Chapter 3
The Rock and Fossil Record
Table of Contents
Section 1 Earth’s Story and Those Who First Listened
Section 2 Relative Dating: Which Came First?
Section 3 Absolute Dating: A Measure of Time
Section 4 Looking at Fossils
Section 5 Time Marches On
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Chapter 3
Section 1 Earth’s Story and Those Who First Listened
The Principle of Uniformitarianism
• Scientist James Hutton, the author of Theory of the
Earth, proposed that geologic processes such as
erosion and deposition do not change over time.
• Uniformitarianism is the idea that the same geologic
processes shaping the Earth today have been at work
throughout Earth’s history.
• The next slide shows how Hutton developed the idea
of uniformitarianism.
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Chapter 3
Section 1 Earth’s Story and Those Who First Listened
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Chapter 3
Section 1 Earth’s Story and Those Who First Listened
The Principle of Uniformitarianism, continued
• Uniformitarianism Versus Catastrophism Hutton’s
theories sparked a scientific debate by suggesting the
Earth was much older than a few thousand years, as
previously thought.
• A few thousand years was not enough time for the
gradual geologic processes that Hutton described to
have shaped the planet.
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Chapter 3
Section 1 Earth’s Story and Those Who First Listened
The Principle of Uniformitarianism, continued
• A Victory for Uniformitarianism Catastrophism
was geology’s guiding principle until the work of
geologist Charles Lyell caused people to reconsider
uniformitarianism.
• Lyell published Principles of Geology in the early
1830s. Armed with Hutton’s notes and new evidence of
his own, Lyell successfully challenged the principle of
catastrophism.
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Chapter 3
Section 1 Earth’s Story and Those Who First Listened
Modern Geology -- A Happy Medium
• During the late 20th century, scientists such as
Stephen J. Gould challenged Lyell’s uniformitarianism.
They believed that catastrophes occasionally play an
important role in shaping Earth’s history.
• Today, scientists realize that most geologic change is
gradual and uniform, but catastrophes that cause
geologic change have occurred during Earth’s long
history.
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Chapter 3
Section 1 Earth’s Story and Those Who First Listened
Paleontology -- The Study of Past Life
• The history of the Earth would be incomplete without
knowledge of the organisms that have inhabited our
planet and the conditions under which they lived.
• The science involved with the study of past life is
called paleontology.
• Paleontologist study fossils, which are the remains of
organisms preserved by geologic processes.
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Chapter 3
Section 2 Relative Dating: Which Came First?
Objectives
• Explain how relative dating is used in geology.
• Explain the principle of superposition.
• Describe how the geologic column is used in relative
dating.
• Identify two events and two features that disrupt rock
layers.
• Explain how physical features are used to determine
relative ages.
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Chapter 3
Section 2 Relative Dating: Which Came First?
The Principle of Superposition
• Geologists try to determine the order in which events
have happened during Earth’s history. They rely on
rocks and fossils to help them in their investigation.
• The process of determining whether an event or
object is older or younger than other events or objects
is called relative dating.
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Chapter 3
Section 2 Relative Dating: Which Came First?
The Principle of Superposition, continued
• Layers of sedimentary rock, such as the ones shown
below, are stacked like pancakes.
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Section 2 Relative Dating: Which Came First?
The Principle of Superposition, continued
• As you move from the top to the bottom in layers of
sedimentary rock, the lower layers are older.
• Superposition is a principle that states that younger
rocks lie above older rocks, if the layers have not been
disturbed.
•http://youtu.be/EadTLGMu3LI
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Section 2 Relative Dating: Which Came First?
The Principle of Superposition, continued
• Disturbing Forces Not all rock sequences are
arranged with the oldest layers on the bottom and the
youngest layers on top.
• Some rock sequences have been disturbed by forces
within the Earth.
• These forces can push other rocks into a sequence,
tilt or fold rock layers, and break sequences into
moveable parts.
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Chapter 3
Section 2 Relative Dating: Which Came First?
The Geologic Column
• The geologic column is an ideal sequence of rock
layers that contains all the known fossils and rock
formations on Earth, arranged from oldest to youngest.
