Elasmosaurus ("Plated Lizard") was a large plesiosaur (the largest of the long necked Plesiosaur kind) from the Late Cretaceous Period related to Liopleurodon and Cryptoclidus. It reached 13-15 meters long, but much of its length was made of its neck, which consisted of 75 vertebra.
Elasmosaurus lived in the waters of the Late Cretaceous period 75-65 million years ago.
Elasmosaurus was the largest plesiosaur ever to roam the seas. They measured 13-15 meters long and weighed as much 10 tons, making them larger than their earlier Jurassic cousins Cryptoclidus, arguably the largest member of the Plesiosaurs.
Elasmosaurus lived in huge numbers and was some of the most exciting reptiles ever discovered. They could save energy like dolphins do. Their small heads on their long necks were a way to deceive prey. Fish would have been frightened by their big bodies, but what the Elasmosaurus did was, like their ancestors Cryptoclidus, Elsamosaurus could sneak those heads right into the middle a shoal, and then strike.