Elasmosaurus. Artwork of two Elasmosaurus marine reptiles swimming underwater. These animals were long-necked members of the Plesiosaur group of carnivorous marine reptiles. They inhabited the seas of the Mesozoic Era from 225-65 million years ago. They had a relatively small head on a long neck, probably catching their fish prey with whip- like darting movements. Plesiosaurs swam like turtles, flapping their flippers up and down.