Mosasaurus (whose name means "lizard of the river Maas") is an extinct genus of mosasaurid marine reptile lived in the Upper Cretaceous, about 70-65 million years ago (Maastrichtian), in Western Europe and in North America. The genus currently has five species: the species of M. hoffmannii, M. missouriensis, M. conodon, M. lemonnieri and M. beaugei.