Precambrian Time: Vast and Enigmatic Box 11.1 Understanding …wps.prenhall.com/wps/media/objects/1516/1553090/ch11.pdf · Precambrian Time: Vast and Enigmatic The Precambrian encompasses

• Box 11.1 Understanding Earth: The Burgess Shale

• Box 11.2 Earth as a System: The Great Paleozoic Extinction

• Box 11.3 Earth as a System: Demise of the Dinosaurs

Weathering and erosion have exposed these petrified logs in the Triassic Chinle Formation in Arizona’s PetrifiedForest National Park. (Photo by David Muench)

Precambrian Time: Vast and Enigmatic

Paleozoic Era: Life Explodes

Mesozoic Era: Age of the Dinosaurs

Cenozoic Era: Age of Mammals

C H A P T E R 11Earth’s History: A Brief Summary

309

310 Unit Four Deciphering Earth’s History

Earth has a long and complex history. Thesplitting and colliding of continents has re-sulted in the formation of new ocean basinsand the creation of Earth’s great mountainranges. Furthermore, the nature of life on our

planet has experienced dramatic changes through time.Many of the changes on planet Earth occur at a

“snail’s pace,” generally too slow for people to perceive.Thus, human awareness of evolutionary change is fair-ly recent. Evolution is not confined to life forms, for allof Earth’s “spheres” have evolved together: the atmos-phere, hydrosphere, solid Earth, and biosphere. Exam-ples are evolutionary changes in the air we breathe,evolution of the world oceans, the rise of mountains,ponderous movements of crustal plates, the comingsand goings of vast ice sheets, and the evolution of a vastarray of life forms. As each facet of Earth has evolved,it has powerfully influenced the others.

As we saw in Chapter 10, geologists have manytools at their disposal for interpreting the clues aboutEarth’s past. Using these tools, and clues that are con-tained in the rock record, geologists have been able tounravel many of the complex events of the geologicalpast. The goal of this chapter is to provide a briefoverview of the history of our planet and its life forms(Figure 11.1). We will describe how our physical worldassumed its present form and how Earth’s inhabitantschanged through time.*

Precambrian Time: Vast and EnigmaticThe Precambrian encompasses immense geologicaltime, from Earth’s distant beginnings 4.5 billion yearsago until the start of the Cambrian period, over 4 bil-lion years later. Thus, the Precambrian spans about 88percent of Earth’s history. To get a visual sense of thisproportion, look at the right side of Figure 11.2, whichshows the relative time span of eras. Our knowledge ofthis ancient time is sketchy, for much of the early rockrecord has been obscured by the very Earth processesyou have been studying, especially plate tectonics, ero-sion, and deposition.

Untangling the long, complex Precambrian rockrecord is a formidable task, and it is far from complete.Most Precambrian rocks are devoid of fossils, whichhinders correlation of rocks. Rocks of this great age aremetamorphosed and deformed, extensively eroded, andobscured by overlying strata. Consequently, this least-understood span of Earth’s history has not been suc-cessfully divided into briefer time units, as have laterintervals. Indeed, Precambrian history is written in scat-

*You may want to refer to the section on the “Early Evolution of Earth” in theIntroduction (p. 2). It sets the stage by providing a brief but useful summary ofthe formation of Earth and the other members of our solar system.

tered, speculative episodes, like a long book with manymissing chapters.

Precambrian RocksLooking at Earth from the Space Shuttle, astronauts seeplenty of ocean (71 percent) and much less land area(29 percent). Over large expanses of the continents, theorbiting space scientists gaze upon many Paleozoic,Mesozoic, and Cenozoic rock surfaces, but fewer Pre-cambrian surfaces. This demonstrates the law of su-perposition: Precambrian rocks in these regions areburied from view beneath varying thicknesses of morerecent rocks. Here, Precambrian rocks peek through thesurface where younger strata are extensively eroded, asin the Grand Canyon and in some mountain ranges.However, on each continent, large core areas of Pre-cambrian rocks dominate the surface, mostly as de-formed metamorphic rocks. These areas are calledshields because they roughly resemble a warrior’sshield in shape.

Figure 11.3 shows these shield areas of Precambri-an rocks worldwide. In North America (includingGreenland), the Canadian Shield encompasses 7.2 mil-lion square kilometers (2.8 million square miles), theequivalent of about 10 states of Texas put together.

Much of what we know about Precambrian rockscomes from mining the ores contained in such rocks.The mining of iron, nickel, gold, silver, copper, chromi-um, uranium, and diamonds has provided Precambrianrock samples for study, and surveys to locate valuableore deposits have revealed much about the rocks.

Noteworthy are extensive iron-ore deposits. Rocksfrom the middle Precambrian (1.2 billion to 2.5 billionyears ago) contain most of Earth’s iron ore, mainly asthe mineral hematite ( ). These iron-rich sedimen-tary rocks probably represent the time when oxygen be-came sufficiently abundant to react with iron dissolvedin shallow lakes and seas. Later, after much of the ironwas oxidized and deposited on lake and sea bottoms,formation of these iron-rich deposits declined and oxy-gen levels in the ocean and atmosphere began to in-crease. Because most of Earth’s free oxygen results fromplant photosynthesis, the formation of extensive Pre-cambrian iron-ore deposits is linked to life in the sea.

Notably absent in the Precambrian are fossil fuels(coal, oil, natural gas). The reason is clear—a virtual ab-sence of land plants to form coal swamps and of certainanimals to form petroleum. Fossil fuels are from a latertime.

Earth’s Atmosphere EvolvesEarth’s atmosphere is unlike that of any other bodyin the solar system. No other planet has the same life-sustaining mixture of gases as Earth.

Today, the air you breathe is a stable mixture of 78percent nitrogen, 21 percent oxygen, about 1 percent

Fe2O3

Figure 11.1 Paleontologists studyancient life. This researcher isexamining the partially cleaned skull ofthe Tyrannosaurus rex known as “Sue”at Chicago’s Field Museum of NaturalHistory. The aim of historical geology isto understand the development ofEarth and its life through time. Fossilsare essential tools in that quest. (Photoby Ira Block)

argon (an inert gas), and trace gases like carbon dioxideand water vapor. But our planet’s original atmosphere,several billion years ago, was far different.

Earth’s very earliest atmosphere probably wasswept into space by the solar wind, a vast stream of par-ticles emitted by the Sun. As Earth slowly cooled, amore enduring atmosphere formed. The molten surfacesolidified into a crust, and gases that had been dissolvedin the molten rock were gradually released, a processcalled outgassing. Outgassing continues today fromhundreds of active volcanoes worldwide. Thus, geolo-gists hypothesize that Earth’s original atmosphere wasmade up of gases similar to those released in volcanicemissions today: water vapor, carbon dioxide, nitrogen,and several trace gases.

As the planet continued to cool, the water vaporcondensed to form clouds, and great rains commenced.At first, the water evaporated in the hot air before reach-ing the ground, or quickly boiled away upon contactingthe surface, just like water sprayed on a hot grill. Thisaccelerated the cooling of Earth’s crust. When the sur-face had cooled below water’s boiling point ( or

), torrential rains slowly filled low areas, formingthe oceans. This reduced not only the water vapor inthe air but also the amount of carbon dioxide, for it be-came dissolved in the water. What remained was anitrogen-rich atmosphere.

If Earth’s primitive atmosphere resulted from vol-canic outgassing, we have a problem, because volca-noes do not emit free oxygen. Where did the verysignificant percentage of oxygen in our present atmos-phere (nearly 21 percent) come from?

The major source of oxygen is green plants. Put an-other way, life itself has strongly influenced the compo-sition of our present atmosphere. Plants did not justadapt to their environment; they actually influenced it,dramatically altering the composition of the entire plan-et’s atmosphere by using carbon dioxide and releasingoxygen. This is a good example of how Earth operatesas a giant system in which living things interact withtheir environment.

