Chapter 5
The Fossil Record
Preview
Section 1 Geologic History
Section 2 Looking at Fossils
Section 3 Time Marches On
Concept Mapping
Chapter 5
Section 1 Geologic History
Bellringer
“The Present is the Key to the Past.”
This phrase was the cornerstone of the
uniformitarianist theory developed by geologist
James Hutton in the late 1700s.
Write a few sentences in your science journal about
how studying the present could reveal the story of
Earth’s history. Use sketches to illustrate processes
that occurred millions of years ago that you can still
see today.
Chapter 5
Section 1 Geologic History
Objectives
• Compare uniformitarianism with catastrophism.
• Describe how the science of geology has changed
over the past 200 years.
• Contrast relative dating with absolute dating.
Chapter 5
Section 1 Geologic History
The Principle of Uniformitarianism, continued
• In Theory of the Earth (1788), James Hutton
introduced the idea of uniformitarianism.
• Uniformitarianism assumes that geologic
processes that are shaping the Earth today have
been at work throughout Earth’s history.
Chapter 5
Section 1 Geologic History
Chapter 5
Section 1 Geologic History
The Principle of Uniformitarianism, continued
• Uniformitarianism Versus Catastrophism
During Hutton’s time, most scientists supported
catastrophism, the principle that all geologic
change occurs suddenly.
• Supporters of catastrophism thought that Earth’s
mountains, canyons, and seas formed during rare,
sudden events called catastrophes.
Chapter 5
Section 1 Geologic History
The Principle of Uniformitarianism, continued
• Most people also believed that Earth was only a
few thousand years old.
• Hutton’s work suggested a very different reality.
• According to his theories, Earth had to be much
older, because gradual geologic processes would
take much longer than a few thousand years.
Chapter 5
Section 1 Geologic History
The Principle of Uniformitarianism, continued
• A Victory for Uniformitarianism Catastrophism
remained the guiding principle of geology in the
early 19th century.
• But uniformitarianism became geology’s guiding
principle after Charles Lyell reintroduced the
concept in his Principles of Geology (1830-1833).
Chapter 5
Section 1 Geologic History
The Principle of Uniformitarianism, continued
• Using Hutton’s notes and evidence of his own, Lyell
successfully challenged the principle of
catastrophism.
• He saw no reason to doubt that major geologic
change happened at the same rate in the past as it
happens in the present—gradually.
Chapter 5
Section 1 Geologic History
Modern Geology—A Happy Medium
• During the late 20th century, scientists such as
Stephen J. Gould challenged the principle of
uniformitarianism.
• They believed that catastrophes sometimes play an
important role in shaping Earth’s history.
• Neither theory completely accounts for all geologic
change.
Chapter 5
Section 1 Geologic History
Modern Geology—A Happy Medium,
continued
• Most geologic change is gradual and uniform.
• But catastrophes that cause geologic change have
occurred during Earth’s long history.
• Asteroid and comet strikes to Earth, for example,
have caused rapid change.
Chapter 5
Section 1 Geologic History
Modern Geology—A Happy Medium,
continued
• Some scientists think an asteroid strike 65 million
years ago caused the dinosaurs to become extinct.
Chapter 5
Section 1 Geologic History
Relative Dating
• Scientists can use two methods to determine the
age of objects in sedimentary rocks.
• One of those methods is known as relative dating.
• Relative dating examines a fossil’s position within
rock layers to estimate its age.
Chapter 5
Section 1 Geologic History
Relative Dating, continued
• The bottom layers of rock are usually the oldest, and
the top layers are usually the youngest.
• Scientists can use the order of these rock layers to
determine the relative age of objects within the
layers.
• For example, fossils in the bottom layers are usually
older than fossils in the top layers.
Chapter 5
Section 1 Geologic History
Relative Dating, continued
• The Geologic Column To make relative dating
easier, geologists combine data from all of the
known rock sequences around the world.
• From this information, geologists create the
geologic column—an ideal sequence of rock layers
that contains all of the known fossils and rock
formations on Earth.
• These layers are arranged from oldest to youngest.
Chapter 5
Section 1 Geologic History
Chapter 5
Section 1 Geologic History
Absolute Dating
• Scientists can use absolute dating to more
precisely determine the age of a fossil or rock.
• In absolute dating, scientists examine atoms to
measure the age of fossils or rocks in years.
• Atoms are the particles that make up all matter.
Chapter 5
Section 1 Geologic History
Absolute Dating, continued
• Some atoms are unstable, and will decay over time.
• When an atom decays, it becomes a different and
more stable kind of atom.
• Each kind of unstable atom decays at its own rate.
