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READINGPASSAGE12Terminated!DinosaurEra!ATheageofdinosaurs,whichendedwiththecataclysmicbangofameteorimpact65millionyearsago,manyalsohavebegunwithone.Researchersfoundrecentlythefirstdirect,thoughtentative,geologicalevidenceofameteorimpact200millionyearsago,coincidingwithamassextinctionthateliminatedhalfofthemajorgroupsoflifeandopenedtheevolutionarydoorforwhatwasthenarelativelysmallgroupofanimals:dinosaurs.BThecauseandtimingoftheascentofdinosaurshashavebeenmuchdebated.Ithasbeenimpossibletodrawanyspecificconclusionsbecausethetransitionbetweentheoriginofdinosaursandtheirascenttodominancehasnotbeensampledindetail.“Thereisprobablyageochemicalsignatureofsomethingimportanthappening,probablyanasteroidimpact,justbeforethetimeinwhichfamiliardinosaur-dominatedcommunitiesappear,”saidDr.PaulE.Olsen,aprofessorofearthandenvironmentalsciencesatColumbiaUniversity’sLamont-DohertyEarthObservatoryinPalisades,N.Y.COlsenandhiscolleaguesstudiedvertebratefossilsfrom80sitesinfourdifferentancientriftbasins,partofachainofriftsthatformedasNorthAmericabegantosplitapartfromthesupercontinentthatexisted230-190millionyearsago.Inthelayerofrockcorrespondingtotheextinction,thescientistsfoundelevatedamountsoftherareelementiridium.Apreciousmetalbelongingtotheplatinumgroupofelements,iridiumismoreabundantinmeteoritesthaninrocks.DOnearth,asimilarspikeofiridiumin65million-year-oldrocksgaveriseinthe1970stothetheorythatameteorcausedthedemiseofthedinosaurs.ThattheoryremainedcontroversialforyearsuntilitwascorroboratedbyotherevidenceandtheimpactsitewasfoundofftheYucatanPeninsula.Scientistswillneedtoexaminethenewiridiumanomalysimilarly.Thelevelsareonlyaboutone-tenthashighasthosefoundatthelaterextinction.Thatcouldmeanthatthemeteorwassmallerorcontainedlessiridiumorthatameteorwasnotinvolved-iridiumcanalsocomefromtheEarth’sinterior,belchedoutbyvolcaniceruptions.Dr.MichaelJ.Benton,aprofessorofvertebratepaleontologyattheUniversityofBristolinEngland,describedthedataas“thefirstreasonablyconvincingevidenceofaniridiumspike”.EThescientistsfoundmoreevidenceofrapidextinctioninadatabaseof10,000fossilizedfootprintsinformerlakebasinsfromVirginiatoNovaScotia.Althoughindividualspeciescannotusuallybeidentifiedsolelyfromtheirfootprints—thetrackofahousecat,forexample,resemblethoseofababytiger—footprintsaremuchmoreplentifulthanfossilbonesandcanprovideamorecompletepictureofthetypesofanimalswalkingaround.“Itmakesitveryeasyforustoelltheveryobvioussignalsofmassivefaunachange,”Dr.Olsensaid.Becausethesedimentpilesupquicklyinlakebasins,theresearcherswereabletoassignadatetoeachfootprint,basedonthelayerofrockwhereitwasfound.TheydeterminedthatthemixofanimalswalkingacrosswhatisnowtheEastCoastofNorthAmericachangedsuddenlyabout200millionyearsago.FThetracksofseveralmajorreptilegroupscontinuealmostuptothelayerofrockmarkingtheendoftheTriassicgeologicperiod202millionyearsago,andthenvanishinyoungerlayersfromtheJurassicPeriod.“Ithinkthefootprintmethodologyisverynovelandveryexciting,”saidDr.PeterD.Ward,aprofessorofgeologyattheUniversityofWashington.Hecalledthedata“veryrequiredmoreresearch.Lastyear,researchersledbyDr.Wardreportedthatthetypesofcarboninrockchangedabruptlyatthistime,indicationgasuddendyingoffofplantsoverlessthan0,000years.Thefootprintresearchreinforcesthehypothesisthattheextinctionwassudden.GSeveralgroupsofdinosaurssurvivedthatextinction,andthefootprintsshowthatnewgroupsemergedsoonafterward.Beforetheextinction,aboutone-fifthofthefootprintswereleftbydinosaur;aftertheextinction,morethanhalfwerefromdinosaurs.Thechanges,theresearcherssaid,occurredwithin30,000years—ageologicalblinkofaneye.ThescientistspostulatethattheasteroidorcometimpactandtheresultingdeathofTriassiccompetitorsallowedafewgroupsofcarnivorousdinosaurstoevolveinsizeveryquicklyanddominatethetopoftheterrestrialfoodchainglobally.HAmongthecreaturesthatdisappearedintheextinctionwerethedominantpredatorsatthetime:15-foot-longrauisuchianswithgreatknife-liketeethandphytosaursthatresembledlargecrocodiles.Dinosaursfirstevolvedabout230millionyearsago,buttheyweresmall,competinginacrowdedecologicalniche.Beforetheextinction200millionyearsago,thelargestofthemeat-eatingdinosaurswereaboutthesizeoflargedogs.Notterriblyimpressive.”Dr.Olsensaid.Thedinosaursquicklygrew.Thetoe-to-heellengthofthefootofameateaterfromtheJurassicperiodwasonaverage20percentlongerthanitsTriassicancestor.Largerfeetcancarrybiggerbodies;thescientistsinferthedinosaursdoubtedinweight,eventuallyevolvingintofearsomevelociraptors,tyrannosaurusrexandotherlargecarnivorousdinosaurs.IThespurtinevolutionissimilartotheriseofmammalsaftertheextinctionofdinosaurs.Mammals,nolargerthansmalldogsduringtheageofdinosaurs,diversifiedintotigers,elephants,whalesandpeopleafterthereptiliancompetitiondiedaway.ThesuccessofthedinosaursaftertheTriassic-Jurassicextinctionmaybewhytheydidnotsurvivethesecondextinction.”Smallanimalsalwaysdobetterincatastrophicsituations,Dr.Olsensaid,becausetheycansurviveonsmalleramountsoffood.”Healsopointedoutthatscientistsnowbelievethesmalldinosaursdidsurvive.“Wejustcallt