A mosasaur was any of a group of giant, carnivorous marine reptiles that lived during the late Cretaceous period, about 85 to 65 million years ago. Mosasaurs, the largest of which was Hainosaurus with a length of up to 17 meters, were the top predators in the sea at the time. Not being fast swimmers, they relied on stalking their prey which included ammonites and turtles, as well as larger animals such as sharks and plesiosaurs. The closest living relatives of the mosasaurs are snakes.