Adam Stuart Smith

PhD Thesis

School of Biology and Environmental Science University College Dublin

Dr Gareth J. Dyke

PhD

Specimen NMING F8785, a large pliosaur from the Toarcian of Yorkshire, is the holotype of the genus Rhomaleosaurus and the family Rhomaleosauridae. The skull of this specimen was prepared, allowing a detailed description to be presented. A strong ectopterygoid boss in Rhomaleosaurus would have been covered by a cartilaginous sheath in life, and abutted against the medial wall of the mandible. The first ever, full-body reconstruction of a rhomaleosaurid plesiosaur, Rhomaleosaurus, shows that the body of this animal is dorso-ventrally flattened and that there is very little curvature along the vertebral column, except for the pectoral and anterior dorsal regions. Many aspects of the postcranial skeleton are robust and reinforced. There is a notable change in the proportions of the terminal caudal vertebrae in Rhomaleosaurus, associated with an irregular vertebra indicating the presence of a vertical caudal fin in this taxon. The anatomical data collected from this specimen, and a number of additional Lower Jurassic pliosaur specimens from the UK and Germany, is incorporated into the first detailed phylogenetic and morphometric analyses dedicated to pliosaurs. Based on the results of the cladistic analyses, the Pliosauroidea forms a monophyletic group in a sister relationship with Plesiosauroidea. Three groups are resolved within the Pliosauroidea: the Rhomaleosauridae, Leptocleidoidea and Pliosauridae. The genus Rhomaleosaurus contains three species, all from the Toarcian of the UK. These include NMING F8785, the holotype of the species Rhomaleosaurus cramptoni; BMNH R4853, the holotype of Rhomaleosaurus thorntoni; and YORYM G503, the holotype of Rhomaleosaurus zetlandicus. Specimen WM 852.S, the holotype of Rhomaleosaurus propinquus, is referred to Rhomaleosaurus cramptoni. �R�. megacephalus and �R�. victor do not belong to Rhomaleosaurus sensu stricto and they are removed from this genus. �R�. megacephalus is referred to Eurycleidus and �R�. victor represents a novel genus. Specimen LEICS G221.1851 was erected as the neotype of �R�. megacephalus by Cruickshank (1992b), but this neotype status is rejected because existing casts of the original holotype are valid. This specimen is here referred to Eurycleidus sp., A new species of Eurycleidus (diagnosed by an elongate mandibular symphysis) is introduced for specimen WARMS G10875 based on the cladistic analysis and morphometric analyses. The genera Macroplata and Archaeonectrus xxi are both valid monospecific taxa within the Rhomaleosauridae, diagnosed by a number of autapomorphies. The following pliosauroid taxa are also supported by the morphometric analysis and/or cladistic analysis: Attenborosaurus, Sthenarosaurus, Hauffiosaurus, �P� longirostris, and an unnamed taxon represented by specimen BMNH 49202. �Plesiosaurus� macrocephalus represents a juvenile rhomaleosaurid plesiosaur. Simolestes resolves as a derived pliosaurid rather than a derived rhomaleosaurid, indicating that a spatulate rostrum is a convergent character amongst pliosauroids. Maresaurus resolves as a rhomaleosaurid and therefore represents the youngest member of this clade. The descriptions and figures presented in this thesis represent the first detailed descriptions of many specimens and provide new information on the anatomy of rhomaleosaurid pliosaurs. Characters shared by many rhomaleosaurid taxa include a dorsal triangular flange of the maxilla, which extends between the orbit and the external naris; a large ectopterygoid that contacts the suborbital vacuity anteriorly; small semi-circular lateral palatine vacuities and large suborbital vacuities; plate-like developments below the basicranium incorporating squared lappets at the base of the quadrate-pterygoid flange; and a medial bump on the retroarticular process. A large dorso-median foramen situated between the external nares, is restricted to Rhomaleosaurus. The identification of gastroliths in Rhomaleosaurus confirms that this taxon ingested stones. In conclusion, this thesis provides a greater understanding of the anatomy, diversity and variation in Lower Jurassic pliosauroids and presents the first detailed systematic framework for the Rhomaleosauridae

English