How did the extinction of dinosaurs create opportunities for mammals during the Cenozoic Era?

The extinction of dinosaurs at the end of the Cretaceous Period—often referred to as the K–T extinction—profoundly altered the course of Earth's geologic history by removing the dominant terrestrial vertebrates that had long constrained the development of other groups.12

During the Mesozoic Era, mammals existed as small, often shrewlike creatures that lived in the shadow of dinosaurs. The catastrophic event 66 million years ago, likely triggered by an asteroid impact, caused widespread ecosystem collapse. Because many small mammals were capable of living in burrows and consuming a wide variety of food sources, they were better equipped to survive the resulting environmental instability than larger, more specialized species.13

The disappearance of dinosaurs left numerous ecological niches vacant. In the subsequent Cenozoic Era, mammals underwent a period of rapid adaptive radiation, diversifying into the vast array of forms seen today. This expansion allowed placental mammals and marsupials to evolve into roles ranging from small foragers to giant herbivores and predators, effectively filling the biological void left by the extinction of the dinosaurs.2