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 known as the Age of Mammals because the land animals that thrived during this period were mammals. The Northern Hemisphere was dominated by eutherian mammals (placentals), while the Southern Hemisphere saw the dominance of metatherian mammals (marsupials).