HUNTING FOR THE LATEST LARGE-BODIED ICHTHYOSAUR IN THE UPPER TRIASSIC LUNING FORMATION, MINERAL COUNTY, NEVADA, USA

The Upper Triassic Luning Formation of central Nevada hosts one of the world’s most significant concentrations of ichthyosaur fossils. The richest and best studied locality within this unit occurs at Berlin-Ichthyosaur State Park (BISP), Shoshone Mountains, Nye County, Nevada (NV) where the large-bodied ichthyosaur Shonisaurus popularis occurs in abundance, concentrated in the latest Carnian Klamathites macrolobatus Zone. An unusual feature of BISP is that, despite the abundance of ichthyosaur material, it appears to be monotaxic, in contrast with most other Triassic marine reptile assemblages. In addition to the famous BISP localities, ichthyosaur fossils are abundant in Luning Formation outcrops in the Pilot and Cedar mountains of Mineral County, NV, but are poorly studied.

We report the results of initial surveys of the Pilot and Cedar mountains, and the Garfield Hills to the South of BISP in Mineral County, NV. Several new localities in the Pilot Mountains were located within the lower member of the Luning Formation, which was deposited in reefal and inter-reefal environments, and is assigned an early Norian age (Stikinoceras kerri Zone). This depositional setting differs from BISP where ichthyosaur bones occur in a moderately deep setting below fair-weather wave base. Ichthyosaur material is abundant throughout this member along the western flank and northern reaches of the Pilot Mountains. 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. We were also able to locate Shonisaurus material in the Cedar Mountains near known UCMP localities. Reconnaissance in the Garfield Hills found no vertebrate material in the primarily Late Triassic rocks exposed there. The stratigraphy for these localities is not well understood due in part to the structurally complex nature of the area. Work will continue in both the Luning and overlying Norian-Rhaetian Gabbs Formation to test whether the upper limit of Shonisaurus popularis is indeed within the Norian, and to relate its demise to possible regional Norian extinction events.