The Taxonomic and Phylogenetic Position of the Plesiosauroidea From the Lower Jurassic Posidonia Shale of South‐west Germany

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.