Plesiosaurus (pronounced plee-zee-oh-SAWR-us) was a genus of flippered marine reptiles that lived from the Mesozoic period. There were many species of Plesiosaurus. They were not dinosaurs. Plesiosaurus means "near lizard." Plesiosaurs are divided into two groups: the Plesiosauroids with long, snake-like necks, tiny heads, and wide bodies, and the Pliosauroids, which had large heads with very strong jaws and short necks. Plesiosaurs ranged in size from 8-46 feet long (2.5-14 m). They had four flippers, sharp teeth in strong jaws, and short, pointed tails. Plesiosaurs may have evolved from the Nothosaurs or Pistosaurus, a mid-Triassic reptile.

Mosasaur:

Family of extinct aquatic lizards closely related to the present-day Oriental and African monitor lizards. They attained a high degree of adaptation to the marine environment and became distributed around the world during Cretaceous time (from 144 to 66.4 million years ago). The mosasaurs competed with other marine reptiles--the plesiosaurs and ichthyosaurs--for food, which consisted largely of ammonites, fish, and cuttlefish. The mosasaurs had snakelike bodies with large skulls and long snouts. Fore and hind limbs were modified as paddles, having shorter limb bones and more numerous finger and toe bones than their precursors. The tail region of the body was long, and its end was slightly downcurved in a manner similar to that of the early ichthyosaurs. The backbone consisted of more than 100 vertebrae. The construction of the skull was very similar to that of the modern monitors. The jaws bore many conical, slightly recurved teeth set in individual sockets. The jawbones are noteworthy in that they were jointed near mid-length, as in some of the advanced monitors, and connected in front by ligaments only. 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.

Some Mosasaurs include the Mosasaurus (33 feet=10 m long with sharp teeth from the North Atlantic), Platecarpus, Tylosaurus (26 feet=8 m long with sharp teeth from the North and South Atlantic), Plotosaurus, Clidastes, Plioplatecarpus, and Globidens (with flat teeth for crushing shellfish). These animals are named after the Meuse River, which passes near the site in the Netherlands where the first fossil remains were found in 1780. The largest mosasaurs may have been 15 m (49 ft) long.