Discriminating terrestrial, semiaquatic and fully aquatic life styles: The unresolved question of Triassic marine archosauromorphs.
Final Report Abstract
The project aimed to describe and name a new fossil find from a Middle Triassic locality in the Augusta Mountains (Nevada, USA). The locality is famous for its diverse ichthyosaur fauna, and together with numerous ammonoids and typical black shale sediments, this area represented in the Middle Triassic an open marine environment, far away from the palaeocoast. The new find was, even when largely unprepared, identified as an archosaur, a group of terrestrial living tetrapod’s. However, although regarded as solely terrestrial, archosaurs are in the Middle Triassic regularly and globally found in marine sediments, but their skeletons do not show obvious secondary aquatic adaptation as that of marine reptiles such as ichthyosaurs and sauropterygians do, leaving the life style of those archosaurs questionable. Thus, as a second objective, the project aimed to clarify this question by establishing mathematical models (i.e., linear discriminant models) that should be able to classify life styles in diapsids. The new fossil was named Benggwigwishingasuchus eremacarminis. Phylogenetic analyses identified it as a member of an expanded Poposauroidea, a group of advanced pseudosuchian archosaurs. The histology, microanatomy, growth rates and growth mark record had been studied in a humeral midshaft section of the specimen. Unexpectedly, neither the morphology nor the histology or microanatomy of Benggwigwishingasuchus shows any secondary aquatic adaptations and Benggwigwishingasuchus was clearly a primarily terrestrial living animal. For an advanced pseudosuchian, it had remarkable low growth rates and the presence of numerous growth marks indicate an advanced ontogenetic stage despite its small body size (~1.5 m reconstructed total body length). For the second objective, a dataset was compiled that consists of 112 specimens of modern and fossil Diapsida, each represented by 13 characters calculated from various skeletal measurements. The life style, classified as terrestrial, semiaquatic, shallowmarine/freshwater, pelagic/fully marine, was a priori assigned to each specimen. The lifestyle of about 75% of all 112 specimens was then correctly predicted by established linear discriminant models when using all 13, and five or four of these characters. When the models made a wrong prediction, the life style was a priori predicted wrongly by us (e.g. for the “marine” iguana), the taxon was in a transformation stage between two life styles (a very important result!) or the taxon (e.g., varanids, crocodiles) was a generalist and could cope with different life styles/environments. In a next step, we tested six Triassic archosaurs with a questionable life style with our models and revealed clear results for their life styles. Thus, our resulting models are informative tools to predict life style in extinct taxa and following up studies are planned.
Publications
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A new pseudosuchian from the Favret Formation of Nevada reveals that archosauriforms occupied coastal regions globally during the Middle Triassic. Biology Letters, 20(7).
Smith, Nathan D.; Klein, Nicole; Sander, P. Martin & Schmitz, Lars
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Diverse growth rates in Triassic archosaurs—insights from a small terrestrial Middle Triassic pseudosuchian. The Science of Nature, 111(4).
Klein, Nicole
