Cranial Anatomy of the Lower Jurassic Pliosaur Rhomaleosaurus megacephalus (Stutchbury) (Reptilia: Plesiosauria)

Abstract

An account is given of the skull of a large pliosauroid plesiosaur from the lowermost Hettangian (Lower Lias; Lower Jurassic) of Barrow upon Soar, Leicestershire, identified as Rhomaleosaurus megacephalus (Stutchbury, 1846). It is proposed as the neotype of the species, as the holotype was destroyed in an air-raid on Bristol in November 1940. Details of the skull allow emendation of the diagnosis of the genus Rhomaleosaurus. Comparison of R. megacephalus with the Upper Liassic species, Rhomaleosaurus zetlandicus, shows that the former has a more gracile snout and a shallower lower jaw symphysis, and lacks squamosal-quadrate foramina. There may also be differences in the number and nature of the palatal grooves associated with presumed underwater olfaction. Lack of iron pyrites in the matrix surrounding the specimen allowed computed axial tomography (CAT)-scan sections to be obtained, which in association with the little-distorted nature of the skull, permitted a confident reconstruction of the skull. It shows a complete ring of circumorbital bones, and a suborbital fenestra. The braincase can be reconstructed from sagittal break-sections allied with CAT-scan sections. A stapes is identified. A poorly preserved dentition comprises conical, striated, teeth with caniniforms on each premaxilla and at the front of each maxilla. Although very similar to the later species, this skull is not so well adapted for apprehending and dismembering large prey, as is R. zetlandicus.