Plesiosaurian relationships have been subject to considerable debate at both higher and lower levels within the clade. Recent fieldwork in the Uppermost Jurassic of the Agardhfjellet Formation on central Spitsbergen has uncovered numerous ichthyosaurians and plesiosaurian remains, including three new plesiosauroids, one new pliosauroid referrable to Pliosaurus, and a previously described, re-classified taxon. The phylogenetic relationships of these five taxa were investigated based on data sets previously constructed for global plesiosaurian relationships. Two specimens from the British Kimmeridge Clay Formation and a Russian Volgian (uppermost Jurassic) taxon referrable to Pliosaurus were added to the data matrix to improve the phylogenetic resolution of this genus. The results yielded a tree topology closely conforming to the traditional plesiosauroid and pliosauroid dichotomy, nesting Leptocleidia within the latter. Pliosaurus forms a monophyletic clade containing all currently recognised species. The three new long-necked taxa form a monophyletic sister group to the Cretaceous Elasmosauridae. These results should, however, be considered preliminary pending the discovery of more complete cranial material and adult specimens.
Phylogenetic relationships of Upper Jurassic (Middle Volgian) plesiosaurians (Reptilia: Sauropterygia) from the Agardhfjellet Formation of central Spitsbergen, Norway