Discovering the last Triassic giant: Insights from the marine Late Triassic of Nevada and a new ichthyosaur locality from New York Canyon, Nevada, USA.

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McGaughey, Gary

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2022

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Extinction , Ichthyosaur , Nevada , Shonisaurus , Triassic

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Ichthyosaurs are a large group of marine reptiles that first appeared in the Early Triassic Period and continued to be an integral part of marine ecosystems into the Middle Cretaceous Period. At the end of the Triassic, ichthyosaurs were devastated by the end-Triassic Extinction, and never again reached the giant scale or diversity of forms seen in the Triassic. The lead up to this extinction is riddled with gaps in the fossil record. The record of ichthyosaurs in Nevada ranges from the early Triassic to the first appearance of Shonisaurus in the latest Carnian, but the rocks of the latest Triassic have been largely underexplored. The Pilot and Cedar Mountains, and the Garfield Hills to the South of Berlin-Ichthyosaur State Park were surveyed for vertebrates to further explore the Triassic marine rocks of Nevada. In the Pilot Mountains, nearly 30 new fossil localities were located within the lower member of the Luning Formation, which is assigned an early Norian age. Most of the material is likely referable to Shonisaurus and consists of partly to completely disarticulated vertebrae, rib, girdle, and limb elements, with rarer cranial material. Isolated bones from the upper member of the Luning Formation show potential for further discoveries into the middle Norian. Outside of the Pilot Mountains, additional Shonisaurus material was located within Luning Formation exposures in the Cedar Mountains near previously reported localities. Reconnaissance in the Garfield Hills found no vertebrate material in the Late Triassic rocks exposed there. Moreover, a recent discovery of in-situ vertebrate fossils from the latest Rhaetian in New York Canyon (NYC) in the Gabbs Valley Range of Nevada, provides clear evidence of the persistence of giant ichthyosaurs into the latest Rhaetian. Previous studies have reported isolated ichthyosaur elements from NYC, but these have never been adequately studied or described. The specimen comprises at least 17 semi-articulated ribs and two centra from a giant ichthyosaur, comparable in size and shape to the largest known examples of Shonisaurus. Strong controls on ammonite biostratigraphy and organic carbon isotope geochemistry confirm that this specimen is the youngest shastasaurid ichthyosaur reported to date and indicates that these giant ichthyosaurs did not go extinct during the Norian. Instead, they persisted until the end-Triassic extinction, likely perishing as a casualty of the mass extinction event.

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Creative Commons Attribution 4.0 United States

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