The Mesozoic and Cenozoic Era

8/12/2019 The Mesozoic and Cenozoic Era

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624

What Youll Learn How plate tectonics

shaped the landscape of western North America.

What the characteristics

of a dinosaur are. How dinosaurs and

many other organismsbecame extinct at theend of the Mesozoic Era.

Why Its ImportantAs the physical geologyof Earth changed, so did

the biosphere. Reptilesruled the land during theMesozoic, but their reignended abruptly with thedawn of the Cenozoic.

The Mesozoic and Cenozoic Eras

The Mesozoic and Cenozoic Eras

2424

Albertosaurus and Centrosaurus from theCretaceous of Alberta, Canada

Albertosaurus and Centrosaurus from theCretaceous of Alberta, Canada

To find out more about theMesozoic and CenozoicEras, visit the Earth ScienceWeb Site at earthgeu.com
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24.1 Mesozoic Paleogeography 6

How can the shape of a micro-fossil help paleontologists determinewhether it floated or whether it livedon the seafloor? In this lab, you willlearn how the shape of a fossil shellaffected the animals buoyancy, andhow this information helps paleontol-ogists determine how the animal lived.

. Fill a 1000-mL beaker with water.

Using plasticene, make two solid

spheres that are each 1.5 cm diameter.Deform one of the spheres by pinch-ing the plasticene into 5 spinesthatextend around the sphere.

Drop both spheres into the beakerat the same time and observe whathappens.

In your science journal, record your observa-tions. What effect does theshape of the spheres have ontheir buoyancy? The photoson this page show marinemicrofossils. Based on your

observations in this lab,which one do you think floated in the water? Whichone lived on the seafloor?Explain your reasoning.

Discovery L ab Discovery L ab

OBJECTIVES

Explain the breakupof Pangaea.

Distinguish between the different tectonic charac-teristics of the Mesozoic Orogenies.

VOCABULARY

Cordillera

The Mesozoic Era consisted of the Triassic, Jurassic, and Cretaceousperiods, as shown in Figure 24-1 on page 626.Movies such as JurasPark and The Lost World have popularized the Mesozoic as the age othe dinosaurs. What was the world like in which dinosaurs such asthose pictured on page 624 lived? Why did this chapter in Earths his-tory end? These and other questions make the Mesozoic one of themost fascinating times of study in Earths history.

THE BREAKUP OF PANGAEAAn important event that occurred during the Mesozoic Era was the

breakup of Pangaea. Because heat causes solid objects to expandthe heat coming from within Earth beneath Pangaea caused thecontinent to expand. By the Late Triassic, the brittle lithosphere ofPangaea had cracked and broken apart. As some of the large cracks,or rifts, widened and as the landmasses spread apart, the oceanflooded the rift valleys. This resulted in the formation of newoceans that divided the newly separated continents.

Mesozoic Paleogeography 24.124.1
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The breakup of Pangaea resulted in the formation of theAtlantic Ocean.As North America rifted away from Europe andAfrica, some of the spreading areas joined together to form along, continuous rift system called the Mid-Atlantic Ridge. TheMid-Atlantic Ridge is still active today.

ACTIVE TECTONISM IN WESTERN NORTH AMERICAIn contrast to the passive margin that existed along easternNorth America during the Mesozoic, active subduction alongthe western coast continued through the Middle Triassic.Geologists refer to the mountain ranges that formed in west-ern North America during this time as the Cordillera, whichmeans mountain range in Spanish.

Deformation along the western margin of North Americaincreased substantially when Pangaea broke apart. Three majorepisodes of orogeniesmountain buildingoccurred along

the western margin of North America during the Mesozoic. Differenttypes of deformation occurred during each of these orogenies.

The oldest orogeny was characterized by a tremendous numberof igneous intrusions. Large bodies of granite called batholiths existthroughout the cordillera. The spectacular exposure of Half-Domeat Yosemite National Park was intruded during this orogeny.

The next orogeny was characterized by low-angle thrust faultingand folding. This was caused by collisional tectonism along the west-ern margin of North America. This type of deformation began in theLate Jurassic and continued through the Late Cretaceous. The thrustfaults run north-south and place older rocks on top of younger rocksin Utah, Idaho, Wyoming, western Canada, and Montana, shown inFigure 24-2 .

Cretaceous Period

M e s o z o i c

E r a

66 M.Y.B.P.

208 M.Y.B.P.

245 M.Y.B.P.

146 M.Y.B.P.

Jurassic Period

Triassic Period

Figure 24-1 Each Mesozoicperiod is distinct. Pangaeabroke apart and new sea-ways formed during theTriassic. Dinosaurs domi-nated the land during theMesozoic. Flowering plantsevolved during the Creta-ceous. Youll learn moreabout these events as youstudy this chapter.

Figure 24-2 These Paleo-zoic limestones in theSawtooth Mountains, MTwere faulted and thrusteastward during the secondMesozoic orogeny.

626 CHAPTER 24


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The third Mesozoic orogenic event was characterized by vertical uplifts. This orogeny mainly affected the area east of the folds and faults caused by the second orogeny.Deformation caused by the third orogeny began during theLate Cretaceous and continued into the Cenozoic.

SEAWAYS AND SAND DUNESThroughout the Early and Middle Triassic, the superconti-nent Pangaea and a single global ocean defined Earths pale-ogeography. As Pangaea began to split apart, numerous riftbasins formed in eastern North America, and large blocks of crust collapsed to form deep valleys. The Triassic ended witha rapid drop in sea level that caused sedimentation in thewestern United States to change dramatically during the LateTriassic and Early Jurassic. Western North America becamemuch more arid, and it was covered by a thick blanket of sand. Strong winds shaped the sand into dunes. Evidence of this ancient desert is preserved in large-scale, cross-beddedsandstone deposits some of which are shown in Figure 24-3.

Sea level rose again in the Jurassic, and a shallow sea cov-ered central North America. The Appalachian Mountains still rosehigh in the east, and the newly formed mountains of the Cordillerarose high in the west. As the mountains continued to uplift in thewest, large river systems transported sediments from the mountainsinto the sea. The deposits of the Late Jurassic river systems are pre-served today as multicolored sandstones, siltstones, and mudstones.They are well known for large numbers of dinosaur fossils.

The ocean continued to rise onto North America duringthe Cretaceous Period, and the Gulf of Mexico flooded the entiresoutheastern margin of North America. As a result, a sea covered theinterior of North America from Texas to Alaska.

24.1 Mesozoic Paleogeography 6

Figure 24-3 The effects ofthe wind is preserved inthese sand dunes in theVermillion Cliffs Wilderness,AZ.

1. Explain why Pangaea rifted apart.2. Describe the difference between the

three major episodes of mountain build-ing that occurred during the Mesozoic.

3. Thinking Critically What might happen ifa continent stops rifting apart?

SKILL REVIEW4. Recognizing Cause and Effect Explain

how heat that is generated in Earthsinterior can cause continents to rift apart,as in the case of Pangaea. For more help,refer to the Skill Handbook.

