Beginning about 65 million years ago, mammals started to become more plentiful. At first, these creatures were small, mostly insect eating creatures.
Illustration of Arctocyon Credit: D. Bogdanov
These animals had the beginnings of hoofs and claws. Over time, mammals became larger.
Illustration of Barylambda faberi, a pantodont from the Paleocene of North America
From 54 to 33 million years ago, ancestors of deer, sheep, zebra, and cows were present. One example of this was a small rabbit sized animal.
From 24 to 5 million years ago, mammals became faster moving species. Rodents, or mammals like today’s squirrels or rats, developed. This coincided with a dryer climate, which caused more open grasslands to develop as well.
