A family of marine lizards, fossils of which have been found in Upper Cretaceous rocks throughout the world. They evolved into many types but all became extinct at the end of the Cretaceous. Mosasaurs were large, some reaching 9 m in length, with a long, slim body, short neck, and long head. The tail was used for swimming and the limbs for steering; the digits may have been webbed. The teeth were set in pits rather than being fused to the jaws. Most mosasaurs fed on fish, although some are believed to have fed on molluscs.