Re-evaluating body size in the Middle Jurassic pliosaur Liopleurodon ferox Sauvage, 1873

Liopleurodon ferox is a macropredatory pliosaurid plesiosaur from the Callovian (Middle Jurassic) of Europe. Several near-complete skeletons are known from the Peterborough Member of the Oxford Clay Formation of eastern England and approximately equivalent strata in north France. Disparate total lengths between 5.7 and 25 m have been reported for L. ferox, influencing paleoecological interpretations and length estimates for large pliosaurs. An updated model of Liopleurodon proportions and measurements is presented here, reconstructed from a near-complete skull and two well-preserved specimens with extensive postcranial remains. The skull comprises a larger proportion of the total skeletal length (24%) than a previously reported skull:body ratio of 1:5, and greater than the skull:body ratio of sympatric pliosaurs, indicating an ability to capture relatively larger prey. Intervertebral spacing (set at 10% based on other well-preserved plesiosaurians) was included in the final length determinations. By isometrically scaling a dataset of 37 specimens attributed to L. ferox, a total length range of 2.46–8.09 m is calculated. Liopleurodon is the largest aquatic tetrapod and apex predator of the Oxford Clay Formation paleoecosystem. Applying the new model to a range of indeterminate Oxford Clay pliosaurids as well as some Late Jurassic taxa, pliosaurs are calculated to reach total lengths up to 12.2 m, evolving gigantism by the Middle Jurassic and maintaining their position as apex predators for ∼80 my until they were superseded by mosasaurs in the late Turonian of the Upper Cretaceous.