University of Illinois Extension

Mama, Look at the Mammals!

Beginning about 65 million years ago, mammals started to become more plentiful.  At first, these creatures were small, mostly insect eating creatures.

Arctocyon

Illustration of Arctocyon Credit: D. Bogdanov

These animals had the beginnings of hoofs and claws.   Over time, mammals became larger.

pantodont

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.