Chapter 5
Section 1 Geologic History
Absolute Dating, continued
• The time it takes for half of the unstable atoms in a
sample to decay is known as the half-life of that
atom.
• Scientists can examine a sample of rock or fossil,
and look at the ratio of unstable to stable atoms.
• Since they know the half-life, they can determine the
approximate age of the sample.
Chapter 5
Section 1 Geologic History
Chapter 5
Section 1 Geologic History
Absolute Dating, continued
• Uranium-238 has a half-life of 4.5 billion years.
Scientists can use uranium-238 to date rocks or
fossils that are millions of years old.
• Carbon-14 has a half-life of only 5,780 years.
• Scientists use carbon-14 to date fossils and
other objects that are less than 50,000 years old,
such as human artifacts.
Chapter 5
Section 1 Geologic History
Radiometric Dating
Click below to watch the Visual Concept.
Visual Concept
Chapter 5
Section 1 Geologic History
Paleontology—The Study of Past Life
• Paleontology is the science involved with the
study of past life.
• Scientists who study past life are called
paleontologists.
• Paleontologists collect data by studying fossils, the
remains of organisms preserved by geological
processes.
Chapter 5
Section 1 Geologic History
Paleontology—The Study of Past Life,
continued
• Vertebrate and invertebrate paleontologists study
the remains of animals.
• Paleobotanists study fossils of plants.
• Other paleontologists reconstruct past ecosystems,
study the traces that animals left behind, and piece
together the conditions under which fossils formed.
Chapter 5
Section 2 Looking at Fossils
Bellringer
Describe the fossil record of your own life that might
be found 65 million years from now. What items, or
artifacts, might be likely to survive? What kinds of
things would decay and disappear? Do you think
your fossil record would produce an accurate picture
of your life? What might be missing?
Write your description in your science journal. Later,
you will share your description with the class.
Chapter 5
Section 2 Looking at Fossils
Objectives
• Describe five ways in which 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.
Chapter 5
Section 2 Looking at Fossils
Fossilized Organisms
• The remains or physical evidence of an organism
preserved by geologic processes is called a fossil.
• Fossils in rocks can form when an organism dies
and is quickly covered by sediment.
• When the sediment becomes rock, hard parts of
the organism are preserved.
Chapter 5
Section 2 Looking at Fossils
Fossilized Organisms, continued
• If an insect is caught in sticky tree sap, the sap
covers its entire body and hardens quickly.
• Fossils in amber are entire organisms preserved
inside hardened tree sap, called amber.
• Some of the best insect fossils, as well as frogs and
lizards, have been found in amber.
Chapter 5
Section 2 Looking at Fossils
Fossilized Organisms, continued
• Organisms can also be preserved by petrifaction.
• Petrifaction is a process in which minerals replace
the organism’s tissues.
• Permineralization and replacement are forms of
petrifaction.
Chapter 5
Section 2 Looking at Fossils
Fossilized Organisms, continued
• In the process of permineralization, pore space in
an organism’s hard tissue (like bone or wood) is
filled up with mineral.
• In the process of replacement, minerals
completely replace the tissues of the organism.
• Some samples of petrified wood are composed
completely of minerals.
Chapter 5
Section 2 Looking at Fossils
Fossilized Organisms, continued
• In some places, asphalt wells up and forms thick,
sticky pools at Earth’s surface.
• These asphalt pools can trap and preserve many
organisms.
• The La Brea asphalt deposits in Los Angeles,
California have preserved organisms for at least
38,000 years.
Chapter 5
Section 2 Looking at Fossils
Fossilized Organisms, continued
• Frozen Fossils In 1999, scientists removed a
20,000-year-old woolly mammoth that was frozen
in the Siberian tundra.
• These mammoths became extinct about 10,000
years ago.
• Because cold temperatures slow down decay, the
mammoth was almost perfectly preserved.
Chapter 5
Section 2 Looking at Fossils
Other Types of Fossils
• Trace fossils are any naturally preserved evidence
of animal activity.
• Tracks are an example of a trace fossil. They form
when animal footprints fill with sediment.
• Tracks can reveal size and speed of an animal, and
whether it traveled in groups.
Chapter 5
Section 2 Looking at Fossils
Other Types of Fossils, continued
• Burrows are another trace fossil.
• Burrows are shelters made by animals that bury
themselves in sediment, such as clams.
• Another type of trace fossil is coprolite, or
preserved animal dung.
Chapter 5
Section 2 Looking at Fossils
Other Types of Fossils, continued
• Molds and casts are two more examples of fossils.
• A cavity in rock where a plant or animal was buried
is called a mold.
• A cast is an object that is created when sediment
fills a mold and becomes rock.