• Geologists use the geologic column to interpret rock
sequences and to identify the layers in puzzling rock
sequences.
•http://youtu.be/B4QrajShwSw
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Chapter 3
Section 2 Relative Dating: Which Came First?
Disturbed Rock Layers
• Geologists often find features that cut across existing
layers of rock.
• Geologists use the relationships between rock layers
and the features that cross them to assign relative ages
to the features and the layers.
• The features must be younger than the rock layers
because the rock layers had to be present before the
features could cut across them.
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Chapter 3
Section 2 Relative Dating: Which Came First?
Disturbed Rock Layers, continued
• Events That Disturb Rock Layers Geologists
assume that the way sediment is deposited to form
rock layers — in horizontal layers — has not changed
over time.
• If rock layers are not horizontal, something must have
disturbed them after they formed.
• The next slide describes four ways that rock layers
may become disturbed.
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Chapter 3
Section 2 Relative Dating: Which Came First?
Disturbed Rock Layers, continued
• A fault is a break in the Earth’s crust along which
blocks of the crust slide relative to one another.
• An intrusion is molten rock from the Earth’s interior
that squeezes into existing rock and cools.
• Folding occurs when rock layers bend and buckle
from Earth’s internal forces.
• Tilting occurs when internal forces in the Earth
slant rock layers.
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Chapter 3
Section 2 Relative Dating: Which Came First?
Gaps in the Record -- Unconformities
• Missing Evidence Sometimes, layers of rock are
missing, creating a gap in the geologic record. Missing
rock layers create breaks in rock-layer sequences
called unconformities.
• An unconformity is a break in the geologic record
created when rock layers are eroded or when sediment
is not deposited for a long period of time.
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Chapter 3
Section 2 Relative Dating: Which Came First?
Types of Unconformities
• Most unconformities form by both erosion and
nondeposition, but other factors may be involved.
• To simplify the study of unconformities, geologists
place them into three major categories: disconformities,
nonconformities, and angular unconformities.
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Section 2 Relative Dating: Which Came First?
Types of Unconformities, continued
• Disconformities exist where part of a sequence of
parallel rock layers is missing.
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Section 2 Relative Dating: Which Came First?
Types of Unconformities, continued
• Nonconformities exist where sedimentary rock
layers lie on top of an eroded surface of nonlayered
igneous or metamorphic rock.
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Section 2 Relative Dating: Which Came First?
Types of Unconformities, continued
• Angular Unconformities exist between horizontal
rock layers and rock layers that are tilted or folded.
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Chapter 3
Section 2 Relative Dating: Which Came First?
Rock-Layer Puzzles
• Rock-layer sequences often have been affected by
more than one geological event or feature.
• For example,
intrusions may
squeeze into rock
layers that contain an
unconformity, as
shown at right.
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Chapter 3
Section 2 Relative Dating: Which Came First?
Rock-Layer Puzzles, continued
• Determining the order events that led to a sequence
that has been disturbed by more than one rockdisturbing feature is like solving a jigsaw puzzle.
• Geologists must use their knowledge of the events
that disturb rock-layer sequences to piece together the
history of the Earth.
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Chapter 3
Section 3 Absolute Dating: A Measure of Time
Objectives
• Describe how radioactive decay occurs.
• Explain how radioactive decay relates to radiometric
dating.
• Identify four types of radiometric dating.
• Determine the best type of radiometric dating to use
to date an object.
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Chapter 3
Section 3 Absolute Dating: A Measure of Time
Radioactive Decay
• Absolute dating is any method of measuring the age
of an event or object in years.
• To determine the absolute ages of fossils and rocks,
scientists analyze isotopes of radioactive elements.
• Atoms of the same element that have the same
number of protons but different numbers of neutrons
are called isotopes.
•http://youtu.be/OszS0fblz6o
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Section 3 Absolute Dating: A Measure of Time
Radioactive Decay, continued
• Most isotopes are stable, meaning that they stay in
their original form.
• Other isotopes are unstable. Scientists call unstable
isotopes radioactive.
