THE IMPACT OF CENOZOIC COOLING ON THE DIVERSITY OF PLANKTONIC FORAMINIFERA

Planktonic foraminifera are calcareous zooplankton found throughout the world’s oceans. They have arguably the most detailed fossil record of any group, making them ideal for studying macroecology through deep time. Using global Recent (coretop) assemblage data, we first modelled how environmental variables shape macroperforate planktonic foraminiferal communities today. Temperature and salinity emerge as the most significant variables for predicting diversity, although productivity and seasonality are also important; the relative importance of these variables differs between ocean basins. We then tested the temporal transferability of the present-day models by comparing the diversity predicted for Eocene conditions with data collated from offshore and onshore sites worldwide throughout the Eocene. The results indicate the modern latitudinal diversity gradient – lower richness towards the poles – developed through the Eocene. By the Late Eocene, environment-diversity relationships were similar to those found today, but the match between statistical model predictions and data are less good in the early Eocene. Three possible causes for this mismatch are considered: the environmental estimates are inaccurate, the statistical model misses relevant variables, or the intercorrelations among facets of diversity – e.g. richness, evenness, functional diversity – have changed over geological time. These analyses shed light on the role of the Eocene global cooling in establishing today's latitudinal diversity gradient – one of the most powerful general rules in biogeography and macroecology.