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1、Passage4(4/63)Thefossilremainsofthefirstflyingvertebrates,thepterosaurs,haveintriguedpaleontologistsformorethantwocenturies.Howsuchlargecreatures,whichweighedinsomecasesasmuchasapilotedhang-gliderandhadwingspansfrom8to12meters,solvedtheproblemsofpoweredflight,and
2、exactlywhatthesecreatureswere—reptilesorbirds—areamongthequestionsscientistshavepuzzledover.Perhapstheleastcontroversialassertionaboutthepterosaursisthattheywerereptiles.Theirskulls,pelvises,andhindfeetarereptilian.Theanatomyoftheirwingssuggeststhattheydidnotevol
3、veintotheclassofbirds.Inpterosaursagreatlyelongatedfourthfingerofeachforelimbsupportedawing-likemembrane.Theotherfingerswereshortandreptilian,withsharpclaws.Inbirdsthesecondfingeristheprincipalstrutofthewing,whichconsistsprimarilyoffeathers.Ifthepterosaurswalkedo
4、nallfours,thethreeshortfingersmayhavebeenemployedforgrasping.Whenapterosaurwalkedorremainedstationary,thefourthfinger,andwithitthewing,couldonlyturnupwardinanextendedinvertedV-shapealongeachsideoftheanimal’sbody.Thepterosaursresembledbothbirdsandbatsintheiroveral
5、lstructureandproportions.Thisisnotsurprisingbecausethedesignofanyflyingvertebrateissubjecttoaerodynamicconstraints.Boththepterosaursandthebirdshavehollowbones,afeaturethatrepresentsasavingsinweight.Inthebirds,however,thesebonesarereinforcedmoremassivelybyinternal
6、struts.Althoughscalestypicallycoverreptiles,thepterosaursprobablyhadhairycoats.T.H.Huxleyreasonedthatflyingvertebratesmusthavebeenwarm-bloodedbecauseflyingimpliesahighrateofmetabolism,whichinturnimpliesahighinternaltemperature.Huxleyspeculatedthatacoatofhairwould
7、insulateagainstlossofbodyheatandmightstreamlinethebodytoreducedraginflight.Therecentdiscoveryofapterosaurspecimencoveredinlong,dense,andrelativelythickhairlikefossilmaterialwasthefirstclearevidencethathisreasoningwascorrect.Effortstoexplainhowthepterosaursbecamea
8、irbornehaveledtosuggestionsthattheylaunchedthemselvesbyjumpingfromcliffs,bydroppingfromtrees,orevenbyrisingintolightwindsfromthecrestsofwaves.Eachhypothesishas
