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.

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.

[3] Aigialosaurids were semi-aquatic lizards that inhabited shallow marine environments in the ancient Tethys Ocean, the only known fossils having been recovered from Croatia.

[5] Despite thorough reviews of the relationships between early Mosasauroidea and redescriptions of the two normally recognized aigialosaurid genera, Aigialosaurus[6] and Opetiosaurus,[7] the status of the family remains uncertain.

This definition highlights the instability of "aigialosaur" phylogeny and ensures that the name Aigialosauridae is only used in the cases when Aigialosaurus dalmaticus and Opetiosaurus bucchichi form a clade to exclusion of more derived mosasauroids.