Aigialosauridae

Aigialosauridae (from Greek, aigialos, meaning "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 and by others as an adaptive grade within the basal mosasauroids, recent molecular and morphological data suggests that they are the oldest known members of the lineage leading to the mosasaurs.

The family is recognized as containing two species, normally classified into two genera; Aigialosaurus and Opetiosaurus. When used as an adaptive grade rather than as an actual monophyletic group, many other basal mosasauroids of uncertain affinities are occasionally referred to as "aigialosaurs", such as the genera Komensaurus, Haasiasaurus, Carsosaurus and Dallasaurus and even entire mosasaur clades, such as the Tethysaurinae. The term "plesiopedal mosasaurs" or "mosasaurs with a plesiopedal limb condition" is a more formal way to refer to such primitive species firmly within the family Mosasauridae.