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1、智课网托福备考资料托福阅读真题分类――推论题（二）TheOriginsofCetaceansItshouldbeobviousthatcetaceans—whales,porpoises,anddolphins—aremammals.Theybreathethroughlungs,notthroughgills,andgivebirthtoliveyoung.Theirstreamlinedbodies,theabsenceofhindlegs,andthepresenceofafluke1andblowhole2cannotdisguisethe
2、iraffinitieswithlanddwellingmammals.However,unlikethecasesofseaottersandpinnipeds(seals,sealions,andwalruses,whoselimbsarefunctionalbothonlandandatsea),itisnoteasytoenvisionwhatthefirstwhaleslookedlike.Extinctbutalreadyfullymarinecetaceansareknownfromthefossilrecord.Howwastheg
3、apbetweenawalkingmammalandaswimmingwhalebridged?Missinguntilrecentlywerefossilsclearlyintermediate,ortransitional,betweenlandmammalsandcetaceans.Veryexcitingdiscoverieshavefinallyallowedscientiststoreconstructthemostlikelyoriginsofcetaceans.In1979,ateamlookingforfossilsinnorth
4、ernPakistanfoundwhatprovedtobetheoldestfossilwhale.ThefossilwasofficiallynamedPakicetusinhonorofthecountrywherethediscoverywasmade.Pakicetuswasfoundembeddedinrocksformedfromriverdepositsthatwere52millionyearsold.Theriverthatformedthesedepositswasactuallynotfarfromanancientocea
5、nknownastheTethysSea.Thefossilconsistsofacompleteskullofanarchaeocyte,anextinctgroupofancestorsofmoderncetaceans.Althoughlimitedtoaskull,thePakicetusfossilprovidespreciousdetailsontheoriginsofcetaceans.Theskulliscetacean-likebutitsjawboneslacktheenlargedspacethatisfilledwithfa
6、toroilandusedforreceivingunderwatersoundinmodernwhales.Pakicetusprobablydetectedsoundthroughtheearopeningasinlandmammals.Theskullalsolacksablowhole,anothercetaceanadaptationfordiving.Otherfeatures,however,showexpertsthatPakicetusisatransitionalformbetweenagroupofextinctflesh-e
7、atingmammals,themesonychids,andcetaceans.IthasbeensuggestedthatPakicetusfedonfishinshallowwaterandwasnotyetadaptedforlifeintheopenocean.Itprobablybredandgavebirthonland.AnothermajordiscoverywasmadeinEgyptin1989.Severalskeletonsofanotherearlywhale,Basilosaurus,werefoundinsedime
8、ntsleftbytheTethysSeaandnowexposedintheSaharadesert.Thiswhale
