Early and middle Tertiary molluscan faunas of the California Coast Ranges are characterized by taxa now living far south in tropical and subtropical latitudes. Unusually large percentages of warm-water molluscan genera in Eocene and Miocene faunas of the San Joaquin basin reflect episodes during which the climate was substantially warmer than at present. Sharp decreases in warm-water genera and in taxonomic diversity during the middle Oligocene represent an intervening climatic deterioration. Parallel faunal trends occur in other Tertiary basins of the California Coast Ranges.

The post-Oligocene climatic amelioration reached a peak during middle Miocene time; tropical and subtropical genera were nearly four times as abundant as during the Oligocene climatic minimum. Percentages of warm-water genera declined during the late Miocene and dropped sharply during the early Pliocene. The last significantly large element of tropical and subtropical molluscan genera is found in the late Miocene of the San Joaquin basin. By the late Pliocene, molluscan assemblages were of temperate aspect and comparable to those now living at that latitude. The Miocene climatic peak indicated by mollusks is reflected by parallel trends in taxonomic diversity of foraminiferal faunas recently reported by other workers.

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