Elasmosaurus was a type of long-necked plesiosaur, belonging to the family of Elasmosauridae, the order of Plesiosauria, and the suborder of Plesiosauroidea. It belongs to the family that consists of the longest-necked plesiosaurs. Elasmosaurus thrived during the Cretaceous Period, and lived on a diet of fish and other small sea creatures of that time. They typically reached around 10–14 meters in total body length, with a neck that counted for over half of its body length.Elasmosaurus is the largest of all plesiosaurs, with a neck that consisted of 70 vertebrae that counted for over half of the massive animal's total body length: fourteen meters. Elasmosaurus is the type species for the family of Elasmosauridae and was closely related to other elasmosaurid plesiosaurs such as Hydrotherosaurus and Thalassomedon.