Pliosaurus was a huge marine reptile of prehistoric times. Adults measured some 30-40 feet in length and could weigh over 15-30 tons. Their huge skulls measured some 6-10 feet in length on their own. Pliosaurus was a close cousin of the also huge Liopleurodon. Pliosaurus is so fearsome that some have referred to one species as "Predator X." Pliosaurus lived about 145-155 million years ago in the oceans around Europe.