BASINS AND RANGES: TESTING PREDICTORS OF REGIONAL EXTINCTION IN PLIOCENE BIVALVES OF THE TEMPERATE PACIFIC

Explanations for marine extinction pulses in periods of global cooling include loss of shelf habitat due to sea level drop, physiological stresses from cooler water temperatures and lower salinities, and community shifts following changes in productivity and anoxia. We evaluate these factors by investigating selectivity in regional extinction of marine bivalves along the California coast and in the inland San Joaquin Basin at the Plio-Pleistocene boundary. We built a taxonomically standardized database of fossil occurrences (335 species) in the northeastern Pacific (28-42°N) from the literature and assessed the effect of geographic range, body size, phylogeny, habitat loss, and functional ecology on survivorship. The San Joaquin Basin, which drained entirely after the onset of glaciation, acts as a control for shelf-area effects, as habitat loss was more extreme there than in coastal basins. Range size strongly predicts survivorship. Additionally, species ranging into the San Joaquin Basin suffer overall extinction intensities similar to those occurring in outer basins at similar latitude. Substratum habitat, which correlates with exposure to environmental changes, also affects survivorship, with higher survivorship in infaunal taxa. However, feeding ecology (suspension feeders, detritivores and chemosymbiotic taxa), which relates to productivity in the environment, does not predict extinction well, and neither does body size or phylogeny (family membership). The extinction dynamics in the San Joaquin Basin appear to rule out shelf habitat loss as a significant factor, and the non-significance of feeding ecology suggests that changes in productivity were not a major factor, an issue to be further elucidated by comparison at equivalent latitudes within a different oceanographic regime in the Western Pacific. These results suggest that environmental tolerance was the most important determinant of survivorship in this fauna.