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24.224.2 Mesozoic Life The Mesozoic is commonly referred to as the Age of Reptiles.However, the Mesozoic also was a time of some other very importantbiological firsts, such as the first mammals, the first birds, and the

first flowering plants. The mass extinction at the end of the PaleozoicEra had left Earths biosphere quite barren, and therefore ripe for theappearance of new organisms.

As Pangaea began to split apart during the Triassic, much of thehabitat on the continental shelves that was lost during the formationof Pangaea became available again. New marine organisms, whichare collectively called the modern fauna, evolved which filled thishabitat. The modern fauna includes crabs, lobsters, shrimps,sponges, sea urchins, modern corals, snails, and clams. The majormarine vertebrate groups include bony fishes, sharks, aquatic rep-tiles, and aquatic mammals.

LIFE IN THE OCEANSMuch attention is given to the large animals of the Mesozoic. It isimportant, however, to remember that then, as now, the base of thefood chain that supported all the large animals consisted of tiny,ocean-dwelling organisms called phytoplankton, shown in Figure 24-4. Phytoplankton float near the surface of oceans and lakes andmake their own food through the process of photosynthesis. Youexamined the relationship between shape and floating ability in theDiscovery Lab on page 625.

Reef Builders Arise Again As you learned in Chapter 23, the Permo-

Triassic Extinction Event wiped outthe reef-building corals of the

Paleozoic. By the end of theTriassic, the modern corals hadevolved to fill this same niche.In addition to corals, a new group of clams called rudists

developed the ability to buildreefs during the Cretaceous.Rudists are important because

the reefs that they built were very porous and today contain some of

the largest Cretaceous oil deposits inareas such as West Texas.

OBJECTIVES

Discuss why many paleontologists theorize

that birds are descended from dinosaurs.

Describe how paleontol-ogists distinguish among reptile, dinosaur, and mammal fossils.

Explain the evidence indicating that a mete-orite impact caused the Cretaceous-Paleogene mass extinction event.

VOCABULARY

modern fauna angiospermdinosaur Ornithischia Saurischia ectotherm

endothermiridium

628 CHAPTER 24 The Mesozoic and Cenozoic Eras

Magnification 48

Figure 24-4 Thesephytoplankton arediatoms; tiny freshwateror marine plants. Upondeath, their silica shellscontribute to the oceanfloor sediment.

EnvironmentalConnection


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Important Index Fossils Ammonitesare related to modern nautiluses, octo-puses, and squids. Ammonites were abun-dant and diverse throughout the LatePaleozoic and Mesozoic, and their abun-dance indicates that they were very success-

ful predators. Several ammonites are shownin Figure 24-5. Ammonite fossils are wide-spread and abundant. Because ammonitespecies were also short-lived, they are excel-lent index fossils.

While the ammonites were successful Mesozoic marine predators,they were by no means the top predators of the Mesozoic. In fact,swimming reptiles ruled the Mesozoic oceans. Ichthyosaurs resem-bled modern dolphins while plesiosaurs were more like walruses andseals. Predatory mosasaurs lived only during the Cretaceous, but forthat time in the history of life, the sea was theirs to rule. TheseMesozoic reptiles are shown in Figure 24-6.

LIFE ON THE LANDLife on land changed dramatically as the cool climate that character-ized the end of the Paleozoic over many parts of Earth came to an endduring the Mesozoic. The large, temperate coal swamps dried up, andthe climate gradually warmed. Fossils of insects that dominated thePaleozoic coal swamps, such as flies, mosquitoes, wasps, and bees,were also present but were not as common in Mesozoic rocks.

24.2 Mesozoic Life 6

Figure 24-5 Theseammonites are from theLower Jurassic of Europe.Their distinct shell shapeand ornamentation addsto their usefulness asindex fossils. The use of

ammonites to determinethe age of Mesozoic rocks isusually much more accuratethan radiometric dating.

Figure 24-6 The

presence in theoceans of aggressivemarine predators,such as ichthyosaurs(A), mosasaurs (B),and plesiosaurs (C),no doubt kept theirintended prey onalert.

AB

C


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The land flora also changed substantially during the Mesozoic.Paleobotanists rightly call the Mesozoic the Age of Cycads. Cycads areseed plants that do not have true flowers.During the Jurassic, tall cycadtrees along with ginkgoes and conifers dominated the landscape, asshown in Figure 24-7.

Angiosperms Evolve A new kind of plant evolved during theCretaceous that would come to dominate the terrestrial landscape.This important group of plants was the angiosperms, which areseed-bearing plants that have flowers. Before the Cretaceous, therewere no flowers. By the Middle Cretaceous, however, flowering plantswere common. By the end of the Cretaceous, the land was coveredwith flowering trees, shrubs, and vines.

Early Mammals Small, primitive mammals evolved during theLate Triassic. Mammals are easily recognizable today because they are warm-blooded, have hair or fur, and mammary glands. How doscientists identify fossils of mammals? One method is to examine thestructure of the lower jaw, middle ear, and teeth. A reptile has one ear

bone and multiple lower jawbones, whereas a mammal has one lower jawbone and three ear bones. Early mammals arose from mammal-like reptiles. The evolution of the mammal jaw is illustrated in Figure 24-8. Mammals have two sets of teeth during their lives, and theirteeth are differentiated as incisors, canines, and molars. In contrast,reptiles generally have only one kind of tooth, and their teeth con-tinually grow and replace older teeth throughout their lives.


Figure 24-7 Cycads havea limited distribution today.They grow only in thesouthern hemisphere, inCentral America and inFlorida. Only one speciesof ginkgo survives today,Ginkgo biloba, and it is

quite common.

Primitive mammal-like reptile

Primitive therapsid

Advanced therapsid

Mammal

Figure 24-8 Fossils clearlyshow the progressiveenlargement of one ofthe lower jawbones inmammal-like reptiles andits evolution into a single

jawbone, which is charac-teristic of mammals. At thesame time, the other jaw-bones decreased in sizeand became ear bones.


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Flying Reptiles Pterosaurs, as shown inFigure 24-9, were flying reptiles that domi-nated the air during the Mesozoic. The ear-liest pterosaurs were small and had longtails and wingspans of less than 60 cm.Their descendents had no tails and weregenerally much larger. One species had awingspan of more than 12 m. Pterosaurshad light, hollow bones, like modern birds.The modification that allowed them to fly was the growth of a membrane froma greatly lengthened fourth finger.

Two groups of reptiles that arose duringthe Mesozoic and still exist are the croco-diles and turtles. Why crocodiles and turtles survived the greatextinction at the end of the Mesozoic, while the mosasaurs, ple-siosaurs, icthyosaurs, and dinosaurs did not, puzzles paleontologiststo this day.

