Upload
vophuc
View
217
Download
0
Embed Size (px)
Citation preview
CaseStudy:Climate,Biomes,andEquidae
Inthisactivity,youwillexplorehowenvironmentalchangescanshapelifeonEarth,usingEquidaeasacasestudy.Bytheendoftheactivity,youshouldbeableto:
1. Describehowbiodiversityincreaseswiththeevolutionofnewspeciesandisdecreasedbyextinction;2. Evaluateevidenceandproposeideasaboutwhychangingclimaticconditionsandanincreasein
grasslandenvironmentsledtochangesinhorsemorphologyanddiversity;and3. Recognizethatscientificideasaresubjecttochangebasedonnewevidence.
Wewillexploresomepatternsusingthefollowingdiagrams.Wewillfocusonthetaxashownwitharrowsbelow:
Figure1.FamilyTreeofNorthAmericanEquidae.FamilytreeafterMihlbacheretal.,2011.Informationondietsfrom
MacFadden,2005andthePaleobiologyDatabase(paleobiodb.org).
Part1:Climate,Grass,andEquidae
TakealookatFigure2belowandanswerthefollowingquestions.
Figure2.TemperatureofPlanetEarth.Temperatureisshownasanomalies(differences)fromthe1960–1990average,sotemperaturesabovethehorizontallinearewarmerthanthe1960–1990averageandtemperaturesbelowthelinearecolder.Variouscoloredlinesindicatedifferentdatasetsusedtoreconstructtemperatures(seeoriginalfilefordetails).Redarrowsshowmassextinctions.Notechangesintimescaleatverticalbreaks.ImagemodifiedfromGlenFergus,http://en.wikipedia.org/wiki/Paleoclimatology#/media/File:All_palaeotemps.png.ReuseispermittedunderaCreativeCommonsAttribution-ShareAlike3.0Unportedlicense.
1. Eohippus,alsoknownbythenameHyracotherium,meansDawnHorseandistheearliestknownhorsegenus.Eohippuswassmall,aboutthesizeofasmalldog,andstoodapproximately1-1.5feettallattheshoulder—tinycomparedtomodernhorses!ItappearedduringtheearlyEoceneandlivedinforests.WhatwasthetemperatureofEarthlikeduringtheearlyEocene?I.e.,howwoulditcomparetotoday?
2. Fossilpollentellsusthatthefirstgrassesappearedasearlyas80MYA.Itisthoughtthatastheydevelopeddroughttolerance,grassesspreadfromforestmarginsintodry,openhabitats.Thegrassfamilywentthroughamajorperiodofdiversificationandnowincludesover10,000species.
a. Accordingtothegraphabove,whenwasthemajorperiodofgrassdiversification?
b. Whatwasthegeneraltemperaturetrendduringthistime—warmingorcooling?
3. Thisperiodofgrassdiversificationcorrespondedwithanexpansioninthearealextentofgrass-dominatedbiomes(grasslands,savannas,andsteppes)andareductioninforestedhabitats.WhathappenstoEquidaediversityshortlyafterthistime?(Hint:examinethenumberofgenerainFigure1).
Part2–EquidaeToothMorphology
Inmammals,molarscomeinavarietyofdifferentshapes.
Brachydontteethhaveshort,lowcrownscoveredinacapofenamel(thehardestsubstanceofwhichteetharecomposed)thatextendsdowntothegumline,andoneormorewell-developedroots.Thechewingsurfacesareusuallypointedandtendtobegoodfortearingorshredding.Humanshavebrachydontteeth.
Hypsodontteethhavehighcrownsthatextendfarabovethegumline,withenamelextendingbelowthegumline.Theyarealsohaveahardprotectivecoatingofcementum.Thisarrangementprovidesextramaterialforwearandiscommoninmammalsthateatfoodthatishardandabrasive.Theytendtohaveflatchewingsurfacesthataregoodforcrushingandgrinding.
TakealookattheillustrationofEquidaeteethinFigure3below.YouwillwanttomatchthemtotheanimalsinFigure1(wheretheyaremarkedbythelargearrows)tobesureyouunderstandwhentheylived.Themoresquareviewsbelowshowthechewingsurfaceofthetooth.Theotherviewsshowtheteethfromtheside.
