Question

What made niches available for the radiation of mammals?

Answer 1

Many million years ago, dinosaurs and other giant animals have lived so almost all niches were taken by them. At that time, there were no many mammals and they were rather smaller groups. But, after dinosaurs extinction, some niches opened. Now, mammals could move to that niches and use all the resources now they are not used by other superior animals, so adaptive radiation of mammals could occur.