REBUILDING BIODIVERSITY AFTER A MASS EXTINCTION: INSIGHTS FROM MOLLUSCAN BENTHIC ECOSYSTEMS ACROSS THE CRETACEOUS-PALEOGENE BOUNDARY IN PATAGONIA

Environmental perturbations preserved in the fossil record offer the opportunity to assess the dynamics of ecosystem recovery. We studied the recovery dynamics of benthic molluscs from the Cretaceous-Paleogene (K-Pg) mass extinction in Patagonia in marginal marine and middle shelf environments and address three questions: (1) How strongly is community diversity affected by the extinction? (2) Is the loss of diversity through extinctions compensated by a gain in diversity via immigrating and originating taxa? (3) Do diversity trends in middle shelf and marginal marine environments match?

Total diversity in the Danian is lower than in the Maastrichtian and this difference is more pronounced in marginal marine habitats. Similarly, Danian community diversity is below that of the Maastrichtian in marginal marine settings but not in the middle shelf. Recovery was more rapid in middle shelf habitats. The most diverse and abundant elements of post-extinction assemblages are genera surviving the K-Pg boundary. Immigrating and newly evolving genera played subordinate roles and did not compensate the diversity loss at the boundary. The progressive accumulation of post-extinction diversity levels off with time. Early during recovery, diversity tends to build up more slowly than expected by chance. In particular, the re-immigration of survivors from near-by localities was delayed. Later on, invasions (marginal marine) and originations (middle shelf) were unusally common.

The shape of the diversity trajectories mirrors that of classical accumulation curves in which the cumulative number of taxa approaches an asymptote. This suggests that, over the studied time interval, available ecospace was filled and on-site diversity approached saturation although at a lower level than before the extinction event. Survivors that dominated Danian assemblages were rare during the Maastrichtian, and therefore probably poor competitors. Once released from competition by the disruption of Maastrichtian communities their competitiveness increased. This suggests a type of incumbency, where numerically subordinate taxa rose to ecological dominance in the aftermath of a major perturbation. Because similar patterns are known from terrestrial ecosystems, this may be a general process across mass extinctions.