Chapter: Geologic TimeTable of ContentsSection 3: Middle and Recent EarthHistorySection 1: Life and Geologic TimeSection 2: Early Earth History
• Trilobites aresmall, hard-shelled organismsthat crawled onthe seafloor.Geologic TimeLife and Geologic Time1• They are consideredto be index fossilsbecause they lived over vast regions ofthe world during specific periods ofgeologic time.
• Paleontologists have been able to divideEarth’s history into time units based on thelife-forms that lived only during certainperiods.• This division of Earth’s history makes up thegeologic time scale.The Geologic Time Scale1Life and Geologic Time
• Four major subdivisions ofgeologic time are used—eons, eras, periods, andepochs.• The longest subdivisions—eons—are based upon theabundance of certainfossils.1Life and Geologic TimeMajor Subdivisions of Geologic Time
• Next to eons, the longestsubdivisions are the eras,which are marked by major,striking, and worldwidechanges in the types offossils present.1Life and Geologic TimeMajor Subdivisions of Geologic Time
• Periods are units of geologic timecharacterized by the types of life existingworldwide at the time.• Periods can be divided into smaller units oftime called epochs.• Eras are subdivided into periods.1Life and Geologic TimeMajor Subdivisions of Geologic Time• Epochs are also characterized by differencesin life-forms, but some of these differencescan vary from continent to continent.
• Sometimes it is possibleto distinguish layers ofrock that formed duringa single year or season.• In other cases, thickstacks of rock that haveno fossils provide littleinformation that couldhelp in subdividinggeologic time.Dividing Geologic Time1Life and Geologic Time
• The fossil record shows that species havechanged over geologic time.• This change through time is known asorganic evolution.Organic Evolution1• Organisms that are not adapted to changesare less likely to survive or reproduce.• Over time, the elimination of individuals thatare not adapted can cause changes to speciesof organisms.Life and Geologic Time
• Life scientists often define a species as a groupof organisms that normally reproduces onlywith other members of their group.Species1Life and Geologic Time
Natural Selection1Life and Geologic Time• Charles Darwin was a naturalist who sailedaround the world from 1831 to 1836 to studybiology andgeology.
Natural Selection1Life and Geologic Time• In his book, he proposed that naturalselection is a process by which organisms withcharacteristics that are suited to certainenvironment have a better chance of survivingand reproducing than organisms that do nothave these characteristics.
• Because many characteristics are inherited, thecharacteristics of organisms that are betteradapted to the environment get passed on tooffspring more often.Natural Selection1• According to Darwin, this can cause a speciesto change over time.Life and Geologic Time
• A new characteristicbecomes commonin a species only ifsome membersalready possess thatcharacteristic and ifthe trait increasesthe animal’s chanceof survival.Natural Selection Within a Species1Life and Geologic Time
• By carefully choosing individuals withdesired characteristics, animal breeders havecreated many breeds of cats, dog, cattle, andchickens.Artificial Selection1• Natural selection explains howcharacteristics change and how new speciesarise.Life and Geologic Time
• The exoskeleton of a trilobite consists ofthree lobes that run the length of the body.Trilobites1• The trilobite’s body also has a head(cephalon), a segmented middle section(thorax), and a tail (pygidium).Life and Geologic Time
• Paleontologists canuse these differentcharacteristics todemonstrate changesin trilobites throughgeologic time.Changing Characteristics ofTrilobites1Life and Geologic Time
Changing Characteristics ofTrilobites1• These changes cantell you about howdifferent trilobitesfrom differentperiods lived andresponded tochanges in theirenvironments.Life and Geologic Time
• Trilobite eyes showthe result of naturalselection.Trilobite Eyes1Life and Geologic Time
Trilobite Eyes1Life and Geologic Time• In most species of trilobites, the eyes werelocated midway on the head—a compromisefor an organism that was adapted for crawlingon the seafloor and swimming in the water.• Over time, the eyes in trilobites changed.• In many trilobite species, the eyes becameprogressively smaller until they completelydisappeared.
Trilobite Eyes1Life and Geologic Time• Blind trilobites might have burrowed intosediments on the seafloor or lived deeper thanlight could penetrate.• In other species,however, theeyes becamemore complex.
Trilobite Eyes1Life and Geologic Time• One kind of trilobite, Aeglina, developed largecompound eyes that had numerous individuallenses.• Some trilobites developed stalks that held theeyes upward.
Trilobite Bodies1Life and Geologic Time• The trilobite body andtail also underwentsignificant changes inform through time.• It is thought thatOlenellus, and otherspecies that have somany body segments,are primitive trilobites.
Fossils Show Changes1Life and Geologic Time• Trilobite exoskeletons changed as trilobitesadapted to changing environments.• Species that could not adapt became extinct.
