Mosasaur

Mosasaurs are a group of successful predatorial marine reptiles that lived throughout the duration of the Late Cretaceous period. Like Dimetrodon, Pterosaurs, and other marine reptiles Mosasaurs are commonly mistaken for dinosaurs but they are actually distantly related.

The ancestry of Mosasaurs is disputed. It was originally thought that the aquatic reptiles originated from snakes, but recent studies show that the lizard  was most likely the precursor of mosasaurs.