Discriminating terrestrial, semiaquatic and fully aquatic life styles: The unresolved question of Triassic marine archosauromorphs.

点击翻译按钮获取中文摘要

Because of an equivocal morphology and find situation (i.e. in marine sediments), the life style of some extinct archosauromorphs is controversially discussed. Archosauromorphs are generally considered terrestrial tetrapods but occur in Permian and surprisingly frequently in Triassic marine sediments of the western and eastern Tethyan realm as well as in the Pacific realm. Due to the presence of terrestrial and aquatic characters alike, the debate focusses around the question if these animals had been washed into the sea or were fully aquatic members of the respective marine faunas. Thus, besides a huge diversity of non-archosauromorph Triassic marine reptiles and a flourishing radiation of terrestrial archosauromorphs during the Triassic, the existence of unambiguous marine archosauromorphs is not clear. It is however, an unresolved but crucial question why such a successful group as the archosauromorphs should not have inhabited the marine environment, i.e. had been fully aquatic, with the exception of the Mesozoic crocodile lineage of Thalattosuchia. The archosauromorph protorosaur Dinocephalosaurus, the archosauriform Litorosuchus, and the archosaur Qianosuchus are all found in shallow marine sediments in the Middle Triassic of China. There is an agreement that these taxa have had at least a semiaquatic life style based on morphological characters but a fully marine life style is not clearly accepted for them. For other Protorosauria such as the Upper Permian Protorosaurus and the Middle Triassic Tanystropheus, both from shallow marine sediments, for the rauisuchian archosaur Ticinosuchus ferox from the Alpine Triassic of Italy as well as for Sikannisuchus husky from the fully marine (i.e. pelagic) Pardonet Formation of British Columbia, the life style is in general discussed controversially. A new find of an archosauriform from the most open marine (i.e. pelagic) sediments from the Middle Triassic of the Augusta Mountains (Nevada, USA), also shows an equivocal morphology in terms of its life style. This new find will be described in detail and its phylogenetic relationships will be tested in the course of this project. To solve the question of life style in archosauromorphs, a dataset based on morphological, histological, and microanatomical characteristics and combinations thereof of archosaurs with a clear life style will be compiled to classify characteristics of terrestrial, semiaquatic, and fully aquatic life styles (e.g. by linear discriminant analysis). Terrestrisuchus and dinosaurs are bench mark taxa for a terrestrial life style, modern crocodiles for the semiaquatic life style and thalattosuchians for the marine life style. The resulting discrimination model will be applied to fossil taxa with a questionable life style. However, once established the model can be applied to any taxon, which is good enough preserved to gather the necessary morphological, histological and microanatomical data.