Download 2 Precambrian Time and the Paleozoic Era

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CHAPTER 9
Class
Date
A View of Earth’s Past
2 Precambrian Time and the Paleozoic Era
SECTION
KEY IDEAS
As you read this section, keep these questions in mind:
• How is evolution related to geologic change?
• What are two characteristics of Precambrian rock?
• What were the major events of the Paleozoic Era?
What Is Evolution?
Earth’s history is recorded in rock layers. The fossils
from each layer show which species of organisms existed
when the layer formed.
Scientists study and compare fossils from different
times in Earth’s history. By studying fossils, scientists
have discovered evidence that species have changed over
time. Scientists use the term evolution to describe this
change. Evolution is the process in which new species
develop from existing species. Scientists find evidence for
evolution when they compare fossils to living organisms.
This huge species existed
from 110 million to 90 million
years ago.
READING TOOLBOX
Outline As you read, make
a chart showing major
events that happened during
Precambrian time and the
Paleozoic Era.
LOOKING CLOSER
1. Analyze How do these
two skulls support the theory
of evolution?
The group that
includes modern
crocodiles has
existed for 65
million years.
Modern crocodiles look very similar to their crocodilian ancestors.
EVOLUTION AND GEOLOGIC CHANGE
Geologic and climatic changes affect the ability of
many organisms to survive. Scientists use evidence from
rocks and fossils to find out how environmental changes
affected organisms in the past.
The fossil record shows that some species survived
environmental changes but that some species disappeared. Scientists study fossils to find out why some
species changed over time and why some stayed almost
the same. They also try to find out why species became
extinct.
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SECTION 2
Class
Date
Precambrian Time and the Paleozoic Era continued
Why Do Scientists Know Relatively Little
About Precambrian Time?
Critical Thinking
2. Analyze Methods Why
do scientists not divide
Precambrian time into
smaller units based on the
fossil record?
Most scientists agree that Earth formed about 4.6
billion years ago. Earth formed as part of a cloud of dust,
or nebula. Over time, particles from the cloud formed
Earth and other planets.
The formation of Earth was the beginning of
Precambrian time. Precambrian time lasted about 4
billion years and makes up about 88% of Earth’s history.
Scientists do not know much about what happened
during Precambrian time. Geologic changes have
deformed the rock layers from the time. The layers are
not in their original order. Thus, the rock record is hard
to interpret, or understand.
PRECAMBRIAN ROCKS
READING CHECK
3. Explain Give one reason
Precambrian rocks do not
have many fossils.
Earth has many areas of exposed, or uncovered,
Precambrian rocks. These areas are called shields.
Precambrian shields result from millions of years of
geologic changes.
Precambrian rocks have very few fossils. Precambrian
organisms did not have hard parts such as bones or shells
that fossilize easily. Precambrian rocks are also very old.
Volcanic eruptions, erosion, and movement of tectonic
plates probably destroyed most Precambrian fossils.
PRECAMBRIAN LIFE
Scientists have discovered a few Precambrian fossils.
The most common fossils are cyanobacteria. These
fossils are found in stromatolites, or reeflike deposits
that form in water. Scientists have also discovered
Precambrian fossils of marine worms and jellyfish.
Precambrian stromatolites
show that shallow seas
covered much of Earth
during Precambrian time.
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SECTION 2
Date
Precambrian Time and the Paleozoic Era continued
What Changes Happened in the Paleozoic
Era?
The Paleozoic Era began 542 million years ago and
ended 251 million years ago. When the Paleozoic Era
began, many small continents were scattered over Earth.
By the end of the Paleozoic Era, the smaller continents
had moved and collided, or crashed into each other.
This movement formed the supercontinent Pangaea. The
collisions lifted large areas of land and formed mountain
ranges.
Paleozoic rocks have many fossils. These fossils show
that the number of plant and animal species increased
during the Paleozoic Era. Scientists have used the
detailed fossil record to divide the Paleozoic Era into
six periods: Cambrian, Ordovician, Silurian, Devonian,
Carboniferous, and Permian.
READING CHECK
4. Explain How did Pangaea
form?
TIMELINE OF THE PALEOZOIC ERA
Carboniferous Period
299 Ma
Permian mass
extinction results
from major
environmental
changes.
Formation
of Pangaea
is complete.
First
reptiles
appear.
Crinoid
318 Ma
359 Ma
Devonian
Period
Mississippian Pennsylvanian
Period
Period
Permian
Period
251 Ma
Silurian
Period
416 Ma
a
Ordovician
Period
444 Ma
Cambrian
Period
488 Ma
First
amphibians
appear.
LOOKING CLOSER
First
arthropods
appear
on land.
5. Identify What was the
earliest period in the Paleozoic Era?
First land
plants appear.
Modern,
oxygen-rich
atmosphere
has formed.
Trilobite
First possible
vertebrates
appear.
