Lesson 4
Reading Guide
Key Concepts
ESSENTIAL QUESTIONS
• What major geologic
events occurred during
the Cenozoic era?
The Cenozoic
Era
• What does fossil evidence
reveal about the Cenozoic
era?
Vocabulary
Holocene epoch p. 625
Pleistocene epoch p. 627
ice age p. 627
glacial groove p. 627
mega-mammal p. 628
g
Multilingual eGlossary
Is this animal alive?
No, this is a statue in a Los Angeles, California, pond
that has been oozing tar for thousands of years. It
shows how a mammoth might have become stuck in
a tar pit. Mammoths lived at the same time as early
humans. What do you think it was like to live alongside
these animals?
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Chapter 17
ENGAGE
1/20/10 11:07:10 AM
Launch Lab
10 minutes
What evidence do you have that you went to kindergarten?
Rocks and fossils provide evidence about Earth’s past. The more recent the era, the more evidence exists. Is this true for you, too?
1 Make a list of items you have, such as a diploma, that
could provide evidence about what you did and what
you learned in kindergarten.
2 Make another list of items that could provide evidence
about your school experience during the past year.
Think About This
1. Which list is longer? Why?
2.
Key Concept How do you think the items on your lists are like evidence from the first
and last eras of the Phanerozoic eon?
Geology of the Cenozoic Era
Have you ever experienced a severe storm? What did your
neighborhood look like afterward? Piles of snow, rushing water,
or broken trees might have made your neighborhood seem like a
different place. In a similar way, the landscapes and organisms of
the Paleozoic and Mesozoic eras might have been strange and
unfamiliar to you. Though some unusual animals lived during
the Cenozoic era, this era is more familiar. People know more
about the Cenozoic era than they know about any other era
because we live in the Cenozoic era. Its fossils and its rock
record are better preserved.
As shown in Figure 20, the Cenozoic era spans the time from
the end of the Cretaceous period, 65.5 mya, to present day.
Geologists divide it into two periods—the Tertiary (TUR shee
ayr ee) period and the Quaternary (KWAH tur nayr ee) period.
These periods are further subdivided into epochs. The most
recent epoch, the Holocene (HOH luh seen) epoch, began 10,000
years ago. You live in the Holocene epoch.
Proterozoic
Phanerozoic
Eons
Eras
Cenozoic
Periods
Quaternary
Make a shutter-fold
book from a vertical
sheet of paper. Label
it as shown. Use it to
record information
about changes during
the Cenozoic era.
Geology
Biology
Present
Tertiary
65.5 mya
Figure 20 The Cenozoic era is Earth’s most
recent era. It began 65.5 mya.
Lesson 4
EXPLORE
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Cenozoic Era
Tertiary Period
65.5 – 2.6 million years ago
Paleocene epoch
Eocene epoch
Oligocene epoch
Figure 21 Mammals dominated the landscapes of the Cenozoic era.
WORD ORIGIN
Cenozoic Mountain Building
Cenozoic
From Greek kainos, means “new”; and zoic,
means “life”
Review
Math Skills
Math Practice
Personal Tutor
Use Percentages
The Cenozoic era began 65.5 mya. What percentage of the Cenozoic era is taken up by the
Quaternary period, which began 2.6 mya? To
calculate the percentage of a part to the
whole, perform the following steps:
a. Express the problem as a fraction.
2.6 mya
65.5 mya
b. Convert the fraction to a decimal. 2.6 mya
divided by 65.5 mya = 0.040
c. Multiply by 100 and add %.
0.040 × 100 = 4.0%
Practice
What percent of the Cenozoic era is represented by the Tertiary period, which lasted
from 65.5 mya to 2.6 mya? [Hint: Subtract to
find the length of the Tertiary period.]
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As shown in the globes in Figure 21, Earth’s
continents continued to move apart during
the Cenozoic era, and the Atlantic Ocean
continued to widen. As the continents
moved, some landmasses collided. Early in
the Tertiary period, India crashed into Asia.
This collision began to push up the
Himalayas—the highest mountains on Earth
today. At about the same time, Africa began
to push into Europe, forming the Alps. These
mountains continue to get higher today.
