Read-and-Say SomethingWork with a partner. As you readeach paragraph of the text, taketurns saying something aboutthe main idea of the paragraph.Help each other understand theinformation in the text.

●A Classify Make thefollowing Foldable to help youorganize geologic time periodsand events into groups based ontheir characteristics.

Study Coach

Geologic Time

section ● Life and Geologic Time1

What You’ll Learn■ how geologic time is

divided■ how plate tectonics and

other changes on Earthaffect species

Read to Learn

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Before You ReadThink about a giraffe you have seen. On the lines below,describe the giraffe and tell why you think it has a long neck.

Geologic TimeA group of students is searching for fossils. By looking in

rocks that are hundreds of millions of years old, they hopeto find fossils of organisms called trilobites (TRI loh bites).Trilobites are small, hard-shelled animals that lived inancient seas. Trilobites are considered to be index fossils.Index fossils lived over vast regions of the world duringspecific periods of geologic time. The students hope that bystudying trilobite fossils, they can help piece together apuzzle. They want to know what caused the trilobites todisappear from Earth millions of years ago.

What is the geologic time scale?The appearance or disappearance of types of organisms

throughout Earth’s history marks important events ingeologic time. Paleontologists, scientists who study theprehistoric world, divide Earth’s history into time unitsbased on life-forms that existed only during certain periods.This division of Earth’s history is known as the geologictime scale. Sometimes few fossils remain from a period.Then paleontologists use other methods to define a divisionof geologic time.

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Reading Essentials 135

Cretaceous Period

QuaternaryPeriod

TertiaryPeriod

HoloceneEpoch

PleistoceneEpoch

Jurassic Period

Triassic Period

Permian Period

Pennsylvanian Period

Mississippian Period

Devonian Period

Silurian Period

Ordovician Period

Cambrian Period

Proterozoic Eon

Archean Eon

Hadean Eon

PlioceneEpoch

Miocene

Epoch

OligoceneEpoch

EoceneEpoch

PaleoceneEpoch

Millions of years ago

Cenozoic EraMesozoic EraPaleozoic EraPrecambrian

Time

0.008

1.8

5.3

23.8

33.7

55.5

65145

213

248

286

325

360

410

440

505

544

2,500

3,800

4 ,500

Phanerozoic Eon

Origin of Earth

First life

First trilobites First flowering plants

Mass extinction

1. Identify What is thelongest subdivision ofgeologic time?

2. Interpret What majorevent occurred at the endof the Paleozoic Era?

What are major subdivisions of geologic time?The fossil record is used to divide Earth’s history into

geologic time periods. The figure below shows the fourmajor subdivisions of geologic time—eons, eras, periods,and epochs. Eons are the longest subdivision and are basedon the abundance of certain fossils.

Eons are divided into smaller time periods called eras. Anera is marked by major worldwide changes in the types offossils present. For example, at the end of the Mesozoic Era,many kinds of invertebrates, birds, mammals and reptilesbecame extinct.

Eras are subdivided into periods. A period is a unit ofgeologic time during which certain types of life-formsexisted all over the world.

Geologic periods are divided into epochs. An epoch isalso characterized by differences in life-forms, but these mayvary from continent to continent. Epochs may be givennames, like those in the Cenozoic Era or may be calledsimply early, middle, or late.

What limits the divisions of geologic time?There is a limit to how finely geologic time can be

subdivided. It depends on the kind of rock record that isbeing studied. Sometimes it is possible to distinguishdifferent layers of rock that formed during a single year. Inother cases, there is little information to help scientistssubdivide geologic time.

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136 Geologic Time

3. Identify What is thename for a group oforganisms that normallyreproduce only with othermembers of their group?

4. Identify Who proposedthe theory of naturalselection?

Organic EvolutionThe fossil record shows that species (SPEE sheez), or types

of life-forms, have changed over geologic time. Organicevolution is the change in species over geologic time. Mostscientists believe that changes in the environment affect anorganism’s survival. Organisms that do not adapt to thechanges are less likely to survive or reproduce. Over time,the disappearance of individuals that are not adapted to thenew conditions can cause changes to species of organisms.

What is a species?There are many ways to define the term species. Life

scientists often define a species as a group of organisms thatnormally reproduce only with other members of theirgroup. For example, horses generally reproduce only withother horses. Sometimes, members of different species canmate and produce offspring. Horses sometimes mate withdonkeys and produce mules. However, the offspring of twodifferent species are often sterile.

