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Short Film Great Transitions: The Origins of Tetrapods Educator Materials IN-DEPTHFILMGUIDE DESCRIPTION IntheHHMIfilmGreatTransitions:TheOriginofTetrapods,wejoinUniversityofChicagoevolutionarybiologist NeilShubinasherelivestheexcitingdiscoveryofTiktaalikroseae,aspeciesthatlivedaround375millionyears ago(mya)andhadcharacteristicsofbothfishandfour-leggedanimals(ortetrapods).Thefilmtakesusona compellingjourneythroughtheprocessofscience—includingtheaskingofimportantquestions,synthesizing knownfactsinnovelways,generatingandtestinghypotheses,andperseveringdespiterepeatedfailures. KEYCONCEPTS A. Speciesdescendfromotherspecies.Evendistantlyrelatedspecies,likehumansandsponges,cantracetheir sharedancestrybacktoacommonancestor. B. ThefossilrecordprovidesahistoryoflifeonEarth.Itincludesfossilswithfeaturesthatareintermediate,or transitional,betweenthoseofmajorgroupsofanimals. C. Whenaseriesoftransitionalfossilsareviewedtogether,theyrevealthegradualsequenceofchange connectingonemajorgrouptoanother. D. Evidencethatlandvertebratesdescendedfromfishincludestransitionalfossils,anatomicalsimilarities amongembryosandadultanimals,andgeneticevidenceofcommonancestry. E. Thelimbsofmammals,amphibians,reptiles,andbirdslookdifferent,buttheyareallbuiltonasharedbasic arrangementof“onebone,twobones,manybones,anddigits.” F. Tofindfossils,scientistsdevelophypothesesaboutthetypesofhabitatsinwhichearlieranimalslivedand whentheylivedthere.Theythenpredictwhichtypesandagesofrockswouldhousefossilsofthoseanimals. CURRICULUMANDTEXTBOOKCONNECTIONS Curriculum NGSS(April2013) APBiology(2012–13) IBBiology(2016) Textbook MillerandLevine,Biology(2010ed.) Reeceetal.,CampbellBiology(9thed.) Standards MS-LS4-1,MS-LS4-2 HS-LS4-1,LS4.A,LS4.B,LS4.C,LS4-2,LS4-4,LS4-5,HS-ESS1-5,HS-ESS2.B 1.A.1,1.A.2,1.A.4,1.B.1,1.C.1,1.C.3,1.D.2,4.B.3,4.B.4 5.1 ChapterSections 16.2,16.3,16.4,17.3,19.1,19.2 22.1,22.2,22.3,25.2,25.5,25.6 PRIORKNOWLEDGE Itwouldbehelpfulforstudentstobefamiliarwiththeconceptsofnaturalselectionandadaptation,andtohave learnedabouttectonicplatemovements. www.BioInteractive.org PublishedNovember2014 RevisedMarch2017 Page1of10 Short Film Great Transitions: The Origins of Tetrapods Educator Materials PAUSEPOINTS Thefilmmaybeviewedinitsentiretyorpausedatspecificpointstoreviewcontentwithstudents.Thetable belowlistssuggestedpausepoints,indicatingthebeginningandendtimesinminutesinthefilm. 1 2 3 Begin End 0:00 5:00 ContentDescription • Darwinpredictedthattransitional organisms’fossilscouldbeintermediate formsbetweendifferentgroups.Many fossilsoftransitionalorganismshave beenfoundandhaveenabledscientists toreconstructtheoriginsofmany groups. • Althoughtheydon’talwayslookor functionsimilarly,thelimbsofmammals, amphibians,reptiles,andbirdshavea commonarchitecture,suggestinga connectionbetweenverydifferent groupsofanimals. • Thehistoryofvertebratesiscapturedin rockthatcanbedated;birdsare youngest,thenmammals,reptiles,and amphibians.At370millionyearsago, therearenofossilsoftetrapods. • Fishandtetrapodsarebothvertebrates, andinearlydevelopmenttheylookvery similar.DNAevidenceindicatesthatfish aretetrapods’closestrelatives. 5:01 13:10 • Findingfossilscanbechallenging.Tofind fossils,scientistsdevelophypotheses aboutthetypesofhabitatsinwhich earlieranimalslivedandwhentheylived there.Theythenpredictwhichtypesand agesofrockswouldhousefossilsof thoseanimals. • Fossilizationhappensonlyundercertain conditions. • Afteryearsofsearching,ateamof paleontologistsdiscoveredTiktaalikata remotefieldsiteinAlaskawithDevonian agerocks. 