First discovered in 1764 in Maastricht, the mosasaur was a fearsome aquatic lizard living during the cretaceous period (145.5 million to 66 million years ago). They were powerful swimmers, and were well-adapted to living in the warm, shallow inland waters that were common in the Late Cretaceous period.

Mosasaurus possessed long snouts, big skulls, and snake-like bodies. Compared to their ancestors, their arms evolved to become paddles, with more finger and toe bones and shorter limb bones. Similar to early ichthyosaurs, the body's tail portion was lengthy and had a modest downcurvature at the end. The skull's anatomy was strikingly similar to that of today's monitor lizards. Numerous conical, somewhat recurved teeth in separate sockets were present in the jaws, which stood out because they were joined in front by merely ligaments and close to mid-length. This arrangement enabled the animals not only to open the mouth by lowering the mandible but also to extend the lower jaws sideways while feeding on large prey.