212°F100°C

How did plants come to alter the atmosphere? Thekey is the way in which plants create their own food.They employ photosynthesis, in which they use light en-ergy to synthesize food sugars from carbon dioxide andwater. The process releases a waste gas: oxygen. Thoseof us in the animal kingdom rely on oxygen to metab-olize our food, and we in turn exhale carbon dioxide asa waste gas. The plants use this carbon dioxide for morephotosynthesis, and so on, in a continuing system.

The first life forms on Earth, probably bacteria, didnot need oxygen. Their life processes were geared to theearlier, oxygenless atmosphere. Even today, many anaer-obic bacteria thrive in environments that lack free oxygen.Later, primitive plants evolved that used photosynthesisand released oxygen. Slowly, the oxygen content ofEarth’s atmosphere increased. The Precambrian rockrecord suggests that much of the first free oxygen did notremain free because it combined with (oxidized) othersubstances dissolved in water, especially iron. Iron hastremendous affinity for oxygen, and the two elementscombine to form iron oxides (rust) at any opportunity.

Then, once the available iron satisfied its need foroxygen, substantial quantities of oxygen accumulated inthe atmosphere. By the beginning of the Paleozoic era,about 4 billion years into Earth’s existence (after seven-eighths of Earth’s history had transpired), the fossil recordreveals abundant ocean-dwelling organisms that requireoxygen to live. Hence, the composition of Earth’s atmos-phere has evolved together with its life forms, from anoxygenless envelope to today’s oxygen-rich environment.

Precambrian FossilsA century ago, the earliest fossils known dated from theCambrian period, about 540 million years ago. Nonewere known from the Precambrian. This created a majorproblem for science at that time: How could complexorganisms like trilobites abruptly appear in the geologicrecord? The answer, or course, was that there were fos-sils in Precambrian rocks, but they were rare, small, ob-scure, and simply had not been discovered yet. Today,

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"Ageof

Reptiles"

"Ageof

Amphibians"

"Ageof

Fishes"

"Ageof

Invertebrates"

Era Period Epoch

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Collectively calledPrecambrian, comprisesabout 88% of thegeologic time scale

Development ofPlants and Animals

Extinction ofdinosaurs and many

other species

Extinction of trilobitesand many othermarine animals

Humans develop

"Age of Mammals"

First flowering plants

First birds

Dinosaurs dominant

First reptiles

Large coal swamps

Amphibians abundant

First insect fossils

Fishes dominant

First land plants

First fishes

Trilobites dominant

First organismswith shells

First multicelledorganisms

Origin of Earth

First one-celledorganisms

Relative TimeSpan of Eras

Figure 11.2 The geologic time scale. Numbers on the time scale represent time in millions of years before the present. Thesedates were added long after the time scale had been established using relative dating techniques. The Precambrian accounts formore than 88 percent of geologic time. (Data from Geological Society of America)

Chapter 11 Earth’s History: A Brief Summary 313

Canadianshield

Greenlandshield

Brazilianshield

Africanshield

Indianshield

Alps

Balticshield

AustralianshieldKey

Continentalshield

Belts offolded strata

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AndesM

ountains

Appalachians

Calded

onian Belt

Himalaya MountainsD

ividing

Range

Ura

ls

Great

Figure 11.3 Today’s continents look very different from the Precambrian. Remnants of Precambrian rocks are the continentalshields, composed largely of metamorphosed igneous and sedimentary rocks.

our knowledge of Precambrian life, although far fromcomplete, is quite extensive.

Precambrian fossils are disappointing if you are ex-pecting to see fascinating plants and large animals, forthese had not yet evolved. Instead, the most commonPrecambrian fossils are stromatolites. These are dis-tinctively layered mounds or columns of calcium car-bonate (Figure 11.4). Stromatolites are not the remainsof actual organisms but are material deposited by algae.They are indirect evidence of algae because they close-ly resemble similar deposits made by modern algae.

Stromatolites did not become common until themiddle Precambrian, around 2 billion years ago. Stro-

matolites are large, but most actual organisms pre-served in Precambrian rocks are microscopic. Well-preserved remains of many tiny organisms have beendiscovered, extending the record of life back beyond 3.5billion years.

Many of these most ancient fossils are preserved inchert, a hard, dense chemical sedimentary rock. Chertmust be very thinly sliced and studied under powerfulmicroscopes to observe bacteria and algae fossils within it.

Microfossils have been found at several locationsworldwide. Two notable areas are in southern Africa,where the rocks date to more than 3.1 billion years, and

A. B.

Figure 11.4 Stromatolites are among the most common Precambrian fossils. A. These Precambrian fossil stromatolites from Argentinaare more than 2 billion years old. (Photo by Sinclair Stammers/Photo Researchers, Inc.) B. Modern stromatolites growing in saline area,Western Australia. (Photo by Bill Bachman/Photo Researchers, Inc.)


in the Gunflint Chert (named for its use in flintlock ri-fles) of Lake Superior, which dates to 1.7 billion years.In both places, bacteria and blue-green algae have beendiscovered. The fossils are of the most primitive or-ganisms, prokaryotes. Their cells lack organized nuclei,and they reproduce asexually.

More advanced organisms, eukaryotes, have cellsthat contain nuclei. Eukaryotes are among billion-year-old fossils discovered at Bitter Springs in Australia, suchas green algae. Unlike prokaryotes, eukaryotes repro-duce sexually, which means that genetic material is ex-changed between organisms. This reproductive modepermits greatly increased genetic variation. Thus, de-velopment of eukaryotes may have dramatically in-creased the rate of evolutionary change.

Plant fossils date from the middle Precambrian, butanimal fossils came a bit later, in the late Precambrian.Many of these fossils are trace fossils, meaning that theyare not of the animals themselves but of their activities,such as trails and worm holes. Areas in Australia andNewfoundland have yielded hundreds of fossil im-pressions of soft-bodied creatures. Most, if not all, ofthe Precambrian fauna lacked shells, which would de-velop as protective armor during the Paleozoic.

As the Precambrian came to a close, the fossil recorddisclosed diverse and complete multicelled organisms.This set the stage for more complex plants and animalsto evolve at the dawn of the Paleozoic era.

Paleozoic Era: Life ExplodesFollowing the long Precambrian, the most recent 540million years of Earth history are divided into three eras:Paleozoic, Mesozoic, and Cenozoic. The Paleozoic era

encompasses about 292 million years and is by far thelongest of the three. Seven periods make up the Paleo-zoic era (see Figure 11.2).

Before the Paleozoic, life forms possessed no hardparts—shells, scales, bones, or teeth. The beginningof the Paleozoic is marked by the appearance of thefirst life forms with hard parts (Figure 11.5). Hardparts greatly enhanced their chance of being pre-served as part of the fossil record. Therefore, ourknowledge of life’s diversification improves greatlyfrom the Paleozoic onward. This diversity is demon-strated in Figure 11.6.

Abundant Paleozoic fossils have allowed geologiststo construct a far more detailed time scale for the lastone-eighth of geologic time than for the precedingseven-eighths, the Precambrian. Moreover, becauseevery organism is associated with a particular envi-ronment, the greatly improved fossil record providedinvaluable information for deciphering ancient envi-ronments. To facilitate our brief tour of the Paleozoic,we divide it into Early Paleozoic (Cambrian, Ordovi-cian, Silurian periods) and Late Paleozoic (Devonian,Mississippian, Pennsylvanian, Permian periods).

A. B.

Figure 11.5 Fossils of common Paleozoic life forms. A. Natural cast of a trilobite. Trilobites dominated the Paleozoic ocean, scavengingfood from the bottom. B. Extinct coiled cephalopods. Like their modern descendants, these were highly developed marine organisms.(Photos by Edward J. Tarbuck)

?S T U D E N T S S O M E T I M E S A S K . . .I know that era names refer to

“ancient,” “middle,” and “recent” life. What is the origin of period names?

There is no overall scheme for naming the periods; rather,these names have diverse origins. Several names refer toplaces that have prominent strata of that age. For example,the Cambrian period is taken from the Roman name for Wales


Invertebrates Vertebrates Plants

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Seedferns Algae

andfungi

Figure 11.6 This chart indicates the times of appearance and relative abundance of major groups of organisms. The wider theband, the more dominant the group.

