DIVERSITY DYNAMICS OF MICROFOSSILS ACROSS THE CRETACEOUS–PALEOGENE AND EOCENE–OLIGOCENE TRANSITIONS

Microfossils are ubiquitous organisms with an exceptional, well-studied fossil record and their diversity fluctuates temporally and spatially. Microfossil diversity has been documented previously, but how this diversity arises and the effect of major events on speciation and extinction rates is uncertain. PyRate, a program operating within Python, calculates speciation and extinction rates from global occurrence data within a Bayesian framework to accurately determine taxon longevity. This method is used here to estimate diversification dynamics in four different microfossil groups: planktic foraminifera, calcareous nannofossils, radiolarians and diatoms, across the Cretaceous-Paleogene boundary and the Eocene-Oligocene transition.

Results from these analyses demonstrate a steep rise in extinction rate at 66 Ma coupled with very low speciation rates within calcareous groups. This coincides temporally with the end-Cretaceous bolide impact that induced a global mass extinction. A decline in net diversification rates and mean species longevity reflect this extinction event, indicating a perturbation in diversity for 4-5 Myrs. Following the impact, disaster fauna with low diversity and short species turnover, recolonised vacant niches and are likely the cause of highly significant changes in speciation rate. This is not reflected in the siliceous radiolarians however, that show no significant extinction in their entire fossil record. Results for the Eocene-Oligocene transition demonstrate periods of significant diversity change, especially at the Middle Eocene Climatic Optimum. However, evidence of an expected gradual biotic turnover due to long-term climate change, is not shown to occur across the Eocene-Oligocene boundary. This analysis further investigates whether microfossil size can impact upon their survivability and promote selectivity during these shifts in diversity dynamics. Additional work with palaeoclimatic modelling will ensure increased understanding of what external factors drive diversity change across major transitions.