Plesiosauria is an order of Mesozoic marine reptiles. Plesiosaurs first appeared in the Early Jurassic Period and became especially common during the Jurassic Period, thriving until the Cretaceous-Paleogene extinction at the end of the Cretaceous Period. The typical plesiosaur had a broad body and a short tail. They retained their ancestral two pairs of limbs, which evolved into large flippers. All plesiosaurs had four paddle-shaped flipper limbs. This is an unusual arrangement in aquatic animals and it is thought that they were used to propel the animal through the water by a combination of rowing movements and up-and-down movements. There appears to have been no tail fin and the tail was most likely used for helping in directional control. They have been discovered with fossils of belemnites (squid-like animals), and ammonites (giant nautilus-like molluscs) associated with their stomachs. They had powerful jaws, probably strong enough to bite through the hard shells of their prey.