Michael W. Maisch

Martin R�cklin

Academic paper

Palaeontology

43

1

29-40

An osteological re-study of the holotype skull of Plesiosaurus brachypterygius leads to a correction of several inaccuracies and misidentifications of the original description by von Huene and reveals additional facts on the cranial anatomy of this plesiosaur. Comparison with the type species, Plesiosaurus dolichodeirus, from the Lower Lias of England shows considerable differences in many parts of the cranial skeleton, even though the two species are generally so similar that their inclusion in the same genus proposed by von Huene, and agreed with by all subsequent authors, can be supported. Plesiosaurus brachypterygius is provisionally considered as a valid species distinguishable from the contemporaneous Plesiosaurus guilelmiimperatoris by its much shorter limbs, a difference that cannot be explained by ontogenetic variation as proposed by Storrs. Plesiosaurus brachypterygius was probably an ichthyophagous form that occurred rarely in the Posidonienschiefer fauna. THE Posidonienschiefer of south-western Germany has been known for more than 200 years for its exquisitely preserved marine vertebrates from the Lower Toarcian (Lower Jurassic), which have been the focus of numerous studies. Best known of the fossil vertebrates of this formation are the ichthyosaurs, which are both most numerous and best preserved. Rhamphorhynchoid pterosaurs, and teleosaurid and metriorhynchid crocodiles are less common. Sphenodontians and dinosaurs are rarities that have received considerable interest. In contrast to this, one group of tetrapods in the Posidonienschiefer has been almost completely neglected in past decades: the plesiosaurs. These are not as common as the ichthyosaurs nor as spectacular as the pterosaurs, but there is a considerable number of almost perfectly preserved and excellently prepared skeletons. Most of these are still undescribed and form a scientific treasure that might aid considerably in understanding the origins, phylogeny, morphology and ecology of the group as a whole. The present paper is concerned exclusively with a re-description of the holotype skull of Plesiosaurus brachypterygius von Huene, 1923, a taxon based on a complete and very well-preserved skeleton in the Institut und Museum fu¨r Geologie und Pala¨ontologie der Universita¨t Tu¨bingen (GPIT). We studied this skull about two years ago and recognized several inaccuracies in the original description. The recent thorough re-evaluation of the type-species of Plesiosaurus, P. dolichodeirus Conybeare, 1824 by Storrs (1997) led to the decision to publish the results of our investigations to provide an improved basis of knowledge on this important Upper Liassic plesiosaur species.

English