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ExtinctionEpisodesofthe過去的事ItwasnotuntiltheCambrianperiod,beginningabout600millionyearsago,thatagreatproliferationofmacroscopicspeciesoccurredonEarthandproducedafossilrecordthatallowsustotracktheriseandfallofbiodiversity.SincetheCambrianperiod,biodiversityhasgenerallyrisen,buttherehavebeensomenotableexceptions.Biodiversitycollapseddramaticallyduringatleastfiveperiodsbecauseofmassextinctionsaroundtheglobe.Thefivemajormassextinctionsreceivemostoftheattention,buttheyareonlyoneendofaspectrumofextinctionevents.Collectively,morespecieswentextinctduringsmallereventsthatwerelessdramaticbutmorefrequent.Thebestknownofthefivemajorextinctionevents,theonethatsawthedemiseofthedinosaurs,istheCretaceous-Tertiaryextinction.，種就是因此而的。在五次大中恐龍的是發(fā)生在白堊—第三紀(jì)，Startingabout280millionyearsago,reptileswerethedominantlargeanimalsinterrestrialenvironments.Inpopularlanguagethiswastheera“whendinosaursruledEarth,”whenawidevarietyofreptilespeciesoccupyingmanyecologicalniches.However,nogrouporspeciescanmaintainitsdominanceindefini y,andwhen,afterover200millionyears, ofdinosaurscametoadramaticendabout65millionyearsago,mammalsbegantoflourish,evolvingfromrelativelyfewtypesofsmallterrestrialanimalsintothemyriadofdiversespecies,includingbatsandwhales,thatweknowtoday.PaleontologistslabelthispointinEarth’shistoryasoftheCretaceousperiodandthebeginningoftheTertiaryperiod,oftenabbreviatedastheK-Tboundary.Thistimewasalsomarkedbychangesinmanyothertypesofanisms.Overall,about38percentofthefamiliesofmarineanimalswerelost,withpercentagesmuchhigherinsomegroups.Ammonoidmolluskswentfrombeingverydiverseandabundanttobeingextinct.Anextremelyabundantsetofplanktonicmarineanimalscalledforaminiferalargelydisappeared,althoughtheyreboundedlater.Amongplants,theK-Tboundarysawasharpbutbriefriseintheabundanceofprimitivevascularplantssuchasferns,clubmosses,horsetails,andconifersandothergymnosperms.Thenumberoffloweringplants(angiosperms)wasreducedatthistime,buttheythenbegantoincrease，K-TK-T邊界時(shí)期的其他生物也發(fā)生了重大變化?？偟膩碚f，大約38%的海洋生物了的菊石軟體動(dòng)物也瀕，Whatcausedthesechanges?Formanyyearsscientistsassumedthoolingoftheclimatewasresponsible,withdinosaursbeingparticularlyvulnerablebecause,likemodernreptiles,theywereectothermic(dependentonenvironmentalheat,orcold-blooded).Itisnowwidelybelievedthatatleastsomespeciesofdinosaurshadametabolicratehighenoughforthemtobeendotherms(animalsthatmaintainarelativelyconsistentbodytemperaturebygeneratingheatinternally).Nevertheless,climaticexplanationsfortheK-Textinctionarenotreallychallengedbytheideasthatdinosaursmayhavebeenendothermic,becauseevenendothermscanbeaffectedbyasignificantchangeintheclimate.如此，恐龍有可能是恒溫動(dòng)物這一論斷未真正K-T的氣候變化原因。ExplanationsfortheK-Textinctionwererevolutionizedin1980whenagroupofphysicalscientistsledbyLuisAlvarezproposedthat65millionyearsagoEarthwasstuckbya10-kilometer-widemeteoritetravelingat90,000kilometersperhour.TheybelievedthatthisimpactgeneratedathickcloudofdustthatenvelopedEarth,shuttingoutmuchofthe ingsolarradiationandreducingplantphotosynthesistoverylowlevels.Short-termeffectsmighthaveincludedhugetidalwavesandextensivefires.Inotherwords,aseriesofeventsarisingfromasingleclysmiceventcausedthemassiveextinctions.Initially,themeteoritetheorywasbasedonasinglelineofevidence.Atlocationsaroundtheglobe,geologistshadfoundanunusuallyhighconcentrationofiridiuminthelayerofsedimentaryrocksthatwasformedabout65millionyearsago.Iridiumisanelementthatisusually nearEarth’ssurface,butitisabundantinsomemeteorites.Therefore,AlvarezandcolleaguesconcludedthatitwaslikelythattheiridiuminsedimentaryrocksdepositedattheK-Tboundaryhadoriginatedinagiantmeteoriteorasteroid.Mostscientistcametoacceptthemeteoritetheoryafterevidencecametolightthatacircularformation,180kilome