A new species of Kimmeridgian pliosaur (Reptilia; Sauropterygia) and its bearing on the nomenclature of Liopleurodon macromerus

‘Tarloʹs Contention’, that a mandible possessing five to six large caniniform teeth characterizes Pliosaurus macromerus is explored and shown to rest on the assumption that only two morphologies of pliosaur mandibular symphysis exist in the Kimmeridge Clay Formation. A new taxon, Pliosaurus portentificus sp. nov., is described based on a third mandibular symphysial morphology from the Kimmeridge Clay near Ely, Cambridgeshire, UK. This specimen invalidates ‘Tarloʹs Contention’, which was the basis for reassigning Stretosaurus macromerus (formerly Pliosaurus macromerus) to the genus Liopleurodon. All material currently assigned to Liopleurodon macromerus should revert to the original combination of Pliosaurus macromerusPhillips, 1871.