BIOGEOGRAPHIC RANGE SHIFTS IN THREE SPECIES OF PLANKTONIC FORAMINIFERA DURING THE INTENSIFICATION OF NORTHERN HEMISPHERE GLACIATION

Every species occupies a preferred geographical range around a specific environmental optimum. If environmental changes provoke a shift in that optimum, the ecological traits that define each species are expected to adjust accordingly. As environmental changes will not be constant throughout a species’ range, we need high-resolution fossil records from multiple locations to study the drivers of species’ geographical range shifts through time.

Here we present a 0.5 million year-long Plio-Pleistocene record of abundance and morphology of 31,000 individuals from six populations across an Atlantic latitudinal transect of the planktic foraminifera species Truncorotalia crassaformis, Globoconella puncticulata and Globoconella inflata. We focus on Earth’s last great climate transition: the intensification of Northern Hemisphere glaciation (iNHG). T. crassaformis and G. inflata survived the onset of glacial-interglacial cyclicity and are still alive today, whereas G. puncticulata became globally extinct in glacial Marine Isotope Stage (MIS) 96, providing an opportunity to study the driving mechanisms of geographic range shifts under contrasting evolutionary scenarios.

T. crassaformis is largest and most abundant at the low latitude sites, and no temporal changes are detected in any of its populations. G. inflata is restricted to the southern hemisphere, with isolated occurrences in the North Atlantic during pronounced glacial events. G. puncticulata peaks in abundance in the mid-latitudes, but the population’s average shell size decreases during MIS G6 (2.72 Ma), and shells remain small until the species’ extinction. Stepwise population extinctions starting at the high northern latitudes imply a southward shift in the species’ optimum range following the intensification of Northern Hemisphere glaciation. Covariation between size and shape, and size and abundance is weak within populations, but is pronounced among the latitudinally differentiated populations, suggesting that the local biotic and abiotic regulators do not scale obviously to ocean-wide variation.