Mosasaurs are an extinct group of large marine reptiles that became the dominant marine predator during the last 20 million years of the Cretaceous as the plesiosaurs were in decline and the ichthyosaurs went to extinction. They ranged in length from less than 3 feet (1 m) to 13 feet (4 m) with the largest reaching 56 feet (17 m) in length. Mosasaurs were powerful swimmers and were adapted to the warm, shallow inland seas that were common during the late Cretaceous when high sea levels resulted in marine transgressions in many parts of the world including a Cretaceous seaway that covered much of the United States from the Gulf Coast and Texas and north to Montana and the Dakotas. The Mosasaur’s flippers were formed by modifying their legs and feet by introducing webbing between their long finger and toe bones. Their main locomotion power, however, was provided by a broad tail. New evidence indicates that many advanced mosasaurs had large crescent-shaped flukes on the ends of their tails similar to those of sharks. Based on the fact that Mosasaurs had double-hinged jaws and round, pebbly-shaped teeth, it seems that mosasaurs commonly swallowed their prey whole. At the time of their extinction, mosasaurs were facing increased competition from sharks which were soon to take over as the primary marine predator.