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TPO-44FromFishtoTerrestrialVertebratesOneofthemostsignificantevolutionaryeventsthatoccurredonEarthwasthetransitionofwater-dwellingfishtoterrestrialtetrapods(four-limbedorganismswithbackbones).Fishprobablyoriginatedintheoceans,andourfirstrecordsofthemareinmarinerocks.However,bytheDevonianPeriod(408millionto362millionyearsago),theyhadradiatedintoalmostallavailableaquatichabitats,includingfreshwatersettings.Oneofthegroupswhosefossilsareespeciallycommoninrocksdepositedinfreshwateristhelobe-finnedfish.ThefreshwaterDevonianlobe-finnedfishrhipidistiancrossopterygianisofparticularinteresttobiologistsstudyingtetrapodevolution.Thesefishlivedinriverchannelsandlakesonlargedeltas.Thedeltarocksinwhichthesefossilsarefoundarecommonlyredduetooxidizedironminerals,indicatingthatthedeltasformedinaclimatethathadalternatewetanddryperiods.Iftherewereperiodsofdrought,anyadaptationsallowingthefishtosurvivethedryconditionswouldhavebeenadvantageous.Intheserhipidistians，severalsuchadaptationsexisted.Itisknownthattheyhadlungsaswellasgillsforbreathing.Crosssectionscutthroughsomeofthefossilsrevealthatthemudfillingtheinteriorofthecarcassdifferedinconsistencyandtexturedependingonitslocationinsidethefish.Thesedifferencessuggestasadlikecavitybelowthefrontendofthegutthatcanonlybeinterpretedasalung.Gillswereundoubtedlythemainsourceofoxygenforthesefish,butthelungsservedasanauxiliarybreathingdeviceforgulpingairwhenthewaterbecameoxygendepleted,suchasduringextendedperiodsofdrought.So,thesefishhadalreadyevolvedoneoftheprimerequisitesforlivingonland:theabilitytouseairasasourceofoxygen.Asecondadaptationofthesefishwasinthestructureofthelobefins.Thefinswerethick,fleshy,andquitesturdy,withamedianaxisofbonedownthecenter.Theycouldhavebeenusedasfeeblelocomotordevicesonland,perhapsgoodenoughtoallowafishtoflopitswayfromonepoolofwaterthatwasalmostdrytoanadjacentpondthathadenoughwaterandoxygenforsurvival.Thesefinseventuallychangedintoshort,stubbylegs.ThebonesofthefinsofaDevonianrhipidistianexactlymatchinnumberandpositionthelimbbonesoftheearliestknowntetrapods,theamphibians.Itshouldbeemphasizedthattheevolutionoflungsandlimbswasinnosenseananticipationoffuturelifeonland.Theseadaptationsdevelopedbecausetheyhelpedfishtosurviveintheirexistingaquaticenvironment.Whatecologicalpressuresmighthavecausedfishestograduallyabandontheirwateryhabitatandbecomeincreasinglyland-dwellingcreatures?ChangesinclimateduringtheDevonianmayhavehadsomethingtodowiththisiffreshwaterareasbecameprogressivelymorerestricted.Anotherimpetusmayhavebeennewsourcesoffood.Theedgesofpondsandstreamssurelyhadscattereddeadfishandotherwater-dwellingcreatures.■Inaddition,plantshademergedintoterrestrialhabitatsinareasnearstreamsandponds,andcrabsandotherarthropodswerealsomembersofthisearliestterrestrialcommunity.■Thus,bytheDevonianthelandhabitatmarginaltofreshwaterwasprobablyarichsourceofproteinthatcouldbeexploitedbyananimalthatcouldeasilyclimboutofwater.■Evidencefromteethsuggeststhattheseearliesttetrapodsdidnotutilizelandplantsasfood;theywerepresumablycarnivorousandhadnotdevelopedtheabilitytofeedonplants.■Howdidthefirsttetrapodsmakethetransitiontoaterrestrialhabitat?Likeearlylandplantssuchasrhyniophytes,theymadeonlyapartialtransition;theywerestillquitetiedtowater.However,manyproblemsthatfacedearlylandplantswerenotapplicabletothefirsttetrapods.Theancestorsoftheseanimalsalreadyhadacirculationsystem,andtheyweremobile,sothattheycouldmovetowatertodrink.Furthermore,theyalreadyhadlungs,whichrhipidistianspresumablyusedforauxiliarybreathing.Theprincipalchangesfortheearliesttetrapodswereintheskeletalsystem—changesinthebonesofthefins,thevertebralcolumn,pelvicgirdle,andpectoralgirdle.P1：OneofthemostsignificantevolutionaryeventsthatoccurredonEarthwasthetransitionofwater-dwellingfishtoterrestrialtetrapods(four-limbedorganismswithbackbones).Fishprobablyoriginatedintheoceans,andourfirstrecordsofthemareinmarinerocks.However,bytheDevonianPeriod(408millionto362millionyearsago),theyhadradiatedintoalmostallavailableaquatichabitats,includingfreshwatersettings.Oneofthegroupswhosefossilsareespeciallycommoninrocksdepositedinfreshwateristhelobe-finnedfish.1.Paragraph1supportswhichofthefollowingstatementsaboutfishevolution?A.Lobe-finnedfishwereamongtheearliesttypesoffishtoappear.B.Fishbeganlivinginfreshwaterhabitatsonlyafteroriginatingelsewhere.C.Lobe-finnedfishradiatedintoalmostallavailableaquatichabitats.D.DuringtheDevonian,lobe-finnedfishweremorecommoninmarinethaninfreshwaterhabitats.P2：ThefreshwaterDevonianlobe-finnedfishrhipidistiancrossopterygianisofparticularinteresttobiologistsstudyingtetrapodevolution.Thesefishlivedinriverchannelsandlakesonlargedeltas.Thedeltarocksinwhichthesefossilsarefoundarecommonlyredduetooxidizedironminerals,indicatingthatthedeltasformedinaclimatethathadalternatewetanddryperiods.Iftherewereperiodsofdrought,anyadaptationsallowingthefishtosurvivethedryconditionswouldhavebeenadvantageous.Intheserhipidistians，severalsuchadaptationsexisted.Itisknownthattheyhadlungsaswellasgillsforbreathing.Crosssectionscutthro