Paleobiology of Archaeohippus (Mammalia; Equidae), a three-toed horse from the Oligocene-Miocene of North America

174 Pages·2002·7.5 MB·English

by  O'Sullivan, Jay Alfred, 1965-

PALEOBIOLOGYOFARCHAEOHIPPUS(MAMMALIA;EQUIDAE),
ATHREE-TOEDHORSEFROMTHEOLIGOCENE-MIOCENE
OFNORTHAMERICA
JAYALFREDO’SULLIVAN
ADISSERTATIONPRESENTEDTOTHEGRADUATESCHOOL
OFTHEUNIVERSITYOFFLORIDAINPARTIALFULFILLMENT
OFTHEREQUIREMENTSFORTHEDEGREEOF
DOCTOROFPHILOSOPHY
UNIVERSITYOFFLORIDA
2002
Copyright2002
by
JayAlfredO’Sullivan
Thisstudyisdedicatedtomywife,Kym. Sheprovidedallofthelove,strength,patience,
andencouragementIneededtogetthisstartedandtoseeitthroughtocompletion. She
alsoprovidedmewiththeincentivetomakethisinvestmentoftimeandenergyinthe
pursuitofmydreamtobecomeascientistandteacher. Thatincentivecomeswitha
varietyofnames-Sylvan,Joanna,Quinn. Thiseffortisdedicatedtothemalso.
Additionally,Iwouldliketorecognizethepeoplewhoplantedthefirstseedsofadream
tfhiantanhcaisalc!)omceamteotforumityiorne-scmuyeaplasroenftrso,mJomsyepohthaenrdpJaoraenn.ts—SupDpootrtO’(Seumloltiivoanna,l,JiamndJaffeand
LeslieSewell,BillandLoisGrigsby,andJerrySewell. Toallofthesepeople,thiswork
isdedicated,withlove.
ACKNOWLEDGMENTS
IthankDr.BruceJ.MacFaddenforsuggestingthatItakealookataninterestinglittle
fossilhorse,foralwayshavingfreshideaswhenmineweredry,andforkeepingme
movingeverforward. IthankalsoDrs.S.DavidWebbandRiehardC.HulbertJr.for
completingtheTripleThreatofFloridaMuseumvertebratepaleontology. Ineachhis
ownway,thesethreemenareaninspirationfortheirprofessionalismandtheirscholarly
devotiontoFloridapaleontology. Theyareseeondtonone. Dr.DouglasS.Jones
emphasizedtometheimportanceofevolutionaryquestionsthatinfluencethe
evolutionaryhistoriesofalltaxa. Dr.JohnF.Eisenberg,withhiseyeeveronthebig
pieture,wasabletoremindmethatthestudyoftheevolutionaryhistoriesofpartieular
taxaisinformativeandfun,and,asdidDr.BrianK.McNab,taughtmethatecologyis
justasinfluentialasphylogenyinshapingthosehistories,andsometimesmoreso. (No
hardeningofthecategoriesinthesepages.) Overtheyears,additionalguidance,
assistance,andfriendshipwereprovidedbymanypeople,includingBrianAhem,Dr.L.
BarryAlbright,BrianBeatty,JerryBond,Dr.J.DanielBryant,Dr.ThureCeding,Walter
Cressler,Dr.JasonCurtis,RuthAnnCzerenda,PhilD’Amo,Dr.DavidDilcher,Dr.Peter
Dodson,Dr.JimEhleringer,Dr.RobertEmry,Dr.RobertEvander,RobertFeranec,Marc
Frank,Dr.DavidFroelich,JeffGage,Dr.GinaGould,DianaHallman,PeterHallman,
Dr.F.GlynnHayes,Dr.GeorgeHecht,C.AndrewHemmings,Dr.PennylynnHiggins,
Dr.DavidHodell,CaseyHoliday,Dr.RobertHunt,SteveHutchens,SueHutchens,Dr.
PatHylton,Dr.Carlosludiea,KenethaJohnson,Dr.WalterJudd,Dr.DonaldKennedy,
IV
Dr.JohnKrigbaum,GregLabbe,Dr.W.DavidLambert,RussMcCarty,JulieMeachan,
MatthewC.Mihlbachler,TangelynMitchell,CyndiMoncrief,GaryMorgan,Irv
Quitmeyer,KarenPallone,Dr.DavidPhilips,JenniferPiascik,RogerPortell,Arthur
Poyer,Dr.DonaldR.Prothero,Dr.JonRieskind,DennisRuez,Dr.EricSargis,Dr.Bruce
J.Shockey,ErikaSimons,Dr.JoshB.Smith,MattSmith,Dr.JohnStorer,Dr.Frederick
S.Szalay,Dr.CarlTerranova,Dr.KentVliet,LaurieWalz,SkyeWhite,Dr.RonWolff.