Figure3.EquidaeTeeth.Molarsshownfromthesideandchewingsurface.Gray=enamel,green=dentine,beige=cementum.Sizesnottoscaletomakedetailsvisible.IllustrationsbyMichelleTribble,https://tribbill.wordpress.com/.ReuseispermittedunderaCreativeCommonsAttribution-Non-Commercial-ShareAlike3.0license.
4. DescribethegeneralchangesinEquidaetoothmorphologyshowninthesixtaxainFigure3.
5. Howdoesthetimingofthesechangesintoothmorphologyrelatetothetimingoftheexpansionofgrasslands?I.e.,whichtypeofteetharemorecommonintheearlyhistoryofEquidaeandwhicharemorecommonafterthediversificationandexpansionofgrasses?
6. EohippusandMesohippuswerebrowsers,meaningtheyatesofterleaves,shoots,buds,andfruits.
Parahippuswasmostlyabrowser,butmayhavealsoeatensomegrasses,whichcontainhighlevelsofabrasivesilicaintheirepidermalcellwalls.Merychippushadsomespeciesthatweremixedfeeders,andothersthatwereprimarilygrazers,meaningtheyategrasses.DinohippusandEquusweregrazers.Whatmightbesomereasonforthechangesinteeththatyoudescribedabove?
Part3–EquidaeFootMorphology
ExaminetheillustrationofEquidaefootmorphologyinFigure4below.Again,youwillwanttomatchthemtotheanimalsinFigure1(wheretheyaremarkedbylargearrows)tobesureyouunderstandwhentheylived.
Figure4.Equidaefootmorphology.Sizesnottoscaletomakedetailsvisible.IllustrationsbyMichelleTribble,https://tribbill.wordpress.com/.ReuseispermittedunderaCreativeCommonsAttribution-Non-Commercial-ShareAlike3.0license.
7. TakealookatthefootofEohippusinFigure4above.Unlikemodernhorses,whichhavebonyhooves,itstoeshadfleshypads.Howmanytoesdidithaveonitsfrontfeet?
8. WhathappenedtothefeetoftheEquidaeshownovertimeintermsof:
a. Toenumber
b. Relativesizesoftheindividualtoes
c. Lengthoffeet
Howmightadrierclimateandchangefromforeststograsslandshaveselectedforthesechanges?ThephotosonthePowerPointslidemayhelpyoutovisualizethetwodifferentenvironments.
9. WhymightfleshytoepadsbeadvantageousintheforestedenvironmentsearlyEquidaelikeEohippusandMesohippuslivedin,butbonyhoovesbebettersuitedtograsslands?(Hint:whatisthegroundlikeinforestsvs.grasslands?Moistandspongyordryandfirmer?)
10. Whymighttheexpansionofgrasslandshaveledtolongerfeet/legsingrasslandenvironments?(Hint:thinkabouthowhorseswouldescapepredatorsineachenvironmentandtheneedtocoverdistance).
Part4-ChangingIdeasaboutHorseEvolution
Wehaveexploredsomeofthechangesinhorsesincludingbodysize,diet,toothcharacteristics,andfoot/legmorphology.Inthepast,horseevolutionwasthoughttohavebeenafairlylinearprocess,withthesechangeshappeninginasingledirection,forexamplefromsmallertolargerbodysize.Withonlyasmallsetoffossils,thismightappeartobethecase,asinthesetofspeciesshowninFigure5below.
Figure5.EquidaefossilsfromtheStateMuseumofNaturalHistoryKarlsruhe,Germany.ImagebyH.Zell,http://commons.wikimedia.org/wiki/File:Equine_evolution.jpg.ReusepermittedunderaCreativeCommonsAttribution-ShareAlike3.0Unportedlicense.
TheEquidaeskeletonsinFigure5showaprogressiveincreaseinbodysizethroughtime.Overtime,moreandmorefossilhorsetaxahavebeendiscovered.WhathappenswhenwelookatamorecompletesetofEquidaesizes?ConsiderFigure6below.