What is natural selection?Charles Darwin was a naturalist—someone who studies

the natural world. Between 1831 and 1836, Darwin sailedaround the world, carefully observing plants and animals.He collected samples of life-forms and studied them tolearn how they were related.

After returning home to England, Darwin explained histheory of natural selection. Darwin defined natural selectionas the process by which organisms with characteristicssuited to a certain environment have a better chance ofsurviving and reproducing than organisms that do not havethese characteristics. Darwin understood that all organismscompete for resources, such as food and living space. He alsoknew that individuals within a species could be different, orshow variation. An individual’s differences might help or hurtits chances of surviving in a changing environment.

Some organisms that were well suited to their environmentlived longer and had a better chance of producing offspring.Organisms that were poorly adapted to their environmentproduced few or no offspring. Because many characteristicsare inherited, the characteristics of organisms that are betteradapted to the environment get passed on to offspring moreoften. According to Darwin, this can cause a species tochange over time.

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Reading Essentials 137

5. Predict What might havehappened to the animals intheir new environment ifnone of them had had alonger neck?

What is natural selection within a species?Suppose that an animal species exists in which a few of

the individuals have long necks, but most have short necks.The main food for the animal is the leaves on trees in thearea. Then suppose the climate changes and the areabecomes dry. The lower branches of the trees might nothave any leaves. Now which of the animals will be bettersuited to survive? In this case, the animals with the longernecks will be better able to eat the leaves. Clearly, thelong-necked animals have a better chance of surviving andreproducing. Their offspring will have a greater chance ofinheriting the important characteristic. Gradually, as thenumber of long-necked animals becomes greater, thenumber of short-necked animals decreases. Over time, thespecies might change so that nearly all of its members havelong necks—just like the giraffe.

It is important to notice that individual, short-neckedanimals did not change into long-necked animals. A newcharacteristic becomes common in a species only under twoconditions. First, some members must already have thatcharacteristic. Second, the trait must increase the animal’schance of survival. If no animal in the species had a longneck in the first place, a long-necked species could not haveevolved by means of natural selection.

What is artificial selection?Humans have long used artificial selection to breed

domestic animals. Animal breeders carefully chooseindividuals animals with desired characteristics to mate.Their offspring also have the desired characteristics. In thisway, animal breeders have created many different breeds ofcats, dogs, cattle, and chickens.

How do new species evolve?Natural selection explains how characteristics change and

how new species arise. Remember the animals with shortnecks? If they had moved, or migrated, to a different area,they may have survived. Then they may have reproduced inthe new area, developing different characteristics from thelong-necked animals. If the short-necked animals weredifferent enough from the long-necked animals that theycould no longer breed, then a new species would haveevolved.

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138 Geologic Time

6. Explain How could theanimals with short neckssurvive without evolving?

TrilobitesRemember the trilobites you read about earlier? A

trilobite is an ancient organism with a three-lobedexoskeleton. An exoskeleton is a hard outer skeleton. Theseancient animals were given the name trilobite because theyhave a three-part shell. The figure above shows the threeparts of the trilobite shell. The three parts, called lobes, runthe length of its body. Trilobites also have a head(cephalon), a middle that is segmented or divided intosections (thorax), and a tail (pygidium).

What do the changing characteristics oftrilobites tell scientists?

Trilobites lived in Earth’s oceans for more than 200million years. All through the Paleozoic Era, some species oftrilobites became extinct and other species of trilobitesevolved. Different periods of the Paleozoic Era had differentspecies of trilobites. Each species of trilobites had its ownparticular characteristics that were different from all otherspecies.

Paleontologists use the differences in trilobite species toexplain how trilobites evolved over geologic time. Thesechanges tell how different trilobites from different periodslived. The changes also tell how trilobites responded tochanges in their environment.

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7. Identify Look carefully atthe picture of a trilobite.With a red pencil, outlinethe trilobite’s lobes. With ablue pencil, circle its head,middle, and tail.

8. Identify What were thetwo things that happenedto trilobite species duringthe Paleozoic Era?

Reading Essentials 139

Cephalon(SE ful lahn)

Thorax(THOR aks)

Pygidium(pi JIH dee uhm)

Side lobe

Middle lobe

Side lobe

What do trilobite eyes reveal?Trilobites may have been the first organisms on Earth

with complex eyes as shown in the figure above. Trilobiteeyes are the result of natural selection.

