Meyerasaurus

Abstract: The exquisitely preserved holotype of the plio- saur 'Rhomaleosaurus' victor (SMNS 12478) is described from the Toarcian Posidonien-Schiefer (Upper Lias, Lower Juras- sic) of Holzmaden (Baden-Wurttemberg), Germany. The specimen presents a novel combination of synapomorphies and unique morphometric proportions separating it from Rhomaleosaurus sensu stricto and warranting the erection of a new genus, Meyerasaurus gen. nov. Historically, the name 'Thaumatosaurus' has been interchangeable with Rhomaleo- saurus and is frequently associated with SMNS 12478 in the literature. However, this is an invalid taxon and cannot be reinstated. The anatomy of Meyerasaurus victor is compared in detail with other pliosaurs, and its taxonomic affinity is reviewed. M. victor belongs to the family Rhomaleosauridae and shares several anatomical characters with Rhomaleosaurus including a short and robust premaxillary rostrum (length- to-width ratio c. 1.0), parallel premaxilla-maxilla sutures anterior to the nares, vomers contacting the maxillae poster- ior to the internal nares, and c. 28 cervical vertebrae minus the atlas-axis. The known geographical distribution of Rhomaleosaurus, which previously extended across the Ger- man and English palaeobiogeographical zones, is reduced to the English zone as a consequence of the referral of SMNS 12478 to a new genus. This is significant because it contrib- utes to an ongoing trend of increasing generic separation between the German and English zones, while increasing the generic diversity within the German zone itself.