Mosasaur
Mosasaurs (from Latin "Mosa" meaning the 'Meuse river', and Greek σαύρος "sauros" meaning 'lizard') are an extinct group of large marine reptiles containing 38 genera in total. Their first fossil remains were discovered in a limestone quarry at Maastricht on the Meuse in 1764. Mosasaurs probably evolved from an extinct group of aquatic lizards known as aigialosaurs in the Early Cretaceous. During the last 20 million years of the Cretaceous period (Turonian-Maastrichtian ages), with the extinction of the ichthyosaurs and pliosaurs, mosasaurs became the dominant marine predators. They became extinct as a result of the K-Pg event at the end of the Cretaceous period, about 66 million years ago.
Prehistory of the United States
The stratigraphic unit known as the Morrison Formation was deposited during the Late Jurassic. These sediments are now exposed in Arizona, New Mexico, Oklahoma, Utah, Colorado, Wyoming, South Dakota, and Montana. At the time, this region of the country was home to forests of conifers, ginkgos, and tree ferns. Coal would later form from the remains of these plants. Local mammals diversified significantly during the Jurassic. The Morrison Formation is the best source of Jurassic mammal fossils in North America. Local dinosaurs included the ornithopod "Camptosaurus", the sauropods "Apatosaurus" and "Diplodocus", and the theropod "Allosaurus". Unlike many periods of geologic history the Jurassic did not end in a mass extinction. There were, however, lesser extinction events going on at the time, with notable losses occurring among ammonoids and dinosaurs.