Giant Late Triassic ichthyosaurs from the Kössen Formation of the Swiss Alps and their paleobiological implications

ABSTRACT

The Late Triassic was populated by the largest ichthyosaurs known to date, reaching lengths of over 20 m. Recent discoveries include the remains of giant ichthyosaurs from the Austroalpine nappes of the eastern Swiss Alps. The finds come from the lower two members of the Kössen Formation (late Norian to Rhaetian). The material consists of a very large tooth lacking most of the crown from the Rhaetian Schesaplana Member, a postcranial bone association of one very large vertebra and ten rib fragments also from the Schesaplana Member, and an association of seven very large vertebral centra from the upper Norian to lower Rhaetian Alplihorn Member. These associations represent the only published partial skeletons of large to giant ichthyosaurs younger than middle Norian. We compare the material with the two largest ichthyosaurs known from partial skeletons, Shonisaurus popularis (15 m) and Shastasaurus sikkanniensis (21 m) from the late Carnian (ca. 230 Ma) of Nevada and the middle Norian (ca. 218 Ma) of British Columbia, respectively. The incomplete tooth confirms that at least some giant ichthyosaurs had teeth. Based on their proportional differences, the two bone associations may represent two different taxa of Shastasaurus-like ichthyosaurs. The larger and geologically younger specimen may have been nearly the size of S. sikkanniensis, and the smaller that of S. popularis. These giant ichthyosaurs from the eastern Swiss Alps indicate that such ichthyosaurs also colonized the western Tethys. The finds also unequivocally document that giant ichthyosaurs persisted to the latest Triassic.

ACKNOWLEDGMENTS

We would like to thank the former director of the Paläontologisches Institut und Museum, Universität Zürich (PIMUZ), H. Rieber, for his great support. U. Oberli and E. Suter (PIMUZ) are thanked for the careful preparation of many specimens. U. Oberli discovered, prepared, and photographed PIMUZ A/III 4625. A. Rohrbach and D. Wurster discovered two important specimens during their geological fieldwork, and the Bündner Naturmuseum Chur (former director J.-P. Müller) gave the permission to extract the fossils. H. Lanz and R. Roth (PIMUZ) took some photographs, and G. Aguirre helped with modifications of figures. We would like to express our appreciation to G. Oleschinski (University of Bonn) for taking photographs of some specimens while they were on loan in Bonn. We would also like to thank N. Froitzheim (University of Bonn) for assisting in understanding the deformation process in the vertebrae. We thank X. Donhauser (University of Bonn) for his help in reconstructing the field arrangement of the vertebrae of PIMUZ A/III 1470. N. Klein (University of Bonn) is thanked for taking numerous photographs of the BISP material at NSM. We are truly grateful for the expert reviews of the manuscript by E. Maxwell and D. Lomax which improved our manuscript greatly.