I’dliketothanktheBiologicalSciencesProgramandDepartmentofZoologyforvarious
formsofsupport. Essentialaccessto,andloansfrom,collectionswereprovidedby
CharlesSchaffandDr.ParishA.JenkinsoftheMuseumofComparativeZoology,
HarvardUniversity;JohnAlexander,Dr.RichardTedford,andDr.JinMengofthe
AmericanMuseumofNaturalHistory,NewYork;RobertPurdyandDr.RobertEmryof
theNationalMuseumofNaturalHistory,Washington,D.C. Additionalfinancialsupport
wasprovidedbyaLucyDickinsonFellowshipfromtheDepartmentofVertebrate
Paleontology,FloridaMuseumofNaturalHistory,aGrinterFellowshipfromthe
DepartmentofZoologyandtheGraduateSchool,UF,anR.JerryBrittAwardfromthe
FLMNH,aGaryMorganAwardfromtheFloridaPaleontologicalSociety,aMitchell
HopeAwardfromtheSouthwestFloridaFossilClub,aTheodoreRooseveltAwardfrom
theAmericanMuseumofNaturalHistory,andtravelgrantsprovidedbytheDepartment
ofZoology,CollegeofLiberalArtsandSciences,GraduateSchool,andGraduate
StudentCounciloftheUniversityofFlorida.
V
TABLEOFCONTENTS
page
ACKNOWLEDGMENTS
iv
ABSTRACT
viii
CHAPTERS
1 SMALLHORSE,BIGQUESTIONS I
Background 1
AbbreviationsandConventionsUsedinThisStudy 8
2 ANEWSPECIESOFARCHAEOHIPPUS(MAMMALIA;EQUIDAE)FROMTHE
ARIKAREEANOFCENTRALFLORIDA 9
Introduetion 9
SystematiePaleontology 11
Discussion 24
Conclusions 30
3 POPULATIONDYNAMICSOFARCHAEOHIPPUSBLACKBERGI(MAMMALIA;
EQUIDAE)FROMTHEMIOCENETHOMASFARMFOSSILSITEOF
FLORIDA 31
Introduction 31
MaterialsandMethods 33
FirstReproductioninModemFemales 37
FirstReproductioninModemMales 39
Results 40
Discussion 44
EstimatesofFirstReproductioninFossils 44
AgeDependentMortality 46
NeonateMortalityEstimates 49
EstimatesofAverageandPotentialLongevity 50
PaleoenvironmentalReconstmctionoftheThomasFarmFossilSiteBasedon
InterpretationsofSocialStructure 51
PaleoenvironmentalReconstructionoftheThomasFarmFossilSiteBasedon
InterpretationsofDietaryPreferences 52
Conclusions 53
VI
4 STABLEISOTOPICANALYSISOFEVOLUTIONARYHETEROCHRONY,
BODYSIZEREDUCTION,ANDDIETARYSPECIALIZATIONIN
ARCHAEOHIPPUSBLACKBERGIYROUTHEMIOCENETHOMASFARM
FOSSILSITE 55
Introduction 55
HeterochronyAffectsMorphology,LifeHistory,andBehavior 58
StableIsotopesinToothEnamelArehiveEnvironmentalandBehavioralData...61
MaterialsandMethods 63
Results 65
Diseussion 69
PatternsofvariationinS'*0 69
EvaluatingDietaryDifferentiationinyl.blackbergiandP.leonensis 76
BodySizeandResoureePartitioning 76
Patternsofvariationin5*^C 78
Conclusions 82
5 CLADISTICANALYSISOFANCHITHEREHORSES 86
Introduction 86
NotesonTwoImportantTaxa 93
MaterialsandMethods 96
CharactersUsedinthisAnalysis 98
TaxaUsedinthisAnalysis 102
Results 103
Discussion 112
Conclusions 119
6 SUPPOSITIONSANDCONCLUSIONS 121
DwarfingandtheGulfCoastalPlain 122
Conclusions 127
APPENDIX-COMPARISONSOFKEYTAXA 131
ObservationsonthePostcranialSkeleton 138
ObservationsonSomeVeryEarlyFloridaEquids 140
ObservationsonMaterialattheAMNH 143
CommentsonMaterialsatUSNM 148
LISTOFREFERENCES 149
BIOGRAPHICALSKETCH 162
vii
.
