61:2.5

On land this was pre-eminently the age of mammalian renovation and expansion. Of the earlier and more primitive mammals, over one hundred species were extinct before this period ended. Even the mammals of large size and small brain soon perished. Brains and agility had replaced armor and size in the progress of animal survival. And with the dinosaur family on the decline, the mammals slowly assumed domination of the earth, speedily and completely destroying the remainder of their reptilian ancestors.