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Approximately 82 to 66 million years ago during the Campanian and Maastrichtian stages of the Late Cretaceous. These are natural matrixes, not reconstructions. Each will be slightly different. 

Mosasaurs comprise a group of extinct, large marine reptiles from the Late Cretaceous. Their first fossil remains were discovered in a limestone quarry at Maastricht on the Meuse in 1764. They belong to the order Squamata, which includes lizards and snakes.

Mosasaurs had a snakelike body with a large skull and a long snout. Their limbs were modified into paddles having shorter limb bones and more numerous finger and toe bones than those of their ancestors. 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 structure of the skull was very similar to that of the modern monitor lizards, to which mosasaurs are related. The jaws bore many conical, slightly recurved teeth set in individual sockets. The jawbones are notable 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.

Along with being a crazy cool extinct terrors of the seas, they're famous! You may seen them in the Jurassic world movies. The big scary guy that eats the Indominus Rex at the end of the movie (sorry for spoilers) this one!  They make Jaws look like a guppy.