DINOSAURS RULE THE LANDNo other group of animals has captured the human imagination asmuch as the dinosaurs have. Dinosaurs were a group of reptilesthat developed an upright posture about 228 million years ago. Eventhough our understanding of these rulers of the Mesozoic land haschanged, our fascination with dinosaurs has remained. Dinosaurscame in all sizes, from the very small to the extraordinarily large,and all were terrestrial. You can learn how to estimate a dinosaursweight in the Science and Math feature on page 644. Although thelargest dinosaurs were most likely slow and plodding animals, many of them were quick and agile. All reptiles other than dinosaurs havea sprawling posture, that is, their legs are not set directly underneaththeir bodies. In contrast, as shown in Figure 24-10, dinosaurs legswere set directly underneath their bodies because their hips andankles were different from those of other reptiles.

Figure 24-10 The charac-teristic sprawling postureof reptiles is shown here.Dinosaurs, with theirstraight shoulders, ankles,and hips, stood apart fromthe reptiles.

Figure 24-9 This pterosaurfossil was found in Jurassic-aged rocks in Germany.The membrane that wasattached to the fourth fin-ger was actually a flap oftough skin that stretchedfrom the finger to the sidesof the pterosaurs body.



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Dinosaur Hips Two major groups of dinosaurs are recognizedbased on their hip structure: Ornithischia and Saurischia.Three bonescomprised the hip, as shown in Figure 24-11. The ischium and pubiswere parallel to one another in ornithischian dinosaurs. This is similarto the orientation of these bones in modern birds, which is why scien-tists named this group of dinosaurs Ornithischia, meaning bird-hipped. This name is unfortunate, however, because birds likely didnot evolve from Ornithischia. The ischium and pubis were at an angleto one another in Saurischia, similar to the orientation observed inmodern lizards. Saurischia means lizard hipped. Scientists hypoth-esize that birds are actually descended from the Saurischia.

There were five different groups of ornithischian dinosaurs:stegosaurs, ankylosaurs, pachycephalosaurs, ceratopsians, andornithopods. All ornithischian dinosaurs were plant eaters, alsocalled herbivores. There were two different groups of saurischiandinosaurs: sauropods and theropods. Although they shared a similarhip structure, these two groups of dinosaurs were quite different. Thesauropods were all quadrupedalwalked on four legsplant eaters,and some grew to enormous sizes.They were the largest land animalsto have ever lived. In contrast to the sauropods, all theropods werebipedalwalked on two legscarnivores. Your research results fromthe GeoLab on page 642 will help you interpret what dinosaurs ate.Figure 24-12 shows some representative Mesozoic dinosaurs.


Ilium

Ilium

Pubis

PubisIschium

Ischium

Ornithischia

Saurischia

Figure 24-12 Herbivoressuch as Parasaurolophus(right) and Triceratops (cen-ter) had to constantly be onthe lookout for carnivoressuch as Tyrannosaurus.

Figure 24-11 The arrange-ment of hip bones in dino-saurs serves as a basis fortheir classification.

To learn more aboutdinosaurs, go to theNational GeographicExpedition on page 892.
http://endmat.pdf/http://endmat.pdf/http://endmat.pdf/http://endmat.pdf/http://endmat.pdf/http://endmat.pdf/http://brieftoc.pdf/


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Dinosaurs to Birds? The idea that birds are related to dinosaursstems from the amazing similarities between theropods and the old-est known bird, Archaeopteryx. Fossils of feather impressions and awishbone provide clear evidence that Archaeopteryx was definitelya bird, even though it did have teeth and a theropod-like skeleton.

Ectotherm or Endotherm? All living reptiles are ectotherms,meaning that their body temperatures vary in response to outsidetemperatures. All living mammals and birds are endotherms, mean-ing that they maintain relatively constant body temperatures, regard-less of temperatures outside. Some paleontologists hypothesize thatat least some groups of dinosaurs were endotherms. One reason isthat bones of endotherms typically have more passageways for bloodthan bones of ectotherms do, and dinosaur bones have numerouspassageways. Critics of the hypothesis that some dinosaurs wereendotherms correctly point out that the bones of crocodiles and tur-tles also have numerous passageways, and yet they are ectotherms.

A recent discovery of a possible fossilized heart in a dinosaur sup-ports the hypothesis that some dinosaurs were endothermic. Theheart has four chambers and one aortal arch, which is a conditionthat exists only in endothermic animals. Whether dinosaurs wereectotherms or endotherms is still controversial, but for now, thereseems to be evidence that perhaps some dinosaurs were endotherms.

M ASS EXTINCTIONSA major mass extinction event ended the Mesozoic. Most majorgroups of organisms were devastated and all known species of

dinosaurs, pterosaurs, ammonites, mosasaurs, and plesiosaurs


Using Numbers Theend of the Permianis marked by anextinction event inwhich 80 percent ofall marine generabecame extinct, andthe end of the Creta-ceous is marked byan extinction eventin which 50 percentof all marine generabecame extinct. Howmany times greaterwas the Permian

extinction event thanthe Cretaceousextinction event?

DEBRISIMPACT SITE

Tampico

Tropic of Cancer

Mexico City

Mexico

Guatamala

Belize

YucatanPeninsula

Gulf of Mexico

Map of Impact Site

Figure 24-13 The circularshape and underlying layerof melted iridium-rich rockprovides evidence of ameteorite impact. It wasnamed the Chicxulub craterafter a nearby village.


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became extinct. Geologists theorize that a large meteorite, at least10 km in diameter, slammed into the Yucatan Peninsula shown inFigure 24-13 on page 633, at a speed of up to 240 000 kph. Such animpact would have blown 25 trillion metric tons of rock and sedi-ment high into the atmosphere. Organisms lucky enough to havesurvived the impact would have faced a millennium of greenhouse

warming and increased levels of UV radiation from the Sun.The chemistry of a clay layer that separates Cretaceous fromPaleogene rocks in Italy provides evidence of such a meteoriteimpact. An unusually high amount of iridium, a metal that is rarein rocks at Earths surface but is relatively common in meteoritesand asteroids, is found not only in Italy but in Cretaceous-Paleogeneboundary sites worldwide. Soot and charcoal, which are evidence of widespread fires, are also common in the sediments.

A buried crater in the Gulf of Mexico contains iridium, which hasa radiometrically dated age of approximately 65 million years. Thisage is close to the time of the Late Cretaceous mass extinctions. It isimportant to note, however, that elevated amounts of iridium arealso present in Earths interior, and thus, volcanic deposits can beenriched with iridium. During the Late Cretaceous, massive volcaniceruptions occurred in India.

Today, most scientists agree that both a large meteorite impactand massive volcanism occurred at the end of the Cretaceous. In themidst of a stressful time of climatic cooling, changing plant popula-tions and a gradual decline in dinosaur diversity and abundance, alarge meteorite struck Earth. The extraordinary stress that theimpact added to an already stressed ecosystem likely caused the cli-

max of the Cretaceous-Paleogene mass extinction.


