Morphospace expansion paces taxonomic diversification after end Cretaceous mass extinction

Highly resolved paleontological records can address a key question about our current climate crisis: How long until the biosphere rebounds from our actions? There are many ways to conceptualize the recovery of the biosphere; here we focus on the global recovery of species diversity. Mass extinction may be expected to be followed by rapid speciation, but the fossil record contains many instances where speciation is delayed, a phenomenon for which we have a poor understanding. A likely explanation for this delay is that extinctions eliminate morphospace as they curtail diversity, and the delay in diversification is a result of the time needed for new innovations to rebuild morphospace which can then be filled out by new species. Here, we test this Morphospace Reconstruction Hypothesis using the morphological complexity of planktic foraminifer tests after the Cretaceous-Paleogene mass extinction. We show that increases in complexity precede changes in diversity, indicating that plankton are colonizing new morphospace and then slowly filling it in. Preliminary diversification is associated with a rapid increase in the complexity of groups refilling relict Cretaceous ecospace. Subsequent jumps in complexity are driven by evolutionary innovations (development of spines and photosymbionts) which opened new niche space. The recovery of diversity was paced by the construction of new morphospace, implying a fundamental speed limit on diversification after an extinction event.