Extinctions of latest Miocene through Late Pleistocene mollusc faunas from Western Atlantic, and Eastern Pacific coastal California are well documented. 60-85% of Pliocene mollusc species from the Gulf of Mexico and Western Atlantic provinces became extinct as compared to 39% of species, 36% of bivalves and 42% of gastropods, in Pliocene Eastern Pacific coastal faunas. It has been suggested that processes driving extinction of molluscs in the Western Atlantic and Gulf of Mexico, ocean water cooling during the Late Pliocene glacial period, or a dramatic productivity decrease in nearshore waters, did not contribute to the extinction Pacific coastal mollusc faunas. In contrast to this, the extinction of molluscs in the Pliocene San Joaquin Basin averaged nearly twice that of the Eastern Pacific with 64% of molluscan species, 55% of bivalves and 78% of gastropods, becoming extinct which approximates Western Atlantic faunas. However, the Pliocene San Joaquin Basin was a shallow inland sea connected to Eastern Pacific Ocean by a narrow strait in its northern reaches. Regional tectonics and global eustasy severed this connection many times during the Pliocene. Thus, endemism, speciation, and extinction of molluscs in the Pliocene San Joaquin Basin were related to paleogeography rather than conditions in the open ocean.