Sea Monsters

Ichthyosaurs were great hunters with streamlined bodies, powerful tails and large eyes to see in the dark depths. They could swim up to 40km per hour.

Plesiosaurs swam by flapping all four of their large, wing-like flippers. It's very efficient but no other creature has ever developed this way of swimming.

Plesiosaurs may have had long necks so they could sneak up on schools of fish without frightening them with their large bodies. Their neck bones were locked together so they couldn't bend them very far. 

Mosasaurs ate anything: ammonites, fish, turtles, plesiosaurs, other mosasaurs and even sea birds. The only thing a mosasaur had to fear was a bigger mosasaur.