13.2 Paleozoic Era: Section 13.2 Life ... - Planet Earth 13.2.pdfEarth’s History 369 13.2 Paleozoic Era: ... you would have seen the familiar blue planet ... and the Siberian landmass

Earth’s History 369

13.2 Paleozoic Era:Life Explodes

Reading StrategyIdentifying Details Copy the table below.As you read the section, fill out the table withnotes.

Key ConceptsWhen was the Paleozoicera?

How did tectonicmovements affect thelocations and formationsof the continents duringthe Paleozoic era?

What kind of life existed inthe early Paleozoic?

How did life evolve duringthe Paleozoic era?

Vocabulary◆ Gondwana◆ Laurasia

As the Precambrian came to a close, the fossil record discloseddiverse and complete multicelled organisms. This set the stage for morecomplex plants and animals to evolve at the dawn of the Paleozoic era.

Following the long Precambrian, the most recent 540 millionyears of Earth’s history are divided into three eras: Paleozoic,Mesozoic, and Cenozoic. The Paleozoic era encompasses about 292 million years and is by far the longest of the three.

Before the Paleozoic, life forms possessed no hard parts, such asshells, scales, bones, or teeth. Hard parts greatly enhanced a life form’schance of being preserved as part of the fossil record. The Paleozoicera contains many more diverse fossils due to the emergence of lifeforms with hard parts.

Abundant Paleozoic fossils have allowed geologists to construct afar more detailed time scale for the last one-eighth of geologic timethan for the preceding seven-eighths, the Precambrian. Moreover,because every organism is associated with a particular environment,the greatly improved fossil record provided invaluable information forlearning about ancient environments. For our brief tour of thePaleozoic, we divide it into Early Paleozoic (Cambrian, Ordovician,Silurian periods) and Late Paleozoic (Devonian, Mississippian,Pennsylvanian, Permian periods).

Continental Plant Animalpositions life life

Early Paleozoic

Middle Paleozoic

Late Paleozoic

FOCUS

Section Objectives13.4 List the periods that make up

the Paleozoic era.13.5 Explain how tectonic

movements affected thelocations of the continentsduring the Paleozoic era.

13.6 Describe some of the life-formsthat existed in the earlyPaleozoic era.

13.7 Explain how life evolvedduring the Paleozoic era.

Build VocabularyGraphic Organizers As they read thissection, have students create a conceptmap or flowchart that includes thevocabulary terms and major conceptsfor this section.

Reading StrategySample answers: Early Paleozoic—Gondwana at the South Pole, otherlandmasses near the equator; confined tothe sea; marine invertebrates such astrilobites, cephalopods, and brachiopodsdeveloped and animals with hard shells;Late Paleozoic—Pangaea formed; landplants developed from leafless spikes tofull trees; fish developed scales, andamphibians became dominant

L2

L2

Reading Focus

1


Section 13.2

HSES_1eTE_C13.qxd 5/16/04 11:52 AM Page 369

Silurian(430 m.y.a.)

Equator

NorthAmerica

Siberia

NorthernEurope

ChinaAustralia

SouthAmerica Africa

India

Antarctica

G O N D W A N A

370 Chapter 13

Early PaleozoicThe early Paleozoic consists of a 123-million-year span that includesthe Cambrian, Ordovician, and Silurian periods. Looking at Earthfrom space at this time, you would have seen the familiar blue planetwith many clouds, but the arrangement of the continents would bevery different from today.

Early Paleozoic History During the Cambrian, Ordovician,and Silurian periods, the vast southern continent of Gondwana encom-passed five continents (South America, Africa, Australia, Antarctica,India, and perhaps China).

North America and several other landmasses were not part ofGondwana. Although the exact position of these northern continentsis uncertain, they are thought to have been near the equator and sep-arated by a narrow sea, as shown on the map in Figure 5.

At the beginning of the Paleozoic, North America was a land withno living things, plant or animal. Soon, a mountain-building eventaffected eastern North America. The final chapter in this story was theformation of the Appalachian Mountains, over 200 million years later.

During the Silurian period, much of North America was once againcovered by shallow seas. This time, large barrier reefs restricted circula-tion between shallow marine basins and the open ocean. Water in thesebasins evaporated, depositing large quantities of rock salt and gypsum.

Why are more fossils found from the Paleozoic erathan Precambrian time?

Figure 5 A landmass calledGondwana and the northerncontinental landmasses existed inearly Paleozoic time. Explaining What continentsmade up Gondwana?

370 Chapter 13

INSTRUCT

Early PaleozoicBuild Reading LiteracyRefer to p. 392D in Chapter 14, whichprovides guidelines for previewing.

