Plesiosaurs (meaning ‘nearly a lizard’) were hunting sea reptiles. True plesiosaurs had long-necks (for example, Elasmosaurus) while their relatives, Pliosaurs, were short-necked and large-headed. Pliosaurs were much more successful hunters because their shorter necks meant they could move more easily. Pliosaurs grew to 15m. Plesiosaurs and pliosaurs lived throughout the Jurassic and Cretaceous periods and lived at the same time as dinosaurs.

Elasmosaurus was a plesiosaur with an extremely long neck. It lived in the late Cretaceous period (up to 65 million years ago). It was the longest plesiosaur – about 14m long – and weighed 2 tonnes. Half of its length was neck – made up of 70 bones – more than any other animal in history. Its limbs had evolved into paddles. It had a small head with many sharp teeth.

Its neck was too weak to allow it to rear up out of the water (as in some popular films) and in fact it probably used the neck as a kind of forward rudder.

It was a slow swimmer which ate fish. The long neck would have allowed Elasmosaurus to have drifted below a school of fish then scooped them up. It swallowed small stones in order to help its digestion.