Plate Tectonics and Earth History• Plate tectonics isone possibleanswer to theriddle of trilobiteextinction.• By the end of the Paleozoic Era, sea levelshad dropped and the continents had cometogether to form one giant landmass, thesupercontinent Pangaea.1Life and Geologic Time
Plate Tectonics and Earth History• Because trilobites lived in the oceans, theirenvironment was changed or destroyed.• Not all scientists accept this explanation forthe extinctions at the end of the PaleozoicEra, and other possibilities—such as climatechange—have been proposed.1Life and Geologic Time
1Question 1Which of these geologic time subdivisions islongest?A. eraB. eonC. epochD. periodSection CheckNC: 5.01
1Section CheckAnswerThe answer is B.Eons are the longestsubdivisions ofgeologic time andare based upon theabundance of certainfossils.NC: 5.01
1Question 2What is organic evolution?Section CheckAnswerOrganic evolution is the changeof species over time.NC: 5.01, 5.02
1Question 3__________ is the process by which organismswith specific characteristics have a greaterchance of surviving and reproducing thanorganisms that do not have those characteristics.A. Artificial selectionB. Natural selectionC. Organic evolutionD. PredominanceSection CheckNC: 5.02
1Section CheckAnswerThe answer is B. Charles Darwin wrote aboutthe theory of evolution by natural selection inhis book, The Origin of Species.NC: 5.02
Precambrian Time• Precambrian time is the longest part ofEarth’s history and includes the Hadean,Archean, and Proterozoic Eons.Early Earth History2
Precambrian Time• Precambrian time lasted from about 4.5billion years ago to about 544 million yearsago.Early Earth History2
Precambrian Time• Although the Precambrian was the longestinterval of geologic time, relatively little isknown about the organisms that lived duringthis time.• One reason is that many Precambrian rockshave been so deeply buried that they havebeen changed by heat and pressure.Early Earth History2
Precambrian Time• In addition, most Precambrian organismsdidn’t have hard parts that otherwise wouldhave increased their chances to be preservedas fossils.Early Earth History2
Early Life• Many studies of theearly history of lifeinvolve ancientstromatolites.• Stromatolites arelayered mats formedby cyanobacteriacolonies.2Early Earth History
Early Life• Cyanobactreia are blue-green algae thoughtto be one of the earliest forms of life onEarth. They contained chlorophyll and usedphotosynthesis.• During photosynthesis they producedoxygen, which helped oxygen become amajor atmospheric gas.2Early Earth History
Early Life• Animals without backbones, calledinvertebrates, appeared toward the end ofPrecambrian time.• Because these early invertebrates were soft-bodied, they weren’t often preserved asfossils. Because of this, many Precambrianfossils are trace fossils.2Early Earth History
Unusual Life-Forms• A group of animals with shapes similar tomodern jellyfish, worms, and soft corals wasliving late in Precambrian time.• This group of organisms has become knownas the Ediacaran fauna.2Early Earth History• Ediacaran animals were bottom dwellers andmight have had tough outer covering like airmattresses.
• The Paleozoic Era, or era of ancient life,began about 544 million years ago and endedabout 248 million years ago.The Paleozoic Era• An abundance of organisms with hard parts,such as shells, marks the beginning of thePaleozoic Era.2Early Earth History
Paleozoic Life• Many of the life-forms scientists know aboutwere marine, meaning they lived in theocean.2Early Earth History• Trilobites were common, especially early inthe Paleozoic.• Other organisms developed shells that wereeasily preserved as fossils.• Vertebrates, or animals with backbones, alsoevolved during this era.
Paleozoic Life• Armored fish with jaws lived during theDevonian Period.2Early Earth History• By the DevonianPeriod, forestshad appeared andvertebrates beganto adapt to landenvironments, aswell.
Life on Land• Paleontologists know that many ancient fishhad lungs as well as gills.2Early Earth History• Lungs enabled these fish to live in water withlow oxygen levels—when needed they couldswim to the surface and breathe air.• One kind of ancient fish had lungs andleglike fins, which were used to swim andcrawl around on the ocean bottom.
Life on Land• Paleontologists hypothesize that amphibiansmight have evolved from this kind of fish.2Early Earth History
• Today amphibianslive in a variety ofhabitats in waterand on land.• They all have atleast one thing incommon, though.They must laytheir eggs in wateror moist places.2Early Earth HistoryLife on Land
• By the Pennsylvanian Period, someamphibians evolved an egg with a membranethat protected it from drying out.• These animals, called reptiles, no longer neededto lay eggs in water.2Early Earth HistoryLife on Land• Reptiles also have skin with hard scales thatprevent loss of body fluids.• This adaptation enables them to survive fartherfrom water in relatively dry climates.
