178-3 Environmental and Biotic Influences on Local Extinction Risk: A Case Study from the Late Neogene San Joaquin Seaway, Central-Southern California

Session: Paleontology, Diversity, Extinction, Origination

Presenting Author:

Authors:

Abstract:

Understanding the traits that influence extinction risk in the fossil record can provide valuable information for evaluating vulnerability of extant species in the face of accelerating global change. However, trait-risk relationships are strongly influenced by local environments and ecosystems, both of which vary across the range of a single species, and can be hard to discern at the global scale. Regional studies are needed to determine the degree to which determinants of extinction risk depend on local environment and biota. The San Joaquin Basin in central-southern California presents an opportunity to study the dynamics of extinction and extirpation in a geographically limited mesocosm. The San Joaquin Basin was flooded by the Pacific Ocean during the Miocene and Pliocene, creating an epeiric seaway known as the San Joaquin seaway. The environment of the seaway was dynamic, characterized by large changes in freshwater input from rivers draining the nearby Sierran highlands and from tectonically and eustatically-driven changes in marine connectivity. Closure of the seaway occurred between 2.3 and 2.0 Ma, transitioning to a non-marine basin. The stepwise restriction and ultimate demise of the San Joaquin Seaway is recorded by a thick sequence of highly fossiliferous sediments that are well-exposed in the Kettleman Hills on the western margin of the San Joaquin Valley. There are approximately 230 molluscan species with a fossil record in the Kettleman Hills, 60% of which are now extinct. Previous studies of the Kettleman Hills have identified seven distinct clusters of last occurrences appearing to coincide with eustatic sea level changes. Studies hypothesized that they represent local extinction pulses driven by changes in salinity, temperature, and productivity, but extinction selectivity patterns have not been studied using modern analytical methods. Using existing museum collections, we compiled a database of species occurrences in the Kettleman Hills and species traits including body size, feeding mode, trophic level, and salinity tolerance. Preliminary analyses suggest that the extinct species with last occurrences in the lower part of the section were preferentially carnivorous gastropods and/or relatively cold-water species. Future work will place turnover patterns in their sequence-stratigraphic context and will account for spatiotemporal variation in preservation and sampling to estimate the temporal pattern and environmental drivers of biodiversity change during the demise of the San Joaquin Seaway.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7447

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