The Mosasaurs This is a mosasaur

At the time when the tyrannosaurs ruled the continents, gigantic predatory lizards roamed the seas. These were the mosasaurs.

Look at a mosasaur: it is a long animal with limbs transformed into flippers, as in turtles and dolphins. The oldest mosasaurs would have swum with snake-like undulating movements, like eels. But those exhibited here were more evolved: only the tail moved, from side to side, in a powerful to-and-fro motion. The flippers were used for balance and steering.