The Cenozoic is also known as the Age of Mammals because the terrestrial animals that dominated both hemispheres were mammalsthe eutherians (placentals) in the northern hemisphere and the metatherians (marsupials, now mainly restricted to Australia and to some extent South America) in the southern hemisphere.
The Cenozoic era is also called the Age of Mammals because the dominant land animals during this period were mammals. In the Northern Hemisphere, it was eutherian mammals (placentals), while in the Southern Hemisphere, it was metatherian mammals (marsupials).