Eutherian morphological disparity across the end‐Cretaceous mass extinction

In the aftermaths of mass extinction events, during radiations of clades, and in several other evolutionary scenarios, there is often a decoupling of taxonomic diversity and morphological disparity. The placental mammal radiation after the end‐Cretaceous mass extinction is one of the archetypal adaptive radiations, but the change in morphological disparity of the entire skeleton has never been quantified across this important boundary. We reconstruct ancestral morphologies of 680 discrete morphological characters onto dated phylogenies of 177 mostly Cretaceous and Palaeogene eutherians (placental mammals and their stem relatives). Using a new approach to incorporate morphologies representing ghost lineages, we assess three measures of morphological disparity (sum of ranges, sum of variances and mean pairwise dissimilarity) across stage‐level time bins within the Cretaceous and Palaeogene. We find that the range‐based metric suggests that eutherian disparity increased immediately after the end‐Cretaceous mass extinction, while both variance‐based metrics declined from the Campanian to the Maastrichtian, but showed no change in disparity from the Maastrichtian to the Puercan – the first North American Land Mammal Age of the Paleocene. Increases in variance‐based metrics lag behind the range‐based metric and per‐lineage accumulation rate, suggesting that the response of mammals to the Cretaceous–Palaeogene event was characterized by an early radiation that increased overall morphospace occupation, followed later by specialization that resulted in increased dissimilarity.