1. How do paleontologists distinguishbetween fossils of mammals and fossilsof reptiles?

2. Discuss how Archaeopteryx supports thehypothesis that birds are descended fromdinosaurs.

3. Explain why a cycad is not an angiosperm.

4. What is the main characteristic that sepa-rated dinosaurs from all other reptiles?

5. Describe the differences betweenSaurischia and Ornithischia.

6. Thinking Critically What conclusions canbe stated, based on tooth characteristics,regarding the differences in diet betweenreptiles and mammals?

SKILL REVIEW

7. Comparing and Contrasting Compare andcontrast the evidence for meteoriteimpact and volcanism at the end of theCretaceous. For more help, refer to theSkill Handbook.

Topic: Meteor Impactsand ExtinctionsTo find out more aboutmass extinction, visit theEarth Science Web Site atearthgeu.com

Activity: Prepare a map ofthe world showing threelarge meteor impactsbesides the Chicxulubimpact. Indicate theapproximate dates on yourmap. Do any of these datescorrespond to other massextinctions?

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OBJECTIVES

24.324.3 Cenozoic Paleogeography OBJECTIVES

Describe the type of tetonism that characterizethe Cenozoic orogeny.

Understand the extentglaciation that occurredin North America.

VOCABULARY

Basin and Range ProvincTethys Sea

24.3 Cenozoic Paleogeography 6

You are living during the Cenozoic, which means recent life. TheCenozoic Era encompasses approximately the last 66 million years of Earths history. The Cenozoic portion of the geologic time scale is

shown in Figure 24-14. The final breakup of Pangaea occurred duringthe Cenozoic. Earths life-forms and surface features continued tochange, evolving into their present form. The Cenozoic has been atime of major climate changes, which have been in part, caused by thepositions of the continents.

The Ice Ages As Australia split apart from Antarctica during theMiddle-to-Late Eocene, the warm climate began to deteriorate.A change in ocean circulation is thought to be the cause. WhenAntarctica and Australia were connected as shown in Figure 23-15 onpage 612, a current of warm water from the north moderated thetemperature of Antarctica.When Antarctica split apart from Australiaduring the Oligocene, it was isolated over the south pole. A cold cur-rent began to flow around Antarctica, and glaciers began to form.

The climate began to warm again during the Early Miocene. Theglaciers on Antarctica began to melt, and the sea rose onto the mar-gin of North America. In the Problem-Solving Lab on page 637, youwill consider glacial melting and the length of time involved. Glaciersreturned to Antarctica during the Middle and Late Miocene. Duringthe Pliocene, the water of the Arctic Ocean began to freeze to forman arctic ice cap, which set the stage for the ice ages of the LatePliocene and the Pleistocene.

During the Late Pliocene through the Pleistocene,the northern hemisphere experienced extensiveglaciation, or an ice age. Glaciers from the arcticadvanced and retreated in at least four stages overNorth America. The paths of the Ohio River and theMissouri River roughly mark the southernmost pointto which glaciers advanced in North America. Duringthe peak of Pleistocene glaciation, glaciers up to 3 km-thick covered some areas north of these rivers. You

will model the effect glaciers have on sediment depo-sition when you complete the MiniLab on page 636.

TECTONIC EVENTSWestern North America had been tectonically activethroughout the Cenozoic. The orogenic events thatoccurred at the end of the Mesozoic uplifted massive

Quaternary Period

Holocene Epoch

Pleistocene Epoch

Pliocene Epoch

Miocene Epoch

Oligocene Epoch

Eocene Epoch

Paleocene Epoch

C e n o z o

i c E r a

Recent

23 M.Y.B.P.

66 M.Y.B.P.

1.6 M.Y.B.P.

1.6 M.Y.B.

5 M.Y.B.P.

23 M.Y.B.P

35 M.Y.B.P

56 M.Y.B.P

66 M.Y.B.P

0.01 M.Y.B

Neogene Period

Paleogene Period

Figure 24-14 Although thelast Ice Age began about2 million years ago duringthe Pleistocene, ice age con-ditions really began about40 million years ago duringthe Oligocene Epoch.


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Glaciers and DepositionModel the deposition of sediment bymelting glaciers.

Procedure1. Pour water into a large, wide-mouthed jar

until it is approximately 3/4 full.2. Add a mixture of clay, silt, sand, and peb-

bles to the jar. Put the lid tightly on the jar.3. Shake the jar for 30 seconds and allow the

contents to settle.4. Finely crush enough ice to fill approxi-

mately three-fourths of a second large,wide-mouthed jar. Stir a mixture of clay,silt, sand, and pebbles into the ice andpour the mixture into the jar.

5. Allow the ice to melt and the particles tosettle overnight.

Analyze and Conclude1. Describe the differences in the way the

sediments settled in the two jars.2. Compare and contrast the sorting of the

grain sizes in the two jars.3. How could geologists use this information

to determine whether sediment had beendeposited by a glacier or by running water?

blocks of crust to form the Rocky Mountains. Large basins that formed adja-cent to the Rocky Mountains were filledwith as much as 3000 m of sediment fromthe uplifted and eroded mountains. The sed-iment that filled these basins contains beau-

tifully preserved fish, insect, frog, plant, andbird fossils. A fossil fish from the mostfamous of these deposits, the Green RiverFormation in Wyoming, is shown in Figure 24-15. The basins in Wyoming that filledwith huge, swampy, river deposits providedan ideal environment for the accumulationof vast amounts of coal. These coal seams areclose to Earths surface, and some are morethan 50 m thick. Wyoming is one of thelargest coal mining regions in the world. Thecoal there is especially valuable because ithas a very low sulfur content, and thus, itburns cleanly.

Subduction in the West Volcanismreturned to the western coast of NorthAmerica at the end of the Eocene. TheCascade Mountains in the Pacific Northwestare the result of the subduction of an oceanicplate beneath the western coast of NorthAmerica. During the Miocene, the NorthAmerican Plate was forced over the EastPacific Rise resulting in the creation of theSan Andreas Fault. Because there is currently

Figure 24-15 Fossil fish, like this50 million year old Phareodus sp.from the Eocene-aged, GreenRiver formation in Wyoming, arepreserved in spectacular detail.


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24.3 Cenozoic Paleogeography 6

New Continental Shorelines? If allthe ice on Earth were to melt, sea levelwould rise to approximately 50 m aboveits current level. Sea level is rising todayat an average rate of 2 mm per year.Assuming that sea level will continue to

rise at this rate, how long will it take forall the ice on Earth to melt and sea levelto rise to its maximum? Use the relation-ship distance rate time. Rememberthat 1000 mm 1 m.

Analysis1. What assumption is this calculation

based on?2. How realistic are the average rates?3. What are some possible shortcomings

of this calculation?4. What major cities would this rise in sea

level endanger?

Thinking Critically5. Where, if at all, would new coral reefs

grow? Why?

