''= lizard) is a family of Late Cretaceous semiaquatic pythonomorph lizards closely related to the mosasaurs. Regarded by some paleontologists as a distinct monophyletic group an ...

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 Hvar (; Chakavian: ''Hvor'' or ''For'', el, Φάρος, Pharos, la, Pharia, it, Lesina) is a Croatian island in the Adriatic Sea, located off the Dalmatian coast, lying between the islands of Brač, Vis and Korčula. Approximately long, wi ...

Croatia , image_flag = Flag of Croatia.svg , image_coat = Coat of arms of Croatia.svg , anthem = "Lijepa naša domovino"("Our Beautiful Homeland") , image_map = , map_caption = , capit ...

, the genus contain one valid species, ''O. bucchichi''. It was a small reptile measuring long. The genus was diagnosed as congenetic with ''

Aigialosaurus ''Aigialosaurus'' 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 age near Hvar, Croatia, the genus conta ...

'' 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