Late Cretaceous, 85-65 million years ago, Elasmosaurus

A bizarre creature whose 15m long body was dwarfed by its long, thin neck and tail, Elasmosaurus swam using four flippers.
It was a carnivorous hunter which used its long neck to get close to prey without them noticing. A swift flick of the neck could catch them unawares. Its small head limited the size of what it could eat.

Elasmosaurus spent all its time in the water, often cruising coastal waters for shoals of fish. It would occasionally dive down to the seabed in shallow areas to find rounded pebbles. In its stomach, these aided digestion and provided ballast.
Elasmosaurus travelled long distances to find mating and breeding grounds. There is evidence it may have given birth to live young which it reared until they were old enough to look after themselves.