Using Math

no subduction beneath southern and centralCalifornia, the volcanoes in most of Cali-fornia are geologically inactive.

The subduction of the East Pacific Risebeneath the North American plate coincideswith pull-apart, or extensional, tectonism inthe southwestern United States and north-central Mexico. A series of mountains thattrend north-to-slightly-northeast are sepa-rated by long, linear valleys and extend fromNevada and western Utah to north-centralMexico. This area, shown in Figure 24-16, iscalled the Basin and Range Province. Asextensional tectonism has pulled the crustapart, large blocks of the crust have droppeddown along normal faults to form the basins,leaving other blocks at higher elevations toform the mountain ranges. This extension is still occurring today.

Hot Spots in the West In Chapter 18, you learned that hot spotsare mantle plumes that rise to Earths surface. Some of the hot spotsthat occur in the western United States are related to the continuingsubduction along the western coast of North America. YellowstoneNational Park is famous for its beautiful geysers and hot springs. Theland that makes up Yellowstone National Park is situated on a hot spotthat has been active since the Early Cenozoic. The rocks at Yellowstone

100 km0

California

Fault G a r

l o c k

S i e r r a N e v a d a L a s V e g a s f a u l t z o n e Arizona

Colorado

Nevada

Utah Colorado

Wyoming

NewMexico

Plateau

D e a t h V a l l e y

f a u l t z o n e

Basin areasMountain areas

Normal fault(hachures ondownthrow side)

Strike-slip(Arrows showmovement)

N

Basin and Range Province

Figure 24-16 The exten-sive development of basinsand mountain ranges inthis area is a clearexample of the resultsof extensional tectonism.


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Figure 24-17 The TethysSea had a strong, westward-flowing current that trans-ported organisms acrosslarge distances. Cretaceous-aged fossils that resemblefossils from the Tethys Seaarea are found as far westas the Hawaiian Islands.

indicate that, in the past, this hot spot generatedtremendous volcanic activity. Volcanic rocks thatare Pleistocene in age record episodes of explosivevolcanism. In fact, the yellow color of the extrusivevolcanic rock rhyolite is responsible for the name of the river and the national park.

Continental Collisions While the finalbreakup of Pangaea occurred during theCenozoic, plate tectonics also brought continentstogether during this time. The spectacular resultof one such collision is the Himalayan Mountains.India traveled north and collided with the south-ern margin of Asia. The force of this tremendouscontinent-to-continent collision resulted in theformation of the Himalayan Mountains, whichcontain the point of highest elevation on Earth,Mt. Everest. The rocks at the top of Mt. Everest are

Ordovician marine limestones. Tectonic forces have pushed whatwas the seafloor during the Ordovician to the top of the world!

Africa also drifted north after the breakup of Pangaea and col-lided with the connected landmass of Europe and Asia, or Eurasia.This continent-to-continent collision formed the Alps. Before Africacollided with Eurasia, a narrow sea called the Tethys Sea separatedthe two continents, as shown in Figure 24-17. The collision betweenAfrica and Eurasia continues today, and it is almost certain that inthe geologic future, the last remnant of the Tethys Sea will dry up.

CordilleranMountains

Tethyansea

SouthPole

Siberian flora

+

NorthPole

+

I n t

e r i

o r

S e a w a y

Shallow seaLandMountains

Tethyan currentTethyan seaSiberian flora

1. What kind of tectonic deformation char-acterized the Cenozoic?

2. What was the southern boundary ofPleistocene glaciation in the CentralUnited States?

3. What is the relationship between the EastPacific Rise, the North American Plate and

the San Andreas Fault?4. Thinking Critically The positioning of

Antarctica over the south pole helpedtrigger the Cenozoic ice ages. There wouldbe a major rise of sea level if all the iceon Antarctica melted. If Antarctica were

to move north of the south pole, as Indiadid, it is likely that all the ice on thecontinent would melt. Explain why themovement of Antarctica should or shouldnot be considered in predicting howa sea level change would affect theclimate today.

SKILL REVIEW5. Recognizing Cause and Effect Discuss the

tectonic activity on the western coast ofNorth America and the effect it had onthe Basin and Range Province. For morehelp, refer to the Skill Handbook.

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OBJECTIVES

24.424.4 Cenozoic Life OBJECTIVES

Describe the landscapeof the Oligocene in Central North America.

Discuss the changes in animals in North America during the Cenozoic.

Identify the characteristics of primates.

Explain what separateshominids from the othe

hominoids.

VOCABULARY

primate hominoid hominid Homo sapiens

Figure 24-18 The Badlandof South Dakota may resem-ble a moon-scape but theywere far from a lifelessplain. In fact, the abundantand diverse mammal fossilsfound here have inspiredsome paleontologists to callthe Oligocene the GoldenAge of Mammals.

24.4 Cenozoic Life 6

The modern marine fauna, including clams, snails, sea urchins, crus-taceans, bony fishes, and sharks, survived the Cretaceous massextinction to populate the modern oceans. Whales and dolphins

evolved during the Cenozoic as completely aquatic mammals.Walruses and sea lions returned to the oceans. These mammals arestill partly terrestrial today, but, like plesiosaurs were, they are muchmore at home in the water than on land.

LIFE ON LANDMost of the currently living groups of mammals had evolved by theEocene. Forests dominated North America during the Paleoceneand Eocene. As the climate cooled during the Late Eocene, however,the forests gave way to open land. Grasses, which were important tomany large mammals, appeared during the Eocene. Grasses spreadout over the plains, and by the Late Oligocene, grassy savannas, likethose in East Africa today, were common from Texas to SouthDakota. The grasslands supported a large diversity of mammals,most of which are members of groups living today. These includeddogs, cats, rodents, rabbits, camels, horses, pronghorn antelope, andmastodons. The rocks in Badlands National Park in South Dakota,shown in Figure 24-18, contain a treasure trove of fossils from theOligocene. The rocks are made of clay, silt, and sand that weredeposited in marshes and lakes and by slowly moving streams.

Pleistocene Mammals As the Pliocene ice age began, the greatsavannas were replaced by more arid land. The change in climatecaused many of the savanna mammals to become extinct. A new group of animals evolved to populate the land as the Late Pliocene-Pleistocene ice age sent a chill across North America. Several of theseanimals are shown in Figure 24-19 on page 640.


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PRIMATES AND HUMANSOne of the most difficult prob-lems to overcome in studies of theorigin of humans and our rela-tionship to other primates is thescarcity of fossils. This makes thisarea of study dynamic, partly because the discovery of a singlenew fossil can dramatically change our understanding.

Primates Primates are distinguished from other mammals by agrasping hand with an opposable thumb and two eyes directed for-ward that result in stereoscopic vision. Such vision allows primatesto judge distance quite accurately. Other primate characteristicsinclude smaller, fewer, and less-specialized teeth than other mam-mals and a relatively large brain.

