The Anisian (Middle Triassic) ammonoids of Nevada - An integrated approach to understand morphological change
Final Report Abstract
The overarching achievement of this project is that we broadened our understanding of evolutionary and developmental processes of ammonoids in relation to their environment. Ammonoid diversity but also disparity is controlled by both ‘internal’ biological processes (e.g., developmental flexibility, intraspecific variability, heterochronic processes) and ‘external’ environmental factors controlling habitat space and geographic distributions, e.g., sea level cycles, tectonic shifts, oceanographic conditions, and climate change. Therefore, a synthetic and quantitative approach to the macroevolution and paleobiogeography of ammonoids is essential to understand the evolutionary dynamics of this highly remarkable group of animals. The Fossil Hill Member is characterised by drastic facies changes at the base and top that were known for a long time. However, the succession within the Member was not known in detail and our carbonate microfacies and geochemical studies revealed a clear difference in facies between both working areas and underlined that the Fossil Hill Member at the Augusta Mountains is a different and discontinuous sedimentary body than that of the Humboldt Range at Fossil Hill. Our detailed field-work found evidence for volcaniclastic deposits at Fossil Hill and a less continuous and complete succession with some minor paleoenvironmental shifts. Nevertheless, no major paleoenvironmental shifts were detected and the investigated sequences proof of a generally stable, but nevertheless hostile paleoenvironment of the Middle Triassic ecosystem with dysoxic to anoxic bottom-waters. Small-scale environmental changes most likely occurred in relation to shortterm oxygenation of the ocean floor. The occurrence of abundant and partially well-preserved bivalve assemblages proves the – at least temporary – dense occupation of the habitat. We could not identify a dominating environmental factor driving the evolution of the studied ammonoids in one direction. The observed evolutionary trends within Ceratitidae seem to be driven by intrinsic factors rather than by adaptation to the few small-scale paleoenvironmental changes. Within the family Ceratitidae, paedomorphosis seems to be more common than peramorphosis. The biologic meaning of the observed evolutionary pattern currently remains elusive. Originally it was assumed that the high taxonomic diversity was highly influenced by taxonomic oversplitting. However, the statistical evaluation of the ontogenetic analysis proved that ammonoid diversity in the Anisian of NV Nevada seems not to be unreasonably inflated. The original idea was to consider paleopathologies of ammonoids to estimate the predatory pressure and to identify possible enemies. Although quite many ammonoids from the Lower Triassic show pathologies, surprisingly only a very low number of our Anisian specimens shows any traces and thus it was impossible to us this as a proxy. In respect to specimen quantity, it was surprising to see that despite of an incredibly large amount of material the specimens that can be potentially used for statistical investigations and particularly cross-sectioning are very limited. Last but not least we were aware of encountering traces of volcanic activity in this setting at an active margin of the North American continent during the Triassic. However, it was a surprise that we were able to date a peak interval of this activity in the Fossil Hill section quite exactly by biostratigraphy.
Publications
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2017. Ceratiten aus Bayern und Österreich, in: Scheer, U., Stottrop, U. (Eds.), Erdgeschichten. Geologie im Ruhr Museum. Verlag der Buchhandlung Walther König, Essen, pp. 142-143
Lehmann, J.
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2020. Ontogenetic analysis of Anisian (Middle Triassic) ptychitid ammonoids from Nevada, USA. - Journal of Paleontology 94, 829-851
Bischof, E.A., Lehmann, J.
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2021. Ontogeny of highly variable ceratitid ammonoids from the Anisian (Middle Triassic). - PeerJ(9:e10931), 1-28
Bischof, E.A., Schlüter, N., Korn, D., Lehmann, J.
