EARLY TRIASSIC NEVADAN AMMONITE ASSEMBLAGES: NEW U-PB ZIRCON AGES FOR KEY INDICATORS OF MARINE BIOTIC RECOVERY AFTER THE PERMIAN-TRIASSIC MASS EXTINCTION

The Permian-Triassic mass extinction, the most devastating mass extinction event in Earth history, was followed by a series of rapid, significant carbon cycle perturbations and climatic oscillations during the Early Triassic. Consequently, species diversity increased and decreased episodically. Ammonites recovered more rapidly than most biota, making them key indicators of marine faunal repopulation. Apparently rapid changes in ammonoid diversity have been used to understand and quantify marine faunal response to these chaotic post-extinction conditions. However, whereas the regional biochronological contexts of these ammonite assemblages have been well studied, there are sparse independent radioisotopic ages of ammonite assemblages to test the global correlation and synchrony of these Early Triassic recovery dynamics.

To address this paucity of radioisotopic dates, we used high precision U-Pb zircon geochronology to date four samples from the upper Weaver Rhyolite of the Koipato Group, located in the Humboldt Range, northeastern Nevada. The Koipato Group is mainly the product of a brief, high-volume pulse of bimodal magmatism during the latest Permian and earliest Triassic. Clastic interbeds from the upper portion of the Weaver Rhyolite contain the late Olenekian ammonite genus Procolumbites, which is the defining genus for a globally significant ammonite biochronozone used to gauge ammonite recovery after the Permian-Triassic extinction. This is the first time that an Early Triassic Nevadan ammonite genus has been radioisotopically calibrated to a numerical timescale.

Our U-Pb zircon ages of ash beds locally intercalcated with Procolumbites fossils are robust and reproducible, ranging from 248.53 ± 0.03 to 248.71 ± 0.04 Ma. Other work has shown that early Triassic ammonite biochronozones were globally synchronous through time; these age constraints thus establish a global chronostratigraphic context for Procolumbites occurrences, which has implications for timescale calibration and evolutionary studies. Specifically, these results indicate that ammonite recovery following the Permian-Triassic mass extinction was more protracted than previously understood.