Our species, Homo sapiens, belongs to a group of primates calledhominids. In turn, hominids are part of a larger group calledhominoids that includes the great apes.The fossil record of hominidsextends back approximately 4.4 million years. What is it that separatesa hominid from the other hominoids? The most recognizable featureis that all hominids are bipedal. That is, they have an upright postureresulting from a modification of the hipbone and they walk on twolegs. Hominids also have larger brains, smaller canine teeth, andsmaller faces than other hominoids, and they use sophisticated toolsand have greater manual dexterity.

The Rise of Homo Sapiens Tracing the ancestry of Homosapiens, the species to which humans belong, to earlier hominidsbegan in the 1850s, when a fossilized skull was discovered inNeander Tal near Dusseldorf, Germany. Tal is the German word forvalley. Since that time, paleoanthropologists have scoured Earthlooking for fossils of hominids in an attempt to piece together theancestry of Homo sapiens .

Figure 24-19 The woollymammoth, dire wolf, andthe sabre-toothed cat area few of the Pleistoceneanimals that evolved abili-ties to withstand the cold.

Update To find outmore about hominidfossil finds, visit the EarthScience Web Site atearthgeu.com
http://earthgeu.com/http://earthgeu.com/http://earthgeu.com/http://earthgeu.com/http://earthgeu.com/http://earthgeu.com/http://earthgeu.com/http://brieftoc.pdf/


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24.4 Cenozoic Life 6

1. What kind of plants dominated the land-scape of Central North America duringthe Oligocene?

2. How did the flora and fauna change inresponse to climatic changes during theCenozoic?

3. What characteristics distinguish primatesfrom other mammals?

4. What separates hominids from the otherhominoids?

5. Discuss why it is difficult to reconstructthe evolutionary history of hominids.

6. Thinking Critically Explain why the evolu-tion of grasses was a significant event forthe mammals of central North America.

SKILL REVIEW7. Communicating Suppose you are a jour-

nalist. In your science journal, write apress-release describing the recent discov-ery of a group of Neanderthal hominids.For more help, refer to the Skill Handbook.

What about the fossils from Neander Tal? Thesehominids are commonly called Neanderthals. Most fossilevidence indicates that Neanderthals were most likely a sidebranch of H. sapiens and not direct ancestors of modernhumans. The Neanderthals were hunters that inhabitedEurope and the Near East approximately 200 000 to 30 000

years ago. Neanderthals differed from modern humans in anumber of ways; differences in their skulls are shown inFigure 24-20. Neanderthals had heavy brows, mouths thatprojected forward, and receding chins.They had short, thick limbs and more massive, muscular bodies. Perhaps mostsurprising is that they had slightly larger brains than mod-ern humans do. Neanderthals did live in caves and used avariety of tools.There is evidence that they buried their deadand placed items such as tools in their graves.

It may seem strange that scientists do not have a morecomplete understanding of the relationships among thehominids. After all, the fossil skeleton of one of the earliestknown hominid species, Australopithecus afarensis, pro-vides evidence that bipedal, upright-walking hominidsexisted at least 3.5 million years ago. However, comparedto the rest of the fossil record, there are relatively few hominid fossils. As a result, our understanding of the evo-lution of many other groups of animals is much betterthan our understanding of the evolution of hominids.Subsequently, each new find yields information that coulddramatically change previous ideas.

Figure 24-20 A characteristic ofNeanderthals is a gap that occursbetween the rear teeth and the

jaw bone (A). Modern humans donot possess this (B).

A

B

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Internet GeoLab 6

1. Complete your data table, includingall information that you think isimportant.

2. Go to the Earth Science Web Site atearthgeu.com to post your data.

3. Visit sites listed on the Earth ScienceWeb Site for more information onthe diets of dinosaurs.

Sharing Your Data Find this InternetGeoLab on the Earth Science Web Siteat earthgeu.com . Post your data in thetable provided for this activity. Use theadditional data from other students tocomplete your chart and answer theConclude and Apply questions.1. What part of a dinosaur skeleton is

most important in determining itsdiet? Why? What is the likelihoodthat this part of a skeleton willbe preserved?

2. What are some other characteristicsassociated with dinosaur skeletons

that help paleontologists determinewhat their diets were like?

3. Which were more abundant, meat-eating dinosaurs or plant-eatingdinosaurs? Why?

4. How did sauropods share foodresources? Describe the evidence usedby paleontologists to determine how sauropods shared food resources.

5. How could the same evidence that isused to determine the diets of dinosaurs be used for other animals?

Procedure

Conclude & Apply

Plan the Experiment

1. Find a resource that describes skeletalcharacteristics of meat-eating and

plant-eating dinosaurs. The EarthScience Web Site lists sites with infor-mation about dinosaurs.

2. Gather information from the links onthe Earth Science Web Site or thelibrary about the environments thatthese two types of dinosaurs lived inand which dinosaurs lived in thesame environments.

3. Design a data table to record yourresearch results. Include categoriessuch as Dinosaur Name, Meat or

Plant Eater, Food Preference, SkeletalCharacteristics, Jaw and Teeth

Characteristics, and so on.

Triceratops was an herbivore.
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and a graduated cylinder large enough to holdyour biggest model.

2. Calculate the scale of each model. Measurethe length of the model. The length of theactual dinosaur divided by the length of themodel is the scale. Example: a model Tyranno- saurus is 30 cm long. A real Tyrannosaurus skeleton is 12 meters (1200 cm) long, sothe scale is 1/40.

3. Fill the empty graduated cylinder with enoughwater to cover the model. Record thatamount. Completely submerge the model andrecord the new level of the water. The differ-ence between the first reading and second

is the volume of the model.4. Multiply the volume of the model by the cube

of the scale. The result is the volume of theliving dinosaur.

5. Multiply the volume of the living dinosaur by the density of living tissue (about 0.9 g/cc).The result is the weight of the living dinosaur,in grams. Convert that number into kilograms.


Weight is a much better indicator of overallsize than length or height. A python may belonger than an elephant, and a giraffe may be

taller, but most people would agree that theheavier elephant was the bigger animal.

How to Weigh a DinosaurTwo methods have been employed to esti-

mate the weights of dinosaurs. The first usesa formula derived from the cross-sectional areaof the leg bones. Since the legs supported theanimal, their strength is an approximation ofthat animals weight. Heavier animals havethicker bones.

The second method estimates the volume ofa dinosaur, then multiplies the volume by the spe-cific gravity. Most living animals have a specificgravity of around 0.9 kilograms per liter, but thiscan vary from 0.8 to just over 1. But how do youobtain the volume of a dinosaur? Calculating thevolumes of simple shapes like cubes and spheresis easy, but dinosaur shapes were not simple.

One way is to measure the volume of anaccurate scale model, then multiply that volumeby the scale cubed (since the real dinosaurincreased in length, width, and height by thescaled amount). The easiest way to obtain thevolume is to immerse the model in water andmeasure how much water is displaced.