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Section 3 Absolute Dating: A Measure of Time
Radioactive Decay, continued
• Radioactive isotopes tend to break down into stable
isotopes of the same or other elements in a process
called radioactive decay.
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Chapter 3
Section 3 Absolute Dating: A Measure of Time
Radioactive Decay, continued
• Because radioactive decay occurs at a steady rate,
scientists can use the relative amounts of stable and
unstable isotopes present in an object to determine
the object’s age.
•http://youtu.be/oFdR_yMKOCw
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Chapter 3
Section 3 Absolute Dating: A Measure of Time
Radioactive Decay, continued
• Dating Rocks — How Does It Work? In radioactive
decay, an unstable radioactive isotope of one element
breaks down into a stable isotope. The stable isotope
may be of the same element or of a different element.
•The unstable radioactive isotope is called the parent
isotope.
• The stable isotope produced by the radioactive decay
of the parent isotope is called the daughter isotope.
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Chapter 3
Section 3 Absolute Dating: A Measure of Time
Radioactive Decay, continued
• The rate of radioactive decay is constant, so scientists
can compare the amount of parent material with the
amount of daughter material to date rock.
• The more daughter material there is, the older the
rock is.
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Chapter 3
Section 3 Absolute Dating: A Measure of Time
Radiometric Dating
• Determining the absolute age of a sample, based on
the ratio of parent material to daughter material is
called radiometric dating.
• If you know the rate of decay for a radioactive element
in a rock, you can figure out the absolute age of the
rock.
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Chapter 3
Section 3 Absolute Dating: A Measure of Time
Radiometric Dating, continued
• A half-life is the time needed for half of a sample of a
radioactive substance to undergo radioactive decay.
• After every half-life, the amount of parent material
decrease by one-half.
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Chapter 3
Section 3 Absolute Dating: A Measure of Time
Types of Radiometric Dating
• Scientists use different radiometric-dating methods
based on the estimated age of an object. There are four
radiometric-dating techniques.
• Potassium-Argon Method Potassium-40 has a halflife of 1.3 billion years, and it decays leaving a daughter
material of argon.
• This method is used mainly to date rocks older than
100,000 years.
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Chapter 3
Section 3 Absolute Dating: A Measure of Time
Types of Radiometric Dating, continued
• Uranium-Lead Method Uranium-238 is a radioactive
isotope with a half-life of 4.5 billion years. Uranium-238
decays in a series of steps to lead-206.
• The uranium-lead method can be used to date rocks
more than 10 million years old.
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Chapter 3
Section 3 Absolute Dating: A Measure of Time
Types of Radiometric Dating, continued
• Rubidium-Strontium Method The unstable parent
isotope rubidium-87 forms a stable daughter isotope
strontium-87.
• The half-life of rubidium-87 is 49 billion years. This
method is used for rocks older than 10 million years.
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Chapter 3
Section 3 Absolute Dating: A Measure of Time
Types of Radiometric Dating, continued
• Carbon-14 Method Carbon is normally found in
three forms, the stable isotopes carbon-12 and
carbon-13, and the radioactive isotope carbon-14.
• Living plants and animals contain a constant ratio of
carbon-14 to carbon-12. Once a plant or animal dies,
no new carbon is taken in. The amount of carbon-14
begins to decrease as the plant or animal decays.
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Chapter 3
Section 3 Absolute Dating: A Measure of Time
Types of Radiometric Dating, continued
• The half-life of carbon-14 is 5,730 years.
• The carbon-14 method of radiometric dating is
used mainly for dating things that lived within the
last 50,000 years.
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Chapter 3
Section 4 Looking at Fossils
Objectives
• Describe five ways that different types of fossils form.
• List three types of fossils that are not part of organisms.
• Explain how fossils can be used to determine the
history of changes in environments and organisms.
• Explain how index fossils can be used to date rock
layers.
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Chapter 3
Section 4 Looking at Fossils
Fossilized Organisms
• The remains or physical evidence of an organism
preserved by geologic processes is called a fossil.
