Mosasaurus is the largest member of the mosasaurid family. Though they are not properly dinosaurs, they were contemporaries. Mosasaurus, like most members of the mosasaurid family, are adapted to aquatic living; they have fish-like bodies, and instead of legs and feet they have flippers, with the front flippers being larger than the hind flippers. However, as they are reptiles they cannot breathe underwater, though they can hold their breath for extended periods.

They are aggressive carnivores, even preying upon sharks, and will attack nearly any living creature they encounter.