Mesozoic Era | geochronology - Encyclopedia Britannica

Mesozoic Era, second of Earth's three major geologic eras of Phanerozoic time. Its name is derived from the Greek term for “middle life.” The Mesozoic Era ... MesozoicEra TableofContents MesozoicEra IntroductionMesozoicgeologyMesozoiclife FastFacts 2-MinSummary RelatedContent Media Images More AdditionalReading MoreArticlesOnThisTopic Contributors ArticleHistory Home Science EarthScience,GeologicTime&Fossils Fossils&GeologicTime MesozoicEra geochronology Actions Cite verifiedCite Whileeveryefforthasbeenmadetofollowcitationstylerules,theremaybesomediscrepancies. Pleaserefertotheappropriatestylemanualorothersourcesifyouhaveanyquestions. SelectCitationStyle MLA APA ChicagoManualofStyle CopyCitation Share Share Sharetosocialmedia Facebook Twitter URL https://www.britannica.com/science/Mesozoic-Era GiveFeedback ExternalWebsites Feedback Corrections?Updates?Omissions?Letusknowifyouhavesuggestionstoimprovethisarticle(requireslogin). 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MesozoicEra-StudentEncyclopedia(Ages11andup) By CarolMarieTang • EditHistory TableofContents KeyPeople: GeorgeGaylordSimpson GideonAlgernonMantell ...(Showmore) RelatedTopics: dinosaur TriassicPeriod CretaceousPeriod JurassicPeriod fossil ...(Showmore) Seeallrelatedcontent→ Summary Readabriefsummaryofthistopic MesozoicEra,secondofEarth’sthreemajorgeologicerasofPhanerozoictime.ItsnameisderivedfromtheGreektermfor“middlelife.”TheMesozoicErabegan252.2millionyearsago,followingtheconclusionofthePaleozoicEra,andended66millionyearsago,atthedawnoftheCenozoicEra.(Seethegeologictimescale.)ThemajordivisionsoftheMesozoicEraare,fromoldesttoyoungest,theTriassicPeriod,theJurassicPeriod,andtheCretaceousPeriod.TheancestorsofmajorplantandanimalgroupsthatexisttodayfirstappearedduringtheMesozoic,butthiseraisbestknownasthetimeofthedinosaurs.Earth’sclimateduringtheMesozoicErawasgenerallywarm,andtherewaslessdifferenceintemperaturebetweenequatorialandpolarlatitudesthanthereistoday.TheMesozoicwasatimeofgeologicandbiologicaltransition.Duringthiserathecontinentsbegantomoveintotheirpresent-dayconfigurations.Adistinctmodernizationoflife-formsoccurred,partlybecauseofthedemiseofmanyearliertypesoforganisms.ThreeofthefivelargestmassextinctionsinEarthhistoryareassociatedwiththeMesozoic:amassextinctionoccurredattheboundarybetweentheMesozoicandtheprecedingPaleozoic;anotheroccurredwithintheMesozoicattheendoftheTriassicPeriod;andathirdoccurredattheboundarybetweentheMesozoicandsubsequentCenozoic,resultinginthedemiseofthedinosaurs. 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Mesozoicgeology AttheoutsetoftheMesozoic,allofEarth’scontinentswerejoinedtogetherintothesupercontinentofPangea(seethemapoftheEarlyTriassic).Bythecloseoftheera,Pangeahadfragmentedintomultiplelandmasses.ThefragmentationbeganwithcontinentalriftingduringtheLateTriassic.ThisseparatedPangeaintothecontinentsofLaurasiaandGondwana.BytheMiddleJurassictheselandmasseshadbegunfurtherfragmentation.AtthattimemuchofPangealaybetween60°Nand60°S,andattheEquatorthewideningTethysSeacutbetweenGondwanaandLaurasia.Whenriftinghadsufficientlyprogressed,oceanicspreadingcentresformedbetweenthelandmasses.DuringtheMiddleJurassic,NorthAmericabeganpullingapartfromEurasiaandGondwana.BytheLateJurassic,AfricahadstartedtosplitofffromSouthAmerica,andAustraliaandAntarcticahadseparatedfromIndia(seethemapoftheLateJurassic).NearthecloseoftheCretaceous,MadagascarseparatedfromAfrica,andSouthAmericadriftednorthwestward(seethemapoftheLateCretaceous).Pangea:EarlyTriassicPeriodPaleogeographyandpaleoceanographyofEarlyTriassictime.Thepresent-daycoastlinesandtectonicboundariesoftheconfiguredcontinentsareshownintheinsetatthelowerright.Adaptedfrom:C.R.Scotese,TheUniversityofTexasatArlingtonPangea:LateJurassicPeriodPaleogeographyandpaleoceanographyofLateJurassictime.Present-daycoastlinesandtectonicboundariesofcontinentsareshownintheinsetatthelowerright.Adaptedfrom:C.R.Scotese,TheUniversityofTexasatArlingtonDistributionoflandmasses,mountainousregions,shallowseas,anddeepoceanbasinsduringLateCretaceoustime.Includedinthepaleogeographicreconstructionarecoldandwarmoceancurrents.Thepresent-daycoastlinesandtectonicboundariesoftheconfiguredcontinentsareshownintheinsetatthelowerright.AdaptedfromC.R.Scotese,TheUniversityofTexasatArlington Asthecontinentsriftedandruptured,thicksequencesofmarinesedimentsaccumulatedinlargelineartroughsalongtheirmargins.OceanbasindepositsofJurassicagearefoundtodayinthecircum-Pacificregion,alongthecoastsofeasternNorthAmericaandtheGulfofMexico,andonthemarginsofEurasiaandGondwana(thatis,alongthenorthernandsouthernboundariesoftheTethysSea). Majormountainbuilding(orogeny)beganonthewesternmarginsofbothNorthandSouthAmericaandbetweentheseparatingfragmentsofGondwana.Forexample,thenorthwesterlymovementofNorthAmericaresultedinacollisionofthewesternedgeoftheNorthAmericancontinentalplatewithacomplexofislandarcsduringtheLateJurassic.So-calledexoticterranes,geologicfragmentsthatdiffermarkedlyinstratigraphy,paleomagnetism,andpaleontologyfromadjoiningcontinentalcrust,wereaccretedtothemarginoftheNorthAmericanplate.Asthrustingoccurredinaneastwarddirection,hugegraniticbatholithsformedinwhatisnowtheSierraNevadarangealongtheCalifornia-Nevadaborder.OthernotableepisodesofmountainbuildingduringtheMesozoicincludetheSevierandLaramideorogenies,whichtookplaceinwesternNorthAmericaduringCretaceoustime.TheseeventscreatedtheRockyMountains. Mesozoicrocksarewidelydistributed,appearinginvariouspartsoftheworld.Alargepercentageoftheserocksaresedimentary.AtvarioustimesduringtheMesozoic,shallowseasinvadedcontinentalinteriorsandthendrainedaway.DuringMiddleTriassictime,amarineincursion—theMuschelkalkSea—coveredthecontinentalinteriorofEurope.SeasagaintransgresseduponthecontinentsbetweentheEarlyandLateJurassicandintheEarlyCretaceous,leavingextensivebedsofsandstone,ironstone,clays,andlimestone(seeSolnhofenLimestone).AlastmajortransgressionofmarinewatersfloodedlargesegmentsofallthecontinentslaterintheCretaceous.Thesesharprisesinsealevelandresultantworldwidefloodingarethoughttohavehadtwocauses.Thefirstwaswarmglobaltemperatures,whichpreventedlargevolumesofwaterfrombeingsequesteredonlandintheformoficesheets.Thesecondwasrelatedtoacceleratedseafloorspreading;theattendantenlargementofoceanridgesdisplacedenormousamountsofoceanwaterontothelandmasses.MarinetransgressionwassoextensivethatinNorthAmerica,forexample,aseawayspreadallthewayfromtheArctictotheGulfofMexicointheCretaceousPeriod.Widespreaddepositionofchalk,clay,blackshales,andmarloccurred.InpartsofNorthAmerica,lakeandriversedimentsrichindinosaurfossilsweredepositedalongsidemarinesediments.