Mosasaurus was a late mosasaur (a type of aquatic, finned, giant predatory lizard from the Cretaceous period). The type species, Mosasaurus hoffmanni, was found in the Netherlands in 1776. It was named in 1822 by W.D. Conybeare.
The type species (M. hoffmanni) is estimated to have been able to attain lengths of up to 56 feet (17m) long. Mosasaurus had four paddle-like limbs on a long, streamlined body and a long, powerful tail. The large head had huge jaws, up to 4 ft (1.2 m long) with many teeth. The jaws could open about 3 feet (1 m). The lower jaw is loosely hinged to the skull with a moveable joint on each side (behind the teeth). This loose joint let it swallow huge prey. They would have hunted fish, turtles, molluscs, and shellfish. Ammonites have been found bearing mosasaur teeth marks.
The type specimen was found in a chalk quarry in Maastricht, Holland. It was found by a German Army surgeon, Johann Hoffmann, who collected fossils for the Haarlem Museum. In the course of a struggle for ownership, the skeletal parts went to the museum, whilst the skull stayed with the owner of the land, who refused to let anyone see it.
The true identity of the monster was decided correctly by the French anatomist Georges Cuvier. In 1795 French troops were outside Maastricht, and Cuvier arranged for the large skull to be saved when they stormed the town. The skull duly went to Cuvier in Paris, fortunately, because he was the leading comparative anatomist of the day. He recognised the skull as that of a giant lizard, from its teeth and skull bones, though not until 1808, and by then the son of a Dutch professor, Adriaan Camper, had already had the same idea. The discovery of the specimen was important in another way, because it helped to convince Cuvier that extinction of some species was a fact. Cuvier later came up with a catastrophism-type theory.
The family Mosasauridae is split into several subfamilies, with Mosasaurus being placed within Mosasaurinae. This subfamily, in turn, is further split into smaller tribes, with Mosasaurus grouped with Clidastes, Moanasaurus, Amphekepubis, and Liodon in the tribe Mosasaurini.
Since the genus was first named in the 19th Century, lots of species have been assigned to Mosasaurus, but just 4 are now seen as valid by most researchers: M. hoffmannii Mantell 1829 (the type species), M. lemonnieri Dollo 1889, M. missouriensis (Harlan 1834), and M. beaugei Armbourg, 1952.
Some more named, but invalid or doubtful species are:
M. copeanus Marsh, 1869
M. crassidens Marsh, 1870
M. dekayi Bronn, 1838
M. giganteus (Somering, 1916)
M. gracilis Owen, 1851
M. hobetsuensis Suzuki, 1985
M. johnsoni (Mehl, 1930)
M. lonzeensis Dollo, 1904
M. lundgreni (Schroder, 1885)
M. meirsii Marsh, 1869
M. mokoroa Welles & Gregg, 1971
M. neovidii von Meyer, 1845
M. prismaticus Sakuai, Chitoku & Shibuya, 1999
M. scanicus Schroder, 1885
M. iguanavus (Cope, 1868)
M. poultneyi Martin,1953
Mosasaurus in The Land Before Time
A Mosasaurus skeleton can be seen in the Mysterious Beyond in The Great Valley Adventure. One also appears in the intro of The Land Before Time IV: Journey Through the Mists. It is pursuing a shoal of fish, jumping out of the water after them.