Preview Have students scan theboldfaced material, topic headings, andgraphics in this section to get an idea ofhow it is organized and what it is about.This preview of the section will helpstudents know where to find theinformation needed to complete thetable for this section’s reading strategy.Visual, Verbal

L1

2

Section 13.2 (continued)

Customize for Inclusion Students

Learning Disabled Graphic organizers areessential to managing the large amount ofinformation presented in this chapter. Helpstudents who are learning disabled use thereading strategy for this section by havingstudents complete this activity in heterogeneous

cooperative groups. Also, remind studentsto refer back to the geologic time scale onp. 365 if they get lost in what they are reading.Tell students to add details to their own versionof the scale as they read. (See Reteach inSection 13.1.)



Early Paleozoic Life Life in early Paleozoic time wasrestricted to the seas. Vertebrates had not yet evolved, so life consistedof several invertebrate groups. The Cambrian period was the goldenage of trilobites. More than 600 types of these mud-burrowing scav-engers flourished worldwide. By Ordovician times, brachiopodsoutnumbered the trilobites. Brachiopods are among the most wide-spread Paleozoic fossils and, except for one modern group, are nowextinct. The adult brachiopods lived attached to the seafloor, but theyoung larvae were free swimming. This mobility accounts for thegroup’s wide geographic distribution.

The Ordovician also marked the appearance of cephalopods—mobile and highly developed mollusks that became the majorpredators of the time. Squid and octopus are descendents of these earlycephalopods. Cephalopods were the first truly large organisms onEarth. Figure 6 shows some cephalopods and other organisms of theOrdovician.

The beginning of the Cambrian period marks an important event inanimal evolution. For the first time, organisms appeared that secretedmaterial that formed hard parts, such as shells. Hard parts clearly servedmany useful purposes and aided adaptations to new ways of life.Mollusks, such as clams and snails, secreted external shells that protectedthem and allowed body organs to function in a more controlled envi-ronment. The successful trilobites developed an exoskeleton of a proteincalled chitin, which permitted them to burrow through soft sediment insearch of food. The fossil in Figure 7 shows the exoskeleton of a trilobite.

How did the formation of hard parts benefit animals?

Figure 7 Trilobite Fossil Inferring When did trilobiteslive?

Figure 6 Cephalopods, trilobites,brachiopods, snails, and coralsinhabited the waters of theOrdovician period.

Integrate BiologyTrilobites Have students research thedifferent varieties of trilobites. Studentsshould then present their findings tothe class, including pictures and acomparison of sizes and structuresof the different varieties.Verbal

Use CommunityResourcesIf possible, bring students to a nearbymuseum to observe fossils andrecreations of ancient environments.Visual, Kinesthetic

L2

L2


Answer to . . .

Figure 5 South America, Africa,Australia, Antarctica, India, andperhaps China

Figure 7 Trilobites lived during theCambrian period.

There were moreorganisms with hard

parts, which greatly enhanced a life-form’s chance of being preserved inthe fossil record.

Hard parts allowedanimals to venture

into different environments.

The thick layers of rock salt and gypsumdeposited during the early Paleozoic are calledevaporite beds because they form as a result ofwater evaporating and leaving previously

dissolved materials behind. These beds are animportant resource for the chemical, rubber,plasterboard, and photographic industries inOhio, Michigan, and western New York State.

Facts and Figures


372 Chapter 13

Late PaleozoicThe late Paleozoic consists of four periods—the Devonian,Mississippian, Pennsylvanian, and Permian—that span about 160 mil-lion years. Tectonic forces reorganized Earth’s landmasses during thistime, creating the supercontinent Pangaea.

Late Paleozoic History As ancestral North America collidedwith Africa, the narrow sea that separated these landmasses began toclose slowly, as compared in Figure 8B and 8C. Strong forces of com-pression from this collision deformed the rocks to produce theAppalachian Mountains of eastern North America.

During the union of North America and Africa, the other north-ern continents began to join, as shown in Figure 8. By the Permianperiod, this newly formed landmass had collided with western Asiaand the Siberian landmass along the line of the Ural Mountains.Through this union, the northern continent of Laurasia was born,made up of present-day North America, Europe, western Asia.

As Laurasia was forming, Gondwana moved northward. By thePennsylvanian period, Gondwana collided with Laurasia, forming amountainous belt through central Europe. By the end of thePaleozoic, all the continents had fused into the supercontinent ofPangaea. With only a single vast continent, the world’s climatechanged dramatically. The interior of this large continent, located farfrom a source of moisture became quite arid. In addition, these regionsbecame very seasonal, having extremes far greater than those we expe-rience today.

