A View from the Other Side of the World: Middle Triassic Ichthyosaurs of Nevada and Their Implications for the Initial Phase of the Mesozoic Marine Revolution in the Eastern Pacific Realm

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The Early and Middle Triassic represent crucial time periods for our understanding of the organization and evolution of ecosystem because it represents the time the modern ecosystem structure was established after the most devastating extinction event in Earth history at the end of the Permian. In the marine realm, secondarily aquatic reptiles, in particular ichthyosaurs and sauropterygians, radiated quickly and are considered key elements of the so called Mesozoic Marine Revolution (MMR) during which a reorganization of marine trophic webs towards their modern structure took place. While much data has been gathered on the shallow-water environments of the Tethys realm, comparatively little is known on the open water fauna of the Pacific realm, leaving a significant gap in our knowledge of this crucial phase of vertebrate evolution. Here we seek to address this and gap in order to gain a comprehensive picture of the faunal differences between the Tethys and Pacific realms. This forms the foundation for a true understanding of the recovery and structuring of marine ecosystems after the P/T extinction events and the associated biogeographic patterns. Our work involves several approaches, field- and collections-based. We propose to systematically prospect the unexplored fossiliferous Middle Triassic Fossil Hill Member outcrops in the Augusta Mountains in Central Nevada which promise to yield further highly relevant finds of marine reptiles of the open water environment in the Pacific realm. Previous field seasons have yielded well-preserved skeletons of a medium-sized pregnant ichthyosaur as well as the anterior third of the skeleton of a giant ichthyosaur (skull length 1.95 m). This spectacular specimen differs from Thalattoarchon, the first macropredator among secondarily aquatic tetrapods, in the even larger size and particularly the piscivorous dentition. In combination with a reevaluation of mixosaurid and cymbospondylid ichthyosaurs from the Fossil Hill Member of both the classical Humboldt Range and the Augusta Mountains, anatomical description of these new specimens will provide important data for understanding morphological disparity and reproductive strategies in early ichthyosaurians. A phylogenetic analysis will provide the foundation for a detailed discussion of early ichthyosaurian evolution as well as further analyses of the biogeography and ecosystem structures. Systematic work will be followed by an analysis of trophic relationships because these play an integral role in the establishment of the ecosystem and the radiation of marine reptiles. The project will integrate closely with another project already funded by the DFG, the investigation of the ammonoid fauna and biostratigraphy of the Fossil Hill Member.