13:11 17:11 • Tiktaalikhasamixoffishlikeand tetrapod-likefeatures.Ithasscalesand finswithfinrays,likeafish.Likea tetrapod,ithasaflatheadwitheyeson www.BioInteractive.org ReviewQuestions • Whatare transitional fossils?Whyare theyimportant? • Whatevidence suggeststhat four-legged animalsevolved fromfish? Standards NGSS(April2013) MS-LS4.A,MSLS4.C,MS-ESS1.C, HS-LS4.C,HSLS4.D APBiology (2012–13) 1.A.1,1.A.4, 1.B.1,1.C.1, 1.C.3,4.B.4 IBBiology(2016) 5.1 • Whydidthe paleontologists decidetohunt forthe transitionalfossil inAlaska? NGSS(April2013) MS-ESS1.C APBiology (2012–13) 1.A.4,1.B.1 IBBiology(2016) 5.1 WhyisTiktaalik considereda transitional fossil? NGSS(April2013) MS-LS4.A,MSLS4.C,MS-ESS1.C, HS-LS4.C,HS- • PublishedNovember2014 RevisedMarch2017 Page2of10 Short Film Great Transitions: The Origins of Tetrapods • • Educator Materials top,interlockingribssuggestingithad • lungs,aneck,ahipbone,andaforelimb withthebeginningofabonepatternlike thatoftetrapods. Thegreattransitionfromfishtotetrapod happenedgraduallyovermanymillions ofyears. Otherfossilshaveprovidedevidenceof othertransitions. Didthetransition fromfishto tetrapodhappen quicklyorslowly? LS4.D APBiology (2012–13) 1.A.1,1.A.2, 1.A.4,1.B.1, 1.C.1,1.C.3 IBBiology(2016) 5.1 BACKGROUND Tiktaalik(pronouncedtic-TAH-lick)means“shallowwaterfish”inthelanguageoftheNunavutpeoplewholive intheCanadianArctic,whereTiktaalikwasfoundin2004.TheteamthatmadethediscoveryincludedNeil Shubin;hisformergraduatestudent,TedDaeschler,whoisnowatTheAcademyofNaturalSciencesofDrexel University;andShubin’sformergraduateadvisor,HarvardUniversityevolutionarybiologistFarishJenkins.The discoveryofTiktaalikisdescribedindetailintheaccompanyingarticle“It’saFishapod!”bySeanB.Carroll http://www.hhmi.org/biointeractive/article-fishapod.Tiktaalikisatetrapod—agroupthatincludesfour-legged animalslikedogsandhorses,aswellaswhalesandsnakes,whichevolvedfromfour-leggedancestorsbutno longerhavefourlegs. HowAreFossilsFound? Oneofthemajorthemesofthefilmishowfossilsarefound.Thefirststepinlocatingfossilsoftherightageand typeistomakepredictionsaboutwhereandwhentheorganismwouldhavelived.Beforeabout385million yearsago(mya),notetrapodsareknownfromthefossilrecord.Thefirstfossilsoftetrapods,Acanthostegaand Ichthyostega,weredatedto365mya.So,Shubinandhiscolleagueshypothesizedthatthefishintheprocessof becomingmoretetrapod-likelivedbetween385and365mya.This20-million-year-spaniswithinageological epochcalledtheUpperDevonian. Figure1.Rocksagedbetween385and365myawerepredictedto containfossilswithtransitionalfeaturesbetweenthoseoffishand tetrapods. Inthefilm,weseeamapofDevonianrocks.ThemapcomesfromatextbookentitledEvolutionoftheEarthby DottandBatten,whichcontainsmanygeologicalmaps.Likeallmaps,geologicalmapsaredesignedtoshow wherecertainthingsarelocated.Whereasthemapsweknowbestshowthedistributionofroads,rivers,or countyboundaries,ageologicalmapshowsthedistributionofgeologicalfeatures,includingdifferentkindsof rocks(e.g.,sedimentary,igneous,andmetamorphic)andthedifferentagesofrocks.Thesemapsaretypically generatedtoguideoilandmineralexpeditions. www.BioInteractive.org PublishedNovember2014 RevisedMarch2017 Page3of10 Short Film Great Transitions: The Origins of Tetrapods Educator Materials