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Short Film Great Transitions: The Origins of Tetrapods Educator Materials

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IN-DEPTHFILMGUIDE

DESCRIPTIONIntheHHMIfilmGreatTransitions:TheOriginofTetrapods,wejoinUniversityofChicagoevolutionarybiologistNeilShubinasherelivestheexcitingdiscoveryofTiktaalikroseae,aspeciesthatlivedaround375millionyearsago(mya)andhadcharacteristicsofbothfishandfour-leggedanimals(ortetrapods).Thefilmtakesusonacompellingjourneythroughtheprocessofscience—includingtheaskingofimportantquestions,synthesizingknownfactsinnovelways,generatingandtestinghypotheses,andperseveringdespiterepeatedfailures.

KEYCONCEPTSA. 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

PRIORKNOWLEDGEItwouldbehelpfulforstudentstobefamiliarwiththeconceptsofnaturalselectionandadaptation,andtohavelearnedabouttectonicplatemovements.

Curriculum StandardsNGSS(April2013) 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.BAPBiology(2012–13) 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.4IBBiology(2016) 5.1 Textbook ChapterSectionsMillerandLevine,Biology(2010ed.) 16.2,16.3,16.4,17.3,19.1,19.2Reeceetal.,CampbellBiology(9thed.) 22.1,22.2,22.3,25.2,25.5,25.6

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PAUSEPOINTSThefilmmaybeviewedinitsentiretyorpausedatspecificpointstoreviewcontentwithstudents.Thetablebelowlistssuggestedpausepoints,indicatingthebeginningandendtimesinminutesinthefilm.

Begin End ContentDescription ReviewQuestions Standards1

0:00

5:00

• Darwinpredictedthattransitionalorganisms’fossilscouldbeintermediateformsbetweendifferentgroups.Manyfossilsoftransitionalorganismshavebeenfoundandhaveenabledscientiststoreconstructtheoriginsofmanygroups.

• Althoughtheydon’talwayslookorfunctionsimilarly,thelimbsofmammals,amphibians,reptiles,andbirdshaveacommonarchitecture,suggestingaconnectionbetweenverydifferentgroupsofanimals.

• Thehistoryofvertebratesiscapturedinrockthatcanbedated;birdsareyoungest,thenmammals,reptiles,andamphibians.At370millionyearsago,therearenofossilsoftetrapods.

• Fishandtetrapodsarebothvertebrates,andinearlydevelopmenttheylookverysimilar.DNAevidenceindicatesthatfisharetetrapods’closestrelatives.

• Whataretransitionalfossils?Whyaretheyimportant?

• Whatevidencesuggeststhatfour-leggedanimalsevolvedfromfish?

NGSS(April2013)MS-LS4.A,MS-LS4.C,MS-ESS1.C,HS-LS4.C,HS-LS4.DAPBiology(2012–13)1.A.1,1.A.4,1.B.1,1.C.1,1.C.3,4.B.4IBBiology(2016)5.1

2

5:01

13:10

• Findingfossilscanbechallenging.Tofindfossils,scientistsdevelophypothesesaboutthetypesofhabitatsinwhichearlieranimalslivedandwhentheylivedthere.Theythenpredictwhichtypesandagesofrockswouldhousefossilsofthoseanimals.

• Fossilizationhappensonlyundercertainconditions.

• Afteryearsofsearching,ateamofpaleontologistsdiscoveredTiktaalikataremotefieldsiteinAlaskawithDevonianagerocks.

• WhydidthepaleontologistsdecidetohuntforthetransitionalfossilinAlaska?

NGSS(April2013)MS-ESS1.CAPBiology(2012–13)1.A.4,1.B.1IBBiology(2016)5.1

3

13:11

17:11

• Tiktaalikhasamixoffishlikeandtetrapod-likefeatures.Ithasscalesandfinswithfinrays,likeafish.Likeatetrapod,ithasaflatheadwitheyeson

• WhyisTiktaalikconsideredatransitionalfossil?

NGSS(April2013)MS-LS4.A,MS-LS4.C,MS-ESS1.C,HS-LS4.C,HS-

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top,interlockingribssuggestingithadlungs,aneck,ahipbone,andaforelimbwiththebeginningofabonepatternlikethatoftetrapods.

