THE LAST TRIASSIC GIANT? A LATE RHAETIAN ICHTHYOSAUR FROM NEW YORK CANYON, NEVADA, USA

Ichthyosaurs achieved their maximum size during the Late Triassic Period. The largest described genus, Shonisaurus, appeared in the latest Carnian and persisted into the middle Norian, though the exact timing of its extinction is not completely clear. In Europe, recently described material demonstrates that giant ichthyosaurs continued into the Rhaetian, and suggest that at least in the Tethys, large-bodied ichthyosaurs may have persisted until the end-Triassic extinction (ETE). This material implies that the absence of late Norian and Rhaetian occurrences in North America may result from sampling bias rather than a true ecological signal. New York Canyon (NYC) in the Gabbs Valley Range of Nevada, U.S.A., presents an excellent location for answering questions on the possible last occurrence of giant ichthyosaurs in Panthalassa because it is a well-studied fossiliferous marine reference section for the ETE and Triassic-Jurassic boundary, and Shonisaurus is already known from the stratigraphically lower Luning Formation in the nearby Pilot Mountains to the south and Shoshone Mountains to the north. Previous studies have reported isolated ichthyosaur elements from NYC, but these have never been adequately studied or described. Here we report new in situ vertebrate fossils from the late Rhaetian of the Gabbs Formation in NYC. The specimen comprises at least six articulated ribs from a giant ichthyosaur, comparable in size and shape to Shonisaurus. The bone-bearing horizon is just 2-4 meters below the negative δ¹³C_org excursion and Hg anomaly that marks the beginning of the ETE in this section. This specimen indicates that giant ichthyosaurs did not go extinct during the Norian in Panthalassa. Instead, they persisted until the ETE, perishing as a casualty of the mass extinction event.