托福TPO真题阅读The origins of cetaceans原文分析.docx

托福TPO真题阅读The origins of cetaceans原文分析.docx

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托福TPO真题阅读Theoriginsofcetaceans原文分析

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Theoriginsofcetaceans--原文分析

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THEORIGINSOFCETACEANS

→Itshouldbeobviousthatcetaceans-whales,porpoises,anddolphins-aremammals.Theybreathethroughlungs,notthroughgills,andgivebirthtoliveyoung.Theirstreamlinedbodies,theabsenceofhindlegs,andthepresenceofafluke1andblowhole2cannotdisguisetheiraffinitieswithlanddwellingmammals.However,unlikethecasesofseaottersandpinnipeds（seals,sealions,andwalruses,whoselimbsarefunctionalbothonlandandatsea）,itisnoteasytoenvisionwhatthefirstwhaleslookedlike.Extinctbutalreadyfullymarinecetaceansareknownfromthefossilrecord.Howwasthegapbetweenawalkingmammalandaswimmingwhalebridged?

Missinguntilrecentlywerefossilsclearlyintermediate,ortransitional,betweenlandmammalsandcetaceans.

→Veryexcitingdiscoverieshavefinallyallowedscientiststoreconstructthemostlikelyoriginsofcetaceans.In1979,ateamlookingforfossilsinnorthernPakistanfoundwhatprovedtobetheoldestfossilwhale.ThefossilwasofficiallynamedPakicetusinhonorofthecountrywherethediscoverywasmade.Pakicetuswasfoundembeddedinrocksformedfromriverdepositsthatwere52millionyearsold.TheriverthatformedthesedepositswasactuallynotfarfromanancientoceanknownastheTethysSea.

→Thefossilconsistsofacompleteskullofanarchaeocyte,anextinctgroupofancestorsofmoderncetaceans.Althoughlimitedtoaskull,thePakicetusfossilprovidespreciousdetailsontheoriginsofcetaceans.Theskulliscetacean-likebutitsjawboneslacktheenlargedspacethatisfilledwithfatoroilandusedforreceivingunderwatersoundinmodernwhales.Pakicetusprobablydetectedsoundthroughtheearopeningasinlandmammals.Theskullalsolacksablowhole,anothercetaceanadaptationfordiving.Otherfeatures,however,showexpertsthatPakicetusisatransitionalformbetweenagroupofextinctflesh-eatingmammals,themesonychids,andcetaceans.IthasbeensuggestedthatPakicetusfedonfishinshallowwaterandwasnotyetadaptedforlifeintheopenocean.Itprobablybredandgavebirthonland.

→AnothermajordiscoverywasmadeinEgyptin1989.Severalskeletonsofanotherearlywhale,Basilosaurus,werefoundinsedimentsleftbytheTethysSeaandnowexposedintheSaharadesert.Thiswhalelivedaround40millionyearsago,12millionyearsafterPakicetus.Manyincompleteskeletonswerefoundbuttheyincluded,forthefirsttimeinanarchaeocyte,acompletehindlegthatfeaturesafootwiththreetinytoes.Suchlegswouldhavebeenfartoosmalltohavesupportedthe50-foot-longBasilosaurusonland.Basilosauruswasundoubtedlyafullymarinewhalewithpossiblynonfunctional,orvestigial,hindlegs.

→Anevenmoreexcitingfindwasreportedin1994,alsofromPakistan.ThenowextinctwhaleAmbulocetusnatans（"thewalkingwhalethatswam"）livedintheTethysSea49millionyearsago.Itlivedaround3millionyearsafterPakicetusbut9millionbeforeBasilosaurus.Thefossilluckilyincludesagoodportionofthehindlegs.Thelegswerestrongandendedinlongfeetverymuchlikethoseofamodernpinniped.Thelegswerecertainlyfunctionalbothonlandandatsea.Thewhaleretainedatailandlackedafluke,themajormeansoflocomotioninmoderncetaceans.Thestructureofthebackboneshows,however,thatAmbulocetusswamlikemodernwhalesbymovingtherearportionofitsbodyupanddown,eventhoughaflukewasmissing.Thelargehindlegswereusedforpropulsioninwater.Onland,whereitprobablybredandgavebirth,Ambulocetusmayhavemovedaroundverymuchlikeamodernsealion.Itwasundoubtedlyawhalethatlinkedlifeonlandwithlifeatsea

Fluke:

thetwopartsthatconstitutethelargetriangulartailofawhale

1.

"Blowhole:

aholeinthetopoftheheadusedforbreathing

→Paragraph1:

Itshouldbeobviousthatcetaceans-whales,porpoises,anddolphins-aremammals.Theybreathethroughlungs,notthroughgills,andgivebirthtoliveyoung.Theirstreamlinedbodies,theabsenceofhindlegs,andthepresenceofafluke3andblowhole4cannotdisguisetheiraffinitieswithland-dwellingmammals.However,unlikethecasesofseaottersandpinnipeds（seals,sealions,andwalruses,whoselimbsarefunctionalbothonlandandatsea）,itisnoteasytoenvisionwhatthefirstwhaleslookedlike.Extinctbut,alreadyfullymarinecetaceansareknownfromthefossilrecord.Howwasthegapbetweenawalkingmammalandaswimmingwhalebridged?

Missinguntilrecentlywerefossilsclearlyintermediate,ortransitional,betweenlandmammalsa