EVOLUTIONARY RESPONSE OF THE PLANKTIC FORAMINIFER ORBULINOIDES BECKMANNI TO CLIMATIC CHANGE DURING THE MIDDLE EOCENE CLIMATIC OPTIMUM (MECO)

Evolution is most likely driven not by any single environmental variable, but by the whole climate system. Analogously, selection on one organismal trait often generates a response in others. Thus, to quantify biotic responses to abiotic drivers it is crucial to study the effects of multivariate climate change on multiple morphological traits. Planktonic foraminifera are particularly well suited to this because their fossil record is extraordinarily complete, well-dated and yields a diverse suite of environmental parameters from geochemical analysis.

Here, we present the first study of a species’ evolutionary response to climate change through its entire duration from origination to extinction, using the Eocene planktonic foraminifera Orbulinoides beckmanni. This species’ short range (40.5 Ma to 40.0 Ma) defines planktonic foraminiferal Zone E12 and includes the Middle Eocene Climatic Optimum (MECO). We measured test size, test shape, final chamber size and shape, and the number and size of sutural and areal apertures for O. beckmanni and its immediate ancestor Globigerinatheka euganea. Cluster analysis of all measured traits is subsequently used to determine the exact timing of the origination of O. beckmanni (and morphological criteria for defining this key species), and so the base of Zone E12.

To quantify evolutionary response of ancestor and descendant species to multivariate environmental change before, during and after the MECO, all traits are compared to existing regional environmental reconstructions. The results will provide new insights in a species’ response to multivariate climate change from origination to extinction, and in particular to transient climate change during the MECO as compared to ‘background’ Eocene environmental variability.