The position of an organism’s eyes tells how it lived. Ifits eyes are in the front of its head, it likely swam activelythrough the sea. If its eyes are located toward the back ofits head, it likely lived on the bottom of the ocean. Mostspecies of trilobites had eyes that were midway between thefront and the back of the head. This clue tells us thattrilobites were adapted to both active swimming andcrawling on the ocean floor.

What changes occurred in trilobite eyes?Over time, the eyes in some trilobites changed. Gradually,

the eyes of many trilobite species became smaller andsmaller. Eventually, their eyes disappeared completely. Theseblind trilobites, shown above, might have burrowed intosediments on the ocean floor. Or they lived in a part of thedeep ocean where there was no light.

Not all trilobite species lost their eyes. Some trilobitespecies developed highly complex eyes. One species oftrilobite had compound eyes—eyes with many individuallenses. These trilobites had excellent vision. Still other trilobitespecies developed complex eyes on stalks that extended fromtheir head. They also could see their world very well.

What changes occurred in trilobite bodies?The trilobite body also changed over geologic time, as

shown in the figure below. Some early trilobite species hadmany segments in the middle part of the body. Latertrilobites had fewer segments.

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140 Geologic Time

9. Identify Whatcharacteristic helpsscientists figure out wheredifferent trilobite specieslived?

Picture This10. Identify Highlight the

body segments of thetrilobites in the figure.

Ordovician Period Devonian Period

Cambrian Period

Silurian Period

Mississippian Period

What evidence do fossils provide?The exoskeletons of trilobites changed as their

environment changed. Each change in the trilobite bodyshows how different species of trilobite adapted to newconditions. Some species of trilobite could not adapt.These species disappeared, or became extinct.

Plate Tectonics and Earth HistoryEarth’s crust is made up of several plates. These plates

are in slow but constant motion. This motion, called platetectonics, caused continents to split apart or to collide. Atthe time the trilobites dominated Earth’s seas, Earth’s plateswere moving together. When all the continents collided, theyformed a single, enormous continent known as Pangaea(pan JEE uh), shown in the figure below. Pangaea was onegiant landmass, or supercontinent. When Pangaea wasforming, sea levels were dropping. Because trilobites lived inthe ocean, they could not survive in the changedenvironment. At the end of the Paleozoic Era, trilobitesbecame extinct.

Some scientists do not accept that the formation ofPangaea caused the extinctions at the end of the PaleozoicEra. Changes in the climate or other conditions may haveled to the Paleozoic extinctions. As in all scientific debates,evidence must be considered carefully, and conclusions mustbe drawn based on the evidence.

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11. Explain What did eachchange in the trilobite bodyshow?

Picture This12. Outline In the figure,

outline the borders of thecontinents that crashedtogether to form thesupercontinent, Pangaea.

PA

NG

A

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Reading Essentials 141

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End ofSection

Visit blue.msscience.com to access your textbook, interactivegames, and projects to help you learn more about geologic timeand natural selection.

142 Geologic Time

1. Review the terms and their definitions in the Mini Glossary. Then write two sentencesabout geologic time and natural selection. Use at least four vocabulary words in yoursentences.

2. Fill in the correct term to show how the Geologic Time Scale is divided.

a.______________________________

b.______________________________

c.______________________________

d.______________________________

After You ReadMini Glossaryeon: largest geologic time division that is based on the

abundance of certain fossilsepoch: geologic time division characterized by differences in

life-forms, which may vary from continent to continentera: geologic time division marked by major worldwide

changes in the types of fossils presentgeologic time scale: divisions of time in Earth’s historynatural selection: process by which organisms with

characteristics suited to a certain environment have abetter chance of surviving and reproducing thanorganisms that do not have these characteristics

organic evolution: change in species over geologic timePangaea: one giant landmass, or supercontinent, that

formed at the end of the Paleozoic Eraperiod: subdivision of geologic time during which certain

types of life-forms existed all over the worldspecies: group of organisms that normally reproduce only

with other members of their grouptrilobite (TRI loh bite) : small, hard-shelled animal that

lived in ancient seas

Cretaceous Period

QuaternaryPeriod

TertiaryPeriod

HoloceneEpoch

PleistoceneEpoch

Jurassic Period

Triassic Period PlioceneEpoch

Miocene

Epoch

OligoceneEpoch

EoceneEpoch

PaleoceneEpoch

Cenozoic EraMesozoic Era

Phanerozoic Eon