Chapter 5
Section 2 Looking at Fossils
Using Fossils to Interpret the Past
• The Information in the Fossil Record The fossil
record gives only a rough sketch of the history of life
on Earth.
• Most organisms never become fossils.
• Many fossils have yet to be discovered.
Chapter 5
Section 2 Looking at Fossils
Using Fossils to Interpret the Past, continued
• Organisms with hard body parts have left more
fossils than those with soft body parts.
• Organisms that lived in areas that favored
fossilization have also left more fossils.
Chapter 5
Section 2 Looking at Fossils
Using Fossils to Interpret the Past, continued
• But fossils can show a history of environmental
change.
• For example, the presence of marine fossils on
mountaintops in Canada means that these
mountains formed at the bottom of the ocean.
• Marine fossils can also help scientists reconstruct
ancient coastlines and detect the presence of
ancient seas.
Chapter 5
Section 2 Looking at Fossils
Using Fossils to Interpret the Past, continued
• Scientists can use fossils of plants and land
animals to reconstruct past climates.
• By examining fossils, scientists can tell whether
the climate of an area was cooler or wetter than
that climate is now.
Chapter 5
Section 2 Looking at Fossils
Using Fossils to Interpret the Past, continued
• History of Changing Organisms Scientists study
the relationships between fossils to interpret how
life has changed over time.
• Since the fossil record is incomplete,
paleontologists look for similarities between fossils
over time to try to track change.
Chapter 5
Section 2 Looking at Fossils
Using Fossils to Date Rocks
• Scientists have found that particular types of fossils
appear only in certain layers of rock.
• By dating rock layers above and below these
fossils, scientists can determine the time span in
which the organism lived.
• If the organism lived for a relatively short period of
time, its fossils would show up in limited layers.
Chapter 5
Section 2 Looking at Fossils
Using Fossils to Date Rocks, continued
• Index fossils are fossils of organisms that lived for
a relatively short, well-defined geologic time span.
• To be index fossils, these fossils must be found
worldwide.
Chapter 5
Section 2 Looking at Fossils
Using Fossils to Date Rocks, continued
• Ammonites of the genus Tropites are index
fossils.
Chapter 5
Section 2 Looking at Fossils
Using Fossils to Date Rocks, continued
• These ammonites were marine mollusks similar to
modern squids.
• Tropites lived between 230 million and 208 million
years ago.
• Fossils of these ammonites are index fossils for that
time period.
Chapter 5
Section 2 Looking at Fossils
Using Fossils to Date Rocks, continued
• Trilobites of the genus
Phacops are also
index fossils.
• Trilobites are extinct.
Their closest living
relative is the
horseshoe crab.
Chapter 5
Section 2 Looking at Fossils
Using Fossils to Date Rocks, continued
• Phacops lived about 400 million years ago.
• When scientists find fossils of trilobites anywhere on
Earth, they assume the rock layers are also
approximately 400 million years old.
Chapter 5
Section 3 Time Marches On
Bellringer
Archaeologists and paleontologists believe that
modern humans have lived on Earth for 150,000 to
200,000 years. If we imagine the history of the Earth
to be the length of one calendar year, on which date
do you think modern humans arrived?
Record your answer in your science journal.
Chapter 5
Section 3 Time Marches On
Objectives
• Explain how geologic time is recorded in layers of
sedimentary rock.
• Explain how the geologic time scale illustrates the
occurrence of processes on Earth.
• Explain how the fossil record provides evidence of
changes that have taken place in organisms over
time.
Chapter 5
Section 3 Time Marches On
Geologic Time
• Earth is about 4.6 billion years old.
• Paleontologists find a record of Earth’s history in
rock formations and fossils around the world.
• Dinosaur National Monument in Utah contains the
remains of thousands of dinosaurs that inhabited
the area about 150 million years ago.
Chapter 5
Section 3 Time Marches On
Geologic Time, continued
• Although 150 million years seems like an incredibly
long period, it is little more than 3% of the time our
planet has existed.
• The Rock Record and Geologic Time One of the
best places in North America to see Earth’s history
recorded in rock layers is Grand Canyon National
Park in northwestern Arizona.
Chapter 5
Section 3 Time Marches On
Geologic Time, continued
• The Colorado River has cut the Grand Canyon
nearly 2 km deep in some places.
• Over the course of 6 million years, the river has
eroded countless layers of rock.
• These layers represent almost half, or nearly 2
billion years, of Earth’s history.
Chapter 5
Section 3 Time Marches On
Geologic Time, continued
• The Fossil Record and Geologic Time
Sedimentary rocks in the Green River formation can
be found in parts of Wyoming, Utah, and Colorado.
• These rocks are thousands of meters thick, and
were once part of a system of ancient lakes that
existed for millions of years.