AbstractofDissertationPresentedtotheGraduateSchool
oftheUniversityofFloridainPartialFulfillmentofthe
RequirementsfortheDegreeofDoctorofPhilosophy
PALEOBIOLOGYOFARCHAEOHIPPUS(MAMMALIA;EQUIDAE),
ATHREE-TOEDHORSEFROMTHEOLIGOCENE-MIOCENE
OFNORTHAMERICA
By
JayAlfredO’Sullivan
May2002
Chairman: Dr.BruceJ.MacFadden
MajorDepartment: Zoology
TheOligocene-Miocenethree-toedhorseArchaeohippus(Mammalia;Equidae)is
abrachydontbrowserthatexhibitsphyleticbodysizereductionandparahippinepedal
adaptations. ThisstudyprovidesasummaryofpreviousworkregardingArchaeohippus,
documentsanewspeciesofArchaeohippus,includesalifetableanalysisof
ArchaeohippusblackbergifromThomasFarm,presentsananalysisofstableisotopes
fromtoothenamelofthreefossilhorsetaxa,anddemonstratesphylogeneticrelationships
ofArchaeohippusinacladisticanalysis.
PreviousstudentsofmiddleCenozoicequidshavebeenpolarizedinthe
phylogeneticplacementofArchaeohippus Some,emphasizingtheimportanceofthe
primitivelow-crownedteeth,havealignedthisgenuswiththeanchitheressensustricto
suchasAnchitherium. Thosewhofocusedonthesharedderivedconditionofthemanus
andpeslinkedArchaeohippuswithParahippus. Anewspeciesfromthemiddle
Arikareeanistheoldestandsmallestknownmemberofthegenus. Itscheekteethhave
viii
relativelyadvancedcharacterssuchascrochets,additionalenamelplications,and
triangularhypostylesenclosingpostfossettes,anditsmanushasreducedsidedigitsand
elongatedmedialphalanges;thesecharacterslinkitwithprimitiveparahippines.
ThelifetableanalysisofthepopulationofArchaeohippusblackbergifromthe
earlyHemingfordianThomasFarmfossilsitedisplaysanage-dependentmortalityspike
intheyoungadultcohortthatmaybeduetomalecombat,andwhichdiffersfromthe
mortalityprofileofsympatricParahippusleonensisandapopulationofMiohippus
obliquidens,acloseoutgroupfromtheOligoceneofWyoming. Analysisofstable
isotopesofcarbonfromtoothenamelfailedtoelucidatedietarydifferentiationofthe
ThomasFarmequids. OxygenisotopesrevealedreducedontogenyinA.blackbergi
relativetoP.leonensisbutnotM.obliquidens. Acladisticanalysisofanchitheregrade
equidssupportedthemonophylyofthegenusArchaeohippusandplaceditwithinthe
primitiveparahippines.
ThisstudyprovidesinsightintothepaleobiologyofArchaeohippus,thesmall
browsingecomorphofthefirstgreatequidadaptiveradiation. Thisradiationproduceda
varietyofequidformsinresponsetointensiveglobalclimatechangethatresultedinthe
fragmentationofforestsandtheappearanceofextensivesavannaecosystemsacross
NorthAmerica.
IX
CHAPTER
1
SMALLHORSE,BIGQUESTIONS
Background
Althoughthefossilremainsarerelativelyfragmentary,thelateOligoceneand
earlyMioceneofFloridacontaintheequidgeneraMiohippus,Archaeohippus,
Anchitherium,andParahippus. Inabroaderphylogeneticcontext,thesehorsesare
importantbecausetheycontainanddefinemanyofthecharacterstatetransitionsthat
occurredbetweenadvancedanchitheriineandprimitiveequinehorses. Althoughmuch
previousresearchregardingArchaeohippushasemphasizeditsuniqueness,thegenusis
equallyinterestingforthosecharactersthatsuggestitsaffinities.
Theaffinities Archaeohippusareobscuredinpartbyacomplicatedtaxonomic
history. Specimensofthissmall,brachydontMiocenehorsewerefirstmentionedin
publicationbyCope(1886)fromtheearlyBarstovianMascallFaunaofOregon. Cope
namedthespeciesultimus,andassignedittoAnchitherium,agenusoflarge,tridactyl
Miocenehorseswithbrachydontteeth. Osborn(1910)placedbothAnchitheriumand
Archaeohippusinthegrosslyparaphyleticsubfamily"Anchitheriinae,"whatIwillrefer
toas"Anchitheriinae"sensulato(ASL). Osborn’s(1910)formulationofthissubfamily
alsoincludedMesohippus,Miohippus,Parahippus,andHypohippus,aswellasthe
EuropeanpalaeothereAnchilophus. Theinclusionofthepalaeothererendersthisconcept
ofthesubfamilypolyphyletic. InamorerecentlookatOsborn’sgrouping(MacFadden
1992),ASLisdefinedasthosehorseswithfullymolarizedP2-M3thatlackthedental