(SeeMorrisonFormation.) AsubstantialamountofigneousrockalsoformedduringtheMesozoic.TheorogeniesoftheJurassicandCretaceousperiodsinvolvedvolcanismandplutonicintrusionsuchasoccurredduringtheemplacementofgranitesandandesitesintheAndesofSouthAmericaduringtheLateJurassic.TwoofthelargestvolcaniceventsinEarth’shistoryoccurredduringtheMesozoic.TheCentralAtlanticMagmaticProvince,ahugevolumeofbasalt,wascreatedattheendoftheTriassicduringtheinitialriftingofPangea.Thesurfaceareaofthisigneousprovinceoriginallycoveredmorethan7millionsquarekm(about3millionsquaremiles),anditsrockscanbefoundtodayfromBraziltoFrance.Despitesuchamassivevolumeofbasalticmaterialextruded,volcanicactivitywasprobablyshort-lived,spanningonlyafewmillionyears.AttheendoftheCretaceous,anotherigneousprovince,thefloodbasaltsoftheDeccanTraps,formedonwhatisnowtheIndiansubcontinent.Somescientistshavesuggestedthatbothoftheselargeigneouseventsmayhaveinjectedsignificantamountsofcarbondioxideandaerosolsintotheatmosphere,triggeringachangeinglobalclimate.ThetimingofthesevolcaniceventsappearstooverlaptheTriassic-JurassicandCretaceous-Tertiary,orCretaceous-Paleogene,massextinctions,andtheymayhaveplayedaroleinthem. Mesozoiclife ThefaunaandfloraoftheMesozoicweredistinctlydifferentfromthoseofthePaleozoic,thelargestmassextinctioninEarthhistoryhavingoccurredattheboundaryofthetwoeras,whensome90percentofallmarineinvertebratespeciesand70percentofterrestrialvertebrategeneradisappeared.AtthestartoftheMesozoic,theremainingbiotabeganaprolongedrecoveryofdiversityandtotalpopulationnumbers,andecosystemsbegantoresemblethoseofmoderndays.Vertebrates,lessseverelyaffectedbytheextinctionthaninvertebrates,diversifiedprogressivelythroughouttheTriassic.TheTriassicterrestrialenvironmentwasdominatedbythetherapsids,sometimesreferredtoas“mammal-likereptiles,”andthethecodonts,ancestorsofdinosaursandcrocodiles,bothofwhichappearedduringtheLateTriassic.Thefirsttruemammals,whichweresmall,shrewlikeomnivores,alsoappearedintheLateTriassic,asdidthelizards,turtles,andflyingpterosaurs.Intheoceans,mollusks—includingammonites,bivalves,andgastropods—becameadominantgroup.Fishes,sharks,andmarinereptilessuchasplesiosaurs,nothosaurs,andichthyosaursalsoswamtheMesozoicseas. AnothermajorextinctioneventstruckatthecloseoftheTriassic,onethatwipedoutasmuchas20percentofmarinefamiliesandmanyterrestrialvertebrates,includingtherapsids.Thecauseofthismassextinctionisnotyetknownbutmayberelatedtoclimaticandoceanographicchanges.Inall,35percentoftheexistinganimalgroupssufferedextinction. IntheoceanstheammonitesandbrachiopodsrecoveredfromtheLateTriassiccrisis,thrivinginthewarmcontinentalseas.AmmonitesrapidlybecameverycommoninvertebratesinthemarinerealmandarenowimportantindexfossilsforworldwidecorrelationofJurassicrockstrata.Manyotheranimalforms,includingmollusks(notablythebivalves),sharks,andbonyfishes,flourishedduringtheJurassic.DuringtheJurassicandCretaceous,theecologyofmarineecosystemsbegantochange,asshownbyarapidincreaseindiversityofmarineorganisms.Itisbelievedthatincreasingpredationpressurescausedmanymarineorganismstodevelopbetterdefensesandburrowmoredeeplyintotheseafloor.Inresponse,predatorsalsoevolvedmore-effectivewaystocatchtheirprey.Thesechangesaresosignificantthattheyarecalledthe“MesozoicMarineRevolution.” Thedominantterrestrialvertebratesweredinosaurs,whichexhibitedgreatdiversityduringtheJurassicandCretaceous.BirdsarebelievedtohaveevolvedfromdinosaurancestorsduringtheLateJurassic.Ancestorsoflivingvertebrates,suchasfrogs,toads,andsalamanders,appearedonlandalongwiththetwoimportantmodernmammalgroups,theplacentalsandthemarsupials.Plantlifealsoexhibitedagradualchangetowardmore-modernformsduringthecourseoftheMesozoic.WhereasseedfernshadpredominatedintheTriassic,forestsofpalmlikegymnospermsknownascycadsandconifersproliferatedunderthetropicalandtemperateconditionsthatprevailedduringtheJurassic.Thefirstfloweringplants,orangiosperms,hadappearedbytheCretaceous.TheyradiatedrapidlyandsupplantedmanyoftheprimitiveplantgroupstobecomethedominantformofvegetationbytheendoftheMesozoic.dinosaurstoscaleAselectionofdinosaursgroupedbythegeologicintervalinwhichtheylived.EncyclopædiaBritannica,Inc. TheMesozoicclosedwithanextinctioneventthatdevastatedmanyformsoflife.Intheoceansalltheammonites,reef-buildingrudistbivalves,andmarinereptilesdiedoff,asdid90percentofthecoccolithophores(single-celledplantlikeplankton)andforaminifera(single-celledanimal-likeplankton).Onlandthedinosaursandflyingreptilesbecameextinct.TheLateCretaceousextinctionshavebeenvariouslyattributedtosuchphenomenaasglobaltectonics,drainingofthecontinentalseas,northwardmigrationofthecontinentsintodifferentandmuchcoolerclimaticzones,intensifiedvolcanicactivity,andacatastrophicmeteoriteorasteroidimpact.TheCretaceousextinctionmayverywellhavehadmultiplecauses.Asthelandmasseswereupliftedbyplatetectonismandmigratedpoleward,theclimateoftheLateCretaceousbegantodeteriorate.Infact,someoftheextinctionswerenotsuddenbutratherspannedmillionsofyears,suggestingthatagradualdeclineofsomeorganismshadalreadybegunbeforetheendoftheCretaceous.However,strongevidencesupportsthecontentionthatalarge-scaleimpactplayedasignificantroleinthemassextinctionsattheendoftheMesozoic,includingthesuddendisappearanceofmanygroups(suchasammoniteandmicrofossilspecies),thepresenceofgeochemicalandmineralogicalsignaturesthatmostlikelycamefromextraterrestrialsources,andthediscoveryoftheChicxulubcraterintheYucatánPeninsula.Itisbelievedthatanasteroidwithadiameterofabout10km(6miles)hittheEarthandcausedwildfires,acidrain,monthsofdarkness(becauseofthelargeamountofashinjectedintotheatmosphere),andcoldtemperatures(causedbyincreasedreflectionofsolarenergybackintospacebyairborneparticles).Anintensewarmingmayhavefollowed,heatbeingtrappedbyatmosphericaerosols.Whateverthecause,thismajormassextinctionmarkstheendoftheMesozoicEra.Theendofthedinosaurs(exceptbirds)andmanyotherformsoflifeallowedthedevelopmentofmodernbiotaintheCenozoicEra.ChicxulubcraterTheChicxulubcrateratthenorthwesternpointoftheYucatánPeninsulawasformedbytheimpactofanasteroid66millionyearsago.Thecloudofdustandcarbongasesthatresultedisthoughtbysomescientiststohavecausedtheextinctionofthedinosaurs.NASA/JPLcomputer-generatedimageofChicxulubcraterChicxulubcrateronthenortherncoastoftheYucatánPeninsulainMexico,inacomputer-generatedimagesynthesizedfromgravityandmagnetic-fielddata.Theburiedstructure,whichmeasuresatleast180km(112miles)across,isthoughttobethescarremainingfromtheimpact66millionyearsagoofanasteroidorcometmeasuringperhaps10km(6miles)indiameter.TheYucatáncoastlinebisectsthecrateralmosthorizontallythroughitscentre.V.L.Sharpton,UniversityofAlaska,Fairbanks;NASA CarolMarieTang