ThemapinthefilmshowsthreeDevoniandepositsinNorthAmerica.Twoofthosedepositshadalreadybeen searchedforfossils.ThethirdDevoniandeposithadbeenexploredbyCanadiangeologicalsurveyorsandafew oilcompanygeologistsbutnofossilhunters.Thisthirdformationcontainedsedimentaryrocklayers—thetype ofrockswherefossilsaremostcommonlyfound.Aftersomeadditionalresearch,ShubinandDaeschler discoveredthattherockshadformedinwhatwereshallow,meanderingstreamsinDevoniantimes. Evenafteridentifyingrocksoftherightageandtype,scientistscannotbesurethattheywillfindfossilsinthose rocks.AlmostallorganismsthathavelivedonEarthleftnotraceoftheirexistence.Mostareeatenordecaybefore theyarefossilized,andsofttissuesdonotfossilizewell.Andevenifanorganismisburiedinsedimentsand becomesfossilized,therocklayerwiththatfossilhastobecomeexposedonEarth’ssurfaceforsomeonetofindit. Sohowcanpaleontologistsexpecttofindfossilsatall,letalonefossilsofspeciesthatexistedataparticular pointintime?Onecriticalfactmakesthispossible.Asinglespeciescanexistformillionsofyearsandconsistof hundredsofmillionsorevenbillionsofindividuals.Theprobabilityoffindingthefossilremainsofanyone individualistiny,butnotimpossible. FindingTiktaalik In2004,Shubinandcolleaguesfoundthefossilofanewspeciesthatshedlightonakeytransitioninthe evolutionoftetrapods:thefin-to-limbtransition.Thecrewwasexcavatingonedayinanareawithlotsof Devonianfishfossils(a“wholeaquarium,”asShubinsaidinthefilm).Stickingoutofarockyedgeinthequarry wasthetipofthesnoutofaflat-headedfish.Whywastheflatheadsuchanimportantclue? Manyfishhaveflatheads.Butwhatissignificantinthiscaseisthattheearliesttetrapodsthathadbeen discovered(i.e.,AcanthostegaandIchthyostega)hadflatheads.Sofindingaflat-headedfishwasagood indicatorthatthisfossilwaspartofthegradualevolutionarysequencefromfishtotetrapods. Oncethefossilwascleaned,itshowedthatithadtwonostrils andtwoeyesmountedontopofitsflathead.Mostfishhave nostrils,buttetrapod-likefishlikeTiktaalikhaveaspecialized kindofnostrilthatconnectstotheinsideofthemouth.Fishthat arenotcloselyrelatedtotetrapodshavenostrilsthatonly connectwiththeexterior. AnotherimportantfeatureofthisfossilisthatTiktaalik’shead wasnotconnecteddirectlytoitsshouldersliketheheadofa fishwouldhavebeen(Figure2).Instead,itsheadsatinfrontof theshoulders,attheendofaflexibleneck—meaningthehead wasfreetomoveupanddownandsideways. Figure2.Tiktaalikhasamixoffishandtetrapodfeatures.One oftheimportanttetrapodcharacteristicsisthepresenceofa neck,whichallowsTiktaaliktomorefreelymoveitshead. (Figuretakenfromtheinteractive“MeetTiktaalikroseae” availableontheUniversityofChicagowebsiteat http://tiktaalik.uchicago.edu/meetTik2.html). www.BioInteractive.org PublishedNovember2014 RevisedMarch2017 Page4of10 Short Film Great Transitions: The Origins of Tetrapods Educator Materials Likeafish,thefossilofTiktaalikshowsevidenceofscales,gills,andfins.Butinsidethefins,Tiktaalikcontainslimb bonesthatarecharacteristicofanearlytetrapod.Onebone(thehumerus)extendsfromtheshoulderand connectstotwoadditionalbones(theradiusandulna),whicharesharedbyalllobe-finnedfish(i.e.,coelacanths, lungfish,tetrapods,andalltheirextinctrelatives).WhatisspecialaboutthefinofTiktaalikisthatthosethree bonesthenconnecttomanysmallbonesthatformwhatDr.Shubindescribesas“aversionofawrist.” ThefossilofTiktaalikalsorevealedinterconnecting(orimbricate)ribsthatsurroundedthelungs.Lungfishhave lungs,buttheydonothaveimbricateribs.Thelungsoflungfishandtetrapodsarehomologous,whichmeans