Early Paleozoic HistoryThe early Paleozoic consists of a 123-million-year spanthat embraces the Cambrian, Ordovician, and Siluri-an periods. Anyone approaching Earth from space atthis time would have seen the familiar blue planetwith plentiful white clouds, but the arrangement ofcontinents would have looked very different fromtoday (Figure 11.7). At this time, the vast southerncontinent of Gondwanaland encompassed five conti-nents (South America, Africa, Australia, Antarctica,India, and perhaps China). Evidence of an extensivecontinental glaciation places western Africa near theSouth Pole!

Landmasses that were not part of Gondwanalandexisted as five separate units and some scattered frag-ments. Although the exact position of these northern

continents is uncertain, ancestral North America andEurope are thought to have been near the equator andseparated by a narrow sea, as shown on the map in Fig-ure 11.7.

As the Paleozoic opened, North America was a landwith no living things, plant or animal. There were noAppalachian or Rocky Mountains; the continent waslargely a barren lowland. Several times during theCambrian and Ordovician periods, shallow seas movedinland and then receded from the interior of the conti-nent. Deposits of clean sandstones, used today to makeglass, mark the edge of these shallow seas in the mid-continent.

Early in the Paleozoic, a mountain-building eventaffected eastern North America from the present-daycentral Appalachians to Newfoundland. The mountainsproduced during this event, known as the TaconicOrogeny, have since eroded away, leaving behind de-formed strata and a large volume of detrital sedimen-tary rocks that were derived from the weathering ofthese mountains.

(Cambria). The Permian is named for the province of Perm inRussia, while the Jurassic period gets its name from the JuraMountains located between France and Switzerland.


Silurian(430 m.y.a.)

Equator

NorthAmerica

Siberia

NorthernEurope

WesternAsia

ChinaAustralia

SouthAmerica Africa

India

Antarctica

G O N D W A N A L A N D

Figure 11.7 Reconstruction of Earth as it may have appeared in early Paleozoic time. The southern continents were joined intoa single landmass called Gondwanaland. Four of the five landmasses that would later join to form the northern continent ofLaurasia lay scattered roughly along the equator. (After C. Scotese, R. K. Bambach, C. Barton, R. VanderVoo, and A. Ziegler)

During the Silurian period, much of North Ameri-ca was once again inundated by shallow seas. This timelarge barrier reefs restricted circulation between shal-low marine basins and the open ocean. Water in thesebasins evaporated, causing deposition of large quanti-ties of rock salt and gypsum. Today these thick evapor-ite beds are important resources for the chemical, rubber,plasterboard, and photographic industries in Ohio,Michigan, and western New York State.

Early Paleozoic LifeLife in early Paleozoic time was restricted to the seas.Vertebrates had not yet evolved, so life consisted of sev-eral invertebrate groups (see Box 11.1). The Cambrianperiod was the golden age of trilobites. More than 600genera of these mud-burrowing scavengers flourishedworldwide. By Ordovician times, brachiopods outnum-bered the trilobites. Brachiopods are among the mostwidespread Paleozoic fossils and, except for one mod-ern group, are now extinct. Although the adults livedattached to the seafloor, the young larvae were free-swimming. This mobility accounts for the group’s widegeographic distribution.

The Ordovician also marked the appearance ofabundant cephalopods—mobile, highly developed mol-lusks that became the major predators of their time. Thedescedents of cephalopods include the modern squid,octopus, and nautilus. Cephalopods were the first trulylarge organisms on Earth (Figure 11.8). Whereas thelargest trilobites seldom exceeded 30 centimeters (12inches) in length and the biggest brachiopods were nomore than about 20 centimeters (8 inches) across, onespecies of cephalopod reached a length of nearly 10 me-ters (30 feet).

The beginning of the Cambrian period marks animportant event in animal evolution. For the first time,organisms appeared that secreted material that formedhard parts, such as shells. Why several diverse life formsbegan to develop hard parts about the same time re-mains unanswered. One proposal suggests that becausean external skeleton provides protection from preda-tors, hard parts evolved for survival. Yet, the fossilrecord does not seem to support this hypothesis. Or-ganisms with hard parts were plentiful in the Cambri-an period, whereas predators such as cephalopods werenot abundant until the Ordovician period, some 70 mil-lion years later.

Whatever the answer, hard parts clearly servedmany useful purposes and aided adaptations to newways of life. Sponges, for example, developed a net-work of fine interwoven silica spicules that allowedthem to grow larger and more erect, capable of ex-tending above the surface in search for food. Mollusks(clams and snails) secreted external shells of calciumcarbonate that protected them and allowed body or-gans to function in a more controlled environment. Thesuccessful trilobites developed an exoskeleton of a pro-tein called chitin, which permitted them to burrowthrough soft sediment in search of food (see Fig-ure 11.5A).

Late Paleozoic HistoryThe late Paleozoic consists of four periods—the De-vonian, Mississippian, Pennsylvanian, and Permian—that span about 160 million years. Tectonic forcesreorganized Earth’s landmasses during this time, cul-minating with the formation of the supercontinent Pan-gaea (Figure 11.9).


UNDERSTANDING EARTH:The Burgess Shale

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The possession ofhard parts great-

ly enhances the likelihood of organismsbeing preserved in the fossil record.Nevertheless, there have been rare oc-casions in geologic history when largenumbers of soft-bodied organisms havebeen preserved. The Burgess Shale isone well-known example. Located inthe Canadian Rockies near the town ofField in southeastern British Columbia,the site was discovered in 1909 byCharles D. Walcott of the SmithsonianInstitution.

The Burgess Shale is a site of excep-tional fossil preservation and records a

diversity of animals found nowhere else(Figure 11.A). The animals of theBurgess Shale lived shortly after theCambrian explosion, a time when therehad been a huge expansion of marinebiodiversity. Its beautifully preservedfossils represent our most complete andauthoritative snapshot of Cambrian life,far better than deposits containing onlyfossils of organisms with hard parts. Todate, more than 100,000 unique fossilshave been found.

The animals preserved in theBurgess Shale inhabited a warm shal-low sea adjacent to a large reef that waspart of the continental margin of North

Forming Pangaea, the Supercontinent. As ancestralNorth America collided with Africa, the narrow seathat separated these landmasses began to close slowly(compare Figure 11.9B and 11.9C). Strong compres-sional forces from this collision deformed the rocks toproduce the original northern Appalachian Moun-tains of eastern North America.

During the fusion of North America and Africa,the other northern continents began to converge (Fig-ure 11.9). By the Permian period, this newly formedlandmass had collided with western Asia and theSiberian landmass along the line of the Ural Moun-tains. Through this union, the northern continent ofLaurasia was born, encompassing present-day NorthAmerica, Europe, western Asia, Siberia, and perhapsChina.

As Laurasia was forming, Gondwanaland mi-grated northward. By the Pennsylvanian period,Gondwanaland collided with Laurasia, forming amountainous belt through central Europe. Simulta-neously, a collision between the African fragment ofGondwanaland and the southeastern edge of NorthAmerica created the southern Appalachian Moun-tains.

By the close of the Paleozoic, all the continents hadfused into the supercontinent of Pangaea (Figure 11.9).With only a single vast continent, the world’s climatebecame very seasonal, having extremes far greater thanthose we experience today. As illustrated in Box 11.2,these altered climatic conditions may have contributedto one of the most dramatic biological declines in all ofEarth history.

America. During the Cambrian, theNorth American continent was in thetropics astride the equator. Life was re-stricted to the ocean, and the land wasbarren and uninhabited.

What were the circumstances that ledto the preservation of the many lifeforms found in the Burgess Shale? Theanimals lived in and on underwatermud banks that formed as sediment ac-cumulated on the outer margins of a reefadjacent to a steep escarpment (cliff). Pe-riodically the accumulation of muds be-came unstable and the slumping andsliding sediments moved down the es-carpment as turbidity currents. Theseflows transported the animals in a tur-bulent cloud of sediment to the base ofthe reef where they were buried. Here,in an environment lacking oxygen, theburied carcasses were protected fromscavengers and decomposing bacteria.This process occurred again and again,building a thick sequence of fossil-richsedimentary layers. Beginning about175 million years ago, mountain-building forces elevated these stratafrom the seafloor and moved themmany kilometers eastward along hugefaults to their present location in theCanadian Rockies.