In North America, the western coast continued to push against the seafloor next to
it, and the Rocky Mountains continued to
grow in height. New mountain ranges—the
Cascades and the Sierra Nevadas—began to
form along the western coast. On the eastern coast, there was little tectonic activity.
The Appalachian Mountains, which formed
during the Paleozoic era, continue to erode
today.
Reading Check Why are the Appalachian
Mountains relatively small today?
Chapter 17
EXPLAIN
1/20/10 11:07:22 AM
Quaternary Period
2.6 million years –
present
Miocene epoch
Pliocene epoch
Pleistocene epoch
Holocene epoch
Pleistocene Ice Age
Like the Mesozoic era, the early part of
the Cenozoic era was warm. In the middle of
the Tertiary period, the climate began to
cool. By the Pliocene (PLY oh seen) epoch,
ice covered the poles as well as many mountaintops. It was even colder during the next
epoch—the Pleistocene (PLY stoh seen).
The Pleistocene epoch was the first epoch
of the Quaternary period. During this time,
glaciers advanced and retreated many times.
They covered as much as 30 percent of
Earth’s land surface. An ice age is a time when
a large proportion of Earth’s surface is covered
by glaciers. Sometimes, rocks carried by glaciers created deep gouges or grooves, as
shown in Figure 22. Glacial grooves are
grooves made by rocks carried in glaciers.
The glaciers contained huge amounts of
water. This water originated in the oceans.
With so much water in glaciers, sea level
dropped. As sea level dropped, inland seas
drained away, exposing dry land. When sea
level was at its lowest, the Florida peninsula
was about twice as wide as it is today.
Pleistocene Ice Age
Figure 22 Glacial grooves
in Ohio are evidence that
glaciers extended far into
North America during the
Pleistocene ice age.
Visual Check Approximately what
percentage of the United States was covered
with ice?
Lesson 4
EXPLAIN
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Figure 23 These mega-mammals lived at different times during the Cenozoic era. They are all extinct
today. The human is included for reference.
Cenozoic Life—The Age of Mammals
The mass extinction event at the end of the Mesozoic era
meant that there was more space for each surviving species.
Flowering plants, including grasses, evolved and began to
dominate the land. These plants provided new food sources.
This enabled the evolution of many types of animal species,
including mammals. Mammals were so successful that the
Cenozoic era is sometimes called the age of mammals.
Mega-Mammals
Figure 24
The sabertoothed cat was a fierce
Pleistocene predator.
Recall that mammals were small during the Mesozoic era.
Many new types of mammals appeared during the Cenozoic
era. Some were very large, such as those shown in Figure 23. The
large mammals of the Cenozoic era are called mega-mammals.
Some of the largest lived during the Oligocene and Miocene
periods, from 34 mya to 5 mya. Others, such as woolly mammoths, giant sloths, and saber-toothed cats, lived during the
cool climate of the Pliocene and Pleistocene periods, from
5 mya to 10,000 years ago. Many fossils of these animals have
been discovered. The saber-toothed cat skull in Figure 24 was
discovered in the Los Angeles tar pits pictured at the beginning
of this lesson. A few mummified mammoth bodies also have
been discovered preserved for thousands of years in glacial ice.
Key Concept Check How do scientists know that
mega-mammals lived during the Cenozoic era?
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Chapter 17
EXPLAIN
6/14/10 4:36:31 PM
Isolated Continents and Land Bridges
The mammals depicted in Figure 23 lived in North America,
South America, Europe, and Asia. Different mammal species
evolved in Australia. This is mostly because of the movement
of Earth’s tectonic plates. You read earlier that land bridges can
connect continents that were once separated. You also read
that when continents are separated, species that once lived
together can become geographically isolated.
Most of the mammals that live in Australia today are marsupials (mar SOO pee ulz). These mammals, like kangaroos, carry
their young in pouches. Some scientists suggest that marsupials
did not evolve in Australia. Instead, they hypothesize that marsupial ancestors migrated to Australia from South America when
South America and Australia were connected to Antarctica by
land bridges, as shown in Figure 25. After ancestral marsupials
arrived in Australia, Australia moved away from Antarctica, and
water covered the land bridges between South America, Antarctica,
and Australia. Over time, the ancestral marsupials evolved into
the types of marsupials that live in Australia today.
ACADEMIC VOCABULARY
hypothesize
(verb) To make an assumption about something that is
not positively known
Reading Check What major geologic events affected the
evolution of marsupials in Australia?
