A role for chance in marine recovery from the end-Cretaceous extinction

Two contrasting ecological models have been proposed for recovery from mass extinctions. The first posits that evolutionary recoveries are structured by trophic interactions alone, resulting in a predictable recovery of species richness and abundance earlier in lower trophic levels¹. The second, the contingent model, holds that both chance and ecology are key to the structure of recoveries², thus precluding inherent predictability. Documented recovery patterns from the Cretaceous-Palaeogene mass extinction could support either model^1,3-5, as most previous studies have lacked the high-resolution records needed to discriminate between the scenarios. Here we use high-resolution marine sediment records to reconstruct pelagic community structure during the Palaeogene recovery in three sites in the South Atlantic and North Pacific Ocean. We document heterogeneity in the timing of recovery between sites from the alternative community structures characteristic in early pelagic ecosystems. We show that the evolution of species richness and abundance is decoupled between two well-represented groups of phytoplankton and zooplankton, as well as between taxa within a single trophic level. Our results favour the contingent recovery model. We suggest that ecological and environmental mechanisms may account for any similarities in community structure among sites and for the eventual transition from early recovery to late recovery communities, whereas chance may explain intersite differences in the timing and recovery path.