The Burgess Shale is one of the mostimportant fossil discoveries of the twen-tieth century. Its layers preserve for usan intriguing glimpse of early animallife that is more than a half billion yearsold.

Figure 11.A Two examples of Burgess ShaleFossils. Thaumaptilon walcotti (at left) was arelatively large (up to 20 centimeters, or 8 inches,long) leaf-like animal. (Photo by National Museumof Natural History) Aysheaia pedunculata (on right)was an ancient relative of modern velvet wormsand may have clung to soft sponges with tinyhooks on its feet. (Photo by Royal Tyrrell Museum)


Figure 11.8 During the Ordovician period (490–443 million years ago), the shallow waters of an inland sea over central North Americacontained an abundance of marine invertebrates. Shown in this reconstruction are straight-shelled cephalopods, trilobites, brachiopods,snails, and corals. ( The Field Museum, Neg. #GEO80820c, Chicago)©

Late Paleozoic LifeDuring most of the late Paleozoic, organisms diversi-fied dramatically. Some 400 million years ago, plantsthat had adapted to survive at the water’s edge beganto move inland, becoming land plants. These earliestland plants were leafless vertical spikes about the sizeof your index finger. However, by the end of the De-vonian, 40 million years later, the fossil record indicatesthe existence of forests with trees tens of meters high.

In the oceans, armor-plated fishes that had evolvedduring the Ordovician continued to adapt. Their armorplates thinned to lightweight scales that increased theirspeed and mobility (Figure 11.10). Other fishes evolvedduring the Devonian, including primitive sharks thathad a skeleton made of cartilage and bony fishes, thegroups to which virtually all modern fishes belong. Be-cause of this, the Devonian period is often called the“age of fishes.”

By late Devonian time, two groups of bony fishes—the lung fish and the lobe-finned fish—became adapt-ed to land environments. Not unlike their modernrelatives, these fishes had primitive lungs that supple-mented their breathing through gills. Lobe-finned fishlikely occupied tidal flats or small ponds. In times ofdrought, they may have used their bony fins to “walk”from dried-up pools in search of other ponds. Throughtime, the lobe-finned fish began to rely more on theirlungs and less on their gills. By late Devonian time, theyhad evolved into true air-breathing amphibians with

fishlike heads and tails. It should be noted that insectshad already invaded the land.

Modern amphibians, like frogs, toads, and sala-manders, are small and occupy limited biological nich-es. But conditions during the remainder of the Paleozoicwere ideal for these newcomers to the land. Plants andinsects, which were their main diet, already were veryabundant and large. Having only minimal competitionfrom other land dwellers, the amphibians rapidly di-versified. Some groups took on roles and forms thatwere more similar to modern reptiles, such as croco-diles, than to modern amphibians.

By the Pennsylvanian period, large tropical swampsextended across North America, Europe, and Siberia(Figure 11.11). Trees approached 30 meters (100 feet),with trunks over a meter across. The coal deposits thatfueled the Industrial Revolution, and which provide asubstantial portion of our electric power today, origi-nated in these vast swamps. Further, it was in the lushcoal swamp environment of the late Paleozoic that theamphibians evolved quickly into a variety of species.

Mesozoic Era: Age of the DinosaursSpanning about 183 million years, the Mesozoic era isdivided into three periods: the Triassic, Jurassic, andCretaceous. The Mesozoic era witnessed the beginning

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C. Permian(260 m.y.a.)

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A. Devonian(410 m.y.a.)

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Figure 11.9 During the late Paleozoic, plate movements were joining together the major landmasses to produce thesupercontinent of Pangaea. (After C. Scotese, R. K. Bambach, C. Barton, R. VanderVoo, and A. Ziegler)


EARTH AS A SYSTEM:The Great Paleozoic Extinction

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The Paleozoicended with the

Permian period, a time when Earth’smajor landmasses joined to form the su-percontinent Pangaea. This redistribu-tion of land and water and changes inthe elevations of landmasses broughtpronounced changes in world climates.Broad areas of the northern continentsbecame elevated above sea level, andthe climate grew drier. These changesapparently triggered extinctions ofmany species on land and sea.

By the close of the Permian, 75 per-cent of the amphibian families had dis-appeared, and plants had declined innumber and variety. Although manyamphibian groups became extinct, theirdescendants, the reptiles, would becomethe most successful and advanced ani-

mals on Earth. Marine life was notspared. At least 80 percent, and perhapsas much as 95 percent, of marine life dis-appeared. Many marine invertebratesthat had been dominant during the Pa-leozoic, including all the remaining trilo-bites as well as some types of corals andbrachiopods, failed to adapt to the wide-spread environmental changes.

The late Paleozoic extinction was thegreatest of at least five mass extinctionsto occur over the past 500 million years.Each extinction wreaked havoc with theexisting biosphere, wiping out largenumbers of species. In each case, how-ever, the survivors formed new biolog-ical communities that were morediverse than their predecessors. Thus,mass extinctions actually invigoratedlife on Earth, as the few hardy survivors

eventually filled more niches than theones left by the victims.

The cause of the great Paleozoic ex-tinction is uncertain. The climate changesfrom the formation of Pangaea and theassociated drop in sea level undoubted-ly stressed many species. In addition, atleast 2 million cubic kilometers of lavaflowed across Siberia to produce what iscalled the Siberian Traps. Perhaps debrisfrom these eruptions blocked incomingsunlight, or perhaps enough sulfuric acidwas emitted to make the seas virtuallyuninhabitable. Some recent research sug-gests that an impact with an extrater-restrial body may have contributed.Whatever caused the late Paleozoic ex-tinction, it is clear that without it a verydifferent population of organisms wouldtoday inhabit this planet.

of the breakup of the supercontinent Pangaea. Also inthis era, organisms that had survived the great Permi-an extinction described in Box 11.2 began to diversify inspectacular ways. On land, dinosaurs became dominantand remained unchallenged for over 100 million years.Because of this, the Mesozoic era is often called the “ageof dinosaurs.”

Early geologists recognized a profound differencebetween the fossils in Permian strata and those inyounger Triassic rocks, as if someone had drawn a boldline to separate the two time periods. Clearly one halfof the fossil groups that occurred in late Paleozoic rockswere missing in Mesozoic rocks. On this basis, it wasdecided to separate the Paleozoic and Mesozoic at thePermian–Triassic boundary.

Mesozoic HistoryThe Mesozoic era began with much of the world’s landabove sea level. In fact, in North America no period ex-hibits a more meager marine sedimentary record thandoes the Triassic period. Of the exposed Triassic stra-ta, most are red sandstones and mudstones that lackfossils and contain features indicating a terrestrialenvironment.

As the second period opened, the Jurassic, the seainvaded western North America. Adjacent to this shal-low sea, extensive continental sediments were deposit-ed on what is now the Colorado Plateau. The mostprominent is the Navajo Sandstone, a windblown, whitequartz sandstone that in places approaches a thicknessof 300 meters (1000 feet). These massive dunes indicate

Figure 11.10 These placoderms or“plate-skinned” fish were abundant duringthe Devonian (417–354 million years ago).(Drawing after A. S. Romer)


Figure 11.11 Restoration of a Pennsylvania coal swamp (323–290 million years ago). Shown are scale trees (left), seed ferns (lower left),and scouring rushes (right). Also note the large dragonfly. ( The Field Museum, Neg. #GEO85637, Chicago. Photographer: JohnWeinstein.)

©

that a major desert occupied much of the AmericanSouthwest during early Jurassic times.

A well-known Jurassic deposit is the Morrison For-mation, within which is preserved the world’s richeststorehouse of dinosaur fossils. Included are fossilizedbones of huge dinosaurs such as Apatosaurus (formerlyBrontosaurus), Brachiosaurus, and Stegosaurus.

