Paleobiogeography of early to middle Paleozoic phytoplankton: a review and synthesis

Phytoplankton form the foundation of marine food webs, and their fossil record provides key insights into ecosystem and climate evolution. We review the biogeography of Paleozoic phytoplankton based on data from the literature and new multivariate analyses of acritarch data that are georeferenced to paleolatitude and paleolongitude. Results show that acritarchs are rather cosmopolitan with wide geographic ranges, their distribution often differentiated into two broad latitudinal realms: a northern warm-water and a southern cold-water assemblage. Provincialism was most pronounced during the Ordovician and Devonian. The Ordovician provincial structure collapsed during the Hirnantian glaciation, resulting in a short-lived cosmopolitan phase that persisted into the early Silurian. Biogeographic differentiation re-emerged in the middle Silurian and was reorganized in the Devonian as plate movements closed ocean basins and enabled new intercontinental exchanges. Overall, climate change and plate tectonics appear to be the primary drivers of phytoplankton provincialism.