6. Identify During which
period did the first land
plants appear?
7. Interpret Diagrams Which
group of organisms evolved
first—amphibians or reptiles?
542 Ma
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A View of Earth’s Past
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SECTION 2
Class
Date
Precambrian Time and the Paleozoic Era continued
THE CAMBRIAN PERIOD
READING CHECK
8. List Identify three kinds of
organisms that existed during
the Cambrian Period.
Many marine, or sea, organisms appeared during the
Cambrian Period. The warm, shallow waters on Earth
were perfect for marine invertebrates, or animals without
backbones. The most common Cambrian invertebrates
were trilobites. Scientists use many kinds of trilobite
fossils as index fossils.
Cambrian invertebrates also included brachiopods, a
kind of animal with a shell. Many kinds of brachiopods
lived during this period, but very few exist today. Worms,
jellyfish, snails, and sponges also existed during the
Cambrian Period.
THE ORDOVICIAN PERIOD
During the Ordovician Period, brachiopods and other
marine organisms, such as graptolites, became more
common. At the same time, the number of trilobite species decreased. By this period, vertebrates had appeared.
Vertebrates are animals with backbones. The earliest
vertebrates were fish. Fish in the Ordovician Period did
not have jaws or teeth. Their bodies had thick, bony plates.
THE SILURIAN PERIOD
Critical Thinking
9. Infer Land animals
evolved only after land plants
evolved. Why do you think
events happened in this
order?
Vertebrate and invertebrate marine life thrived during
the Silurian Period. Corals and relatives of modern sea
stars became more common. Eurypterids, shown below,
also lived during this period. Near the end of the Silurian
Period, early land plants and animals evolved.
Scientists have
found fossils of giant
eurypterids that are
about 2 m long.
THE DEVONIAN PERIOD
The Devonian Period is called the Age of Fishes.
Rocks from this period hold fossils of many bony fish.
One group of fishes, lungfish, could breathe air. Another
group of fishes, rhipidistians, may have used their
strong fins to crawl on land. Early amphibians, such as
the ancestors of modern frogs, probably evolved from
rhipidistians. Land plants, such as ferns, also appeared
during this period.
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Holt McDougal Earth Science
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A View of Earth’s Past
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SECTION 2
Class
Date
Precambrian Time and the Paleozoic Era continued
THE CARBONIFEROUS PERIOD
During the Carboniferous Period, Earth’s climate was
generally warm. Humidity was often very high. Forests
and swamps covered much of the land. Over many
millions of years, these forests and swamps became coal
deposits. These coal deposits exist today in places such
as Pennsylvania and Ohio. In North America, scientists
divide the Carboniferous Period into the Mississippian
Period and the Pennsylvanian Period.
Mississippian Pennsylvanian
Period
Period
Carboniferous Period
LOOKING CLOSER
299 Ma
318 Ma
10. Analyze Graphics About
when did reptiles first appear
on Earth?
First reptiles appear.
Amphibians and fish thrived during the Carboniferous
Period. Crinoids were common ocean animals. Insects
such as giant cockroaches and dragonflies were common
land animals. Near the end of the period, early reptiles,
which looked like large lizards, appeared.
THE PERMIAN PERIOD
The Permian Period is the last period in the Paleozoic
Era. Many environmental changes happened during this
period. By the start of the Permian Period, the continents
had joined to form Pangaea. New mountain ranges
formed, and areas of dry land developed. The warm,
shallow seas became smaller.
The environmental changes caused a mass extinction
of Paleozoic organisms. In a mass extinction, a very
large number of species becomes extinct at about the
same time. As the seas became smaller, there were
fewer habitats for marine species. During the mass
extinction at the end of the Permian Period, most marine
invertebrates, such as trilobites, died out.
However, fossils show that many reptiles and
amphibians survived the changes. Animals from these
groups dominated Earth for millions of years after the
Paleozoic Era.
READING CHECK
11. Identify What group of
organisms was most affected
by changing environments
at the end of the Permian
Period?
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A View of Earth’s Past
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Section 2 Review
SECTION VOCABULARY
evolution the process of change by which new
species develop from preexisting species over
time
Paleozoic Era the geologic era that followed
Precambrian time and that lasted from 542
million years ago to 251 million years ago
Precambrian time the interval of time in the
geologic time scale from Earth’s formation to
the beginning of the Paleozoic era, from 4.6
billion to 542 million years ago
1. Summarize Complete the table below to describe the major developments in the
six periods of the Paleozoic Era.
Period
When Period
Began (Ma)
Major Events and Biological Developments
Cambrian
Ordovician
488
Silurian
Devonian
Carboniferous
Permian
2. Identify What are two characteristics of Precambrian rock?
3. Analyze Relationships How do geologic changes affect evolution?
Copyright © Holt McDougal. All rights reserved.
Holt McDougal Earth Science
132
A View of Earth’s Past