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