A new and unusual pliosaurid from the Peterborough Member, Oxford Clay Formation (Callovian, Jurassic) of Cambridgeshire, UK, exhibits a type of pachyostosis and bone thickening of the rib cage and vertebrae. Pachycostasaurus dawni gen. et sp. nov. was most probably a benthic feeding carnivore with a generalist diet that included fish and invertebrates, and was a slow swimming underwater flyer with ventrally placed ballasting of the skeleton to resist roll. The apparent rarity of this pliosaur may indicate that it is allochthonous in the Oxford Clay biota.
From a palaeoecological viewpoint, the find is significant for two reasons. Firstly, as an air breathing animal that is thought to have fed on benthos or nektobenthos, Pachycostasaurus may have been important in transferring resources from the benthic food web to the surface food web. In a more general sense, the new find provides further insights into the possible trophic structure of aquatic animal communities during deposition of the Oxford Clay.