The Rise of the Age of Mammals? Total Evidence tip-dated trees and disparity models to assess the effect of the K-Pg extinction on mammalian evolution

Popular science accounts state that after the extinction of the non-avian dinosaurs at the Cretaceous-Paleogene (K-Pg) boundary 66 million years ago, mammals rapidly diversified to fill newly empty ecological niches.However, evidence for this is mixed. Paleontological analyses suggest that mammals radiated in response to the K-Pg extinction event, whereas neontological analyses suggest that the mammal radiation began before K-Pg and were not greatly affected by it.Here we aim to shed new light on this debate by looking at living and fossil taxa simultaneously.We tested the effect of the K-Pg extinction event on mammalian morphological diversity (disparity) based on a new Total Evidence tip-dated tree.Using a novel, continuous time-slicing method for measuring changes in disparity-through-time, along with null models of disparity evolution, we found mixed support for a change in disparity after the K-Pg boundary between a gradual and a punctuated model of evolution. This implies that the extinctions at the end of the Cretaceous had a much more complex effect on mammalian morphological evolution than previously thought.Our findings contradict the popular theory that the non-avian dinosaurs and other Mesozoic tetrapods were simply restricting mammalian evolution, and that their extinction liberated ecological niches for mammals to fill through evolution.