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10.18: History of Cenozoic Life

Difficulty Level: At Grade Created by: CK-12
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Why are Pleistocene animals so large?

A large surface area-to-volume ratio is better for keeping warm, so many ice age mammals were huge. Although the dominant animals were mammals, you might not recognize the Pleistocene Earth any more than the Mesozoic Earth.

Cenozoic Life

The extinction of so many species at the end of the Mesozoic again left many niches available to be filled. Although we call the Cenozoic the age of mammals, birds are more common and more diverse. Early in the era, terrestrial crocodiles lumbered around along with large, primitive mammals and prehistoric birds.

Diversification of the Mammals

Their adaptations have allowed mammals to spread to even more environments than reptiles. The success of mammals is due to several of their unique traits. Mammals are endothermic and have fur, hair, or blubber for warmth. Mammals can swim, fly, and live in nearly all terrestrial environments. Mammals initially filled the forests that covered many early Cenozoic lands. Over time, the forests gave way to grasslands, which created more niches for mammals to fill.

Pleistocene Megafauna

As climate cooled during the ice ages, large mammals were able to stand the cold weather, so many interesting megafauna developed. These included giant sloths, saber-toothed cats, wooly mammoths, giant condors, and many other animals that are now extinct (Figure below).

The saber-toothed cat lived during the Pleistocene.

A lecture from Yale University on the effect of life on Earth and Earth on life during 4.5 billion years. Glaciations appear at minute mark 23:30-26:20 and then the video goes into mass extinctions (6c): http://www.youtube.com/watch?v=K6Dl_Vs-ZkY&feature=player_profilepage (47:10).

"The Evolution of Life in 60 Seconds" scales all 4.6 billion years of Earth history into one minute. Don’t blink at the end (1i - I&E Stand.): http://www.youtube.com/watch?v=YXSEyttblMI&feature=related (1:03).

Many of the organisms that made up the Pleistocene megafauna went extinct as conditions warmed. Some may have been driven to extinction by human activities.

Imagine a vast grassy plain covered with herds of elephants, bison and camels stretching as far as the eye can see. Lions, tigers, wolves and later, humans, hunt the herds on their summer migration.This was the San Francisco Bay Area at the close of the last Ice Age.

Learn more at http://www.kqed.org/quest/television/ice-age-bay-area2.


  • With the extinction of the dinosaurs, mammals diversified and took over the available niches.
  • Many of the organisms of the Pleistocene were enormous, probably in have a low surface area to body ratio.
  • Many of the Pleistocene megafauna have gone extinct but some remain.


Use this resource to answer the questions that follow.


1. When were the dinosaurs killed off?

2. What killed off the dinosaurs?

3. How long did it take for mammals to evolve?

4. Where did mammals live?

5. What is the Eocene?

6. What animals ruled the Earth after the dinosaurs?

7. What advantages do mammals have?


1. What are the Pleistocene megafauna and why were they so large?

2. What characteristics do mammals have that allow them to fill so many niches?

3. How does climate affect evolution?

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Difficulty Level:
At Grade
Date Created:
Feb 24, 2012
Last Modified:
Jul 15, 2016
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