Download 3 The Fossil Record - Earth Science > Home

Name
CHAPTER 8
Class
Date
The Rock Record
3 The Fossil Record
SECTION
KEY IDEAS
As you read this section, keep these questions in mind:
• What are four ways whole organisms can be
preserved as fossils?
• What are examples of fossilized traces of organisms?
• How can scientists use index fossils to determine the
age of rocks?
What Are Fossils?
Fossils are the body remains or traces of organisms
that lived in the past. The study of fossils is called
paleontology. Scientists can learn many things from
fossils, including
• the ages of rock layers
• information about past climates
• how life on Earth has changed over time
Almost all fossils that scientists find are in sedimentary rock. Fossils are rare in igneous and metamorphic
rock because those rocks form under conditions of high
heat and pressure. Such conditions generally destroy
existing fossils or material that could become fossils.
The fossil record is made up of all fossils that scientists have found so far. The fossil record shows how
organisms have changed over time. These changes give
scientists information about Earth’s past environments
and how these environments have changed. For example,
scientists have found fossils of marine plants and animals
in areas far from present oceans. The fossils tell us that
these areas were once covered by ocean.
READING TOOLBOX
Outline Make an outline of
this section. Use the heading
questions to help you develop your outline. Be sure to
include the vocabulary terms
and answers to the Key Ideas
questions.
READING CHECK
1. Explain Why are fossils
uncommon in igneous and
metamorphic rock?
How Can Fossils Form?
Normally, the bodies of dead plants and animals are
eaten by animals or decomposed by bacteria. An organism can become a fossil only if it is preserved before it
is eaten or decays. In most cases, the only parts of an
organism that are preserved are hard parts. The table on
the next page describes ways parts of an organism’s body
can fossilize, or become fossils.
READING CHECK
2. Explain Most organisms
never become fossils. Why?
Copyright © Holt McDougal. All rights reserved.
Holt McDougal Earth Science
119
The Rock Record
Name
SECTION 3
Class
Date
The Fossil Record continued
How Fossils Form
Method
Example
Mummification Most bacteria, which
cause decay, cannot survive in very dry
places. Thus, mummified remains are
often found in dry places.
Amber Hardened tree sap is called
amber. Insects can be trapped in sap.
When the sap hardens, the insect is
preserved in the amber. In rare cases,
scientists have been able to collect
DNA from animals preserved in amber.
Tar Seeps In some places, petroleum
oozes to Earth’s surface, where it
becomes a tar seep. Water often covers
tar seeps. Animals that come to drink
the water may fall in and become
trapped by the tar. The tar preserves
their bodies.
Freezing Most bacteria cannot
survive very low temperatures. Thus,
organisms that are buried in frozen soil
or ice do not decay.
LOOKING CLOSER
3. Compare How does petrification differ from the other
methods listed in the table?
READING CHECK
4. Define What is a trace
fossil?
Petrification Minerals in groundwater
may seep into the tissue of a dead
organism buried in sediment. The
minerals may replace the tissues.
When minerals replace the tissues of a
dead organism, the minerals can form
a mineral copy of the organism. This is
the most common way fossils form.
What Are Trace Fossils?
Trace fossils are any evidence other than body parts
that an organism once existed. Examples of trace fossils
include tracks, footprints, and burrows. These traces can
fossilize when sediments cover them and harden. Trace
fossils can give scientists clues about how an animal
looked and how it lived.
Copyright © Holt McDougal. All rights reserved.
Holt McDougal Earth Science
120
The Rock Record
Name
SECTION 3
Class
Date
The Fossil Record continued
Types of Trace Fossils
Type of Trace Fossil
Example
Carbon Films When organic
material decays partially, it leaves
behind a thin carbon-rich layer, or
film. The film shows the surface
features of the organism.
Molds and Casts Sediment may
bury a shell. If the shell dissolves,
it leaves a space in the sediment
called a mold. If mud or sand fills
a mold and hardens, a cast forms.
A cast is a copy of the original
organism.
Coprolites Fossilized solid wastes
from animals are called coprolites.
Scientists can look at sections of
a coprolite under a microscope to
discover what an animal ate.
LOOKING CLOSER
5. Classify Many scientists
consider molds and casts to
be trace fossils. Why do you
think this is so?
Gastroliths Some dinosaurs
swallowed stones to help grind
their food. These smooth stones,
which are called gastroliths, can
survive as fossils.
What Are Index Fossils?
Index fossils are fossils found only in rock layers of a
certain geologic age. To be an index fossil, a fossil must
• be present in rocks found over a large region
• have features that make it clearly different from other
fossils
• be from an organism that lived during a short span of
geologic time
• be found with many other fossils of the same organism
Because index fossils meet very specific requirements,
scientists can use them to determine the age of rocks.
For example, suppose scientists find the same index fossils in different parts of the world. They can conclude
that the rock in these areas formed at about the same
time. Recall that the original organisms of index fossils
lived during short spans of time. Scientists can use this
information to determine the absolute age of the rock.
Critical Thinking
6. Apply Concepts Scientists think horseshoe crabs
have existed unchanged for
hundreds of millions of years.
Could a fossilized horseshoe
crab be an index fossil?
Explain.
Copyright © Holt McDougal. All rights reserved.
Holt McDougal Earth Science
121
The Rock Record
Name
Class
Date
Section 3 Review
SECTION VOCABULARY
fossil the trace or remains of an organism that
lived long ago, most commonly preserved in
sedimentary rock
index fossil a fossil that is used to establish the
age of rock layers because it is distinct, abundant, and widespread and existed for only a
short span of geologic time
paleontology the scientific study of fossils
trace fossil a fossilized mark that formed in
sedimentary rock by the movement of an animal on or within soft sediment
1. Describe Identify and describe four processes in which whole organisms may be
fossilized.
2. Identify List four types of fossils that scientists can use as indirect evidence that
an organism existed.
3. Compare How are the processes of mummification and petrification similar? How
are they different?
4. Apply Concepts Suppose a rock layer in Mexico and a rock layer in Australia con-
tain the same index fossil. What do you know about the absolute age of the layer
in both places? Explain your answer.
Copyright © Holt McDougal. All rights reserved.
Holt McDougal Earth Science
122
The Rock Record