LONG-TERM ORIGINATION-RATES ARE RE-SET AT MASS EXTINCTIONS

Diversification during recovery intervals is rapid relative to background rates, yet the impact of recovery dynamics on long-term evolutionary patterns is poorly understood. Backwards survivorship curves (BSCs), which depict the age distribution for all genera within a cohort that originated prior to a window of observation, allow for robust calculations of origination rates and for inferences into the processes affecting those rates. Using a heavily revised version of Sepkoski’s genus-level compendium, we analyzed BSC’s for cohorts of bivalve genera coexisting at each stage boundary from the Pleistocene to the Middle Jurassic, and find that only a few inflection points and shifts in origination rates were supported. Maximal support for an inflection point in all Cenozoic BSCs occurs at the base of the Maastrichtian, with origination rates increasing in the Cenozoic. Mesozoic cohorts consistently support two additional inflection points, at the base of the Triassic and at the Triassic-Jurassic boundary, with origination rates declining after each post-extinction burst. Therefore, though shifts in background origination rates consistently coincide with mass extinctions, the rate shift can be either positive or negative.

The lack of significant variation in origination rates between mass extinction events must be related to the macroecological or macroevolutionary dynamics of recovery. Every supported inflection point is associated with the origin of significantly long-lived genera, usually within recovery intervals. Species richness and geographic range promote survivorship and potentially control rates through ecospace utilization, and both richness and range would expand more rapidly in recovery versus background states. For the K-Pg extinction, where sample sizes are sufficient to calculate order-level rates, the magnitude of the rate shift is inversely related to the number of long-lived genera originating within each order. Recovery genera likely set long-term rates by rapidly filling ecospace through either range expansion or rapid speciation, and maintain these rates owing to the increased survivorship conferred by these variables. Post-Paleozoic origination rates, then, are directly tied to recovery dynamics following each mass extinction event.