A New Genus of Plesiosaur (Reptilia: Sauropterygia) from the Pliensbachian (Early Jurassic) of England, and a Phylogeny of the Plesiosauria

Two new species of plesiosaur have been recognised from the Pliensbachian (Early Jurassic) of England. They share derived characters and so are referred to the same new genus, Raptocleidus. Raptocleidus blakei is from Blockley Station Quarry, Gloucestershire (holotype specimen LEICT G1.2002) while Raptocleidus bondi is from the coast of Dorset (holotype specimen NHMUK R16330). The two new species share characters with different family-level groups of plesiosaur; the pliosaurids, leptocleidians and rhomaleosaurids. This makes classification of the new taxa problematic using purely comparative anatomical techniques. To further investigate their systematic position within the Plesiosauria, a phylogenetic analysis was performed. This is the largest analysis of plesiosaurian relationships ever attempted. The new species were found to be basal members of a paraphyletic assemblage which form successive sister groups to the Leptocleidia but which lie outside of this node-based taxon. The new stem-based taxon Leptocleidomorpha has been erected to encompass the new clade. The Leptocleidomorpha formed a novel sister group relationship with the Pliosauridae with the branching point situated at the very base of the Jurassic or earlier. The new clade Eupliosauria has been erected to encompass this new group. Finally two clades (Cryptoclidinae Williston, 1925 and Muraenosaurinae White, 1940) have been newly defined to describe diversity within the plesiosauroid clade Cryptoclididae. Additional specimens have been identified that may prove to be new taxa pending further work.