Relative Dating

Geologists can relatively date thefossils they uncover with respect totheir positions in the ground.Assume the playing cards shown arelayers of rocks viewed from above.Using the cards, answer thequestions below.

Analyze and Conclude1. Observing List the order, first

(oldest) to last (youngest), thatthe cards were laid down.

2. Inferring Were you able to place all of thecards in sequence? If not, which one(s) couldnot be “relatively” dated and why not?

3. Applying What geologic process wouldbe represented by slipping a card in fromthe side?

♣♣

♣

2

22♣

♥

♥

♥

8

88♥

♦

♦

♦

6

6

6♦

♦♦ ♦3 33♦

♠

♠

♠

7

7

7♠

♥

♥

♥

10

10

10♥

♠

♠

♠

5

5

5♠

♣

♣

♣

A

A

1♣

♥

♥

♥

4

4

4♥

♣

♣

♣

9

9

9♣

Youngest (last)

Oldest (first)

372 Chapter 13

Late PaleozoicBuild Reading LiteracyRefer to p. 362D in Chapter 13, whichprovides guidelines for using priorknowledge.

Use Prior Knowledge Review the lawof superposition with students. (Theoldest rock layers will always be foundon the bottom of an outcrop unless thatoutcrop has been overturned orintruded.) Ask students to apply this lawduring the Quick Lab.

Relative DatingObjective After completing thisactivity, students will be able to applythe law of superposition to put layers ina sequence.

Skills Focus Observing, Inferring

Prep Time none

Class Time 10 minutes

Teaching Tip Give students about5 minutes to complete the activity, thenspend a few minutes discussing theresults with the class.

Expected Outcome Students willcorrectly apply the law of superpositionto determine the sequence in which thecards were laid down.

Analyze and Conclude1. 8 of hearts, 6 of diamonds, 3 ofdiamonds, 7 of spades, 10 of hearts,5 of spades, ace of clubs, 4 of hearts,9 of clubs2. All the cards, except the 2 of clubs,could be put in sequence. The 2 of clubswas not in the stack, so it was notpossible to determine where it wouldhave gone in the sequence.3. An intrusion would be like slipping acard in from the side.Visual, Logical

L2

L1


In the early Paleozoic, a mountain-buildingevent affected eastern North America from thepresent-day Appalachians to Newfoundland.These mountains have since eroded away,

leaving behind deformed strata and a largevolume of sedimentary rocks from theweathering of these mountains.

Facts and Figures



Permian(260 m.y.a.)

Equator

NorthAmerica

Siberia

EuropeChina

Australia

SouthAmerica Africa

India

Antarctica

Devonian(410 m.y.a.)

Equator

NorthAmerica

Siberia

Europe China Australia

SouthAmerica Africa

India

Antarctica

L A U R A S I A

G O N D W A N A

L A U R A S I A

G O N D W A N AMississippian(330 m.y.a.)

Equator

NorthAmerica

Siberia

Europe

China

Australia

SouthAmerica Africa

IndiaAntarctica

Tethys Sea

PA

NG

A

EA

Figure 8 During the latePaleozoic, plate movements werejoining together the majorlandmasses to produce thesupercontinent of Pangaea.A

B

C

Late Paleozoic Plate MovementsBuild Reading LiteracyRefer to p. 186D in Chapter 7, whichprovides guidelines for relating text andvisuals.

Relate Text and Visuals Remindstudents to look back at the geologictime scale on p. 365 as they read to seewhen the major events happened andto review the sequence of epochs andperiods.Visual

Use VisualsFigure 8 This diagram shows themovement of landmasses during the latePaleozoic. Ask: During the latePaleozoic, how did Gondwana moverelative to the equator? (toward theequator) How would this movementhave affected the climate ofGondwana? (It would have gottenwarmer.) During the late Paleozoic,how did Siberia move relative to theequator? (away from the equator) Howwould this movement have affectedthe climate of Siberia? (It would havegotten colder.)Visual

L1

L1


In the Mississippian period, when Laurasia andGondwana were forming, ancestral NorthAmerica and Africa collided, creating strongcompressional forces that deformed rocks,producing the northern AppalachianMountains of eastern North America. As

Pangaea was forming in the Pennsylvanianperiod, the African fragment of Gondwanaand the southeastern edge of North Americacollided, creating the southern AppalachianMountains.

