Evolutionary rates of a rapid radiation; the Paleogene planktic foraminifera

Population growth models of taxonomic radiations of clades from their origins have rarely been tested at the species level on an evolutionary time scale. The diversification of Paleogene planktic Foraminifera following the mass extinctions at the Cretaceous-Tertiary boundary is tested for correspondence with the logistic growth model and paleoenvironmental record. An explosive initial diversification rate lasted less than 1 ma. A period of slow, fluctuating growth to a mid-Eocene maximum of 44 species followed. A large-diversity fluctuation at the Paleocene/Eocene boundary is caused by an extinction pulse and low origination rate. The rest of the Paleogene was marked by a very slow decline in species richness. There is no correlation between the record of temperature fluctuations and global diversity, except for the correspondence of the final phase of slow decline with a cooling of oceanic surface waters. During this last period, with time partialled out, originations and extinctions are correlated (P le 0. 10), weakly supporting a hypothesis of diversity-dependent equilibrium. Average durations for species originating within these three phases doubled with each phase. Diversity data better fit a simple asymptotic rather than logistic or sigmoid curve; initial exponential growth occurs much more rapidly than does the slowed growth to equilibrium. more » Comparisons of diversification between radiating clades must be done with a diversity-dependent curve in mind; there can be as much difference among different phases of a clade's history as among different groups. « less