Land Bridges
Figure 25 At the beginning of the Cenozoic era, Australia was linked to South America via
Antarctica, which was then warm. This provided a route for animal migration.
Migration route
N
e
Am
h
t
or
rica
Eurasia
h
ut ca
S o er i
Am
Africa
India
Australia
Antarctica
North
America
65.5 million years ago
Europe
Asia
Africa
South
America
Australia
Antarctica
Present day
Lesson 4
EXPLAIN
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Rise of Humans
The oldest fossil remains of human ancestors have been
found in Africa, where scientists think humans first evolved.
These fossils are nearly 6 million years old. A skeleton of a 3.2million-year-old human ancestor is shown in Figure 26.
Modern humans—called Homo sapiens—didn’t evolve until
the Pleistocene epoch. Early Homo sapiens migrated to Europe,
Asia, and eventually North America. Early humans likely
migrated to North America from Asia using a land bridge that
connected the continents during the Pleistocene ice age. This
land bridge is now covered with water.
Pleistocene Extinctions
Climate changed at the close of the Pleistocene epoch 10,000
years ago. The Holocene epoch was warmer and drier. Forests
replaced grasses. The mega-mammals that lived during the
Pleistocene became extinct. Some scientists suggest that megamammal species could not adapt fast enough to survive the
environmental changes.
Key Concept Check How did climate change at the end of
the Pleistocene epoch?
Future Changes
Figure 26 Lucy is the name
scientists have given this
3.2-million-year-old human
ancestor.
There is evidence that present-day Earth is undergoing a
global-warming climate change. Many scientists suggest that
humans have contributed to this change because of their use
of coal, oil, and other fossil fuels over the past few centuries.
MiniLab
20 minutes
What happened to the Bering land bridge?
Pleistocene animals and humans likely crossed into North America from Asia using the Bering
land bridge. Why did this bridge disappear?
1 Read and complete a lab safety form
Asia
2 Form two pieces of modeling clay into
continents, each with a continental shelf.
3 Place the clay models into a watertight container
with the continental shelves touching. Add water,
leaving the continental shelves exposed. Place
a dozen or more ice cubes on the continents.
4 During your next science class, observe the
container and record your observations.
Bering
Land Bridge
North
America
Analyze and Conclude
Key Concept How does your model represent what happened at the end of the
Pleistocene epoch?
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Chapter 17
EXPLAIN
1/20/10 4:04:11 PM
Lesson 4 Review
Visual Summary
Online Quiz
Assessment
Use Vocabulary
.
1 Gouges made by ice sheets are
The mega-mammals
that lived during
most of the Cenozoic
era are extinct.
2 You live in the
epoch.
Understand Key Concepts
3 Which organism lived during the
Cenozoic era?
A. Brachiosaurus
B. Dunkleosteus
C. saber-toothed cats
D. trilobites
4 Classify Which terms are associated with
Glaciers extended
well into North
America during the
Pleistocene ice age.
the Cenozoic era: Homo sapiens,
mammoth, dinosaur, grass?
Interpret Graphics
5 Determine The map below shows
Lucy is a 3.2-millionyear-old human
ancestor.
coastlines of the southeastern U.S. at three
times during the Cenozoic era. Which
choice represents the coastline at the
height of the Pleistocene ice age?
Choice A
Choice B
Choice C
Use your lesson Foldable to review the lesson.
Save your Foldable for the project at the end
of the chapter.
6 Summarize Copy and fill in the graphic
organizer below to list living mammals that
might be considered mega-mammals today.
What do you think
Critical Thinking
You first read the statements below at the
beginning of the chapter.
7. Mammals evolved after dinosaurs became
7 Suggest what might happen if the
Australian continent crashed into Asia.
extinct.
8. Ice covered nearly one-third of Earth’s land
surface 10,000 years ago.
Did you change your mind about whether you
agree or disagree with the statements? Rewrite
any false statements to make them true.
Math Skills
Review
Math Practice
8 The Cenozoic era began 65.5 mya. The
Oligocene and Miocene epochs extended
from 34 mya to 5 mya. What percentage
of the Cenozoic era is represented by the
Oligocene and Miocene epochs?
Lesson 4
EVALUATE
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