Facts and Figures


374 Chapter 13

Late Paleozoic Life During most of the late Paleozoic, organ-isms diversified dramatically. Some 400 million years ago, plantsthat had adapted to survive at the water’s edge began to move inland,becoming land plants. These earliest land plants were leafless verticalspikes about the size of your index finger. However, by the end of theDevonian, 40 million years later, the fossil record indicates the exis-tence of forests with trees tens of meters high.

In the oceans, armor-plated fishes that had evolved during theOrdovician continued to adapt. Their armor plates thinned to light-weight scales that increased the organisms’ speed and mobility, asshown in Figure 9. Other fishes evolved during the Devonian, includ-ing primitive sharks that had a skeleton made of cartilage and bonyfishes—the groups to which virtually all modern fishes belong. Becauseof this, the Devonian period is often called the “age of fishes.”

By late Devonian time, several fish became adapted to land envi-ronments. The fishes had primitive lungs that supplemented theirbreathing through gills. Lobe-finned fish likely occupied tidal flats andsmall ponds. Through time, the lobe-finned fish began to use theirlungs more than their gills. By the end of the Devonian period, theyhad developed lungs and eventually evolved into true air-breathingamphibians with fishlike heads and tails.

Modern amphibians, like frogs, toads, and salamanders, are smalland occupy limited biological niches. However, conditions during theremainder of the Paleozoic were ideal for these newcomers to the land.Plants and insects, which were their main diet, already were very abun-dant and large. The amphibians rapidly diversified because theyhad minimal competition from other land dwellers. Some groupstook on roles and forms that were more similar to modern reptiles,such as crocodiles, than to modern amphibians.

Figure 9 Armor-plated fish werecommon during the Devonianperiod. Inferring What steps wereinvolved in the evolution ofarmor-plated fish into modernday fish?

374 Chapter 13

Integrate BiologySurvival of the Fittest Evolution islargely based on the idea of naturalselection, or survival of the fittest. Thisidea states that any mutation thatincreases the chance of an organism’ssurvival, and therefore reproductivesuccess, would be passed along tofuture generations, making eachsuccessive generation more fit than thelast. Have students consider why thefollowing adaptations would be passedon. Ask: Why would a fish withspecialized organs for lungs be morefit than fish with gills? (The fish withlung structures could obtain oxygen fromair. It could survive if the water dried up orthe tide went out, leaving it on dry land.)Why would organisms able toreproduce on land be more fit thanother organisms? (Organisms able toreproduce on land could survive in a morearid environment.)Verbal, Logical

L2


The lobe-finned fish that developed during thelate Devonian may have used their bony finsto “walk” from dried-up pools in search ofother ponds. As they moved farther across aridPangaea to find water, a lobe-finned fish withan evolved lung structure would be more likely

to survive the trek. Eventually, these hardysurvivors evolved into true amphibians. Somelobe-finned fish with only primitive lungs didmanage to survive this difficult time, andthese became today’s fish. Today’s fish stillhave primitive lungs in addition to gills.

Facts and Figures



By the Pennsylvanian period, large tropical swamps extendedacross North America, Europe, and Siberia. Trees approached 30 meters, with trunks over a meter across. The coal deposits that weuse today for fuel originated in these swamps. See Figure 10. Theselush swamps allowed the amphibians to evolve quickly into a varietyof species.

The Great Paleozoic ExtinctionThe Paleozoic ended with the Permian period, a time when Earth’smajor landmasses joined to form the supercontinent Pangaea. Thisredistribution of land and water and changes in the elevations of land-masses brought pronounced changes in world climates. Broad areas ofthe northern continents became elevated above sea level, and the cli-mate became drier. These climate changes are believed to havetriggered extinctions of many species on land and sea.

By the close of the Permian, 75 percent of the amphibian familieshad disappeared, and plants had declined in number and variety.Although many amphibian groups became extinct, their descendants,the reptiles, would become the most successful and advanced animalson Earth. Much of the marine life did not adapt and survive. At least80 percent, and perhaps as much as 95 percent, of marine life disap-peared. Many marine invertebrates that had been dominant duringthe Paleozoic, including all the remaining trilobites as well as sometypes of corals and brachiopods, could not adapt to the widespreadenvironmental changes.

Figure 10 Model of aPennsylvanian Coal SwampShown are scale trees (left), seedferns (lower left), and scouringrushes (right). Note the largedragonfly.

The Great PaleozoicExtinctionBuild Reading LiteracyRefer to p. 446D in Chapter 16, whichprovides guidelines for sequencing.