• Thegreattransitionfromfishtotetrapodhappenedgraduallyovermanymillionsofyears.

• Otherfossilshaveprovidedevidenceofothertransitions.

• Didthetransitionfromfishtotetrapodhappenquicklyorslowly?

LS4.DAPBiology(2012–13)1.A.1,1.A.2,1.A.4,1.B.1,1.C.1,1.C.3IBBiology(2016)5.1

BACKGROUNDTiktaalik(pronouncedtic-TAH-lick)means“shallowwaterfish”inthelanguageoftheNunavutpeoplewholiveintheCanadianArctic,whereTiktaalikwasfoundin2004.TheteamthatmadethediscoveryincludedNeilShubin;hisformergraduatestudent,TedDaeschler,whoisnowatTheAcademyofNaturalSciencesofDrexelUniversity;andShubin’sformergraduateadvisor,HarvardUniversityevolutionarybiologistFarishJenkins.ThediscoveryofTiktaalikisdescribedindetailintheaccompanyingarticle“It’saFishapod!”bySeanB.Carrollhttp://www.hhmi.org/biointeractive/article-fishapod.Tiktaalikisatetrapod—agroupthatincludesfour-leggedanimalslikedogsandhorses,aswellaswhalesandsnakes,whichevolvedfromfour-leggedancestorsbutnolongerhavefourlegs.

HowAreFossilsFound?

Oneofthemajorthemesofthefilmishowfossilsarefound.Thefirststepinlocatingfossilsoftherightageandtypeistomakepredictionsaboutwhereandwhentheorganismwouldhavelived.Beforeabout385millionyearsago(mya),notetrapodsareknownfromthefossilrecord.Thefirstfossilsoftetrapods,AcanthostegaandIchthyostega,weredatedto365mya.So,Shubinandhiscolleagueshypothesizedthatthefishintheprocessofbecomingmoretetrapod-likelivedbetween385and365mya.This20-million-year-spaniswithinageologicalepochcalledtheUpperDevonian.

Figure1.Rocksagedbetween385and365myawerepredictedtocontainfossilswithtransitionalfeaturesbetweenthoseoffishandtetrapods.

Inthefilm,weseeamapofDevonianrocks.ThemapcomesfromatextbookentitledEvolutionoftheEarthbyDottandBatten,whichcontainsmanygeologicalmaps.Likeallmaps,geologicalmapsaredesignedtoshowwherecertainthingsarelocated.Whereasthemapsweknowbestshowthedistributionofroads,rivers,orcountyboundaries,ageologicalmapshowsthedistributionofgeologicalfeatures,includingdifferentkindsofrocks(e.g.,sedimentary,igneous,andmetamorphic)andthedifferentagesofrocks.Thesemapsaretypicallygeneratedtoguideoilandmineralexpeditions.

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ThemapinthefilmshowsthreeDevoniandepositsinNorthAmerica.Twoofthosedepositshadalreadybeensearchedforfossils.ThethirdDevoniandeposithadbeenexploredbyCanadiangeologicalsurveyorsandafewoilcompanygeologistsbutnofossilhunters.Thisthirdformationcontainedsedimentaryrocklayers—thetypeofrockswherefossilsaremostcommonlyfound.Aftersomeadditionalresearch,ShubinandDaeschlerdiscoveredthattherockshadformedinwhatwereshallow,meanderingstreamsinDevoniantimes.

Evenafteridentifyingrocksoftherightageandtype,scientistscannotbesurethattheywillfindfossilsinthoserocks.AlmostallorganismsthathavelivedonEarthleftnotraceoftheirexistence.Mostareeatenordecaybeforetheyarefossilized,andsofttissuesdonotfossilizewell.Andevenifanorganismisburiedinsedimentsandbecomesfossilized,therocklayerwiththatfossilhastobecomeexposedonEarth’ssurfaceforsomeonetofindit.

Sohowcanpaleontologistsexpecttofindfossilsatall,letalonefossilsofspeciesthatexistedataparticularpointintime?Onecriticalfactmakesthispossible.Asinglespeciescanexistformillionsofyearsandconsistofhundredsofmillionsorevenbillionsofindividuals.Theprobabilityoffindingthefossilremainsofanyoneindividualistiny,butnotimpossible.

