Early-Triassic Nevadan Ammonite Assemblages: Key Indicators of Marine Biotic Recovery After the End Permian Mass Extinction

Document Type

Student Presentation

Presentation Date



College of Arts and Sciences


Department of Geosciences

Faculty Sponsor

Dr. Mark Schmitz


The end-Permian mass extinction occurred approximately 252 million years ago, and is the most devastating mass extinction in earth history. An estimated 80-96% of marine faunal species went extinct. Ammonites, a mollusk preferentially preserved in the fossil record, evolved rapidly through the Permian and into the early Triassic, making them useful indicators of post-extinction marine fauna repopulation.

A series of rapid, significant carbon cycle perturbations and climatic oscillations in the early Triassic hindered biotic recovery. Ammonites were especially vulnerable to these cycles, and the cyclical increase and decrease in ammonoid diversity has been used to understand and quantify marine faunal response to these chaotic post-extinction conditions.

In this study, we measured uranium-lead decay in the mineral zircon to radioisotopically date a volcanic formation located in northwestern Nevada that contain ammonites used as a global marker for marine biotic recovery. This marks the first time that an early-Triassic Nevadan assemblage has been radioisotopically calibrated to an absolute timescale.

Creating a robust numerical timeline focused on the tempo of ammonite migration and evolution within the early Triassic recovery interval is necessary to add resolution to the biotic recovery timeline, and allow its correlation to tumultuous environmental conditions.

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