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.
Plotosaurus
Plotosaurus ("swimmer lizard") is an extinct genus of mosasaur from the Upper Cretaceous (Maastrichtian) of Fresno County, California. Originally named "Kolposaurus" (meaning "bay lizard") by Berkeley paleontologist Charles Lewis Camp in 1942, it was changed to "Plotosaurus" in 1951 when Camp discovered the name had already been assigned to a type of nothosaur.