theyevolvedfromacommonancestorthathadlungs.TheimbricateribsofTiktaalikandlatertetrapodswould haveprovidedextrabodyrigidityinconditionsinwhichthesurroundingwatercouldnotsupporttheweightof theanimal(i.e.,inshallowwaterorsubaerialconditions).Onehypothesisisthattheribsprovidedanadvantage inthoseenvironmentsbykeepingtheweightoftheanimalfromputtingpressureonthelungsandpreventing themfromfillingwithair. SowhyisTiktaalikreferredtoasatransitionalcreature?Itsmixofcharacteristicsoffishandoftetrapods suggestthatitrepresentsoneofthemomentsinthetransitionbetweenfishandtetrapods. DISCUSSIONPOINTS • • • • Studentsmayincorrectlyassumethatdiscoveringfossilsismostlyaboutluck.Askyourstudentsifanyof themhavefoundfossils.Askthosestudentswhofoundfossilsiftheirfossilfindswereluckyoraproductof deliberatesearching.Explainthatpaleontologistsdonotaccidentallystumbleuponthefossilstheydiscover, butthatpaleontologyisaprocessofhypothesistesting.Throughcarefulplanning,readingscientificarticles, andstudyinggeologyandgeologicaltime,paleontologistsformhypothesesaboutthetimeperiodsand habitatsoccupiedbycertainorganisms.Theythendeveloppredictionsaboutwhatkindsofrockscertain fossilsofthoseorganismsshouldbeburiedwithinandwhererocksofthosetypesandagesarefoundon Earth. Studentsmaywonderwhywedonotfindmorefossils—fossilsofeverytypeoforganismthateverlived.Ask studentstoconsideramousethatisrunningneartheschoolatthisverymoment.Askthemtoconsiderthe probabilitythatthatsinglemousewilldietoday,becomefossilized,andberediscoveredamillionyearsfrom now.Theprobabilityisnearlyzero.Tobecomefossilized,themousemustnotbeeatenrightafteritdiesand notdecomposewithinafewdays.Instead,themouse’sbodymustbequicklyandcompletelyburiedin eithervolcanicashorsomekindofsediment.Itmustremainburieduntilthesedimentandthemouse remainsturnintorock—aprocessthatmaytakethousandsorevenmillionsofyears.Enoughtimehas passedinEarth’shistoryforsomeindividuals,fromthehundredsofthousandswithinanyonespecies,to havebeenintherightconditionstobecomefossilized. Aclassicquestionstudentsmayaskaboutevolutionis,“Ifweevolvedfromfish,whyaretherestillfish?” Explaintostudentsthathumansandmodern-dayfishshareacommonancestorthatnolongerexists.The aquatichabitatsinwhichfishthrivetodayhavebeenaroundforoverhalfabillionyears,andfisharewell adaptedtoliveinthoseenvironments.Whensomefishpopulationsevolvedstronger,limblikestructuresin fins,theywereabletoexploitnewtypesofhabitatsandreducecompetitionwithotherfishforresources. Butfishcontinuetoexistbecausetherearestillenvironmentsinwhichfishcanthrive. Askyourstudentswhatadvantageaneckmightprovide.Thelackofbonesconnectingtheheadtothebody providesflexibilitytomovetheheadwithoutmovingtherestofthebody.Aflexibleneckcouldhavebeen beneficialinmanypossiblescenarios.However,thisflexibilitymayhavealsocomeatacost—forexample, thelossoftheprotectivegillcover.Wedon’tknowexactlyhowTiktaalikuseditsneck. www.BioInteractive.org PublishedNovember2014 RevisedMarch2017 Page5of10 Short Film Great Transitions: The Origins of Tetrapods • • • • Educator Materials Studentsmayaskforexamplesofothertransitionsfromearliertolaterforms.Theevolutionofwhalesfrom terrestrialmammalsandbirdsfromdinosaursarebothexcellentexampleswithrichfossilevidenceand manytransitionalforms.Whalesevolvedfromterrestrialmammalsthatwalkedandranonfourlegs.Afun factthatsupportsthisancestryisthataswimmingwhaleflexesitsbackboneupanddownlikearunning