As the Jurassic period gave way to the Cretaceous,shallow seas once again invaded much of westernNorth America, the Atlantic, and Gulf coastal regions.This created great swamps like those of the Paleozoicera, forming Cretaceous coal deposits that are very im-portant economically in the western United States andCanada. For example, on the Crow Indian reservationin Montana, there exists nearly 20 billion tons of high-quality coal of Cretaceous age.

A major event of the Mesozoic era was the breakupof Pangaea (Figure 11.12). A rift developed betweenwhat is now the eastern United States and westernAfrica, marking the birth of the Atlantic Ocean. It alsorepresents the beginning of the breakup of Pangaea, aprocess that continued for 200 million years, throughthe Mesozoic and into the Cenozoic.

As Pangaea fragmented, the westward-movingNorth American plate began to override the Pacificplate. This tectonic event began a continuous wave ofdeformation that moved inland along the entire westernmargin of the continent. By Jurassic times, subductionof the Pacific plate had begun to produce the chaoticmixture of rocks that exists today in the Coast Rangesof California. Further inland, igneous activity was wide-spread, and for nearly 60 million years huge masses ofmagma rose to within a few miles of the surface, where

they cooled and solidified. The remnants of this intru-sive activity include the granitic rocks of the SierraNevada, the Idaho batholith, and the Coast Rangebatholith of British Columbia.

Tectonic activity that began in the Jurassic contin-ued throughout the Cretaceous, ultimately forming thevast mountains of western North America (Figure11.13). Compressional forces moved huge rock units ina shinglelike fashion toward the east. Throughout muchof the western margin of North America, older rockswere thrust eastward over younger strata, for a distanceexceeding 150 kilometers (90 miles).

Toward the end of the Mesozoic, the middle andsouthern ranges of the Rocky Mountains formed. Thismountain-building event, called the Laramide Orogeny,resulted when tectonic forces uplifted large blocks ofPrecambrian rocks in what is today Colorado andWyoming.

Mesozoic LifeAs the Mesozoic era dawned, its life forms were sur-vivors of the great Paleozoic extinction. These survivorsdiversified in many new ways to fill the biological voidscreated at the close of the Paleozoic. On land, condi-tions favored those that could adapt to drier climates.Among plants, the gymnosperms were one such group.Unlike the first plants to invade the land, the seed-bear-ing gymnosperms did not depend on free-standingwater for fertilization. Consequently, these plants werenot restricted to a life near the water’s edge.

The gymnosperms quickly became the dominanttrees of the Mesozoic. They included the cycads, the

321


NorthAmerica Europe Asia

Australia

SouthAmerica

Africa

India

Antarctica

A. 200 Million Years Ago (Early Jurassic)

B. 150 Million Years Ago (Late Jurassic)

C. 100 Million Years Ago (Cretaceous)

D. 50 Million Years Ago (Tertiary)

NorthAmerica Europe Asia

Australia

SouthAmerica

Africa India

Antarctica

PA

NG

A

EA

G O N

DW

AN

A L A N D

L A U R A S I A

Figure 11.12 The breakup of the supercontinent of Pangaea began and continued throughout the Mesozoic era. (AfterD. Walsh and C. Scotese)


Figure 11.13 Uplifted and deformed sedimentary strata in the Canadian Rockies near Lake Louise, Alberta, Canada. (Photo by CarrClifton/Minden Pictures)

conifers, and the ginkgoes. The cycads resembled alarge pineapple plant. The ginkgoes had fan-shapedleaves, much like their modern relatives. Largest werethe conifers, whose modern descendants include thepines, firs, and junipers. The best-known fossil occur-rence of these ancient trees is in northern Arizona’s Pet-rified Forest National Park. Here, huge petrified logslie exposed at the surface, having been weathered fromrocks of the Triassic Chinle Formation (see chapter-opening photo).

The Shelled Egg. Among the animals, reptilesreadily adapted to the drier Mesozoic environment.They were the first true terrestrial animals. Unlikeamphibians, reptiles have shell-covered eggs that canbe laid on land. The elimination of a water-dwellingstage (like the tadpole stage in frogs) was an impor-tant evolutionary step. Of interest is the fact that thewatery fluid within the reptilian egg closely resemblesseawater in chemical composition. Because the reptileembryo develops in this watery environment, theshelled egg has been characterized as a “privateaquarium” in which the embryos of these land verte-brates spend their water-dwelling stage of life.

Dinosaurs Dominate. With the perfection of theshelled egg, reptiles quickly became the dominantland animals. They continued this dominance formore than 160 million years (Figure 11.14). Mostawesome of the Mesozoic reptiles were the dino-saurs. Some of the huge dinosaurs were carnivorous(Tyrannosaurus), whereas others were herbivorous(like the ponderous Apatosaurus, formerly Bronto-saurus). The extremely long neck of Apatosaurus mayhave been an adaptation for feeding on tall conifertrees. However, not all dinosaurs were large. In fact,

certain small forms closely resembled modern fleet-footed lizards. Further, evidence indicates that somedinosaurs, unlike their present-day reptile relatives,were warm blooded.

The reptiles made one of the most spectacular adap-tive radiations in all of Earth history (Figure 11.15). Onegroup, the pterosaurs, took to the air. These “dragons of

Figure 11.14 Fossil skull of Sarcosuchus imperator, or “fleshcrocodile emperor.” Remains of this large crocodile have beenuncovered in the desert of Niger. These animals were riverdwellers, indicating that the climate of this region during theCretaceous period was very different from today. For moreabout this amazing animal, see the Students Sometimes Ask onpage 324. (Photo by Project Exploration)


MesozoicEra

65m.y.a.

Crocodilians

PlesiosaursIchthyosaurs

Pterosaurs

Snakes

Dinosaurs

Lizards

Tobirds

Tomammals

248m.y.a.

Stemreptiles

Therapsids

TurtlesCenozoic

Era

PaleozoicEra

Figure 11.15 Origin and development of the major reptile groups and their descendants. Dinosaurs, pterosaurs (flyingreptiles), and large marine reptiles all became extinct by the end of the Mesozoic.

the sky” possessed huge membranous wings that al-lowed them rudimentary flight (Figure 11.16). Anothergroup of reptiles, exemplified by the fossil Archae-opteryx, led to more successful flyers: the birds. Where-as some reptiles took to the skies, others returned to thesea, including the fish-eating plesiosaurs and ichthyo-

saurs. These reptiles became proficient swimmers butretained their reptilian teeth and breathed by means oflungs.

At the close of the Mesozoic, many reptile groupsbecame extinct. Only a few types survived to recenttimes, including the turtles, snakes, crocodiles, andlizards. The huge land-dwelling dinosaurs, the marineplesiosaurs, and the flying pterosaurs all are knownonly through the fossil record. What caused this greatextinction? (See Box 11.3.)

Figure 11.16 Fossils of the great flying reptile Pteranodon havebeen recovered from Cretaceaous chalk deposits located inKansas. Pteranodon had a wingspan of 7 meters (22 feet), butflying reptiles with twice this wingspan have been discovered instrata of west Texas.

?S T U D E N T S S O M E T I M E S A S K . . .Many dinosaurs were very large. Were they the only large reptiles?

No, indeed. One well-publicized example is a crocodileknown as Sarcosuchus imperator (see Figure 11.14). This hugeriver dweller lived in Africa about 110 million yearsago (Cretaceous period). At maturity (50–60 years) the ani-mal weighed 8 metric tons (more than 17,600 pounds!) andwas about 12 meters (40 feet) long—as long as Tyrannosaurusrex and much heavier. Its jaws were roughly as long as anadult human. This animal has appropriately been dubbed“supercroc.” Paleontologists indicate that the teeth and jawsuggest a diet of large vertebrates, including fish anddinosaurs.