• Several mountain-building episodesoccurred during thePaleozoic Era.2• The AppalachianMountains, forexample, formedduring this time.Early Earth HistoryMountain Building
• The first mountain-building episodeoccurred as the ocean separating NorthAmerica from Europe and Africa closed.• Several volcanic island chains that hadformed in the ocean collided with the NorthAmerican Plate.2• The collision of the island chains generatedhigh mountains.Mountain BuildingEarly Earth History
• The next mountain-building episode was aresult of the African Plate colliding with theNorth American Plate.• When Africa and North America collided,rock layers were folded and faulted.2• Sediments were uplifted to form an immensemountain belt, part of which still remainstoday.Mountain BuildingEarly Earth History
• At the end of thePaleozoic Era, morethan 90 percent of allmarine species, and70 percent of all landspecies died off.2End of an EraEarly Earth History
• Near the end of the Permian Period, thecontinental plates came together andformed the supercontinent Pangaea.2• Mountain-building processes caused seas toclose and deserts to spread over NorthAmerica and Europe.• Many species, especially marine organisms,couldn’t adapt to these changes, and becameextinct.End of an EraEarly Earth History
• During the late Paleozoic Era, volcanoes wereextremely active. If the volcanic activity wasgreat enough, it could have affected the entireglobe.2• Perhaps a large asteroid or comet collidedwith Earth some 248 million years ago.• Perhaps the extinction was caused by severalor all of these events happening at about thesame time.Other HypothesesEarly Earth History
2Section CheckQuestion 1Which was the longest period ofgeologic time?A. Paleozoic EraB. Pennsylvanian PeriodC. Phanerozoic EonD. Precambrian TimeNC: 5.01
2Section CheckAnswerThe answer is D.Precambrian Time wasthe longest period ofgeologic time, lastingabout 4 billion years.NC: 5.01
2Section CheckQuestion 2_________ are blue-green algae and are thoughtto be one of the earliest forms of life on Earth.A. Amphibious plantsB. CyanobacteriaC. DickensoniaD. TrilobitesNC: 5.01, 5.03
2Section CheckAnswerThe answer is B. Cyanobacteria arephotosynthetic organisms containingchlorophyll.NC: 5.01, 5.03
2Section CheckQuestion 3Animals without backbones are called__________.A. exvertebratesB. invertebratesC. neovertebratesD. nonvertebratesNC: 5.01
2Section CheckAnswerThe answer is B. The proper term for animalswithout backbones is “invertebrates”.NC: 5.01
• The Mesozoic Era, or era of middle life,was a time of many changes on Earth.3• At the beginning of the Mesozoic Era, allcontinents were joined as a single landmasscalled Pangaea.Middle and Recent Earth HistoryThe Mesozoic EraThe Breakup of Pangaea
• Pangaea separated into two large landmasses.3• The northern mass was Laurasia, andGondwanaland was the southern landmass.Middle and Recent Earth HistoryThe Mesozoic EraThe Breakup of PangaeaClick image to view movie.
• Reptile’s skin helps it retain bodily fluids.3• This characteristic, along with their shelledeggs, enabled reptiles to adapt readily to thedrier climate of the Mesozoic Era.Middle and Recent Earth HistoryThe Mesozoic EraThe Breakup of Pangaea• Reptiles became the most conspicuousanimals on land by the Triassic period.
3• Dinosaurs ranged inheight from lessthat 1 m toenormous creatureslike Apatosaurusand Tyrannosaurus.• Throughout the Mesozoic Era, new speciesof dinosaur evolved and other speciesbecame extinct.Middle and Recent Earth HistoryDinosaurs
3• Some dinosaur tracks indicate that theseanimals were much faster runners than youmight think.• Gallimimus could reach speeds of 65 km/h.Middle and Recent Earth HistoryDinosaurs Were Active
3• Some studies also indicate that dinosaursmight have been warm blooded, not coldblooded like present-day reptiles.Middle and Recent Earth HistoryDinosaurs Were Active• Slices through some cold-blooded animalbones show rings similar to growth rings intrees.• The bones of some dinosaurs don’t show thisring structure.
3• The fossil record also indicates that somedinosaurs nurtured their young and traveledin herds in which the adults surrounded theiryoung.Middle and Recent Earth HistoryGood Mother Dinosaurs
3• One such dinosaur is Maiasaura.Middle and Recent Earth HistoryGood Mother Dinosaurs• This dinosaur built nests in which it laid eggsand raised its offspring.• Nests have been found in relatively closeclusters, indicating that more than one familyof dinosaurs built in the same areas.• Some fossils of hatchlings have been foundnear adult animals, leading paleontologists tothink that some dinosaurs nurtured theiryoung.
