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1、阅读原文：?Itshouldbeobviousthatcetaceans-whales,porpoises,anddolphins-aremammals.Theybreathethroughlungs,notthroughgills,andgivebirthtoliveyoung.Theirstreamlinedbodies,theabsenceofhindlegs,andthepresenceofafluke1andblowhole2cannotdisguisetheiraffinitieswithland
2、dwellingmammals.However,unlikethecasesofseaottersandpinnipeds(seals,sealions,andwalruses,whoselimbsarefunctionalbothonlandandatsea),itisnoteasytoenvisionwhatthefirstwhaleslookedlike.Extinctbutalreadyfullymarinecetaceansareknownfromthefossilrecord.Howwastheg
3、apbetweenawalkingmammalandaswimmingwhalebridged?Missinguntilrecentlywerefossilsclearlyintermediate,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
4、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。Veryexcitingdiscoverieshavefinallyallowedscientiststoreconstructthemostlikelyoriginsofcetaceans.In1979,ateamlookingforfossilsin
5、northernPakistanfoundwhatprovedtobetheoldestfossilwhale.ThefossilwasofficiallynamedPakicetusinhonorofthecountrywherethediscoverywasmade.Pakicetuswasfoundembeddedinrocksformedfromriverdepositsthatwere52millionyearsold.Theriverthatformedthesedepositswasactual
6、lynotfarfromanancientoceanknownastheTethysSea.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.Thefossilconsistsofacompleteskullofanarchaeocyte,anextinctgroupofancesto
7、rsofmoderncetaceans.Althoughlimitedtoaskull,thePakicetusfossilprovidespreciousdetailsontheoriginsofcetaceans.Theskulliscetacean-likebutitsjawboneslacktheenlargedspacethatisfilledwithfatoroilandusedforreceivingunderwatersoundinmodernwhales.Pakicetusprobablyd
8、etectedsoundthroughtheearopeningasinlandmammals.Theskullalsolacksablowhole,anothercetaceanadaptationfordiving.Otherfeatures,however,showexpertsthatPakicetusisatransitionalformbetweenagroupofextinctfles
