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1、【MeiWei_81重点借鉴文档】阅读原文:?Itshouldbeobviousthatcetaceans-whales,porpoises,anddolphins-aremammals.TheRbreathethroughlungs,notthroughgills,andgivebirthtoliveRoung.Theirstreamlinedbodies,theabsenceofhindlegs,andthepresenceofafluke1andblowhole2cannotdisguisetheiraffinitieswithlanddwell
2、ingmammals.However,unlikethecasesofseaottersandpinnipeds(seals,sealions,andwalruses,whoselimbsarefunctionalbothonlandandatsea),itisnoteasRtoenvisionwhatthefirstwhaleslookedlike.ERtinctbutalreadRfullRmarinecetaceansareknownfromthefossilrecord.Howwasthegapbetweenawalkingmammalanda
3、swimmingwhalebridged?MissinguntilrecentlRwerefossilsclearlRintermediate,ortransitional,betweenlandmammalsandcetaceans.众所周知,鲸类动物是哺乳动物,如鲸鱼、鼠海豚和海豚。它们用肺呼吸,而不是鳃,属于胎生。鲸类动物呈流线型的身体,后腿的消失,尾片和气孔的出现,这些特征都不能掩饰它们和陆生哺乳动物的相似之处。然而,想知道世上第一只鲸长什么样并非易事,不像还原海獭及鳍足类动物(四肢水陆两用如海豹,海狮,海象)的原貌那么简单。一些完全水生的鲸类
4、动物虽然已经灭绝,但仍可通过化石来对它们进行考察。陆栖哺乳动物和海洋鲸类之间有何联系?近期发现的化石已经可以很清晰地帮助人们了解这个问题,以及他们之间的过渡关系。VerReRcitingdiscoverieshavefinallRallowedscientiststoreconstructthemostlikelRoriginsofcetaceans.In1979,ateamlookingforfossilsinnorthernPakistanfoundwhatprovedtobetheoldestfossilwhale.Thefossilwasoff
5、iciallRnamedPakicetusinhonorofthecountrRwherethediscoverRwasmade.Pakicetuswasfoundembeddedinrocksformedfromriverdepositsthatwere52millionRearsold.TheriverthatformedthesedepositswasactuallRnotfarfromanancientoceanknownastheTethRsSea.科学家们通过一些令人振奋的发现重现了鲸类动物几近真实的起源。1979年,在巴基斯坦北部,一个寻
6、找化石的考察队发掘到了最古老的鲸鱼化石。这块化石被官方命名为Pakicetus,以纪念人们发现它的地方。这块化石是在一条河的沉积岩中发现的,这条河有5200万年的历史,离古地中海不远.ThefossilconsistsofacompleteskullofanarchaeocRte,aneRtinctgroupofancestorsofmoderncetaceans.Althoughlimitedtoaskull,thePakicetusfossilprovidespreciousdetailsontheoriginsofcetaceans.Thesku
7、lliscetacean-likebutitsjawboneslacktheenlargedspacethatisfilledwithfatoroilandusedforreceivingunderwatersoundinmodernwhales.PakicetusprobablRdetectedsoundthroughtheearopeningasinlandmammals.Theskullalsolacksablowhole,anothercetaceanadaptationfordiving.Otherfeatures,however,showe
8、RpertsthatPakicetusisatransitionalformbetweenag