FindingTiktaalik

In2004,Shubinandcolleaguesfoundthefossilofanewspeciesthatshedlightonakeytransitionintheevolutionoftetrapods:thefin-to-limbtransition.ThecrewwasexcavatingonedayinanareawithlotsofDevonianfishfossils(a“wholeaquarium,”asShubinsaidinthefilm).Stickingoutofarockyedgeinthequarrywasthetipofthesnoutofaflat-headedfish.Whywastheflatheadsuchanimportantclue?

Manyfishhaveflatheads.Butwhatissignificantinthiscaseisthattheearliesttetrapodsthathadbeendiscovered(i.e.,AcanthostegaandIchthyostega)hadflatheads.Sofindingaflat-headedfishwasagoodindicatorthatthisfossilwaspartofthegradualevolutionarysequencefromfishtotetrapods.

Oncethefossilwascleaned,itshowedthatithadtwonostrilsandtwoeyesmountedontopofitsflathead.Mostfishhavenostrils,buttetrapod-likefishlikeTiktaalikhaveaspecializedkindofnostrilthatconnectstotheinsideofthemouth.Fishthatarenotcloselyrelatedtotetrapodshavenostrilsthatonlyconnectwiththeexterior.

AnotherimportantfeatureofthisfossilisthatTiktaalik’sheadwasnotconnecteddirectlytoitsshouldersliketheheadofafishwouldhavebeen(Figure2).Instead,itsheadsatinfrontoftheshoulders,attheendofaflexibleneck—meaningtheheadwasfreetomoveupanddownandsideways.

Figure2.Tiktaalikhasamixoffishandtetrapodfeatures.Oneoftheimportanttetrapodcharacteristicsisthepresenceofaneck,whichallowsTiktaaliktomorefreelymoveitshead.(Figuretakenfromtheinteractive“MeetTiktaalikroseae”availableontheUniversityofChicagowebsiteathttp://tiktaalik.uchicago.edu/meetTik2.html).

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Likeafish,thefossilofTiktaalikshowsevidenceofscales,gills,andfins.Butinsidethefins,Tiktaalikcontainslimbbonesthatarecharacteristicofanearlytetrapod.Onebone(thehumerus)extendsfromtheshoulderandconnectstotwoadditionalbones(theradiusandulna),whicharesharedbyalllobe-finnedfish(i.e.,coelacanths,lungfish,tetrapods,andalltheirextinctrelatives).WhatisspecialaboutthefinofTiktaalikisthatthosethreebonesthenconnecttomanysmallbonesthatformwhatDr.Shubindescribesas“aversionofawrist.”

ThefossilofTiktaalikalsorevealedinterconnecting(orimbricate)ribsthatsurroundedthelungs.Lungfishhavelungs,buttheydonothaveimbricateribs.Thelungsoflungfishandtetrapodsarehomologous,whichmeanstheyevolvedfromacommonancestorthathadlungs.TheimbricateribsofTiktaalikandlatertetrapodswouldhaveprovidedextrabodyrigidityinconditionsinwhichthesurroundingwatercouldnotsupporttheweightoftheanimal(i.e.,inshallowwaterorsubaerialconditions).Onehypothesisisthattheribsprovidedanadvantageinthoseenvironmentsbykeepingtheweightoftheanimalfromputtingpressureonthelungsandpreventingthemfromfillingwithair.

SowhyisTiktaalikreferredtoasatransitionalcreature?Itsmixofcharacteristicsoffishandoftetrapodssuggestthatitrepresentsoneofthemomentsinthetransitionbetweenfishandtetrapods.

DISCUSSIONPOINTS• Studentsmayincorrectlyassumethatdiscoveringfossilsismostlyaboutluck.Askyourstudentsifanyof

themhavefoundfossils.Askthosestudentswhofoundfossilsiftheirfossilfindswereluckyoraproductofdeliberatesearching.Explainthatpaleontologistsdonotaccidentallystumbleuponthefossilstheydiscover,butthatpaleontologyisaprocessofhypothesistesting.Throughcarefulplanning,readingscientificarticles,andstudyinggeologyandgeologicaltime,paleontologistsformhypothesesaboutthetimeperiodsandhabitatsoccupiedbycertainorganisms.TheythendeveloppredictionsaboutwhatkindsofrockscertainfossilsofthoseorganismsshouldbeburiedwithinandwhererocksofthosetypesandagesarefoundonEarth.

