Liopleurodon is a large Pliosaurid from the Jurassic sea's of Europe. three species have been named. L. ferox, the Type species and L. pachydeirus lived during the Callovian stage of the Middle Jurassic Period (c. 160 to 155 mya), while the third, L. rossicus, lived during the Late Jurassic.
Estimating the maximum size of Liopleurodon has become a controversial subject. The palaeontologist L. B. Tarlo derived that the total body length of a pliosaur (including Liopleurodon) can be estimated from skull length, in which the skull is approximately one seventh of the entire body. The largest known skull belongs to L. ferox (1.5 metres in length), and according to Tarlo's estimation, this individual would be about 11 m (38 ft) long. However, as with its relative Kronosaurus, there is some uncertainty as to whether Tarlo's estimates are correct.
Recent studies on pliosaurs have cast doubt on Tarlo's size estimates, and indicate that pliosaur skulls were about one-fifth of the total body length. Hence, the average size of the L. ferox would have ranged from 7 metres (23 ft) to 10 metres (33 ft) long.
Pliosaur remains excavated from Kimmeridge Clay Formation of England indicate a much larger taxon, possibly up to 15 metres (49.2 feet long), existed, however they have not been identified as belonging to Liopleurodon. A mandible on display in the Oxford University Museum of Natural History estimated over 3 metres (preserved 2.875m) was at one time classified as Liopleurodon macromerus. When the mandible was described, it was originally assigned to Stretosaurus (as Stretosaurus macromerus). The genus Stretosaurus later became a junior synonym of Liopleurodon. However, it has been re-classified as Pliosaurus macromerus.
The discovery of a very large pliosaur was announced in 2002, from Mexico, nicknamed the 'Monster of Aramberri'. A cautious estimate placed this juvenile at about 15 metres (49.2 ft) long. It was widely reported belonging to Liopleurodon, however no taxonomic conclusions could be made due to poor preservation and fact that the remains were of a partial vertebral column (non-diagnostic). The specimen was dated to the Kimmeridgian age of the La Caja Formation.