Franziska Großmann

PhD Thesis

Geowissenschaftlichen Fakultät der Eberhard-Karls-Universität Tübingen

Dekan: Prof. Dr. P. Grathwohl 1. Berichterstatter: Prof. Dr. W. E. Reif 2. Berichterstatter: Prof. Dr. J. H. Nebelsick

PhD

135

The plesiosauroids from the Posidonia shale (Toarcian, Lower Jurassic) of Holzmaden (south west Germany) were examined in detail, their taxonomic and phylogenetic position was analysed and revised and their palaeoecology studied. Ten complete specimens were studied, including the type specimens of Plesiosaurus guilelmiimperatoris Dames, 1895, Plesiosaurus brachypterygius v. Huene, 1923 and Plesiopterys wildii O�Keefe, 2004, as well as a described specimen of P. guilelmiimperatoris (Fraas 1910). Where possible, the six undescribed specimens were assigned to one of the existing species, on the basis of morphological characters and a statistical analysis. P. brachypterygius and P. guilelmiimperatoris differ sufficiently from the type species of the genus Plesiosaurus, Plesiosaurus dolichodeirus, and from each other to be assigned to separate genera. The genus Seeleyosaurus White, 1940 was re-established for P. guilelmiimperatoris, and the new genus Hydrorion was introduced for P. brachypterygius. Plesiopterys wildii turned out to be a junior synonym of P. guilelmiimperatoris. A phylogenetic analysis placed both German genera within the family Elasmosauridae. Seeleyosaurus guilelmiimperatoris resembles strongly the English genus Muraenosaurus. Hydrorion brachypterygius forms a monophyletic group with Microcleidus and Occitanosaurus, which is the sister taxon to the cretaceous elasmosaurs. The cranial musculature of both German taxa was reconstructed and analysed. It could be shown that Hydrorion was able to exert higher biting forces than Seeleyosaurus, whereas the latter could close his jaws faster. A comparison of the flipper morphology indicated that Hydrorion was a highly manoeuvrable swimmer. In contrast to this, Seeleyosaurus was better adapted for stamina and speed. These qualities were compared to the possible prey from the Posidonia shale. Hydrorion probably specialised on the swarm-fish Leptolepis, whereas Seeleyosaurus is more likely to have caught coleoids (squid and cuttlefish). The comparison of Liassic plesiosaurs from Europe showed that palaeobiogeographic zonation was present at species and genus level.

English