Pliosaur

The pliosaurs were a group of large marine plesiosaur reptiles. They lived in the Mesozoic era (later Jurassic period). They had short necks and large heads, with large hind flippers. Their sizes ranged from two to 15 metres. They were predators of large fish and other marine reptiles. 
The first specimen was found by Mary Anning, on the Jurassic Coast of Lyme Regis, England, in the 1820s. Many of her finds are on display in the Natural History Museum, London.
Their body shape suggests they swam and ate under water.p29 It was suggested by D.M.S. Watson that plesiosaurs fell into two groups. The pliosaurs were large predators which ate large prey in deep water. The other plesiosaurs were long-necked with smaller heads. Their method was as surface swimmers, mostly eating with their head above water, darting down to snatch smaller fish which were feeding on plankton.