• What does fossil evidence reveal about the Paleozoic era?
Visual Check1. Sequence Which is the youngest period of the Paleozoic era?
Early PaleozoicThe Phanerozoic eon has three eras. The Paleozoic
(pay lee uh ZOH ihk) era is the oldest era of the Phanerozoic eon. The Mesozoic (mez uh ZOH ihk) era is the middle era of the Phanerozoic eon. The Cenozoic (sen uh ZOH ihk) era is the youngest era of the Phanerozoic eon.
As shown below, the Paleozoic era lasted for more than half of the Phanerozoic eon. The Paleozoic era is divided into three parts: early, middle, and late. The Cambrian and Ordovician periods make up the Early Paleozoic.
What do you think? Read the two statements below and decide whether you agree or disagree with them. Place an A in the Before column if you agree with the statement or a D if you disagree. After you’ve read this lesson, reread the statements to see if you have changed your mind.
Before Statement After
3. North America was once on the equator.
4. All of Earth’s continents were part of a huge supercontinent 250 million years ago.
Answer Questions Write each Key Concept question on half a sheet of paper. As you read, write phrases or sentences that answer each question. Use your answers to review the lesson.
The Age of InvertebratesThe organisms from the Cambrian explosion were
invertebrates (ihn VUR tuh brayts). Invertebrates are animals without backbones.
The invertebrates of the Cambrian lived only in the oceans. So many kinds of invertebrates lived in the Early Paleozoic oceans that this time is often called the age of invertebrates.
Geology of the Early PaleozoicIf you could have visited Earth during the Early Paleozoic,
it would have seemed unfamiliar to you. There was no life on land. All life was in the oceans.
The shapes and locations of Earth’s continents also would have been unfamiliar to you if you could have visited Earth during the Early Paleozoic. The landmass that would become North America was on the equator.
Earth’s climate was warm during the Early Paleozoic. Rising seas flooded the continents. The water formed many shallow, inland seas. An inland sea is a body of water formed when ocean water floods continents. Most of North America was covered by an inland sea.
Middle PaleozoicThe Early Paleozoic ended with a mass extinction event,
but many invertebrates survived. Huge coral reefs along the edges of the continents were home to new forms of life during the Middle Paleozoic. The Middle Paleozoic consists of the Silurian (suh LOOR ee un) period, 441–416 mya, and the Devonian (dih VOH nee un) period, 416–359 mya.
Soon, vertebrates evolved. Vertebrates are animals with backbones.
The Age of FishesSome of the earliest vertebrates were fishes. So many
types of fishes lived during the Middle Paleozoic that this time is often called the age of fishes. Bony armor covered the bodies of some fishes, such as the Dunkleosteus. This fish was a top Devonian predator.
On land, cockroaches, dragonflies, and other insects evolved. Earth’s first plants grew. These early plants were small and lived in water.
Reading Check 2. Explain How do inland seas form?
3. Identify When was the age of fishes? (Circle the correct answer.)a. the Early Paleozoicb. the Middle Paleozoicc. the Late Paleozoic
Make a horizontal three-tab book to record information about changes during the Paleozoic era.
Geology of the Middle PaleozoicMiddle Paleozoic rocks contain evidence of major
collisions between moving continents. These collisions created mountain ranges. When several landmasses collided with the eastern coast of North America, the Appalachian (ap uh LAY chun) Mountains began to form. By the end of the Paleozoic era, these mountains were probably as high as the Himalayas are today.
Late PaleozoicLike the Early Paleozoic, the Middle Paleozoic ended with
a mass extinction event. Many of the invertebrates that lived in the seas and some land animals disappeared.
The Age of AmphibiansIn the Late Paleozoic, some fishlike organisms spent part
of their lives on land. Amphibians had lungs and could breathe air. Tiktaalik (tihk TAH lihk) was one of the earliest of amphibians. Amphibians were so common in the Late Paleozoic that this time is known as the age of amphibians.
Amphibians were adapted to land in several ways. They had lungs and could breathe air. They had thick skin, which kept them from drying out too quickly. They also had strong limbs, which helped them move around on land. These early amphibians, like amphibians today, had to return to the water to mate and lay eggs.