EARTH AS A SYSTEM:Demise of the Dinosaurs

BO

X11

.3

Review for a mo-ment the geolog-

ic time scale, Figure 11.2. You can seethat its divisions represent times of sig-nificant geological and/or biologicalchange. Of special interest is the bound-ary between the Mesozoic era (“middlelife”) and Cenozoic era (“recent life”),about 65 million years ago. Around thistime, more than half of all plant and an-imal species died out in a mass extinc-tion. This boundary marks the end ofthe era in which dinosaurs and otherreptiles dominated the landscape, andthe beginning of the era when mam-mals become very important (Fig-ure 11.B). Because the last period of theMesozoic is the Cretaceous (abbreviat-ed K to avoid confusion with other “C”periods), and the first period of theCenozoic is the Tertiary (abbreviated T),the time of this mass extinction is calledthe Cretaceous-Tertiary or KT boundary.

The extinction of the dinosaurs isgenerally attributed to this group’s in-ability to adapt to some radical changein environmental conditions. Whatevent could have triggered the rapid ex-tinction of the dinosaurs—one of themost successful groups of land animalsever to have lived?

The most strongly supported hy-pothesis proposes that about 65 millionyears ago a large meteorite about 10

kilometers in diameter collided withEarth (Figure 11.C). The speed at impactwas about 70,000 kilometers per hour!The force of the impact vaporized themeteorite and trillions of tons of Earth’scrust. Huge quantities of dust and othermetamorphosed debris was blastedhigh into the atmosphere. For monthsthe encircling dust cloud would havegreatly restricted the sunlight reachingEarth’s surface. With insufficient sun-light for photosynthesis, delicate foodchains would have collapsed. Largeplant-eating dinosaurs would be affect-ed more adversely than smaller lifeforms because of the tremendous vol-ume of vegetation they consumed. Nextwould come the demise of large carni-vores. When the sunlight returned,more than half of the species on Earth,including numerous marine organisms,had become extinct.

This period of mass extinction ap-pears to have affected all land animalslarger than dogs. Supporters of thiscatastrophic-event scenario suggest thatsmall ratlike mammals were able to sur-vive a breakdown in their food chainslasting perhaps several months. Largedinosaurs, they contend, could not sur-vive this event. The loss of these largereptiles opened habitats for the smallmammals that remained. These newhabitats, along with evolutionary

forces, led to the development of thelarge mammals that occupy our mod-ern world.

What evidence points to such a cat-astrophic collision 65 million years ago?First, a thin layer of sediment nearly1 centimeter thick has been discoveredat the KT boundary, worldwide. Thissediment contains a high level of the el-ement iridium, rare in Earth’s crust butfound in high proportions in stony me-teorites. Could this layer be the scat-tered remains of the meteorite that wasresponsible for the environmentalchanges that led to the demise of manyreptile groups?

Despite its growing support, somescientists disagree with the impact hy-pothesis. They suggest instead thathuge volcanic eruptions led to thebreakdown in the food chain. To sup-port their hypothesis, they cite enor-mous outpourings of lavas in theDeccan Plateau of northern India about65 million years ago.

Whatever caused the KT extinction,we now have a greater appreciation ofthe role of catastrophic events in shap-ing the history of our planet and the lifethat occupies it. Could a catastrophicevent having similar results occurtoday? This possibility may explainwhy an event that occurred 65 millionyears ago has captured the interest ofso many.

Figure 11.B Dinosaurs dominated the Mesozoic landscape until their extinction atthe close of the Cretaceous period. This skeleton of Tyrannosaurus stands on display inChicago’s Field Musuem of Natural History. (Photo by Don and Pat Valenti/DRK Photo)

United States

Mexico

Chicxulubcrater

Yucatán

Gulf of Mexico

Figure 11.C Employing gravitationalmeasurements, scientists have located agiant impact crater that formed about 65million years ago and has since been filledwith sediment. About 180 kilometers indiameter, Chicxulub crater is regarded bysome researchers as the impact site thatresulted in the demise of the dinosaurs.


Cenozoic Era: Age of MammalsThe Cenozoic era, or “era of recent life,” encompassesthe past 65 million years of Earth history. It is the “post-dinosaur” era, the time of mammals, including humans.It is during this span that the physical landscapes andlife forms of our modern world came into being. TheCenozoic era represents a much smaller fraction of ge-ologic time than either the Paleozoic or the Mesozoic.Although shorter, it nevertheless possesses a rich his-tory, because the completeness of the geologic recordimproves as time approaches the present. The rock for-mations of this time span are more widespread and lessdisturbed than those of any preceding time.

The Cenozoic era is divided into two periods ofvery unequal duration, the Tertiary period and the Qua-ternary period. The Tertiary period includes five epochsand embraces about 63 million years, practically all ofthe Cenozoic era. The Quaternary period consists of twoepochs that represent only the last 2 million years of ge-ologic time.

Cenozoic North AmericaMost of North America was above sea level through-out the Cenozoic era. However, the eastern and westernmargins of the continent experienced markedly con-trasting events, because of their different relationshipswith plate boundaries. The Atlantic and Gulf coastal re-gions, far removed from an active plate boundary, weretectonically stable. In contrast, western North Americawas the leading edge of the North American plate. Asa result, plate interactions during the Cenozoic gave riseto many events of mountain building, volcanism, andearthquakes in the West.

Eastern North America. The stable continental mar-gin of eastern North America was the site of abundantmarine sedimentation. The most extensive depositionsurrounded the Gulf of Mexico, from the YucatanPeninsula in present-day Mexico to Florida. Here, thegreat buildup of sediment caused the crust to down-warp and produced numerous faults. In many in-stances, the faults created traps in which oil andnatural gas accumulated. Today, these and other pe-troleum traps are the most economically importantresource in Cenozoic strata of the Gulf Coast, asevidenced by the Gulf’s well-known off-shore drillingplatforms.

By early Cenozoic time, most of the original Ap-palachians had been eroded to a low plain. Then, by themid-Cenozoic, isostatic adjustments raised the regiononce again, changing its orientation to base level andrejuvenating its rivers.* Streams eroded with renewedvigor, gradually sculpturing the surface into its present-day topography (Figure 11.17). Sediments from all of

*To review the concept of base level, see Chapter 4.

this erosion were deposited along the eastern marginof the continent, where they attained a thickness ofmany kilometers. Today, portions of the strata deposit-ed during the Cenozoic are exposed as the gently slop-ing Atlantic and Gulf coastal plains.

Western North America. In the West, the formationof the Rocky Mountains was coming to an end. Aserosion lowered the mountains, the basins betweenuplifted ranges filled with sediments. Eastward, agreat wedge of sediment from the eroding Rockieswas building the Great Plains.

Beginning in the Miocene epoch, a broad regionfrom northern Nevada into Mexico experienced crustalmovements that formed more than 150 fault-blockmountain ranges. They rise abruptly above the adjacentbasins, creating the Basin and Range Province (seeChapter 9).

As the Basin and Range Province was forming, theentire western interior of the continent was graduallyuplifted. This uplift reelevated the Rockies and causedmany of the West’s major rivers to vigorously down-cut. As the rivers became entrenched, many spectaculargorges were formed, including the Grand Canyon ofthe Colorado River, the Grand Canyon of the SnakeRiver, and the Black Canyon of the Gunnison. The pres-ent topography of the Rocky Mountains is in largemeasure the result of this late Tertiary uplift and thesubsequent excavation of the early Tertiary basin de-posits by reinvigorated streams.

Volcanic activity was common in the West duringmuch of the Cenozoic. Beginning in the Miocene epoch,great volumes of fluid basaltic lava flowed from fissuresin portions of present-day Washington, Oregon, andIdaho. These eruptions built the extensive (1.3 millionsquare kilometers) Columbia Plateau (see Figure 8.19).Immediately west of the Columbia Plateau, volcanic ac-tivity was quite different. Here, thicker magmas with ahigher silica content erupted explosively, creating theCascade Range, a chain of stratovolcanoes from north-ern California to the Canadian border, some of whichremain active—like Washington’s Mount St. Helens (seeFigures 8.1 and 8.30).

A final episode of folding occurred in the West inlate Tertiary time, creating the Coast Ranges, whichstretch along the Pacific Coast. Meanwhile, the SierraNevada became fault-block mountains as they were up-lifted along their eastern flank, creating the imposingmountain front we know today.