• Studentsmaywonderwhywedonotfindmorefossils—fossilsofeverytypeoforganismthateverlived.Askstudentstoconsideramousethatisrunningneartheschoolatthisverymoment.Askthemtoconsidertheprobabilitythatthatsinglemousewilldietoday,becomefossilized,andberediscoveredamillionyearsfromnow.Theprobabilityisnearlyzero.Tobecomefossilized,themousemustnotbeeatenrightafteritdiesandnotdecomposewithinafewdays.Instead,themouse’sbodymustbequicklyandcompletelyburiedineithervolcanicashorsomekindofsediment.Itmustremainburieduntilthesedimentandthemouseremainsturnintorock—aprocessthatmaytakethousandsorevenmillionsofyears.EnoughtimehaspassedinEarth’shistoryforsomeindividuals,fromthehundredsofthousandswithinanyonespecies,tohavebeenintherightconditionstobecomefossilized.

• Aclassicquestionstudentsmayaskaboutevolutionis,“Ifweevolvedfromfish,whyaretherestillfish?”Explaintostudentsthathumansandmodern-dayfishshareacommonancestorthatnolongerexists.Theaquatichabitatsinwhichfishthrivetodayhavebeenaroundforoverhalfabillionyears,andfisharewelladaptedtoliveinthoseenvironments.Whensomefishpopulationsevolvedstronger,limblikestructuresinfins,theywereabletoexploitnewtypesofhabitatsandreducecompetitionwithotherfishforresources.Butfishcontinuetoexistbecausetherearestillenvironmentsinwhichfishcanthrive.

• Askyourstudentswhatadvantageaneckmightprovide.Thelackofbonesconnectingtheheadtothebodyprovidesflexibilitytomovetheheadwithoutmovingtherestofthebody.Aflexibleneckcouldhavebeenbeneficialinmanypossiblescenarios.However,thisflexibilitymayhavealsocomeatacost—forexample,thelossoftheprotectivegillcover.Wedon’tknowexactlyhowTiktaalikuseditsneck.

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• Studentsmayaskforexamplesofothertransitionsfromearliertolaterforms.Theevolutionofwhalesfromterrestrialmammalsandbirdsfromdinosaursarebothexcellentexampleswithrichfossilevidenceandmanytransitionalforms.Whalesevolvedfromterrestrialmammalsthatwalkedandranonfourlegs.Afunfactthatsupportsthisancestryisthataswimmingwhaleflexesitsbackboneupanddownlikearunningcheetah,notsidetosidelikeaswimmingfish.Aswhaleancestorstransitionedfromaterrestrialtoamarinelife,manycharacteristicschanged.Forexample,legscanbealiabilityinanaquaticenvironmentratherthanabenefit.Asaresult,whaleancestorsshowaprogressivedecreaseinthesizeoftheirhindlimbs.Allthatremainsoftheselimbsinsomeoftoday’swhalespeciesisapairoftinybonesthatareburiedinsidethebodywallandarenotevenattachedtothepelvis.

• Discusswithstudentswhytheterm“missinglink”isinaccurate.Thetermhasbeenpopularinthemediaandhasbeenusedtoincorrectlyimplythatanunbrokenchainoforganismsisrequiredtodemonstratethatallspeciesarerelated.Theterms“transitionalform”and“transitionalfossil”moreaccuratelydescribespeciesthatillustratethesequenceofchangesthatoccurredduringtheevolutionofanewgroupoforganisms.

