The biggest plesiosaur was known to be the Liopleurodon (pronunciation: LIE-oh-PLOOR-oh-don) which existed during the Jurassic period 165 -150 million years ago. They were not a dinosaur, but marine reptile that swam using their four paddle-like flippers. Like our modern day sea turtles, Liopleurodon were able to move a bit on land and may have laid eggs in nest that they dug into the sand.
Liopleurodon had large heads that were 3 meters long containing a very powerful jaw. It had a short neck and a pointed tail. They were meat eaters which caught their prey with its long, sharp teeth. Their main diet consists of fish, other swimming animal such as ichthyosaurs, and other plesiosaurs. They lived in the open oceans and breathed air. They can grow anywhere from 12-15 meters long and weight up to 150 tons.
Palaeontologists believed that Liopleurodon was very common as there were many well preserved fossils from several marine deposits throughout Europe, with main deposits found in the United Kingdom, France, and Germany.
The biggest plesiosaur was known to be the Liopleurodon (pronunciation: LIE-oh-PLOOR-oh-don) which existed during the Jurassic period 165 -150 million years ago. They were not a dinosaur, but marine reptile that swam using their four paddle-like flippers. Like our modern day sea turtles, Liopleurodon were able to move a bit on land and may have laid eggs in nest that they dug into the sand. 
Palaeontologists believed that Liopleurodon was very common as there were many well preserved fossils from several marine deposits throughout Europe, with main deposits found in the United Kingdom, France, and Germany.