资源描述:
《托福阅读题材分类――词汇题》由会员上传分享,免费在线阅读,更多相关内容在行业资料-天天文库。
1、智课网托福备考资料托福阅读题材分类――词汇题Itshouldbeobviousthatcetaceans—whales,porpoises,anddolphins—aremammals.Theybreathethroughlungs,notthroughgills,andgivebirthtoliveyoung.Theirstreamlinedbodies,theabsenceofhindlegs,andthepresenceofafluke1andblowhole2cannotdisguisetheiraffinitieswith
2、landdwellingmammals.However,unlikethecasesofseaottersandpinnipeds(seals,sealions,andwalruses,whoselimbsarefunctionalbothonlandandatsea),itisnoteasytoenvisionwhatthefirstwhaleslookedlike.Extinctbutalreadyfullymarinecetaceansareknownfromthefossilrecord.Howwasthegapbetwee
3、nawalkingmammalandaswimmingwhalebridged?Missinguntilrecentlywerefossilsclearlyintermediate,ortransitional,betweenlandmammalsandcetaceans.Veryexcitingdiscoverieshavefinallyallowedscientiststoreconstructthemostlikelyoriginsofcetaceans.In1979,ateamlookingforfossilsinnorth
4、ernPakistanfoundwhatprovedtobetheoldestfossilwhale.ThefossilwasofficiallynamedPakicetusinhonorofthecountrywherethediscoverywasmade.Pakicetuswasfoundembeddedinrocksformedfromriverdepositsthatwere52millionyearsold.Theriverthatformedthesedepositswasactuallynotfarfromananc
5、ientoceanknownastheTethysSea.Thefossilconsistsofacompleteskullofanarchaeocyte,anextinctgroupofancestorsofmoderncetaceans.Althoughlimitedtoaskull,thePakicetusfossilprovidespreciousdetailsontheoriginsofcetaceans.Theskulliscetacean-likebutitsjawboneslacktheenlargedspaceth
6、atisfilledwithfatoroilandusedforreceivingunderwatersoundinmodernwhales.Pakicetusprobablydetectedsoundthroughtheearopeningasinlandmammals.Theskullalsolacksablowhole,anothercetaceanadaptationfordiving.Otherfeatures,however,showexpertsthatPakicetusisatransitionalformbetwe
7、enagroupofextinctflesh-eatingmammals,themesonychids,andcetaceans.IthasbeensuggestedthatPakicetusfedonfishinshallowwaterandwasnotyetadaptedforlifeintheopenocean.Itprobablybredandgavebirthonland.AnothermajordiscoverywasmadeinEgyptin1989.Severalskeletonsofanotherearlywhal
8、e,Basilosaurus,werefoundinsedimentsleftbytheTethysSeaandnowexposedintheSaharadesert.Thiswhalelivedaround40millionyear