·noun ·pl An order of large, extinct, marine reptiles, found in the Cretaceous rocks, especially in America. They were serpentlike in form and in having loosely articulated and dilatable jaws, with large recurved tteth, but they had paddlelike feet. Some of them were over fifty feet long. They are, essentially, fossil sea serpents with paddles. Called also Pythonomarpha, and Mosasauria.