Mosasaurus (“lizard of the Meuse River”) is a genus of mosasaurs, extinct carnivorous aquatic lizards. It existed during the Maastrichtian age of the late Cretaceous period, between about 70 and 66 million years ago, in western Europe and North America. The name means “Meuse lizard”, as the first specimen was found near the Meuse River (Latin Mosa + Greek sauros lizard).
Mosasaurus was among the last of the mosasaurids, and among the largest. As with most mosasaurids, the legs and feet of Mosasaurus were modified into flippers, and the front flippers were larger than the hind flippers. The largest known species, M. hoffmannii reached lengths up to 17 m (56 ft), slightly longer than its relatives Tylosaurus and Hainosaurus. Mosasaurus was also more robust than related mosasaurids. The skull was more robust than in other mosasaurids, and the lower jaws (mandibles) attached very tightly to the skull. They had deep, barrel-shaped bodies, and with their fairly large eyes, poor binocular vision, and poorly developed olfactory bulbs, experts believe that Mosasaurus lived near the ocean surface, where they preyed on fish, turtles, ammonites, smaller mosasaurs, birds, pterosaurs, and plesiosaurs. Although they were able to dive, they evidently did not venture into deeper waters.
The skull of Mosasaurus tapered off into a short, conical tip. The jaws were armed with massive conical teeth. Their paddle-like limbs had five digits in front and four in back. The body ended in a strong tail, which other mosasaurid fossils suggest had a fluke similar to those of sharks and some ichthyosaurs. The body probably remained stiff to reduce drag through the water, while the end of the tail provided strong propulsion.
Mosasaurus was the first genus of mosasaurs to be named. The first remains known to science were a fragmentary skull from a chalk quarry in the St Pietersberg, a hill near Maastricht, the Netherlands, found in 1764 and collected by lieutenant Jean Baptiste Drouin in 1766. It was procured for the Teylers Museum at Haarlem in 1784 by Martinus van Marum, the first director of the museum, who published its description only in 1790. He considered it to be a species of “big breathing fish” (Pisces cetacei, in other words: a whale). It is still part of the collection as TM 7424.
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 being grouped with Clidastes, Moanasaurus, and Liodon in the tribe Mosasaurini.
Since the genus was first named in the early 19th century, numerous species have been assigned to Mosasaurus, both from North America and Europe. Many researchers have suggested that certain European species are actually synonyms of their American counterparts, though because many of the specimens involved are incomplete, scientists have gone back and forth regarding which species are synonyms and which are not. For example, the giant American species M. maximus is regarded by most researchers as a junior synonym of the European M. hoffmannii, though a few scientists maintain that the two can be distinguished by features of their skull bones.
Five species are generally recognized as valid today: M. hoffmannii Mantell, 1829 (the type species), M. conodon Cope, 1881,M. lemonnieri Dollo, 1889, M. beaugei Arambourg, 1952, and M. missouriensis (Harlan, 1834).
Source: wiki.org, NatGeo.com