Late Cenozoic cooling restructured global marine plankton communities

The geographic ranges of marine organisms, including planktonic foraminifera, diatoms, dinoflagellates, copepods and fish are already shifting poleward due to anthropogenic climate change. However, the extent to which species will move and whether these poleward range shifts represent precursor signals which lead to extinction is unclear. Understanding the development of marine biodiversity patterns over geological time and the factors that influence them are key to contextualizing these current trends. The fossil record of the macroperforate planktonic foraminifera provides a rich and phylogenetically resolved dataset that provides unique opportunities for understanding marine biogeography dynamics and how species distributions have responded to ancient climate changes. Here, we employ a bipartite network approach to quantify group diversity, latitudinal specialization, and latitudinal equitability for planktonic foraminifera over the last 8 Ma using Triton, a recently developed high-resolution global dataset of planktonic foraminiferal occurrences. Results depict a global, clade-wide equatorward shift in ecological and morphological community equitability across the last ~8 million years in response to temperature changes during late Cenozoic bipolar ice sheet formation. Collectively, the Triton data indicate the presence of a “latitudinal equitability gradient” (LEG) amongst planktonic foraminiferal functional groups which, importantly, is coupled to the latitudinal biodiversity gradient only through the geologically recent past (~2 Ma - Recent). Prior to this time, LEGs indicate that higher latitudes promoted community equitability across ecological and morphological groups. Observed range shifts amongst marine planktonic microorganisms in the recent and geological past suggest significant poleward expansion of marine communities under even the most conservative future global warming scenarios.