• IsTiktaalikourancestor?Tiktaalikisrepresentativeofananimalthatwasanancestortomoderntetrapods,includinghumans.Butwecannotsaywhetheritwasadirectancestor.Acommonmisconceptionisthatallfossils,includingtransitionalfossilslikeTiktaalik,aredirectancestorsofmodernspecies.Explainthatfossils,likeTiktaalik,maybedirectancestors,buttheyaremuchmorelikelyrepresentativeofwhattheactualancestorslookedlikeandnottheancestorsthemselves.Infact,thedescendantsofTiktaalikmayhavegoneextinctlongago,butanotherspecies,relatedtoTiktaalik,andyettobediscovered,maybethecommonancestortoallmoderntetrapods.Wemayneverknow,butwecanbecertainthattheancestorlookedalotlikeTiktaalik.

• StudentsmaybeconfusedaboutwhyfossilanimalslikeTiktaalikthatlivedinwarmhabitatsendupinbarrenlocationsliketheCanadianArcticIslands.RemindstudentsthatEarth’sbiologicalandgeologicalhistoriesareconnected.Earth’scrustisdynamic.Platetectonicsmovelandmasses,andfossilsthatwereonceattheequatorcanbemovedtodistantlocations.

RELATEDBIOINTERACTIVERESOURCESIt’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-students-recognize-transitional-fossil)

Thisshortvideodemonstratesthepowerofobservationandtheimportanceoffossilevidence.WhenNeilShubinshowsaTiktaalikfossiltosmallchildren,theyimmediatelynoticethepresenceofbothfishandtetrapodcharacteristics.

ExploreYourInnerAnimals

(http://www.hhmi.org/biointeractive/explore-your-inner-animals)

Thisinteractiveexploresdifferentanatomicalfeaturesofthehumanbodyandwhattheyrevealabouttheevolutionaryhistorywesharewithotherorganisms,includingearlier,long-extinctspecies.

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USINGTHEQUIZThequizisdesignedasasummativeassessmentthatprobesstudentunderstandingofthekeyconceptsaddressedinthefilm.However,someteachersusethequizbeforeandduringthefilmtoassessstudents’priorknowledgeandtoguidestudentsastheywatchthefilm.Teachersareencouragedtochoosetheusethatbestfitstheirlearningobjectivesandtheirstudents’needs.Moreover,becausethevocabularyandconceptsarecomplex,teachersareencouragedtomodifythequiz(e.g.,onlyasksomeofthequestions,explaincomplicatedvocabularyforELLstudents)asneeded.Thelasttwoquestionsareintendedforstudentswithpriorknowledgeaboutmutations,genesexpression,anddevelopment.

QUIZQUESTIONSANDANSWERSThestudentversionofthisquizisavailableasaseparatefile.Keyconceptscoveredbyeachquestionarenotedhere.

1. (KeyConceptB)WhenCharlesDarwinconsideredsomeoftheuniquestructuresfoundinmodernanimals,likethefeatheredwingsofbirds,hefamouslyproposedthatmodernanimalsmusthaveevolvedfromearlierformsthatlackedthosestructures.Healsopredicted.(Fillintheblankwithoneofthestatementsbelow.)

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,andscientificprocessesusedduringtheTiktaalikexpeditions.

Location Maps/Photos ScientificProcessa. CanadianArctic Geologicalmaps Hypothesestestingb. Iceland Roadmaps Predictingc. Alaska Aerialphotos Questioningd. ArcticCircle Animaltrackmaps Developingexplanations

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4. (KeyConceptE)Whichstatementbelowisevidencethatalltetrapodssharedasinglecommonancestor?

a. Alltetrapodslivepartlyinwaterandpartlyonland.

b. Alltetrapodsarewarm-blooded.

c. Alltetrapodlimbsconsistofrearrangedfishfinrays.

d. Alltetrapodlimbshaveacommonpatternofonebone,twobones,manybones,thendigits.

5. (KeyConceptC)Thetransitionfromfishtotetrapodsisbestdescribedas:

a. Likemostofthegreattransitionsinevolutionaryhistory,ithappenedinveryfewbigstepssothatthereareveryfewintermediateforms.

b. Likemostofthegreattransitionsinevolutionaryhistory,ithappenedinmanysmallstepsleadingtomanyintermediateforms.

c. Likenoothertransitioninevolutionaryhistory,ithappenedinmanysteps,sotherearemanyintermediateforms.

d. Likemostofthegreattransitionsinevolutionaryhistory,ithappenedinasinglestepwithnointermediateforms.

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.”Justifyyouranswerinoneortwosentences.

