Elasmosaurus dinosaur. Artwork of Elasmosaurus, a marine dinosaur (plesiosaur) coming ashore. Elas- mosaurus ('plated reptile') had a long neck with a relatively small head and a mouth lined with teeth. It was probably a fish-eater, using its neck to dart its head at prey. It was propelled through the water by four powerful paddle-like limbs which it flapped up and down like the paddles of a modern-day turtle. Elasmosaurus prob- ably came ashore to lay its eggs. Dinosaurs lived during the Mesozoic era (65-225 million years ago). Elasmosaurus was described in 1868, although the first reconstruction mistakenly placed its head on the end of its relatively short tail.