Case Study: Climate, Biomes, and Equidae · 2017-01-14 · Case Study: Climate, Biomes, and Equidae...
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Case Study: Climate, Biomes, and Equidae In this activity, you will explore how environmental changes can shape life on Earth, using Equidae as a case study. By the end of the activity, you should be able to: 1. Describe how biodiversity increases with the evolution of new species and is decreased by extinction; 2. Evaluate evidence and propose ideas about why changing climatic conditions and an increase in grassland environments led to changes in horse morphology and diversity; and 3. Recognize that scientific ideas are subject to change based on new evidence. We will explore some patterns using the following diagrams. We will focus on the taxa shown with arrows below: Figure 1. Family Tree of North American Equidae. Family tree after Mihlbacher et al., 2011. Information on diets from MacFadden, 2005 and the Paleobiology Database (paleobiodb.org).
Transcript of Case Study: Climate, Biomes, and Equidae · 2017-01-14 · Case Study: Climate, Biomes, and Equidae...
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.Variousshaded/patternedlinesindicatedifferentdatasetsusedtoreconstructtemperatures(seeoriginalfilefordetails).Arrowsalongthetopshowmassextinctions.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.Darkgray=enamel,striped=dentine,lightgray=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,horseevolutionwasthoughttohavebeenafairlylinearprocesswiththesechangeshappeninginasingledirection,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,withthediscoveryofmorefossilhorsetaxa,scientistsnowknowthatchangeshavenotbeenunidirectionalthroughtime.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.
