Mosasaurus (/ˌmoʊzəˈsɔrəs/ ; lizard of the Meuse River ) is a genus of mosasaur, extinct carnivorous aquatic lizards.It existed during the Maastrichtian age of the late Cretaceous period, between about 70 and 66 million years ago, in western Europe and North America.The name means Meuse lizard, as the first specimen was found near the Meuse River (Latin Mosa + Greek sauros lizard).osasaurus was among the last of the mosasaurids, and among the largest. As with most mosasaurids, the legs and feet of Mosasaurus were modified into flippers, and the front flippers were larger than the hind flippers.