• Fossils of plants and animals are common in these
rocks, and very well preserved.
Chapter 5
Section 3 Time Marches On
Chapter 5
Section 3 Time Marches On
The Geologic Time Scale
• The geologic column represents the billions of
years that have passed since the first rocks formed
on Earth.
• Geologists study a total of 4.6 billion years of
Earth’s history!
• To make their job easier, geologists have created
the geologic time scale, a scale that divides Earth’s
history into distinct intervals of time.
Chapter 5
Section 3 Time Marches On
Chapter 5
Section 3 Time Marches On
The Geologic Time Scale, continued
• Geologists have divided Earth’s history into
sections of time.
• The largest divisions of time are eons.
• The four eons are the Hadean eon, the Archean
eon, the Proterozoic eon, and the Phanerozoic eon.
Chapter 5
Section 3 Time Marches On
The Geologic Time Scale, continued
• The Phanerozoic eon is divided into three eras,
which are the second-largest divisions of geologic
time.
• The three eras are further divided into periods,
which are the third-largest divisions of geologic
time.
• Periods are divided into epochs, the fourth-largest
divisions of geologic time.
Chapter 5
Section 3 Time Marches On
Earth-History Clock
Click below to watch the Visual Concept.
Visual Concept
Chapter 5
Section 3 Time Marches On
The Geologic Time Scale, continued
• The boundaries between geologic time intervals
represent shorter intervals in which visible changes
took place on Earth.
• Some changes are marked by the disappearance
of index fossil species.
• Other changes can be recognized only by detailed
paleontological studies.
Chapter 5
Section 3 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.
• A sudden increase in species is often a result of a
relatively sudden increase or decrease in
competition between species.
Chapter 5
Section 3 Time Marches On
The Geologic Time Scale, continued
• Hallucigenia sparsa appeared during the Cambrian
period, when the number of marine species greatly
increased.
Chapter 5
Section 3 Time Marches On
The Geologic Time Scale, continued
• The number of species can dramatically decline
over a relatively short period of time as a result of a
mass extinction event.
• Extinction is the death of every member of a
species.
• Gradual events such as global climate change and
changes in ocean currents can cause mass
extinctions.
Chapter 5
Section 3 Time Marches On
The Geologic Time Scale, continued
• The Paleozoic Era—Old Life The Paleozoic era
lasted from about 543 million to 248 million years
ago.
• The Paleozoic era is the first era well represented
by fossils.
• Marine life flourished at the beginning of the
Paleozoic era. However there were few land
animals.
Chapter 5
Section 3 Time Marches On
The Geologic Time Scale, continued
• By the middle of the Paleozoic era, all modern
groups of land plants had appeared.
• By the end of the era, amphibians and reptiles lived
on the land, and insects were abundant.
• The following slide shows what life might have
looked like in the late Paleozoic era.
Chapter 5
Section 3 Time Marches On
The Late Paleozoic Era
Chapter 5
Section 3 Time Marches On
The Geologic Time Scale, continued
• The Paleozoic era came to an end with the largest
mass extinction in Earth’s history.
• Some scientists believe that ocean changes were a
likely cause of this extinction.
• The event killed nearly 90% of all species.
Chapter 5
Section 3 Time Marches On
The Geologic Time Scale, continued
• The Mesozoic Era—The Age of Reptiles The
Mesozoic era began about 248 million years ago.
• The Mesozoic era is called the Age of Reptiles
because reptiles such as dinosaurs dominated the
land.
Chapter 5
Section 3 Time Marches On
The Geologic Time Scale, continued
• Small mammals appeared about the same time
that the dinosaurs did.
• Birds appeared in the late Mesozoic era.
• Many scientists think that birds developed directly
from a type of dinosaur.
Chapter 5
Section 3 Time Marches On
The Geologic Time Scale, continued
• At the end of the Mesozoic era, about 15% to 20%
of all species on Earth became extinct.
• This mass extinction event wiped out the
dinosaurs.
• Global climate change may have caused this
extinction.
Chapter 5
Section 3 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.
• During the Mesozoic era, mammals had to compete
with dinosaurs and other animals for food and
habitat.
Chapter 5
Section 3 Time Marches On
The Geologic Time Scale, continued
• After the mass extinction at the end of the Mesozoic
era, mammals flourished.
• Unique traits may have helped these mammals
survive the environmental changes that probably
caused the extinction of the dinosaurs.
• These traits include the ability to regulate internal
body temperature and to develop young inside the
mother.
Chapter 5
The Fossil Record
Concept Mapping
Use the terms below to complete the concept map
on the next slide.
rock layers
relative dating
decay
atoms
fossils
absolute dating
Chapter 5
The Fossil Record
Chapter 5
The Fossil Record