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# 12.32: Evolution of Modern Mammals

Difficulty Level: At Grade Created by: CK-12
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Practice Evolution of Modern Mammals
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Why so many different types of large mammals?

Elephants, zebras, gazelles, giraffes, rhinoceros, lions, and hippopotamuses, to name just some of the animals of Africa. Each species must have its own niche, otherwise they could not coexist. So, there must be a role for each species within their ecosystem.

### Evolution of Modern Mammals

The Cretaceous Period ended with another mass extinction. This occurred about 65 million years ago. All of the dinosaurs went extinct at that time. Did the extinction of the dinosaurs allow mammals to take over?

Scientists have long assumed that the extinction of the dinosaurs opened up many niches for mammals to exploit. Presumably, this led to an explosion of new species of mammals early in Cenozoic Era. Few mammalian fossils from the early Cenozoic have been found to support this theory. Even so, it was still widely accepted until recently.

#### View from the Mammalian Supertree

In 2007, an international team of scientists compared the DNA of almost all known species of living mammals. They used the data to create a supertree of mammalian evolution. The supertree shows that placental mammals started to diversify as early as 95 million years ago.

What explains the diversification of mammals long before the dinosaurs went extinct? What else was happening at that time? One change was a drop in Earth’s temperature. This may have favored endothermic mammals over ectothermic dinosaurs. Flowering plants were also spreading at that time. They may have provided new and plentiful foods for small mammals or their insect prey.

The supertree also shows that another major diversification of mammals occurred about 50 million years ago. Again, worldwide climate change may have been one reason. This time Earth’s temperature rose. The warmer temperature led to a greater diversity of plants. This would have meant more food for mammals or their prey.

### Summary

• The mammalian supertree shows that placental mammals started to diversify as early as 95 million years ago.

### Practice I

Use this resource to answer the questions that follow.

• http://www.hippocampus.org/Biology \begin{align*}\rightarrow\end{align*} Biology for AP* \begin{align*}\rightarrow\end{align*} Search: Vertebrate Diversity
1. What are the three groups of mammals? Give examples of each.
2. Draw a cladogram depicting the evolution of mammals.

### Review

1. What is the mammalian supertree?

2. Explain why the extinction of most therapsids at the end of the Permian Period may have allowed mammals to evolve.

3. Relate the extinction of dinosaurs to the diversification of modern mammals.

### Vocabulary Language: English Spanish

ectothermic

Having an internal body temperature that is dependent on the temperature of the environment.

endothermic

The ability to regulate body temperature from the inside, through metabolic or other physical changes.

supertree

A single phylogenic tree assembled from smaller phylogenic trees that share some but not necessarily all taxa; may be assembled from phylogenic trees developed with different types of data.

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