mals into new environments. During this time, rodents, the largest group of
mammals, appeared in the fossil record. During the rest of the Cenozoic,
mammalian brains gradually grew larger in proportion to their bodies. Intel-
ligent activity is generally the key to mammalian success, implying a certain
degree of freedom of action.With their superior brains, mammals could com-
pete successfully with much stronger animals.
When the dinosaurs left the stage at the end of the Cretaceous, the mam-
mals were waiting in the wings, poised to conquer Earth. Because the dinosaurs
represented the largest group of animals, their departure left the world wide
open to invasion by the mammals. After the extinction of the dinosaurs, mam-
mals began to radiate into dazzling arrays of new species.In less than 10 mil-
lion years, all 18 modern orders of mammals were established. In addition, all
orders of hoofed mammals emerged in full bloom.The small, nocturnal mam-
mals evolved into larger animals, some of which were evolutionary dead ends.
Of the 30 or so orders of mammals that existed during the early Cenozoic,Figure 173Fossil
mammal skeletons on
displayat the Museum of
Geology, South Dakota
School of Mines at
Rapid City.
Historical Geology