A new Cretaceous Pliosaurid (Reptilia, Plesiosauria) from the Carlile Shale (middle Turonian) of Russell County, Kansas

Abstract

ABSTRACT The Eulert pliosaurid remains (FHSM VP-321) housed at the Sternberg Museum of Natural History (Kansas, U.S.A.) include one of the world's best examples of a Cretaceous pliosaurid plesiosaur skull. The specimen's original assignment to Brachauchenius lucasi was based solely upon the skull (dorsal surface) and left lower jaw (lateral view) because the specimen was embedded in a plaster mount. The history of B. lucasi is similarly problematic, because the type and a referred skull were formerly visible only in ventral and dorsal views, respectively. Further preparation and comparison of these specimens reveal new data about the arrangement of cranial elements. The Eulert pliosaurid bears several distinct autapomorphies as compared with B. lucasi, including cranial proportions (pretemporal length of palate longer, shorter temporal fenestrae), configuration of skull roof elements (frontals participate in premaxilla-parietal suture, suture occurs further forward), and configuration of the palate (posterior vomers not masked by medial alar extensions of the palatines, caudal vomerian fenestrae positioned further posterior, long slit-like anterior pteryoid vacuity present). Furthermore, FHSM VP-321 possesses double-headed cervical ribs, a feature previously unknown in Cretaceous pliosaurids. This combination of characters merits separation of the Eulert pliosaurid and a referred specimen to a new taxon, Megacephalosaurus eulerti. The type and paratype skulls of M. eulerti are 1.5 m and 1.75 m in length, respectively, and thus 50% and 75% larger than the known 1-m-long skulls of B. lucasi, suggesting that M. eulerti may attain larger size than B. lucasi.