Reptile species evolved toward the end of the Paleozoic era. Reptiles were the first animals that did not need water for reproduction. Like the eggs of reptiles today, their eggs had tough, leathery shells that kept them from drying out. Because of this, the entire life of a reptile could be spent on land.
Coal SwampsDuring the Late Paleozoic, dense, tropical forests grew in
swamps along shallow inland seas. When trees and other plants died, they sank into the swamps. A coal swamp is an oxygen-poor environment where, over time, plant material changes into coal. The coal swamps of the Carboniferous (car buhn IF er us) period, 359–299 mya, and the Permian period, 299–251 mya, are the major sources of coal that we use today.
Key Concept Check 4. Explain How did the Appalachian Mountains form?
Key Concept Check 5. Explain What adaptations enabled amphibians to live on land?
6. Contrast Describe one major difference between reptiles and amphibians.
Formation of PangaeaGeologic evidence shows that many continental
collisions occurred during the Late Paleozoic. As continents moved closer together, new mountain ranges formed. By the end of the Paleozoic era, Earth’s continents had formed a giant supercontinent called Pangaea. A supercontinent is an ancient landmass that separated into present-day continents. Pangaea formed from land masses that came together close to Earth’s equator, as shown below. As Pangaea formed, coal swamps dried up. Earth’s climate became cooler and drier.
The Permian Mass ExtinctionThe largest mass extinction in Earth’s history occurred at
the end of the Paleozoic era. Fossil evidence indicates that 95 percent of life-forms in the oceans and 70 percent of all life on land became extinct. This extinction event is called the Permian mass extinction.
Scientists debate what caused this mass extinction. The formation of Pangaea likely decreased the amount of space where ocean organisms could live. The formation of Pangaea also would have changed ocean currents, making the center part of Pangaea drier. But Pangaea formed over many millions of years. The extinction event occurred more suddenly.
Some scientists hypothesize that the impact of a large meteorite caused an extreme climate change. Other scientists propose that huge volcanic eruptions changed Earth’s climate. Both a meteorite impact and erupting volcanoes would have caused large amounts of ash, dust, and rock to enter the atmosphere. This debris would have blocked sunlight, reducing temperatures and destroying food webs.
Whatever the cause, Earth had fewer species after the Permian mass extinction. Only a few species adapted to the changes and survived this mass extinction.
7. Define What was Pangaea? (Circle the correct answer.)a. a supercontinentb. a type of coal swampc. a Paleozoic amphibian
Visual Check8. Sketch Use a highlighter to outline the area that is the continent of Africa today.
Key Concept Check 9. Specify What does fossil evidence reveal about the end of the Paleozoic era?
Reread the statements at the beginning of the lesson. Fill in the After column with an A if you agree with the statement or a D if you disagree. Did you change your mind?
What do you think
END OF LESSON
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ConnectED
Cenozoic (sen uh ZOH ihk) era: the youngest era of the
Phanerozoic eon
coal swamp: an oxygen-poor environment where, over time,
plant material changes into coal
inland sea: a body of water that forms when oceans flood
continents
Mesozoic (mez uh ZOH ihk) era: the middle era of the
Phanerozoic eon
Paleozoic (pay lee uh ZOH ihk) era: the oldest era of the
Phanerozoic eon
supercontinent: an ancient landmass that separated into
present-day continents
1. Review the terms and their definitions in the Mini Glossary. Put the terms Mesozoic era, Cenozoic era, and Paleozoic era in order from oldest to youngest.
2. Use what you have learned about the Paleozoic era to complete the table.
Paleozoic Event When It Happened—Early, Middle, or Late
Pangaea formed.
Insects evolved on land. Middle
Vertebrates began to evolve.
Inland seas covered much of Earth. Early
Reptiles appeared on Earth.
The Appalachian Mountains formed. Middle
This time is called the age of fishes.
Coal swamps formed in many places. Late
The largest mass extinction occurred.
Amphibians appeared on Earth. Late
3. How did answering the Key Concept questions help you learn about the Paleozoic era?
