THERMAL GRADIENTS AND GEOGRAPHIC RANGES: BIODIVERSITY ACROSS THE TEMPERATE PACIFIC 5 MA-PRESENT

Shallow-marine systems experienced many environmental changes over the past 5 Ma in association with climate shifts, from steepening of latitudinal thermal gradients to sea level drops. Local and global extinction occurred in concert with these changes, but causal mechanisms are poorly understood. The draining of inland basins and restriction of Sea of Japan surface waters allow the evaluation of potential extinction drivers such as habitat alteration, where taxa restricted to the altered sites should be most at risk, against climate changes, where geographic range, representing extratropical species’ thermal niche breadth, may be a key determinant, with more intense change expected at higher latitudes. We compiled a taxonomically-standardized database of shallow-marine bivalve species with Pliocene occurrences in the Northeast and Northwest Pacific (24-42° N) and cross-tabulated these fossil occurrences with a database of their global modern occurrences. We found that range size outweighs body size or shelf-area effects as a predictor of local extinction, with causes and magnitude similar on both coasts. However, since the Pliocene, the Northeast and Northwest Pacific have strongly diverged in range size. In the NW Pacific, the median range size increased, as did the ranges of Pliocene survivors, while in the NE Pacific overall ranges were static, with Pliocene survivors losing range. These results suggest that suggest that although climate changes, in particular temperature, played similar roles during the Plio-Pleistocene extinction pulse on both coasts, thermal and other barriers have evolved differently to affect recovery and yield modern biogeographic patterns.