• Fossils are most often preserved in sedimentary
rock, but other materials can also preserve evidence
of past life.
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Chapter 3
Section 4 Looking at Fossils
Fossilized Organisms, continued
• Fossils in Rocks When an organism dies, it either
begins to decay or is consumed by other organisms.
Sometimes dead organisms are quickly buried by
sediment, which slows down decay.
• Shells and bones are more resistant to decay than
soft tissues, so when sediments become rock, the
harder structures are more commonly preserved.
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Chapter 3
Section 4 Looking at Fossils
Fossilized Organisms, continued
• Fossils in Amber Organisms occasionally become
trapped in soft, sticky tree sap, which hardens and
becomes amber.
• Insect fossils have often been preserved in this way,
but frogs and lizards have also been found in amber.
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Chapter 3
Section 4 Looking at Fossils
Fossilized Organisms, continued
• Petrifaction is a process in which minerals replace
and organism’s tissues.
• One form of petrifaction is called
permineralization, a process in which the pore
space in an organism’s hard tissue is filled up with
mineral.
• Replacement is a process in which an organism’s
tissues are completely replaced by minerals.
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Chapter 3
Section 4 Looking at Fossils
Fossilized Organisms, continued
• Fossils in Asphalt There are places where asphalt
wells up at the Earth’s surface. These thick, sticky
pools can trap and preserve organisms.
• Frozen Fossils Since cold temperatures slow
down decay, many types of fossils have been found
preserved in ice.
•http://youtu.be/LHWGJiUFCH0
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Chapter 3
Section 4 Looking at Fossils
Other Types of Fossils
• Trace Fossils are naturally preserved evidence of
animal activity. Preserved animal tracks are an example
of a trace fossil.
• Other types of trace fossils include preserved burrows
or shelters that were made by animals, and coprolite,
which is preserved animal dung.
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Chapter 3
Section 4 Looking at Fossils
Other Types of Fossils, continued
• Molds and Casts are two more examples of fossils.
• A mold is a mark or cavity made in a sedimentary
surface by a shell or other body.
• A cast is a type of fossil that forms when sediments fill
the cavity left by a decomposed organism.
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Chapter 3
Section 4 Looking at Fossils
Using Fossils to Interpret the Past
• The Information in the Fossil Record The fossil
record offers only a rough sketch of the history of life on
Earth. The fossil record is incomplete because most
organisms never became fossils.
• Scientists know more information about organisms
that had hard body parts and that lived in environments
that favored fossilization.
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Chapter 3
Section 4 Looking at Fossils
Using Fossils to Interpret the Past, continued
• History of Environmental Changes The fossil
record reveals changes in an area’s climate over time.
By using the fossils of plants and land animals,
scientists can reconstruct past climates.
• History of Changing Organisms By studying the
relationships between fossils, scientists can interpret
how life has changed over time.
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Chapter 3
Section 4 Looking at Fossils
Using Fossils to Date Rocks
• Scientists have learned that particular types of
fossils appear only in certain layers of rock.
• By dating the rock layers above and below these
fossils, scientists can determine the time span in
which the organisms that formed the fossils lived.
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Chapter 3
Section 4 Looking at Fossils
Using Fossils to Date Rocks, continued
• If a type of organism existed for only a short period
of time, its fossils would show up in a limited range
of rock layers. These fossils are called index fossils.
• Index fossils are fossils that are found in the rock
layers of only one geologic age, and can be used to
establish the age of the rock layers.
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Chapter 3
Section 4 Looking at Fossils
Using Fossils to Date Rocks, continued
• Ammonites An example of an index fossil is the
fossil of a genus of ammonites called Tropites.
• Tropites, a marine mollusk similar to a modern squid,
lived between 230 million and 208 million years ago.
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Chapter 3
Section 4 Looking at Fossils
Using Fossils to Date Rocks, continued
• Trilobites Fossils of a genus of trilobites called
Phacops are another example of an index fossil.
• Trilobites are extinct and lived approximately 400
million years ago. When scientists find Phacops in a
rock, they assume that the rock is approximately 400
million years old.
