Cenozoic coccolith size changes—Evolutionary and/or ecological controls?

Size is one of the most important characteristics of any organism and can readily be used in quantitative analyses of patterns in physiology, ecology, and evolution. One of the best archives for the history of organism evolution are deep-sea sediments, because (1) microfossils are one of their major constituents, (2) they can be accurately dated, and (3) they are available from almost all areas of the world. We present new data on size variations of entire oval to circular coccolith assemblages during the last 65 million years from a number of globally distributed deep-sea cores. We document the generally decreasing size patterns of these assemblages from the early Cenozoic to the Holocene and show that higher size variability and thus largest sizes can be observed in high latitudes. However, the documented size trends are not directly related to available proxies for paleoenvironmental conditions. Taxonomic investigations of the measured assemblages indicate that the evolution, abundance, and extinction of large-sized species in different lineages, such as Calcidiscus, Coccolithus, Chiasmolithus, Cruciplacolithus, Helicosphaera, and Reticulofenestra, and the newly developed small-coccolith producing taxa in the Neogene, such as Emiliania, Gephyrocapsa, Reticulofenestra haqii, R. minutula and Umbilicosphaeraceae, are responsible for the observed assemblage size variations during the last 65 million years.