Reassessment of extinction patterns of Pliocene molluscs from California and environmental forcing of extinction in the San Joaquin Basin

Abstract Thirty latest Miocene through Late Pleistocene molluscan faunas from coastal California and the San Joaquin Basin (SJB), central California, were reviewed to better understand the pattern of Pliocene mollusc extinction in California and particularly in the Etchegoin Group (informal SJB nomenclature). Species lists were compiled for each fauna, taxonomy reviewed to eliminate synonyms and uncertain identifications, and the extinct versus living status of each species determined. Only 34% of molluscan species, 40% of bivalves and 21% of gastropods, in the Etchegoin Group are extant as compared to 61% of molluscs, 64% of bivalve and 56% of gastropod species, in Pacific coastal faunas. The Etchegoin Group was deposited in a marginal basin connected to the Pacific Ocean through a long and narrow silled strait subject to its connection being cut by eustatic regression and regional tectonism. Seven major regional extinctions affected the Etchegoin Group molluscan faunas where > 40% species became extinct: two in the Early Pliocene upper Etchegoin Formation at 4.4 and 4.0 Ma and five in the Early–Late Pliocene San Joaquin Formation at ∼4.0, 2.9, 2.6, and 2.4 Ma and that coincident with the final ocean connection closure at 2.3 Ma. Peak diversities corresponded with periods of highest sea-level at ∼4.5, 4.2, 3.1, 2.7, 2.5 and 2.4 Ma when immigrant faunas became established during periods of warm climate and normal-marine conditions. Upon sea-level fall the basin became cooler, brackish, and faunas adapted to warmer and normal marine conditions became extinct with slow recovery of diversity afterwards. Low-diversity faunas characterize periods of low and rising sea level when circulation through the connecting strait was insufficient to maintain normal marine conditions thus hindering establishment of most immigrants from coastal faunas. Restricted circulation with the Pacific substantially reduced the nutrient supply to the basin leading to a long-term productivity collapse that exacerbated the effects of a deteriorating environment thus leading to the major extinction event observed at the Etchegoin–San Joaquin Formations contact at 4.0 Ma. Increasing restriction from the Pacific Ocean during the Pliocene limited immigration of coastal species into the San Joaquin Basin to those opportunistic species best able to adapt to the environment inside the basin while species unable to adapt to conditions inside the SJB were filtered-out in the strait. Stenotopy of endemic species precluded range expansion through the connecting strait into the Pacific Ocean. Thus, abrupt regression-driven hydrologic change, productivity collapse from coincident geochemical and sedimentary change, and climatic change led to the major extinction events in the Pliocene SJB. Speciation events following extinctions suggest diversification of surviving faunas into habitats created by changed environmental conditions. Despite the number and wide geographic distribution of faunas reviewed in this paper, only 50–90% of extant mollusc species found in Pliocene Etchegoin Group faunas are also found in coastal California Pliocene faunas demonstrating the incompleteness of the California fossil record.