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
Final Report Abstract
The Early and Middle Triassic represent crucial time periods for our understanding of the organization and evolution of marine ecosystems as these periods represent the time the modern ecosystem structure was established after the most devastating extinction event in Earth’s 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 Marine Mesozoic Revolution 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 about the open water faunas of the Pacific realm, leaving a significant gap in our knowledge of this crucial phase of vertebrate evolution. The focus of this project is this 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 event and the associated biogeographic patterns. Specifically, we aimed to systematically prospect the unexplored fossiliferous Middle Triassic Fossil Hill Member outcrops in the Augusta Mountains which promised to yield further highly relevant finds of marine reptiles of the open water environment in the Pacific realm. The fossils from this member are collectively known as the Fossil Hill Fauna (FHF). We described the well-preserved skeletons of a medium-sized gravid ichthyosaur as a new species of the genus Cymbospondylus, C. duelferi. The specimen is the second-oldest record of a gravid ichthyosaur and suggests that life birth in the clade evolved as an aquatic adaptation. The highlight of the project, and indeed of our decades-long research in the Middle Triassic of Nevada, was the description of a new giant ichthyosaur, again pertaining to the genus Cymbospondylus. The skull of this animal was nearly two meters in length, and we estimated total body length at 17.5 m and body mass at 45 metric tons. The find represents the first of the giant ichthyosaurs of the Triassic, and indeed the first giant animal to have evolved, predating giant dinosaurs by 40 million years and giant whales by 200 million years. Strikingly, gigantism in ichthyosaurs evolved much faster than in whales. Our analysis of the FHF ecosystem using energy flux modeling indicates that the ecosystem was stable and could even have supported another giant taxon. In conclusion, our work in Nevada results in a fundamentally different perspective of marine ecosystem recovery and marine amniote evolution after the end-Permian mass extinction than what was previously inferred from fossil deposits left behind by the Tethys ocean. The FHF inhabited the eastern equatorial realm of the World’s ocean, Panthalassa, with ichthyosaurs being the dominant elements. This clades must have radiated amazingly fast after its invasion of the seas in an unknown region in the earliest Triassic (there is no Permian record of the clade, whatsoever), dispersing around the globe almost instantaneously and diversifying into wide array of life styles and body sizes. The project resulted in several publications, one of which was a 14-page research article in Science. Furthermore, one master's thesis and one bachelor's thesis were written within the framework of the project. The published results, especially on Cymbospondylus youngorum, received extensive and first-rate coverage in all manner of media and social networks.
Publications
- (2018). Structural, functional, and physiological signals in ichthyosaur vertebral centrum microanatomy and histology. Geodiversitas, 40, 161-170
Houssaye, A., Nakajima, Y. & Sander, P. M.
(See online at https://doi.org/10.5252/geodiversitas2018v40a7) - (2020). A new cymbospondylid ichthyosaur (Ichthyosauria) from the Middle Triassic (Anisian) of Nevada, USA. Journal of Systematic Palaeontology, 18, 1167-1191
Klein, N., Schmitz, L., Wintrich, T. & Sander, P. M.
(See online at https://doi.org/10.1080/14772019.2020.1748132) - (2021). Early giant reveals faster evolution of large size in ichthyosaurs than in cetaceans. Science, 374, eabf5787 (15 pages, 97-page supplement)
Sander, P. M., Griebeler, E. M., Klein, N., Juarbe, J. V., Wintrich, T., Revell, L. J. & Schmitz, L.
(See online at https://doi.org/10.1126/science.abf5787) - (2021). Histology of Ichthyosauria. In Vertebrate Skeletal Histology and Paleohistology (ed. V. de Buffrénil, A. de Ricqlès, L. Zylberberg and K. Padian), pp. 458-466. CRC Press, Boca Raton
Sander, P. M.
(See online at https://doi.org/10.1201/9781351189590-24)
