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 characterized by the dominance of mammals, specifically eutherians in the northern hemisphere and metatherians in the southern hemisphere.