Procedure1. Obtain several waterproof models of dino-

saurs. You will also need a ruler, a calculator,

Weighing a DinosaurH ow do you weigh a dinosaur? Since the last one died 66 million

years ago, you cant exactly ask one to step on a scale.

Model of Parasaurolophus, actualdinosaur was 10 m tall.

To find out more about dinosaur sizes andweights, visit the Earth Science Web Site atearthgeu.com
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SummaryVocabularyCordillera (p. 626

Vocabularyangiosperm (p. 63dinosaur (p. 631)

ectotherm (p.633)endotherm (p. 633iridium (p. 634)modern fauna

(p. 628)Ornithischia (p. 6Saurischia (p. 632

VocabularyBasin and Range

Province (p. 63Tethys Sea (p. 63

Main Ideas Geologists hypothesize that Pangaea broke apart as heat built

up beneath it. Expansion occurred and ultimately resulted in therifting apart of Pangaea.

The first orogeny is characterized by igneous intrusions, whereasthe second orogeny is characterized by thrust faulting and folding.

Main Ideas The modern marine fauna include crabs, lobsters, shrimps,

sponges, sea urchins, modern corals, snails, and clams. The major

vertebrate groups of the modern fauna include bony fishes,sharks, aquatic reptiles, and aquatic mammals. The oceans contained vast numbers of ammonites that are now

index fossils. The most common land plants were cycads, and thedominant land animals were dinosaurs.

An upright posture distinguishes dinosaurs from other reptiles.

Main Ideas The Cenozoic tectonism is characterized by vertical normal

faulting.

During the Pleistocene, glaciers extended as far south as thecourses of the Ohio and Missouri Rivers.

SECTION 24.1

Mesozoic

Paleogeography

SECTION 24.2

Mesozoic Life

SECTION 24.3

CenozoicPaleogeography

Study Guide 6

Vocabularyhominid (p. 640)hominoid (p. 640Homo sapiens

(p. 640)primate (p. 640)

Main Ideas Large mammals evolved to feed on the abundant grasslands

that developed during the Cenozoic. As the ice ages approached,many of the mammals that lived on these grasslands becameextinct and were replaced by large mammals that were adaptedto the cold and arid land south of the glaciers.

Primates are mammals that developed specialized traits, includ-ing a grasping hand with an opposable thumb; stereoscopicvision; smaller, fewer, and less-specialized teeth, and a relativelylarge brain.

SECTION 24.4

Cenozoic Life

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Assessment 6

1. What characterizes the second orogeny thataffected western North America in theMesozoic Era?a. low-angle faults c. igneous intrusionsb. block faults d. clastic wedges

INTERPRETING DATA Use the table below toanswer questions 2 and 3.

2. How does the presence of iridium at theCretaceous-Paleogene boundary support thetheory of massive volcanic activity?a. Iridium is deposited after a large fire

is extinguished.b. Iridium is a common byproduct of

combustion reactions.c. Iridium is found in abundance in

Earths core.d. All of the above.

3. Underneath the Chicxulub crater is a largelayer of melted rock and a layer of jumbledrocks.Why are these rocks jumbled?a. They have been broken up by water

erosion.b. They contain a higher level of iridium.c. They are pieces of the meteorite that

broke off upon impact.d. They fell into the crater after the impact.

11. What mountain range formed as a result of thecollision of India with Asia?a. Rocky Mountainsb. Andes Mountainsc. Himalayan Mountainsd. Alps

12. What genus do Neanderthals belong to?a. Australopithecus c. Smilodon b. Ardipithicus d. Homo

13. Discuss how and when the Mid-Atlantic Ridge

formed.14. What evidence indicates that during the Late

Triassic and Early Jurassic, western North Americawas arid?

15. What evidence suggests that dinosaurs wereendothermic?

16. How did the paleogeography of Antarctica andAustralia affect the climate during the Oligocene?

17. What characteristics separate primates fromother mammals?

18. Compare the body shapes of ichthyosaurs andplesiosaurs to those of dolphins and whales. Whatcan you conclude about these body shapes?

19. If a meteorite struck Earth at the end of theCretaceous, does this mean that the impactcaused the extinction of the dinosaurs? Explainwhy or why not.

20. Cite several reasons why the evolutionary historyof ammonites is better supported than theoriesabout hominid evolution.

21. Do the characteristics of todays coral reefs sug-gest that they might become oil reservoirs asthe rudist reefs did? Explain.

Thinking Critically

Applying Main Ideas

Standardized Test Practice

Evidence for Unusually high levels of iridiumeteor impact in Cretaceous-Paleogene bound

ary sediments; Discovery ofChicxulub crater

Evidence for Volcanic eruptions during themassive volcanic Late Cretaceous in India; Unusactivity ally high levels of iridium, soot

and charcoal in Cretaceous-Paleogene boundary sediments

Mass Extinction Theories

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Fossils and the Rock RecordGeologists have separated Earths history into divi-sions based upon the fossil record and organizedthese divisions into the geologic time scale.Thedivisions found on the geologic time scaleindescending order of lengthare eons, eras, periods,and epochs.The principles of uniformitarianism,original horizontality, superposition, and cross-cutting relationships are used to interpret Earthsrock recordand thus describe our planets history.Unconformities caused by weathering and erosionor by periods of nondeposition indicate missing lay-ers in the rock record. Fossils are the remains andevidence of plants and animals that once lived onEarth. Fossils preserved in the rock record provideinformation about past environmental conditions

and evolutionary changes in life-forms. They thushelp to correlate rock layers from one area toanother.

Absolute Age of Rocks Absolute age dat-ing measures the actual age of an object such as amineral, rock, or fossil. Radioactive dating uses thedecay rates of various types of radioactive mineralsto determine the actual age of a rock or fossil.Radioactive decay is the random emission of parti-

cles from the nucleus of a radioactive atom at aconstant rate. The time it takes a radioactive iso-tope to decay fifty percent of its original mass to anonradioactive element is known as the mineralshalf-life. Tree rings and varves are other methods of determining the dates of events and changes in theenvironment. Key beds in the rock record that mark

648 UNIT 6

the time of their occurrence include volcanic ashand meteorite-impact debris that spread out overlarge areas of Earth in a small amount of time.

The PrecambrianGeologists have used radiometric dating to showthat Earth must be at least 4.2 billion years old.Because all of the objects in the solar systemformed at the same time, and Moon rocks andmeteorites are dated at 4.6 billion years old, Earthis assumed to be 4.6 billion years old also. EarlyEarth was hot because of abundant radioactive iso-topes, bombardment by meteorites, and gravita-tional contraction. Earths crust formed when theuppermost portion of the mantle cooled.The earlycrust weathered and formed sediments. Sediment-

covered slabs of early crust were subducted andgenerated magmas with granitic compositions.During the Archaean Eon, granitic crust formedmicrocontinents. The microcontinents collided withone another throughout the Proterozoic Eon toform the cores of todays continents. Earths early

Geologic Time

For apreview of geologic time, study this GeoDigest before you read the chapters.After you have studied geologic time, use the GeoDigest toreview the unit.

