Early Cenozoic increases in mammal diversity cannot be explained solely by expansion into larger body sizes

A prominent hypothesis in the diversification of placental mammals after Cretaceous–Palaeogene (K/Pg) boundary suggests that extinction non-avian dinosaurs resulted ecological release mammals, which were previously constrained to small body sizes and limited species richness. This ‘dinosaur incumbency hypothesis’ may therefore explain increases mammalian diversity via expansion into larger size niches, occupied by dinosaurs, but does not directly predict other classes. To evaluate this, we estimate sampling-standardized patterns terrestrial North American fossil within classes, during Cretaceous Palaeogene. We find strong evidence for post-extinction all Increases small-bodied (less than 100 g, common class much smaller smallest non-avialan (c. 400 g)) similar those species. propose had access greater energetic resources or able partition more finely K/Pg mass extinction. is likely be result a combination widespread niche clearing due extinctions, alongside suite biotic abiotic changes occurred Late across boundary, such as shifting floral composition, novel key innovations among eutherian mammals.