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Chapter 3
Section 5 Time Marches On
Objectives
• Explain how geologic time is recorded in rock layers.
• Identify important dates on the geologic time scale.
• Explain how changes in climate resulted in the
extinction of some species.
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Chapter 3
Section 5 Time Marches On
Geologic Time
• The Rock Record and Geologic Time Grand
Canyon National Park is one of the best places in
North America to see Earth’s history recorded in
rock layers.
• These rock layers represent almost half, or nearly
2 billion years, of Earth’s history.
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Chapter 3
Section 5 Time Marches On
Geologic Time, continued
• The Fossil Record and Geologic Time Fossils of
plants and animals are common in sedimentary
rocks that belong to the Green River formation.
• These fossils are well preserved. Burial in the finegrained lake-bed sediments preserved even the
most delicate structures.
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Chapter 3
Section 5 Time Marches On
The Geologic Time Scale
• The geologic column represents the 4.6 billion
years that have passed since the first rocks formed
on the Earth. To aid in their study, geologists have
created the geologic time scale.
• The geologic time scale is the standard method
used to divide the Earth’s long natural history into
manageable parts.
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Chapter 3
Section 5 Time Marches On
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Section 5 Time Marches On
The Geologic Time Scale, continued
• Divisions of Time Geologists have divided the
Earth’s history into sections of time.
• An eon is the largest division of geologic time.
• The four eons are the Hadean eon, the Archean
eon, the Proterozoic eon, and the Phanerozoic eon
•http://youtu.be/FXpeovWDi9k
•.
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Section 5 Time Marches On
The Geologic Time Scale, continued
• Eons are divided into eras. For example, the
Phanerozoic Eon is divided into three eras.
• Periods are the third-largest divisions of geologic
time and are the units into which eras are divided.
• Periods are divided into epochs, the fourth-largest
division of geologic time.
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Chapter 3
Section 5 Time Marches On
The Geologic Time Scale, continued
• The Appearance and Disappearance of Species
At certain times during Earth’s history, the number of
species has increased or decreased dramatically.
• An increase or decrease in the number of species
often comes as a result of a relatively sudden increase
or decrease in competition among species.
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Section 5 Time Marches On
The Geologic Time Scale, continued
• The number of species decreases dramatically over a
relatively short period of time during a mass extinction
event.
• Extinction is the death of every member of a
species.
• Events such as global climate change can cause
mass extinctions.
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Chapter 3
Section 5 Time Marches On
The Geologic Time Scale, continued
• The Paleozoic Era — Old Life This era lasted from
about 543 million to 248 million years ago. It is the first
era that is well represented by fossils.
• Marine life flourished at the beginning of the era and
the oceans became home to a diversity of life.
However, there were few land organisms.
• By the middle of the Paleozoic era, most modern
groups of land plants had appeared.
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Chapter 3
Section 5 Time Marches On
The Geologic Time Scale, continued
• By the end of the Paleozoic era, amphibians and
reptiles lived on the land, and insects were abundant.
• The era came to an end with the largest mass
extinction in Earth’s history.
• Some scientists believe that changes in seawater
circulation were a likely cause of this extinction, which
killed nearly 90% of all species.
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Chapter 3
Section 5 Time Marches On
The Geologic Time Scale, continued
• The Mesozoic Era — The Age of Reptiles This era
began about 258 million years ago. During this era,
reptiles, such as dinosaurs, dominated the land.
• Small mammals appeared about the same time as
dinosaurs, and birds evolved late in the era.
• At the end of the Mesozoic era, about 15% to 20% of
all species on Earth, including the dinosaurs, became
extinct. Global climate change may have been the
cause.
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Chapter 3
Section 5 Time Marches On
The Geologic Time Scale, continued
• The Cenozoic Era — The Age of Mammals The
Cenozoic era began about 65 million years ago and
continues to the present. This era is known as the
“Age of Mammals.”
• After the mass extinction at the end of the Mesozoic
era, mammals flourished. Mammals were able to
survive the environmental changes that probably
caused the extinction of the dinosaurs.
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