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.
List of North American dinosaurs
The earliest record of dinosaurs in North America comes from rare, unidentified (possibly theropod) footprints and teeth in the Middle-Late Triassic Pekin Formation of North Carolina. Later in the Triassic period, dinosaurs left more recognizable remains, and could thus be identified as specific genera. Examples of later Triassic North American dinosaur genera include "Coelophysis", "Chindesaurus", "Gojirasaurus", and "Tawa". Fossils of "Tawa"-like dinosaurs have also been found in South America, which has important indications about paleogeography. During the Early Jurassic Period, dinosaurs such as "Dilophosaurus", "Anchisaurus", "Coelophysis" (formerly known as "Megapnosaurus"), and the early thyreophoran "Scutellosaurus" lived in North America. The latter is believed to have been the ancestor of all stegosaurs and ankylosaurs. The Middle Jurassic is the only poorly represented time period in North America, although several Middle Jurassic localities are known from Mexico. Footprints, eggshells, teeth, and fragments of bone representing theropods, sauropods, and ornithopods have been found, but none of them are diagnostic to the genus level. 
The Late Jurassic of North America, however, is the exact opposite of the Middle Jurassic. The Late Jurassic Morrison Formation is found in several U.S. states, including Colorado, Utah, Wyoming, Montana, New Mexico, Oklahoma, South Dakota, and Texas. It is notable as being the most fertile single source of dinosaur fossils in the world. The roster of dinosaurs from the Morrison is impressive. Among the theropods, "Allosaurus", "Saurophaganax", "Torvosaurus", "Ceratosaurus", "Coelurus", "Ornitholestes", "Tanycolagreus", "Stokesosaurus", and "Marshosaurus" are found in the Morrison. An abundance of sauropods has been found there, including "Apatosaurus", "Diplodocus", "Barosaurus", "Brachiosaurus", "Camarasaurus", "Brontosaurus" and "Amphicoelias". Three genera of stegosaurs, "Alcovasaurus", "Stegosaurus" and "Hesperosaurus", have been found there. Finally, ornithopods found in the Morrison include "Dryosaurus", "Camptosaurus", "Drinker", "Othnielia", and "Othnielosaurus".