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20.3: Evolution and Classification of Mammals

Created by: CK-12

Lesson 20.3: True or False

Name___________________ Class______________ Date________

Write true if the statement is true or false if the statement is false.

_____ 1. Ancestors of mammals evolved close to 30 million years ago.

_____ 2. The positioning of legs under the body instead of along the sides was an early adaptation in mammal evolution.

_____ 3. The ability to regulate body temperature would allow nocturnal animals to remain active at night.

_____ 4. A good sense of vision would be more useful than good hearing when hunting in the dark.

_____ 5. Cynodonts were early ancestors to mammals, and were about the size of a rat.

_____ 6. Of all the mammals, placental mammals were probably the first to evolve.

_____ 7. Unlike modern monotremes, early monotremes did not lay eggs.

_____ 8. The earliest placental mammals were tree climbers and probably ate insects and worms.

_____ 9. Dinosaurs went extinct 65 million years ago.

_____ 10. To this day, marsupials remain the most common and diverse mammals found only in Africa.

_____ 11. The extinction of the dinosaurs allowed mammals to flourish.

_____ 12. The most widely accepted classification of mammals divides living placental mammals into 17 families.

_____ 13. Whales are mammals, but seals are not.

_____ 14. Humans and rats are grouped into the same superorder.

_____ 15. Though not mammals, cynodonts evolved many mammalian traits.

Lesson 20.3: Critical Reading

Name___________________ Class______________ Date________

Read these passages from the text and answer the questions that follow.

Evolution of Early Mammals

The earliest mammals evolved from cynodonts. But the evolution of mammals didn’t end there. Mammals continued to evolve. Monotreme mammals probably split off from other mammals first. They were followed by marsupials. Placental mammals probably evolved last.

Evolution of Monotremes

The first monotremes may have evolved about 150 million years ago. Early monotreme fossils have been found in Australia. An example is a genus called Steropodon. It may have been the ancestor of the platypus. Early monotremes retained some of the traits of their therapsid ancestors. For example, they laid eggs and had a cloaca. These traits are still found in modern monotremes.

Evolution of Marsupials

The first marsupials may have evolved about 130 million years ago. One of the earliest was the extinct genus Sinodelphys. A fossil of this mammal is shown in the FlexBook. It is a remarkable fossil find. It represents a nearly complete animal. Even tufts of hair and imprints of soft tissues were preserved.

Sinodelphys was about 15 centimeters (6 inches) long. Its limb structure suggests that it was a climbing animal. It could escape from predators by climbing into trees. It probably lived on a diet of insects and worms.

Evolution of Placental Mammals

The earliest placental mammals may have evolved about 110 million years ago. The ancestor of placental mammals may be the extinct genus Eomaia. Fossils of Eomaia have been found in what is now China. It was only about 10 centimeters (4 inches) long. It was a tree climber and probably ate insects and worms. Eomaia had several traits of placental mammals. The illustration below shows how Eomaia may have looked.

Probable Ancestor of Placental Mammals: Eomaia. Eomaia lived a little over 100 million years ago. (Image courtesy of Mateus Zica and under the Creative Commons license CC-BY-SA 3.0.)

The placental mammal descendants of Eomaia were generally more successful than marsupials and monotremes. On most continents, placental mammals became the dominant mammals, while marsupials and monotremes died out. Marsupials remained the most common and diverse mammals only in Australia. The debate over the reasons for their success there is not yet resolved.


1. Describe an early monotreme mammal.


2. Describe an early marsupial mammal.


3. Describe an early placental mammal.


4. What is the order of evolution of the three types of mammals? How long ago did they evolve?


5. Which type of mammals became the most successful?


Lesson 20.3: Multiple Choice

Name___________________ Class______________ Date________

Circle the letter of the correct choice.

  1. When did the earliest mammal live?
    1. over 150 million years ago
    2. 150 million years ago
    3. 130 million years ago
    4. 110 million years ago
  2. Pelycosaurs
    1. had sprawling legs and walked like a lizard.
    2. had teeth of different types.
    3. was a synapsid.
    4. all of the above
  3. Therapsids
    1. had a good sense of hearing.
    2. had the ability to regulate their body temperature.
    3. had legs positioned under the body instead of along the sides.
    4. all of the above
  4. By the end of the Triassic Period, cynodonts had
    1. three tiny bones in the middle ear.
    2. ectothermy.
    3. a diaphragm for eating.
    4. all of the above.
  5. Place the following in their correct evolutionary order.
    1. marsupials - placental mammals - monotremes
    2. monotremes - marsupials - placental mammals
    3. marsupials - monotremes - placental mammals
    4. placental mammals - marsupials - monotremes
  6. The most widely accepted traditional classification of mammals divides living placental mammals into ____________ orders.
    1. 7
    2. 12
    3. 17
    4. 22
  7. Traits of primates include
    1. five digits on their hands and feet.
    2. rubbery pads on their feet.
    3. long, pointed canine teeth.
    4. all of the above.
  8. The most successful mammals are the
    1. marsupials, who practically have a whole continent to themselves.
    2. monotremes, who have a very specific niche without competitors.
    3. placental mammals, who have become dominant on most continents.
    4. none of the above

Lesson 20.3: Vocabulary I

Name___________________ Class______________ Date________

Match the vocabulary word with the proper definition.


_____ 1. active at night

_____ 2. may be the ancestor of the platypus

_____ 3. one of the earliest marsupials

_____ 4. amniotic ancestors of mammals

_____ 5. have long pointed canine teeth, like the coyote

_____ 6. have small sharp teeth, like the mole

_____ 7. have feet with fins, like the seal

_____ 8. have five digits on hands and feet, like the monkey

_____ 9. have incisor teeth grow continuously, like the mouse

_____ 10. have tusks, like the elephant

_____ 11. the most common land vertebrates during the first half of the Permian Period

_____ 12. the ancestor of placental mammals

_____ 13. became the most common and diverse land vertebrates during the second half of the Permian Period

_____ 14. flourished worldwide during the first half of the Triassic Period


a. Carnivora

b. cynodonts

c. Eomaia

d. Insectivora

e. nocturnal

f. pelycosaurs

g. Pinnipedia

h. Primates

i. Proboscidea

j. Rodentia

k. Sinodelphys

l. Steropodon

m. synapsids

n. therapsid

Lesson 20.3: Vocabulary II

Name___________________ Class______________ Date________

Fill in the blank with the appropriate term.

1. Ancestors of mammals evolved close to ____________ million years ago.

2. Ancestors of mammals were amniotes called ____________ .

3. Pelycosaurs evolved some mammalian traits, including ____________ of different types.

4. Therapsids evolved ____________ positioned under the body instead of along the sides.

5. A nocturnal niche was one of the few niches that ____________ did not take over in the Triassic Period.

6. ____________ had the ability to regulate their body temperature.

7. Cynodonts probably gave rise to mammals about ____________ million years ago.

8. Placental mammals can be divided into ____________ orders.

9. Carnivora, like the coyote, have long pointed canine ____________.

10. Chiroptera, like the bat, have digits support membranous ____________.

11. Perissodactyla, like the horse, have odd-toed ____________.

12. Cetacea, like the whale, have paddlelike ____________.

Lesson 20.3: Critical Writing

Name___________________ Class______________ Date________

Thoroughly answer the question below. Use appropriate academic vocabulary and clear and complete sentences.

Summarize the evolution of modern mammals.

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