cheetah,notsidetosidelikeaswimmingfish.Aswhaleancestorstransitionedfromaterrestrialtoamarine life,manycharacteristicschanged.Forexample,legscanbealiabilityinanaquaticenvironmentratherthan abenefit.Asaresult,whaleancestorsshowaprogressivedecreaseinthesizeoftheirhindlimbs.Allthat remainsoftheselimbsinsomeoftoday’swhalespeciesisapairoftinybonesthatareburiedinsidethe bodywallandarenotevenattachedtothepelvis. Discusswithstudentswhytheterm“missinglink”isinaccurate.Thetermhasbeenpopularinthemediaand hasbeenusedtoincorrectlyimplythatanunbrokenchainoforganismsisrequiredtodemonstratethatall speciesarerelated.Theterms“transitionalform”and“transitionalfossil”moreaccuratelydescribespecies thatillustratethesequenceofchangesthatoccurredduringtheevolutionofanewgroupoforganisms. IsTiktaalikourancestor?Tiktaalikisrepresentativeofananimalthatwasanancestortomoderntetrapods, includinghumans.Butwecannotsaywhetheritwasadirectancestor.Acommonmisconceptionisthatall fossils,includingtransitionalfossilslikeTiktaalik,aredirectancestorsofmodernspecies.Explainthatfossils, likeTiktaalik,maybedirectancestors,buttheyaremuchmorelikelyrepresentativeofwhattheactual ancestorslookedlikeandnottheancestorsthemselves.Infact,thedescendantsofTiktaalikmayhavegone extinctlongago,butanotherspecies,relatedtoTiktaalik,andyettobediscovered,maybethecommon ancestortoallmoderntetrapods.Wemayneverknow,butwecanbecertainthattheancestorlookedalot likeTiktaalik. StudentsmaybeconfusedaboutwhyfossilanimalslikeTiktaalikthatlivedinwarmhabitatsendupin barrenlocationsliketheCanadianArcticIslands.RemindstudentsthatEarth’sbiologicalandgeological historiesareconnected.Earth’scrustisdynamic.Platetectonicsmovelandmasses,andfossilsthatwere onceattheequatorcanbemovedtodistantlocations. RELATEDBIOINTERACTIVERESOURCES It’saFishapod(http://www.hhmi.org/biointeractive/article-fishapod) ThisarticlebySeanB.CarrolltellsthestoryofthesearchforanddiscoveryofTiktaalik. GreatTransitionsInteractive (http://www.hhmi.org/biointeractive/great-transitions-interactive) InthisClickandLearn,studentsexploreseveraltransitionalfossilsinthetransitionfromfishtotetrapods, includingTiktaalik. YoungStudentsRecognizeaTransitionalTetrapod(http://www.hhmi.org/biointeractive/young-studentsrecognize-transitional-fossil) Thisshortvideodemonstratesthepowerofobservationandtheimportanceoffossilevidence.WhenNeil ShubinshowsaTiktaalikfossiltosmallchildren,theyimmediatelynoticethepresenceofbothfishandtetrapod characteristics. ExploreYourInnerAnimals (http://www.hhmi.org/biointeractive/explore-your-inner-animals) Thisinteractiveexploresdifferentanatomicalfeaturesofthehumanbodyandwhattheyrevealaboutthe evolutionaryhistorywesharewithotherorganisms,includingearlier,long-extinctspecies. www.BioInteractive.org PublishedNovember2014 RevisedMarch2017 Page6of10 Short Film Great Transitions: The Origins of Tetrapods Educator Materials USINGTHEQUIZ Thequizisdesignedasasummativeassessmentthatprobesstudentunderstandingofthekeyconcepts addressedinthefilm.However,someteachersusethequizbeforeandduringthefilmtoassessstudents’prior knowledgeandtoguidestudentsastheywatchthefilm.Teachersareencouragedtochoosetheusethatbest fitstheirlearningobjectivesandtheirstudents’needs.Moreover,becausethevocabularyandconceptsare complex,teachersareencouragedtomodifythequiz(e.g.,onlyasksomeofthequestions,explaincomplicated vocabularyforELLstudents)asneeded.Thelasttwoquestionsareintendedforstudentswithpriorknowledge aboutmutations,genesexpression,anddevelopment. QUIZQUESTIONSANDANSWERS Thestudentversionofthisquizisavailableasaseparatefile.Keyconceptscoveredbyeachquestionarenotedhere. 