Evolutionary Signature of the End-Cretaceous Extinction in Modern Marine Biota

The biogeography of living genera reflects their evolutionary history, but how deep is the history detectable in modern distributions? Backwards survivorship curves (BSCs, also called prenascence curves) plot the proportion of taxa within a cohort that originated prior to some time of observation, and are exponential probability functions with a parameter p representing the origination rate of the cohort. Combining a revised version of Sepkoski's genus-level compendium with a geographic database of living marine bivalves, we analyzed BSCs for extant bivalve genera globally and for the each of the 28 modern biogeographic provinces. Global BSCs show a statistically significant inflection point at the Cretaceous-Tertiary (KT) boundary, with an increase in p following the mass extinction event. This inflection point and subsequent rate increase are visible in curves derived from all of the modern provinces, but are most striking in the tropics. At this finer spatial scale p represents the rate of both origination and immigration, rather than simply in situ origination. Pre-KT and post-KT provincial rates tend to converge in high-latitude provinces, a pattern driven predominantly by shallowing of the Cenozoic portions of the curve with latitude. The similarity of provincial BSCs to the global curve, combined with the large number of shared genera in tropical and extratropical seas, suggests that poleward decreases in Cenozoic rates are dictated by high tropical origination rates and the lower probability of their expansion to (or retention in) polar regions. Remarkably, 65 Myr of evolution, geographic range shifts, tectonics movement, climatic fluctuations, and ecological sorting have failed to erase the evolutionary signature of the last great mass extinction event, which continues to resonate through modern biogeography.