Mosasaurus was a genus of mosasaur, giant carnivorous, aquatic lizards, somewhat resembling flippered crocodiles, with big elongated jaws.
This genus existed during the Maastrichtian age of the Cretaceous period (Mesozoic era), around 70–66 million years ago in the area of modern Western Europe and North America in a western interior sea and was one of the last mosasaurs. Its name means "Meuse river lizard", as its fragmentary skull was found near the Meuse River in 1764 by lieutenant Jean Baptiste Drouin. It was similar to the North American Tylosaurus, but at 18.5m (61 feet) in length, it was even smaller and bigger.
It was initially thought to be a species of whale or crocodile considering its large teeth but De Saint-Fond still assumed the specimen represented a crocodile. In 1798 the son of Petrus Camper, Adriaan Gilles Camper, studied the fossil indirectly by reconsidering the description by his father. He was the first to reach the conclusion that the remains were those of a giant monitor lizard, which result in 1799 he corresponded to Georges Cuvier who formally identified it as a completely new but extinct creature (at the time extinct animals were assumed to be like extant animals but giant).Mosasaurus had unusual skull anatomy which is actually double-hinged like a snake (albeit aquatic and with fins). How it did this is when it was biting down on something large, its jaws would dislocate and ''unhinge'' so it would be able to take bigger bites. Its fins were probably used for steering and balance when swimming, not like plesiosaurs who used their fins as their primary means of locomotion. It was very elongated and thin in the body so it was more streamlined so it could move faster for longer periods of time than plesiosaurs who could only accelerate for short periods. Mosasaurus probably had a strong bite force due to eating prey like giant turtles in its time. There are some other species of mosasaur that specialised in eating this kind of prey so they had stronger bites than Mosasaurus who was a generalist predator so it would take anything that fit into its mouth, not unlike tiger sharks today. It is assumed that mosasaurs swam alone since their bite marks have been found on other mosasaurs indicating aggression and territoriality among the species. Mosasaurus more than likely used the same tactics sharks use today to hunt large prey such as plesiosaurs and ichthyosaurs by going down far beneath the prey and getting one devastating surprise attack to avoid injury. To achieve this tactic, It was probably counter shaded so its top was dark to camouflage against the dark ocean below the target and light underneath to camouflage against the sun's bright light. It was also possible that mosasaurus actively chased down its food by using explosive speed and stamina to hunt prey like ichthyosaurs. It has been recently discovered that the mosasaur genus Platecarpus has a tail fluke on its tail which would aid with swimming. It is not unlikely that other mosasaurs if not all mosasaurs had this tail fluke to achieve even better swimming capability.