1. (KeyConceptB)WhenCharlesDarwinconsideredsomeoftheuniquestructuresfoundinmodernanimals, likethefeatheredwingsofbirds,hefamouslyproposedthatmodernanimalsmusthaveevolvedfrom earlierformsthatlackedthosestructures.Healsopredicted.(Fillintheblankwith oneofthestatementsbelow.) a. thatitwouldbeimpossibletofindfossilevidenceforthisideabecausethefossilrecordissoincomplete. b. thatgeneticevidencewouldshowthatallorganismsshareacommonancestor. c. thatfossilswouldbefoundwithstructuresthatareintermediatebetweenearlyandmodernforms. d. thatfossilevidencewouldinsteadshowthatallmodernanimalshavealwaysexistedintheirpresentform. 2. (KeyConceptsBandC)WhichofthefollowingfeaturesdescribeTiktaalik? i. Neck ii. Lungs iii. Roundhead iv. Fins a. iiandivonly b. i,ii,andiv c. i,iii,andiv d. i,ii,iii,andv 3. (KeyConceptF).Examinethetablebelowandselecttherowthatbestdescribesthesetting,resources,and scientificprocessesusedduringtheTiktaalikexpeditions. a. b. c. d. Location CanadianArctic Iceland Alaska ArcticCircle Maps/Photos Geologicalmaps Roadmaps Aerialphotos Animaltrackmaps ScientificProcess Hypothesestesting Predicting Questioning Developingexplanations www.BioInteractive.org PublishedNovember2014 RevisedMarch2017 Page7of10 Short Film Great Transitions: The Origins of Tetrapods Educator Materials 4. (KeyConceptE)Whichstatementbelowisevidencethatalltetrapodssharedasinglecommonancestor? a. Alltetrapodslivepartlyinwaterandpartlyonland. b. Alltetrapodsarewarm-blooded. c. Alltetrapodlimbsconsistofrearrangedfishfinrays. d. Alltetrapodlimbshaveacommonpatternofonebone,twobones,manybones,thendigits. 5. (KeyConceptC)Thetransitionfromfishtotetrapodsisbestdescribedas: a. Likemostofthegreattransitionsinevolutionaryhistory,ithappenedinveryfewbigstepssothatthereare veryfewintermediateforms. b. Likemostofthegreattransitionsinevolutionaryhistory,ithappenedinmanysmallstepsleadingtomany intermediateforms. c. Likenoothertransitioninevolutionaryhistory,ithappenedinmanysteps,sotherearemanyintermediate forms. d. Likemostofthegreattransitionsinevolutionaryhistory,ithappenedinasinglestepwithnointermediate forms. 6. (KeyConceptsAandD)Whichevidencesupportsthefactthattetrapodsandfisharecloselyrelated? i. Theembryosofmodernfishandtetrapodlooksimilar. ii. Bothmodernfishandtetrapodscanswim. iii. Bothmodernfishandtetrapodsarevertebrates. iv. TheDNAofmodernfishandtetrapodssuggeststhattheyhaveacommonancestor. v. Modernfishhavelimbbonesthatsupporttheirbodies. a. i,ii,andvonly b. i,iii,andivonly c. iiiandvonly d. itovareallsupportingevidence 7. (KeyConceptsB)TrueorFalse.“Transitionalorganismsarenotactualspecies.”Justifyyouranswerinoneor twosentences. False.Transitionalformsarespeciesintheirownright.Theyarecalledtransitionalformsbecausethey possesstraits,likefins,thatarefoundinearlierfossilanimals,butalsopossesstraits,likelimbbones,thatare foundinlaterfossilanimals. www.BioInteractive.org PublishedNovember2014 RevisedMarch2017 Page8of10 Short Film Great Transitions: The Origins of Tetrapods Educator Materials 8. (KeyConceptsF)Studythegraphicbelowofrocklayerswithfossilsinthem.ExplainhowNeilShubinandhis teampredictedthattheywouldfindafossilanimallikeTiktaalikinrocksaround375millionyearsold. Studentsshouldexplainthatfromthefossilrecord,Shubinandhisteamknewthatbefore385myathefossil recorddoesnotcontaintetrapods.Around365myathefossilrecordshowsthefirstevidenceoforganisms withtetrapodlimbcharacteristics,includingdigits.Shubinandhisteamhypothesizedthattheevolutionofa functionaltetrapodlimbwasagradualprocess,andfossilsshowingintermediatestepsinthatprocessshould belocatedinrocksofagesinbetween385millionand365millionyears. 