CLIMATICALLY-DRIVEN MACROEVOLUTIONARY TRAJECTORIES IN THE SIZE OF MARINE PHYTOPLANKTON

The size structure of marine primary producers strongly influences food web interactions and the biogeochemical cycling of carbon. We present a macroevolutionary record of the size of the dominant fossilized marine diatoms over the Cenozoic and dinoflagellate cysts over the last 215 Ma. The minimum and maximum size of the diatom frustule is correlated with species diversity, but the median area of the marine diatom frustule has decreased by ~2.5-fold over the last half of the Cenozoic. The change in the mean size of both groups is strongly correlated with the equator-to-pole and surface-to-deep oceanic thermal gradients inferred from the oxygen isotopes of foraminiferal calcite over the Cenozoic, indicating that climatically altered changes in physical mixing may have driven macroevolutionary decreases in the size of marine pelagic plankton through changes in nutrient availability in the euphotic zone. Hence, climatic changes have shaped the size distribution of primary producers in the ocean and potentially altered both the rate of export of carbon in the ocean and marine food web structure.