Why did mammals diversify rapidly after the end-Cretaceous mass extinction?

Mammals underwent rapid diversification following the end-Cretaceous mass extinction primarily because the event eliminated dominant competitors and predators, leaving numerous ecological niches vacant.1

During the preceding Mesozoic Era, dinosaurs dominated terrestrial environments, while mammals remained relatively small and shrewlike. The K–T extinction, which wiped out non-avian dinosaurs and various large marine reptiles, fundamentally restructured Earth's ecosystems.12

In the resulting Paleocene Epoch, mammals experienced substantial adaptive radiation as they moved into these newly available roles. This period saw the proliferation of diverse groups, including early marsupials and placental mammals, the latter of which eventually became the most diverse group of mammals. By filling these empty niches, mammals evolved into a wide array of forms, ranging from early hoofed animals to the ancestors of modern primates and carnivores, setting the stage for the modern biota of the Cenozoic Era.13