9. (KeyConceptE)Thediagramsbelowillustratethebonesintheforelimbsoffourdifferentorganisms. Althoughtheselimbsalllookdifferent,theysharesomecommonpatterns.Thesecommonpatternssuggest that a. Theseorganismsaremembersofthesamespecies. b. Theorganismsexistedataboutthesamepointintime. c. Theseorganismsshareacommonancestor. d. Theseorganismshaveexactlythesamegenes. 10. (KeyConceptsF)ExplainwhyNeilShubinandhiscolleagueshadtotraveltoanareaabovetheArcticCircle tofindthefossilofananimalthatoncelivedinawarmswamplikehabitat. WhenTiktaaliklivedandbecamefossilized,thecontinentitwasonwaslocatedclosertoEarth’sequator wherewarm,freshwaterhabitatswereabundant.Overtime,piecesofEarth’scrust,andallthefossilsinthe rocksontopofthecrust,movedtodifferentlocationsduetocontinentaldrift.Studentsmayalsomention www.BioInteractive.org PublishedNovember2014 RevisedMarch2017 Page9of10 Short Film Great Transitions: The Origins of Tetrapods Educator Materials thatShubinandcolleaguesknewthattheArcticcontainsrocksfromtheDevonianperiodthatwereexplored atthesurface. 11. (KeyConceptsBandF)Inthefilm,NeilShubinisshownhikinguparockyhillsidewhilerecallingforusoneof theexcitingmomentsduringthehuntforaTiktaalik-likefossilanimal.Neilsays,“Beneathourfeetwere fossilfishbones,fragmentsoffossilfishbones,manythousandsofpieces.Itwasn’tjustonefish;itwasa wholeaquarium,itwasdifferentspecies.”Whatdidthisdiscoverymeanforthefossil-huntingteam? Thefossilizedfishboneswereevidencethattheteamhaddiscoveredalayerofrockthathadformedinthe kindofhabitatwhereananimallikeTiktaalikcouldalsohavelived.Therocklayercontainedmanyfish species,soitmightalsocontainananimalthatexistedduringthetransitionfromwatertoland. KEYREFERENCES Daeschler,E.B.,Shubin,N.H.,andJenkinsJr.,F.A.(2006).ADevoniantetrapod-likefishandtheevolutionofthe tetrapodbodyplan.Nature440:757-763. Daeschler,Ted.http://clade.ansp.org/vert_zoology/people/daeschler/. Downs,J.P.,Daeschler,E.B.,JenkinsJr.,F.A.,andShubin,N.H.(2008).ThecranialendoskeletonofTiktaalik roseae.Nature455:925-929. Shubin,Neil.http://pondside.uchicago.edu/oba/faculty/shubin_n.html. Shubin,N.H.,Daeschler,E.B.,andJenkins,F.A.(2006).ThepectoralfinofTiktaalikroseaeandtheoriginofthe tetrapodlimb.Nature440:764-771. Tiktaalikroseaehttp://tiktaalik.uchicago.edu/. AUTHORS WrittenbyPaulStrode,PhD,FairviewHighSchool,Boulder,andLauraBonetta,PhD,HHMI ReviewedbyJustinLemberg,PhD,TheUniversityofChicago FieldtestedbyKimberlySaltsburg,WestminsterHighSchool,Westminster,MD;ZachMurray,PoncaCityHighSchool, PoncaCity,OK;JackSaffer,CentralIslipHighSchool,CentralIslip,NY;MinuBasu,DoughertyValleyHighSchool,San Ramon,CA;JudithPinto,BergenCountyAcademies,Hackensack,NJ;SteveMeyer,LakotaWestHighSchool,WestChester, OH;ClarissaFurlong,FranklinHeightsHighSchool,Columbus,OH;CharlotteSmith,LakesideHighSchool,Evans,GA; VandanaGudi,RobertMorganEducationalCenter,Miami,FL;BarryGreenwald,HardingHighSchool,St.Paul,MN www.BioInteractive.org PublishedNovember2014 RevisedMarch2017 Page10of10