Osteology of Rhomaleosaurus Thorntoni (Sauropterygia: Rhomaleosauridae) from the Lower Jurassic (Toarcian) of Northamptonshire, England

ABSTRACT This paper describes the holotype of Rhomaleosaurus thorntoni (NHMUK PV R4853) from the upper part of the Whitby Mudstone Formation (Toarcian) of Kingsthorpe Hollow, Northamptonshire, England. Rhomaleosaurus thorntoni possesses the following autapomorphies: (1) dorsomedian foramen with mediolaterally narrow slit-like morphology situated well anterior to the external nares; (2) humerus long relative to skull length and body length; (3) distal end of humerus greatly expanded both preaxially and postaxially. Several other characters present in Rhomaleosaurus thorntoni cannot be determined in other Rhomaleosaurus species, so these may also be autapomorphies of R. thorntoni. A summary of previous phylogenetic analyses of Plesiosauria shows that the relationships among the three main clades (Rhomaleosauridae, Pliosauridae, Plesiosauroidea) are unclear and further investigation is required to understand the evolution of early plesiosaurians.