Sequence As students read LatePaleozoic Life, ask them to create aflowchart showing how the “age offishes” became the “age of amphibians.”(Sample answer: fish, the dominant life-form, developed primitive lungs and couldstay on land longer → lobe-finned fishspent more and more time on land, solobe-finned fish with stronger lungs weremore likely to survive → lobe-finned fishlungs developed further, making them trueair-breathing amphibians → with verylittle competition on land, amphibianstook off as the dominant life-forms)Visual, Verbal

L1


Large bodies of water moderate the climate ofnearby land areas. Since water heats and coolsslower than land does, coastal areas thatbenefit from sea breezes are warmer in thewinter and cooler in the summer. When all thecontinents merged to form Pangaea, many

areas that had been coastal became landlockedin the continent’s interior. Interior regions donot benefit from the moderating effects oflarge bodies of water. So these areas hadmuch hotter summers and much colderwinters.

Facts and Figures

Answer to . . .

Figure 9 The armor plates thinnedand became light-weight scales, whichincreased the speed and mobility ofthe fish.


376 Chapter 13

Section 13.2 Assessment

Reviewing Concepts1. What are the seven periods that make up

the Paleozoic era?

2. Which present-day continents made upGondwana, Laurasia, and Pangaea?

3. Which life forms dominated the early,middle, and late parts of the Paleozoic era?

4. What allowed amphibians to flourish onland?

Critical Thinking5. Comparing and Contrasting Compare

and contrast the life that existed at thebeginning of the Paleozoic era with the lifethat existed at the end of the era.

6. Applying Concepts Explain how life madethe transition from water to land.

The late Paleozoic extinction was the greatest of at least five massextinctions to occur over the past 500 million years. Each of the massextinctions drastically changed the existing biosphere and wiped outlarge numbers of species. In each case, however, the survivors formednew biological communities that were more diverse than their prede-cessors. Thus, mass extinctions actually allowed life on Earth toflourish, as the few hardy survivors eventually filled more niches thanthe ones left by the victims.

The cause of the great Paleozoic extinction is uncertain. The cli-mate changes from the formation of Pangaea and the associated dropin sea level undoubtedly stressed many species. In addition, at least 2million cubic kilometers of lava flowed across Siberia to produce whatis called the Siberian Traps. Perhaps debris from these volcanic erup-tions blocked incoming sunlight, or perhaps enough sulfuric acid wasemitted to make the seas virtually unfit to live in. Some recent researchsuggests that an impact with an extraterrestrial body may have con-tributed to the mass extinction. Whatever caused the late Paleozoicextinction, it is clear that without it a very different population oforganisms would exist today.

Descriptive Paragraph Imagine you areuncovering rocks and fossils from a sitethat was formed during the Paleozoic era.Write a paragraph describing what kinds offossils you would expect to find as you dugfrom the surface and moved downward.

376 Chapter 13

ASSESSEvaluateUnderstandingHave students review with the class byputting them in cooperative groups toshare the tables they created for theReading Strategy in this section.Encourage students to modify theirtables as needed based on discussionwith their group.

ReteachReview the essential content of thissection with students by using thegeologic time scale on p. 365. Thisfigure clearly shows the order of periodsin the Paleozoic as well as the life-formspresent during each period. Then useFigure 8 to review how landmassesmoved during the late Paleozoic andhow these movements would haveaffected life at that time.

Answers should follow the logicalprogression of life described in thechapter, but should begin with latePaleozoic life (closer to the surface) andend with early Paleozoic life (deeper).Recommend that students use thegeologic time scale on p. 365 or thetable they created in this section to helpthem.

L1

L2

3


5. Life at the beginning of the Paleozoic wasmostly restricted to water. By the end of thePaleozoic, life had begun to inhabit land at arapid pace.6. Life moved onto land as plants developedseeds and other characteristics that allowedthem to live without depending on water.Animals developed hard parts and ways toreproduce without standing water. Oncesome plants and animals were able to live onland, the lack of competition allowed them togrow and diversify rapidly.

Section 13.2 Assessment

1. Cambrian, Ordovician, Silurian, Devonian,Mississippian, Pennsylvanian, Permian2. Gondwana—South America, Africa, Australia,Antarctica, India, and perhaps China;Laurasia—North America, Europe, westernAsia; Pangaea—all the present-day continents3. early—invertebrates; late—amphibians4. Amphibians were able to colonize areasthat other animals were not. The lack of com-petition for resources allowed amphibians tobe successful.


Documents

13.2 Paleozoic Era: Section 13.2 Life ... - Planet Earth 13.2.pdfEarth’s History 369 13.2 Paleozoic Era: ... you would have seen the familiar blue planet ... and the Siberian landmass