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▉托福TPO2阅读Passage2原文文本：

TheOriginsofCetaceans

Itshouldbeobviousthatcetaceans-whales,porpoises,anddolphins-aremammals.
Theybreathethroughlungs,notthroughgills,andgivebirthtoliveyoung.Their
streamlinedbodies,theabsenceofhindlegs,andthepresenceofaflukeandblowhole
cannotdisguisetheiraffinitieswithlanddwellingmammals.However,unlikethe
casesofseaottersandpinnipeds(seals,sealions,andwalruses,whoselimbsare
functionalbothonlandandatsea),itisnoteasytoenvisionwhatthefirstwhales
lookedlike.Extinctbutalreadyfullymarinecetaceansareknownfromthefossil
record.Howwasthegapbetweenawalkingmammalandaswimmingwhalebridged?Missing
untilrecentlywerefossilsclearlyintermediate,ortransitional,betweenland
mammalsandcetaceans.

Veryexcitingdiscoverieshavefinallyallowedscientiststoreconstructthemost
likelyoriginsofcetaceans.In1979,ateamlookingforfossilsinnorthernPakistan
foundwhatprovedtobetheoldestfossilwhale.Thefossilwasofficiallynamed
Pakicetusinhonorofthecountrywherethediscoverywasmade.Pakicetuswasfound
embeddedinrocksformedfromriverdepositsthatwere52millionyearsold.Theriver
thatformedthesedepositswasactuallynotfarfromanancientoceanknownasthe
TethysSea.

Thefossilconsistsofacompleteskullofanarchaeocyte,anextinctgroupof
ancestorsofmoderncetaceans.Althoughlimitedtoaskull,thePakicetusfossil
providespreciousdetailsontheoriginsofcetaceans.Theskulliscetacean-like
butitsjawboneslacktheenlargedspacethatisfilledwithfatoroilandusedfor
receivingunderwatersoundinmodernwhales.Pakicetusprobablydetectedsound
throughtheearopeningasinlandmammals.Theskullalsolacksablowhole,another
cetaceanadaptationfordiving.Otherfeatures,however,showexpertsthatPakicetus
isatransitionalformbetweenagroupofextinctflesh-eatingmammals,the
mesonychids,andcetaceans.IthasbeensuggestedthatPakicetusfedonfishin
shallowwaterandwasnotyetadaptedforlifeintheopenocean.Itprobablybred
andgavebirthonland.

AnothermajordiscoverywasmadeinEgyptin1989.Severalskeletonsofanother
earlywhal