Why were mammals able to become the dominant species at the beginning of the Cenozoic?

Question

Quizplus · Accepted Answer

The extinction of the non-avian dinosaurs at the end of the Cretaceous period provided an opportunity for the small mammalian survivors to diversify and fill the open ecological niches. As a result, mammals were able to become the dominant species during the beginning of the Cenozoic era, leading to the evolution of new forms of mammals that eventually dominated the planet.

Why Were Mammals Able to Become the Dominant Species at the Beginning