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atmosphere and the oceans formed through theprocess of outgassing. Nearly all of the oxygen in

the atmosphere is a result of photosynthesis.Certain minerals oxidize, or rust, in the presence of free oxygen. Proterozoic red beds are sedimentaryrock deposits that contain oxidized iron. They areevidence that there was free oxygen in the atmo-sphere during the Proterozoic Eon.

Early Life on Earth All the ingredients werepresent on the early Earth to form proteins, whichare the building blocks of life. Experiments havedemonstrated that amino acids, the molecules thatmake up proteins, were likely abundant on the sur-face of early Earth. The first life was likely prokary-otic cells, which are small and contain no nuclei.Eukaryotic cells, which contain nuclei and are gen-erally larger and more complex than prokaryoticcells, emerged later. The first evidence of multicellu-lar organisms are fossils of eukaryotic algae from a2.1 billion year old banded iron formation in north-ern Michigan. By about 670 million years ago, themulticellular Ediacaran organisms began to flourishthroughout the world.

The Paleozoic EraEarly in the Paleozoic, a shallow sea covered theancient North American continent of Laurentia,which was located near the equator. Laurentiascontinental margin was passiveno tectonic activ-ity was occurring. The fauna included many new

organisms that evolved during the Cambrian explo-sion. Fossils of trilobites and articulate brachiopodsare common. Sea level changes cause depositionalenvironments to change position laterally. Thisresults in adjacent depositional sequences overlyingeach other in vertical succession.

Middle Paleozoic Deposits of evaporite min-erals such as gypsum, and halite formed in the mid-Paleozoic as areas of ocean water were isolatedand evaporated. Collisional tectonism occurred,causing mountain building along active continentalmargins. Clastic wedges formed as a result of mountain building as deposits of sedimentary rocksformed adjacent to uplifted areas in wedge-shapedformations. Articulate brachiopods, corals, and manyother organisms dominated the seas. Fishes evolvedas top ocean predators and plants moved ontoland. At the end of the Ordovician and in the LateDevonian, large numbers of organisms became

extinct in relatively short periods of time. Thisphenomenon is called a mass extinction.

Late Paleozoic In the Late Paleozoic, conti-nents collided to form Pangaea. Cycles of glacial/interglacial periods during the Pennsylvanian arerepresented by cyclothemsstacked deposits of

Geologic Time

GeoDigest 64

Half-Lives of Radioactive IsotopesCarbon 14 5730 (+/- 30) yearsUranium 235 700 million yearsRubidium 87 4.9 billion yearsUranium 238 4.5 billion years

Vital Statistics

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alternating transgressive and regressive rocksequences.The ocean shrank during glacial periods

and expanded during interglacial periods. Seedsand the amniote egg developed during the LatePaleozoic, developments that allowed plants andreptiles to move onto dry land. Regression of theocean and climate change led to a mass extinctionat the end of the Permian Period.

The Mesozoic andCenozoic ErasMesozoic Era In the Mesozoic, Pangaea brokeup. Geologists hypothesize that the size of thesupercontinent led to a heat buildup and expansionof the landmass, resulting in rifting that broke it up.Two orogenies occurred. The first orogeny is charac-terized by igneous intrusions whereas the secondorogeny is characterized by thrust faulting and fold-ing. Ammonites were prevalent in the Mesozoicocean; the most common land plants were cycads;and the dominant land animals were dinosaurs.High levels of iridium, and the Chicxulub structurein the rocks at the Cretaceous-Paleogene boundaryindicate that a meteorite hit Earth at the end of theCretaceous Period. This meteorite may have led tothe extinction of the dinosaurs. Dinosaurs are dis-tinguished from reptiles by their upright posture.Birds may be descended from dinosaurs. The fossilArchaeopteryx , which contains features of bothdinosaurs and birds, is cited as evidence for therelationship.The fossils of reptiles are distinguishedfrom the fossils of mammals by tooth shape, andthe number of lower jawbones.

Cenozoic Era The major orogeny of theCenozoic is characterized by vertical normal fault-ing. During the Pleistocene ice ages, glaciersextended as far south as the courses of the Ohioand Missouri rivers. By the Oligocene, grassy savan-nas covered much of Central North America. Large

mammals evolved to feed in these abundant grass-lands. As the ice ages approached, many grazersbecame extinct and were replaced by large mam-mals that were adapted to cold and arid climates.Primates emerged during the Cenozoic. Primatesare mammals that developed specialized traits suchas a grasping hand with an opposable thumb;smaller, fewer, and less specialized teeth; stereo-

scopic vision; and a relatively large brain.

650 UNIT 6

Geologic Time

F O C U S O N C A R E E R S

leoecologistPaleoecologists study the ecologyof ancient animals and plants.Where did a fossil specimen live?What did it eat? What ate it?What were the limiting factors in

its environment? Paleontology,biology, and ecology are puttogether to complete pictures ofancient environments. Curiosityand patience are essential to apaleoecologist, but advanceddegrees are also required.

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Understanding Main Ideas1. What feature, caused by weathering and

erosion or by periods of nondeposition, indi-cates missing layers in the rock record?a. uniformitarianismb. unconformityc. horizontalityd. cross-cutting

2. What is the random emission of particlesfrom the nucleus of a radioactive atom ata constant rate called?a. radioactive mineralsb. half-lifec. radioactive decayd. uranium

3. What formed when the uppermost portion of Earths mantle cooled?a. meteoritesb. radioactive isotopes

c. sedimentd. Earths crust

4. Earths early atmosphere and the oceansformed through what process?a. photosynthesisb. free oxygenc. outgassingd. Proterozoic red beds

5. The first life on Earth was a small cell withno nucleus called what?a. a prokaryotic cellb. a eukaryotic cellc. algaed. Ediacaran organisms

6. What forms when areas of ocean water are isolatedand evaporate?a. collisional tectonismb. clastic wedgesc. evaporite mineralsd. brachiopods

7. What sequence represents alternatingglacial/interglacial periods during thePennsylvanian?a. cyclothems c. clastic wedgesb. amniote egg d. microcontinent

8. What were the most common land plants of the Mesozoic?a. ammonites c. cycadsb. conifers d. embayments

9. Which of these land features was prominentin Central North America during theOligocene?

a. grasslands c. primatesb. forests d. glaciation

10. Which of the following is a trait of primates?a. more-specialized teethb. smaller brainc. hoovesd. stereoscopic vision

Thinking ritically1. Put the following geologic time divisions in

order from shortest to longest: period, eon,epoch, era.

2. Describe why early Earth was hot.3. Discuss how paleontologists distinguish rep-

tiles from mammals in the fossil record.

A S S E S S M E N T

Geologic Time

Fossilconifer cone

Documents

The Mesozoic and Cenozoic Era