• Birds appeared during the Jurassic Period.3• Some paleontologists think that birds evolvedfrom small, meat-eating dinosaurs.• The earliest bird, Archaeopteryx, had wingsand feathers.Middle and Recent Earth HistoryBirds
• Mammalsfirst appearedin the TriassicPeriod.3• The earliestmammalswere small,mouselikecreatures.Middle and Recent Earth HistoryMammals
• Mammals are warm-blooded vertebratesthat have hair covering their bodies.3• The females produce milk to feed theiryoung.Middle and Recent Earth HistoryMammals• These two characteristics have enabledmammals to survive in many changingenvironments.
• During most of the Mesozoic Era,gymnosperms dominated the land.3• Gymnosperms are plants that produce seedsbut not flowers.Middle and Recent Earth HistoryGymnosperms• These include pines and ginkgo trees.
• Angiosperms, or flowering plants, firstevolved during the Cretaceous Period.3• Angiosperms produce seeds with hard outercoverings.Middle and Recent Earth HistoryAngiosperms• Because their seeds are enclosed andprotected, angiosperms can live in manyenvironments.• Angiosperms are the most diverse andabundant land plants today.
• The Mesozoic Era ended about 65 millionyears ago with a major extinction of land andmarine species.3Middle and Recent Earth HistoryEnd of an Era• Many paleontologists hypothesize that acomet or asteroid collided with Earth,causing a huge cloud of dust and smoke torise in the atmosphere, blocking out the Sun.
• Without sunlight the plants died, and allanimals that depended on these plants alsodied.3Middle and Recent Earth HistoryEnd of an Era• All the organisms that you see around youtoday are descendants of the survivors of thegreat extinction at the end of the MesozoicEra.
• The Cenozoic Era, or era of recent life,began about 65 million years ago andcontinues today.3Middle and Recent Earth HistoryThe Cenozoic Era• The Cenozoic Era is subdivided into twoperiods.• The first of these is the Tertiary period.• The present-day period is the QuaternaryPeriod. It began about 1.8 million years ago.
• Many mountain rangesformed during theCenozoic Era.3Middle and Recent Earth HistoryTimes of Mountain Building• These include theAlps in Europe andthe Andes in SouthAmerica.
• The Himalayaformed asIndia movednorthward andcollided withAsia.3Middle and Recent Earth HistoryTimes of Mountain Building• The collision crumpled and thickened Earth’scrust, raising the highest mountains presentlyon Earth.
• Throughout much of the Cenozoic Era,expanding grasslands favored grazing planteaters like horses, camels, deer, and someelephants.3Middle and Recent Earth HistoryFurther Evolution of Mammals• Many kinds ofmammals becamelarger.
• Not all mammals remained on land.3Middle and Recent Earth HistoryFurther Evolution of Mammals• Ancestors of the present-day whales anddolphins evolved to live in the sea.
• As Australia and South America separatedfrom Antarctica during the continuingbreakup of the continents, many speciesbecame isolated.3Middle and Recent Earth HistoryFurther Evolution of Mammals• They evolved separately from life-forms inother parts of the world.
• Evidence of this can be seen today inAustralia’s marsupials.3Middle and Recent Earth HistoryFurther Evolution of Mammals• Marsupials are mammals such as kangaroos,koalas, and wombats that carry their young ina pouch.
• Your species, Homo sapiens, probablyappeared about 140,000 years ago.3Middle and Recent Earth HistoryFurther Evolution of Mammals• Some people suggest that the appearance ofhumans could have led to the extinction ofother mammals.• As their numbers grew, humans competed forfood that other animals relied upon.
3Section CheckQuestion 1During which timeperiod did Pangaeaseparate?A. DevonianB. MississippianC. PermianD. TriassicNC: 5.01,5.02, 5.03
3Section CheckAnswerThe answer is D. Pangaea separated into twolarge landmasses, Laurasia and Gondwanaland,during the Triassic Period.NC: 5.01,5.02, 5.03
3Section CheckQuestion 2The era of recent life is the __________.A. Cenozoic EraB. Mesozoic EraC. Paleozoic EraD. Precambrian TimeNC: 5.01
3Section CheckAnswerThe answer is A. The Cenozoic Era beganabout 65 million years ago.NC: 5.01
3Section CheckQuestion 3What were the dominant land animals of theMesozoic Era?A. DinosaursB. GymnospermsC. Predatory fishD. TrilobitesNC: 5.01, 5.03
3Section CheckAnswerThe answer is A. Dinosaurs were thedominant land animals of the Mesozoic Era.Gymnosperms were the dominant plants.NC: 5.01, 5.03
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