The rise and fall of species: Temporal patterns of geographic spread from the world of microfossils

Knowing where a species is found and the extent to which it occupies physical space contributes vitally to our understanding of how it uses resources available in that space, who its neighbors are and how it may change evolutionarily.  Macroecological and macroevolutionary studies are particularly influenced by our understanding of the generalities of temporal changes in species geographic spread.  Species geographic ranges are often viewed as approximately constant in size, even though historical and modern species geographic ranges have been observed to be fluid.  The fossil record allows us to observe temporal changes in species geographic spread more directly and completely than any other approach.  Microfossils are abundant and exceptionally well preserved.  Hence we use an extensive global microfossil database, NEPTUNE, assembled from deep sea drilling data, to explore the temporal trajectories of geographic ranges over the entire lifespans of species of four groups of marine plankton, namely nannoplankton, diatoms, planktic foraminifers and radiolarians.  Using a model selection approach, we show that geographic spread is not static over geologic time-scales and commonly follow a trajectory that is shaped like a hat.  This hat trajectory fits all four microfossil groups tested better than other models previously alluded to in the paleontological literature.  This result has important implications for estimating origination and extinction probabilities, information commonly sought after in macroevolutionary studies, but it also has repercussions for fields as distant as community ecology and molecular clock studies.