LIOPLEURODON was the biggest plesiosaur. Liopleurodon was not a dinosaur, but a short-necked plesiosaur (a pliosaur), an extinct, swimming reptile. It lived during the late Jurassic period in Europe and Eastern Europe.
Plesiosaur - long necked marine reptile, steered itself with it's four paddles as it had no tail-fin - went extinct at the same time as the dinosaurs (c. million years ago). Rumored to be living in Loch Ness, Scotland!