As the Tertiary period drew to a close, the effects ofmountain building, volcanic activity, isostatic adjust-ments, and extensive erosion and sedimentation hadcreated a physical landscape very similar to the config-uration of today. All that remained of Cenozoic timewas the final 2-million-year episode called the Quater-nary period. During this most recent (and current)phase of Earth history, in which humans evolved, the


Appalachianplateau Valley and ridge Great

valley

BlueRidgeMtns.

A.

B.

Piedmont Coastalplain

Figure 11.17 The formation of the modern Appalachian Mountains. A. The original Appalachians eroded to a low plain. B. Therecent upwarping and erosion of the Appalachians began nearly 30 million years ago to produce the present topography.

action of glacial ice and other erosional agents addedthe finishing touches.

Cenozoic LifeMammals replaced reptiles as the dominant land ani-mals in the Cenozoic. Angiosperms (flowering plantswith covered seeds) replaced gymnosperms as the dom-inant land plants. Marine invertebrates took on a mod-ern look. Microscopic animals called foraminifera becameespecially important. Today foraminifera are among themost intensely studied of all fossils because their wide-spread occurrence makes them invaluable in correlatingTertiary sediments. Tertiary strata are very important tothe modern world, for they yield more oil than do rocksof any other age.

The Cenozoic is often called the “age of mammals,”because these animals came to dominate land life. Itcould also be called the “age of flowering plants,” forthe angiosperms enjoy a similar status in the plantworld. As a result of advances in seed fertilization anddispersal, angiosperms experienced rapid developmentand expansion as the Mesozoic drew to a close. Thus, asthe Cenozoic era began, angiosperms were already thedominant land plants.

Development of the flowering plants, in turn,strongly influenced the evolution of both birds andmammals. Birds that feed on seeds and fruits, for ex-ample, evolved rapidly during the Cenozoic in close as-sociation with the flowering plants. During the middleTertiary, grasses developed rapidly and spread over theplains. This fostered the emergence of herbivorous(plant-eating) mammals that were mainly grazers. Inturn, the development and spread of grazing animalsestablished the setting for the evolution of the carnivo-rous mammals that preyed upon them.

Mammals Replace Reptiles. Back in the Mesozoic,an important evolutionary event was the appearanceof primitive mammals in the late Triassic, about thesame time the dinosaurs emerged. Yet throughout theperiod of dinosaur dominance, mammals remained inthe background as small and inconspicuous animals.By the close of the Mesozoic era, dinosaurs and otherreptiles no longer dominated the land. It was onlyafter these large reptiles became extinct that mammalscame into their own as the dominant land animals.The transition is a major example in the fossil recordof the replacement of one large group by another.


Mammals are distinct from reptiles in important re-spects. Mammalian young are born live, and mammalsmaintain a steady body temperature—that is, they are“warm-blooded.” This latter adaptation allowed mam-mals to lead more active and diversified lives than rep-tiles because they could survive in cold regions andsearch for food during any season or time of day. Othermammalian adaptations included the development ofinsulating body hair and more efficient heart and lungs.

It is worth noting that perfect boundaries cannot bedrawn between mammalian and reptilian traits. Oneminor group of animals, the monotremes, still lay eggs.The two species in this group, the duck-billed platypusand the spiny anteater, are found only in Australia.Moreover, although modern reptiles are “cold-blood-ed,” some paleontologists believe that dinosaurs mayhave been “warm-blooded.”

With the demise of most Mesozoic reptiles, Ceno-zoic mammals diversified rapidly. The many forms thatexist today evolved from small primitive mammals thatwere characterized by short legs, flat five-toed feet, andsmall brains. Their development and specialization tookfour principal directions: (1) increase in size, (2) increasein brain capacity, (3) specialization of teeth to better ac-commodate a particular diet, and (4) specialization oflimbs to better equip the animal for life in a particularenvironment.

Marsupials, Placentals, and Diversity. Followingthe reptilian extinctions at the close of the Mesozoic,two groups of mammals, the marsupials and theplacentals, evolved and expanded to dominate theCenozoic. The groups differ principally in their modesof reproduction. Young marsupials are born live butat a very early stage of development. After birth, thetiny and immature young crawl into the mother’sexternal stomach pouch to complete their develop-ment. Examples are kangaroos, opossums, and koalabears.

Placental mammals, in contrast, develop within themother’s body for a much longer period, so that birthoccurs after the young are relatively mature and inde-pendent. Most mammals are placental, including hu-mans.

Today marsupials are found primarily in Australia,where they went through a separate evolutionary ex-pansion during the Cenozoic, largely isolated from pla-cental mammals.

In South America, both primitive marsupials andplacentals coexisted before that landmass became com-pletely isolated during the breakup of Pangaea. Evolu-tion and specialization of both groups continuedundisturbed for approximately 40 million years untilthe close of the Pliocene epoch, when the Central Amer-ican land bridge emerged, connecting the two American

A. B.

Figure 11.18 La Brea tar pits in Los Angeles, California. A. Fossils being excavated from the thick tar in 1914. B. Skeleton of an extinctsaber-toothed cat. These bones came from the La Brea tar pits. (Photo courtesy of The George C. Page Museum)


Chapter Summary

?S T U D E N T S S O M E T I M E S A S K . . .What are the La Brea tar pits?

The La Brea tar pits, located in downtown Los Angeles, arefamous because they contain a rich and very well preservedassemblage of Pleistocene vertebrate fossils (see Figure 11.18).These organisms roamed southern California from 40,000 to8000 years ago. The collection includes 59 species of mam-mals and more than 130 species of birds. Hundreds of inver-tebrate and plant fossils are also preserved. Tar pits formwhen crude oil seeps to the surface and the light portion evap-orates, leaving behind sticky pools of heavy tar.

• The Precambrian spans about 88% of Earth history, begin-ning with the formation of Earth about 4.5 billion years agoand ending 540 million years ago with the diversification oflife that marks the start of the Paleozoic era. It is the least un-derstood span of Earth’s history because most Precambrianrocks are buried from view. However, on each continent thereis a “core area” of Precambrian rocks called the shield. Theiron-ore deposits of Precambrian age represent the time whenoxygen became abundant in the atmosphere and combinedwith iron to form iron oxide.

• Earth’s primitive atmosphere consisted of such gases aswater vapor, carbon dioxide, nitrogen, and several trace gasesthat were released in volcanic emissions, a process called out-gassing. The first life forms on Earth, probably anaerobic bac-teria, did not require oxygen. As life evolved, plants, throughthe process of photosynthesis, used carbon dioxide and waterand released oxygen into the atmosphere. Once the availableiron on Earth was oxidized (combined with oxygen), sub-stantial quantities of oxygen accumulated in the atmosphere.About 4 billion years into Earth’s existence, the fossil recordreveals abundant ocean-dwelling organisms that require oxy-gen to live.

• The most common middle Precambrian fossils are stroma-tolites. Microfossils of bacteria and blue-green algae, bothprimitive prokaryotes whose cells lack organized nuclei, havebeen found in chert, a hard, dense, chemical sedimentary rockin southern Africa (3.1 billion years of age) and near Lake Su-perior (1.7 billion years of age). Eukaryotes, with cells con-taining organized nuclei, are among billion-year-old fossilsdiscovered in Australia. Plant fossils date from the middle

Precambrian, but animal fossils came a bit later, in the latePrecambrian. Many of these fossils are trace fossils, and not ofthe animals themselves.

• The Paleozoic era extends from 540 million years ago toabout 248 million years ago. The beginning of the Paleozoicis marked by the appearance of the first life forms with hard parts,such as shells. Therefore, abundant Paleozoic fossils occur,and a far more detailed record of Paleozoic events can be con-structed. During the early Paleozoic (the Cambrian, Ordovi-cian, and Silurian periods) the vast southern continent ofGondwanaland existed. Seas inundated and receded fromNorth America several times, leaving thick evaporite beds ofrock salt and gypsum. Life in the early Paleozoic was re-stricted to the seas and consisted of several invertebrategroups. During the late Paleozoic (the Devonian, Mississip-pian, Pennsylvanian, and Permian periods), ancestral NorthAmerica collided with Africa to produce the original north-ern Appalachian Mountains, and the northern continent ofLaurasia formed. By the close of the Paleozoic, all the conti-nents had fused into the supercontinent of Pangaea. Duringmost of the Paleozoic, organisms diversified dramatically.Insects and plants moved onto the land, and amphibiansevolved and diversified quickly. By the Pennsylvanian peri-od, large tropical swamps, which became the major coal de-posits of today, extended across North America, Europe, andSiberia. At the close of the Paleozoic, altered climatic condi-tions caused one of the most dramatic biological declines inall of Earth history.

