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.