Figure6.Bodysizeofhorses.FigureredrawnfromdatafromMacFaddan(1986).HorseillustrationsbyMichelleTribble,https://tribbill.wordpress.com/.ReuseispermittedunderaCreativeCommonsAttribution-Non-Commercial-ShareAlike3.0license.
11. DoesFigure6suggestacompletelylineartrendtoincreasingbodysize?Explain.
Figure5,andmanylikeitinoldertextbooksandmuseumdisplays,appearstoshowonetaxaturningintoanotherinalinearpattern.Indeed,horseevolutionwaspreviouslythoughttohavebeenessentiallyunidirectional,withchangesprogressinginastraightlinefromEohippustothemodernhorseEquus.
12. CompareandcontrastFigure5andFigure1.DoesFigure1suggestevolutionprogressedlinearlyfromEohippustoEquus??Whyorwhynot?(Hint:doestheEquidaefamilytreeinFigure1looklikeatree
trunkthatfollowsastraightlineliketheoneshownbyarrowsinFigure5?Ordoesitlookmorelikeabushwithlotsofbranches?)
So,althoughwedoseeoveralltrendsinEquidaeadaptationsthroughtimethatcanbecorrelatedwithchangingglobalclimatesandhabitats,withthediscoveryofmorefossilhorsetaxascientistsnowknowthatchangeshavenotbeenunidirectionalthroughtime.Forexample,eventhoughincreasesinsizewerecommoninmostEquidaegroups,decreasesinsizeoccurredinotherssuchasNannippus.WealsoseethattheEquidaefamilytreeisnotstraight,buthasmanybranches.Akeypointisthatscientificideasarenotfixed,butcananddochangeasnewevidenceisdiscovered.
Part5–ABriefHistoryofEquus(modernhorses)
OnereasonscientistspreviouslythoughthorseevolutionwasfairlylinearwasthelimiteddiversityoflivingEquidaetoday.Howdidthiscometobe?
13. WhathappenstothenumberofhorsegenerafromthelateMiocenethroughthepresentdayinNorthAmerica?(Hint:refertofromFigure1)
14. WhilemuchoftheevolutionaryhistoryofhorsestookplaceinNorthAmerica,horseswentextincthereandinSouthAmerica,leavingEquusinEurasiaasthelastsurvivinggenusEquidae.AccordingtoFigure1,approximatelywhendoesthisoccur?
ThistimingcorrespondstotheretreatofthemostrecentglacialadvanceaswellasthearrivalinNorthAmericanofhumans.Theglacial-interglacialcyclescanbeseeninthesaw-toothtemperaturepatterninFigure2.Equidaepersistedthroughoutmanypreviousglacial-interglacialcycles,soalthoughitmayhaveplayedarole,climatealoneisunlikelytohavecausedtheextinctionwithoutthenewplayerinthegame—humanhunters.Overhuntingbyhumansisthoughttohaveplayedakeyroleinthisextinction,whichincluded73%ofmegafaunainNorthAmerica.ItwasnotuntilColumbusthathorseswerereintroducedtoNorthAmericafromsurvivingEquusinEurasia.
ThefactthathorseswentextinctinNorthandSouthAmericaandwereabsentuntilreintroducedbySpanishexplorersisimportantbecausewildhorses,whichdescendedfromthesereintroduceddomestichorses,are
generallyconsiderednon-nativebymoststateandfederalwildlifeagenciesintheUnitedStates.Todaytherearemorethan40,000wildhorsesonpubliclandsinthewesternUnitedStates.Becausetheyarenotconsidered“native,”theyarenotprotectedinthesamewayasnativespeciesandaresubjecttoroundupsandremoval.
TheBureauofLandManagementarguesthattherearemorehorsesonrangelandsthanthelandcansupport,whileanimalrightsactivistsarguetheyshouldbeprotectedlikenativespecies.Whatdoyouthink?
Reflection(tobedoneindividually)
15. Listfivethingsyoulearnedabouthorsestoday.