Mosasaurus is the last and the largest of the mosasaurs, massive marine lizards that eventually became the dominant predators of the oceans and seas during the Late Cretaceous Period. These animals are best described as being like monitor lizards mixed with orcas, possessing elongated, streamlined bodies and two pairs of flippers. It was originally believed that mosasaurs were eel-shaped and thus moved like an eel or a sea snake.