• The Mesozoic era, often called the “age of dinosaurs,” beganabout 248 million years ago and ended approximately 65 mil-

continents. Then an invasion of advanced carnivoresfrom North America brought the extinction of manyhoofed mammals that had persisted in South Americafor millions of years. The marsupials, except for opos-sums, also could not compete and became extinct. BothAustralia and South America provide excellent exam-ples of how isolation caused by the separation of conti-nents increased the diversity of animals in the world.

Large Mammals and Extinction. As we have seen,mammals diversified quite rapidly during the Ceno-zoic era. One tendency was for some groups to becamevery large. For example, by the Oligocene epoch ahornless rhinoceros that stood nearly 5 meters (16 feet)high had evolved. It is the largest land mammal knownto have existed. As time approached the present, manyother types evolved to a large size as well—more, infact, than now exist. Many of these large forms werecommon as recently as 11,000 years ago. However, awave of late Pleistocene extinctions rapidly elimi-nated these animals from the landscape.

In North America, the mastodon and mammoth,both huge relatives of the elephant, became extinct. Inaddition, saber-toothed cats, giant beavers, large groundsloths, horses, camels, giant bison, and others died out

(Figure 11.18). In Europe, late Pleistocene extinctionsincluded woolly rhinos, large cave bears, and the Irishelk. The reason for this recent wave of large animal ex-tinctions puzzles scientists. These animals had survivedseveral major glacial advances and interglacial periods,so it is difficult to ascribe these extinctions to climaticchange. Some scientists believe that early humans has-tened the decline of these mammals by selectively hunt-ing large forms. Although this hypothesis is preferredby many, it is not yet accepted by all.


lion years ago. Early in the Mesozoic much of the land wasabove sea level. However, by the middle Mesozoic, seas in-vaded western North America. As Pangaea began to breakup, the westward-moving North American plate began tooverride the Pacific plate, causing crustal deformation alongthe entire western margin of the continent. Organisms thathad survived extinction at the end of the Paleozoic began todiversify in spectacular ways. Gymnosperms (cycads, conifers,and ginkgoes) quickly became the dominant trees of the Meso-zoic because they could adapt to the drier climates. Reptilesquickly became the dominant land animals, with one groupeventually becoming the birds. The most awesome of theMesozoic reptiles were the dinosaurs. At the close of the Meso-zoic, many reptile groups, including the dinosaurs, becameextinct.

• The Cenozoic era, or “era of recent life,” began approximate-ly 65 million years ago and continues today. It is the time ofmammals, including humans. The widespread, less disturbedrock formations of the Cenozoic provide a rich geologicrecord. Most of North America was above sea level through-out the Cenozoic. Because of their different relations with tec-

tonic plate boundaries, the eastern and western margins ofthe North American continent experienced contrasting events.The stable eastern margin was the site of abundant sedimen-tation as isostatic adjustment raised the eroded Appalachi-ans, causing the streams to downcut with renewed vigor anddeposit their sediment along the continental margin. In thewest, building of the Rocky Mountains was coming to an end,the Basin and Range Province was forming, and volcanic ac-tivity was extensive. The Cenozoic is often called “the age ofmammals” because these animals replaced the reptiles as thedominant land life. Two groups of mammals, the marsupialsand the placentals, evolved and expanded to dominate the era.One tendency was for some mammal groups to become verylarge. However, a wave of late Pleistocene extinctions rapidlyeliminated these animals from the landscape. Some scientistsbelieve that humans hastened the decline of these animals byselectively hunting the larger species. The Cenozoic could alsobe called the “age of flowering plants.” As a source of food, flow-ering plants strongly influenced the evolution of both birdsand herbivorous (plant-eating) mammals throughout theCenozoic era.

Key Termsoutgassing (p. 311) shields (p. 310) stromatolites (p. 313)

Review Questions1. Explain why Precambrian history is more difficult to de-

cipher than more recent geological history.

2. What is the major source of free oxygen in Earth’satmosphere?

3. Match the following words and phrases to the most ap-propriate time span. Select among the following: Pre-cambrian, early Paleozoic, late Paleozoic, Mesozoic,Cenozoic.(a) Pangaea came into existence.(b) Bacteria and blue-green algae preserved in chert.(c) The era that encompasses the least amount of time.(d) Shields.(e) Age of dinosaurs.(f) Formation of the original northern Appalachian

mountains.(g) Mastodons and mammoths.(h) Extensive deposits of rock salt.(i) Triassic, Jurassic, and Cretaceous.(j) Coal swamps extended across North America, Eu-

rope, and Siberia.(k) Gulf Coast oil deposits formed.(l) Formation of most of the world’s major iron-ore

deposits.

(m) Massive sand dunes covered large portions of theColorado Plateau region.

(n) The “age of fishes” occurred during this span.(o) Cambrian, Ordovician, and Silurian.(p) Pangaea began to break apart.(q) “Age of mammals.”(r) Animals with hard parts first appeared in abundance.(s) Gymnosperms were the dominant trees.(t) Columbia Plateau formed.

(u) Stromatolites are among its more common fossils.(v) “Golden age of trilobites” occurred during this span.

(w) Fault-block mountains form in the Basin and RangeProvince.

4. Briefly discuss two proposals that attempt to explain whyseveral groups developed hard parts at the beginning ofthe Cambrian period. Do these proposals appear to pro-vide a satisfactory explanation? Why or why not?

5. List some differences between amphibians and reptiles.List differences between reptiles and mammals.

6. Describe a hypothesis that attempts to explain the ex-tinction of the dinosaurs.

7. Contrast the eastern and western margins of North Amer-ica during the Cenozoic era in terms of their relationshipsto plate boundaries.


Examining the Earth System1. The Earth system has been responsible for both the con-

ditions that favored the evolution of life on this planetand for the mass extinctions that have occurred through-out geological time. Describe the role of the biosphere,hydrosphere, and solid Earth in forming the current levelof atmospheric oxygen. How did Earth’s near-space en-vironment interact with the atmosphere and biosphereto contribute to the great mass extinction that marked theend of the dinosaurs?

2. Most of the vast North American coal resources locatedfrom Pennsylvania to Illinois began forming during thePennsylvanian and Mississippian periods of Earth histo-ry. (This time period is also referred to as the Carbonif-erous period.) Using Figure 11.11, a restoration of aPennsylvanian period coal swamp, describe the climatic

and biological conditions associated with this unique en-vironment. Next, examine Figure 11.9B, C. The mapsshow the formation of the supercontinent of Pangaea andillustrate the geographic position of North America dur-ing the period of coal formation. Where, relative to theequator, was North America located during the time ofcoal formation? What role did plate tectonics play in de-termining the conditions that eventually produced NorthAmerica’s eastern coal reserves? Why is it unlikely thatthe coal-forming environment will repeat itself in NorthAmerica in the near future? (You may find it helpful tovisit the Illinois State Museum Mazon Creek Fossilsexhibit at http://www.museum.state.il.us/exhibits/mazon_creek, and/or the University of California TimeMachine Exhibit at http://www.ucmp.berkeley.edu/help/timeform.html.

Web ResourcesThe Earth Science Website uses the resourcesand flexibility of the Internet to aid in yourstudy of the topics in this chapter. Writtenand developed by Earth science instructors,

this site will help improve your understanding of Earth sci-ence. Visit http://www.prenhall.com/tarbuck and click onthe cover of Earth Science 10e to find:

• Online review quizzes.• Web-based critical thinking and writing exercises.• Links to chapter-specific Web resources.• Internet-wide key term searches.

http://www.prenhall.com/tarbuck

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