Plesiosauria

Plesiosauria includes the large, carnivorous, marine reptiles the Plesiosaurs and Pliosaurs. They used their flippers to propel themselves through the water, rather like a turtle except that they used both the front and rear flippers. Scientists were intrigued to find that using all four flippers didn't make them faster. In fact, the opposite was true, as turbulence from the front flippers interfered with the efficiency of the rear flippers. However, four flippers did make it easier to get going and brake. So while Plesiosaurs and Pliosaurs may not have been ultra-speedy, they would have been agile and quick to react to passing prey.