The taxonomic and phylogenetic position of the Plesiosauroidea from the Lower Jurassic Posidonia shale of south-west Germany

摘要

The two plesiosauroid species from the Posidonia shale of Holzmaden, 'Plesiosaurus' guilelmiimperatoris and 'Plesiosaurus' brachypterygius, do not belong to Plesiosaurus but form new monotypic genera. The new genus Hydrorion is erected for 'P.' brachypterygius, and the genus Seeleyosaurus is re-established for 'P.' guilelmiimperatoris. The recently described species Plesiopterys wildii is regarded as a junior synonym of S. guilelmiimperatoris. A short phylogenetic analysis shows that S. guilelmiimperatoris and Muraenosaurus are basal elasmosaurs. H. brachypterygius, Occitanosaurus tournemirensis from France and Microcleidus homalospondylus from England form a monophyletic clade, which is the sister taxon to the Cretaceous elasmosaurs. A palaeobiogeographical comparison of plesiosaur localities in the Lower Jurassic shows distinct palaeobiogeographical zones for the Toarcian, with different plesiosaur taxa in England, Germany and France.