Mammals had been around for millions of years before the end of the Cretaceous period; in fact, the first group of true mammals, the morganucodontids, evolved in the late Triassic period. They were a successful group of animals for about 150 million years before the dinosaurs became extinct.

By the end of the Cretaceous period, some mammals had developed many innovations vital to their survival. Many stopped laying eggs and were able to deliver live young. Various mammal species eventually grew specialized teeth for a variety of tasks, such as cutting, gnawing, and grinding, for the better processing of food. They developed better ways to compete for food, such as having more energy in proportion to their size, or adapting to changing diets by becoming omnivores (plant and meat-eating animals).

The therian mammals (marsupials and placentals) became the apparent heirs to the land the dinosaurs and other organisms left behind. Some mammal subgroups had already disappeared before the demise of the dinosaurs; others made it through the end of the Cretaceous period; and some even survive to this day.