False.Transitionalformsarespeciesintheirownright.Theyarecalledtransitionalformsbecausetheypossesstraits,likefins,thatarefoundinearlierfossilanimals,butalsopossesstraits,likelimbbones,thatarefoundinlaterfossilanimals.

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8. (KeyConceptsF)Studythegraphicbelowofrocklayerswithfossilsinthem.ExplainhowNeilShubinandhisteampredictedthattheywouldfindafossilanimallikeTiktaalikinrocksaround375millionyearsold.

Studentsshouldexplainthatfromthefossilrecord,Shubinandhisteamknewthatbefore385myathefossilrecorddoesnotcontaintetrapods.Around365myathefossilrecordshowsthefirstevidenceoforganismswithtetrapodlimbcharacteristics,includingdigits.Shubinandhisteamhypothesizedthattheevolutionofafunctionaltetrapodlimbwasagradualprocess,andfossilsshowingintermediatestepsinthatprocessshouldbelocatedinrocksofagesinbetween385millionand365millionyears.

9. (KeyConceptE)Thediagramsbelowillustratethebonesintheforelimbsoffourdifferentorganisms.Althoughtheselimbsalllookdifferent,theysharesomecommonpatterns.Thesecommonpatternssuggestthat

a. Theseorganismsaremembersofthesamespecies.

b. Theorganismsexistedataboutthesamepointintime.

c. Theseorganismsshareacommonancestor.

d. Theseorganismshaveexactlythesamegenes.

10. (KeyConceptsF)ExplainwhyNeilShubinandhiscolleagueshadtotraveltoanareaabovetheArcticCircletofindthefossilofananimalthatoncelivedinawarmswamplikehabitat.

WhenTiktaaliklivedandbecamefossilized,thecontinentitwasonwaslocatedclosertoEarth’sequatorwherewarm,freshwaterhabitatswereabundant.Overtime,piecesofEarth’scrust,andallthefossilsintherocksontopofthecrust,movedtodifferentlocationsduetocontinentaldrift.Studentsmayalsomention

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thatShubinandcolleaguesknewthattheArcticcontainsrocksfromtheDevonianperiodthatwereexploredatthesurface.

11. (KeyConceptsBandF)Inthefilm,NeilShubinisshownhikinguparockyhillsidewhilerecallingforusoneoftheexcitingmomentsduringthehuntforaTiktaalik-likefossilanimal.Neilsays,“Beneathourfeetwerefossilfishbones,fragmentsoffossilfishbones,manythousandsofpieces.Itwasn’tjustonefish;itwasawholeaquarium,itwasdifferentspecies.”Whatdidthisdiscoverymeanforthefossil-huntingteam?

ThefossilizedfishboneswereevidencethattheteamhaddiscoveredalayerofrockthathadformedinthekindofhabitatwhereananimallikeTiktaalikcouldalsohavelived.Therocklayercontainedmanyfishspecies,soitmightalsocontainananimalthatexistedduringthetransitionfromwatertoland.

KEYREFERENCESDaeschler,E.B.,Shubin,N.H.,andJenkinsJr.,F.A.(2006).ADevoniantetrapod-likefishandtheevolutionofthetetrapodbodyplan.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).ThecranialendoskeletonofTiktaalikroseae.Nature455:925-929.Shubin,Neil.http://pondside.uchicago.edu/oba/faculty/shubin_n.html.Shubin,N.H.,Daeschler,E.B.,andJenkins,F.A.(2006).ThepectoralfinofTiktaalikroseaeandtheoriginofthetetrapodlimb.Nature440:764-771.Tiktaalikroseaehttp://tiktaalik.uchicago.edu/.

AUTHORSWrittenbyPaulStrode,PhD,FairviewHighSchool,Boulder,andLauraBonetta,PhD,HHMIReviewedbyJustinLemberg,PhD,TheUniversityofChicagoFieldtestedbyKimberlySaltsburg,WestminsterHighSchool,Westminster,MD;ZachMurray,PoncaCityHighSchool,PoncaCity,OK;JackSaffer,CentralIslipHighSchool,CentralIslip,NY;MinuBasu,DoughertyValleyHighSchool,SanRamon,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