the pliocene pronghorn hexobelomeryx fricki (mammalia ... · hexobelomeryx fricki from the pliocene...

Hexobelomeryx fricki from the Pliocene of Guanajuato 533

ThePliocenepronghornHexobelomeryx fricki (Mammalia:Artiodactyla)fromSanMigueldeAllende,Guanajuato,centralMexico

Eduardo Jiménez-Hidalgo1,* and Oscar Carranza-Castañeda2

1 Laboratorio de Paleobiología, Instituto de Recursos, Campus Puerto Escondido, Universidad del Mar, Km. 2.5 Carretera Puerto Escondido-Oaxaca, Puerto Escondido, Oaxaca 71980, México. 2 Centro de Geociencias, Universidad Nacional Autónoma de México, Campus Juriquilla,

Juriquilla, Querétaro 76230, México.* [email protected]

ABSTRACT

Hexobelomeryx fricki is recorded in the San Miguel de Allende graben since the late early Hemphillian to the Blancan III, from about 7.0 million years to 3.0 million years. The studied specimens were recovered from floodplain and point bar deposits of the informal Rancho Viejo beds. The mortality profile of the H. fricki teeth sample from the study area is an attritional one, with few juveniles and a large majority of old individuals. The estimated body mass of H. fricki specimens range from around 10 to 30 kilograms, according to their age at death. The record of this pronghorn in Guanajuato during the late early Hemphillian and the Blancan III, are the oldest and the youngest occurrence of the species in North America and extends its geographic distribution from northwestern Mexico to central Mexico during the late Hemphillian.

Key words: Hexobelomeryx, Hemphillian, Blancan, Pliocene, San Miguel de Allende, Mexico.

RESUMEN

Hexobelomeryx fricki está presente en el graben de San Miguel de Allende desde la parte tardía del Henfiliano temprano hasta el Blancano III, desde hace aproximadamente 7.0 millones de años hasta hace 3.0 millones de años. Los ejemplares estudiados fueron recolectados de depósitos de llanura de inundación y barras de punta de la unidad informal capas Rancho Viejo. El perfil de mortalidad de la muestra de dientes de H. fricki del área de estudio es atricional, con pocos individuos jóvenes y una gran mayoría de individuos viejos. La masa corporal de los ejemplares de H. fricki tiene un rango entre 10 kilogramos y 30 kilogramos, dependiendo de su edad al momento de morir. El registro de este antilocáprido en Guanajuato durante la parte tardía del Henfiliano temprano y el Blancano III, representa la ocurrencia más antigua y más reciente de la especie en Norteamérica y extiende su distribución geográfica desde el noroeste de México hasta el centro del país durante el Henfiliano tardío.

Palabras clave: Hexobelomeryx, Henfiliano, Blancano, Plioceno, San Miguel de Allende, México.

Revista Mexicana de Ciencias Geológicas, v. 28, núm. 3, 2011, p. 533-543

Jiménez-Hidalgo,E.,Carranza-Castañeda,O.,2011,ThePliocenepronghornHexobelomeryx fricki(mammalia:artiodactyla)fromSanMigueldeAllende,Guanajuato,centralMéxico:RevistaMexicanadeCienciasGeológicas,v.28,núm.3,p.533-543.

Jiménez-Hidalgo and Carranza-Castañeda534

INTRODUCTION

Formorethan30years,Hexobelomeryx frickiFurlong1941 was the only antilocaprid identified in the Pliocene faunasofMexico.ThespecieswaserectedbaseduponspecimensfromthelatestHemphillianYepómera/RincónandMatachiclocalfaunasofChihuahua(Lindsayet al.,2006).

IncentralMexico,thespecimensHexobelomeryxsp.orHexobelomeryxcf.H. frickihavebeenreportedfromthelateearlyHemphillianofHidalgostate(Zietla-Tehuichilafauna)andthelateHemphillianofJaliscostate(TecolotlánandTeocaltichelocalfaunas)(Montellano-Ballesteros,1997;Carranza-Castañeda,2006).

AtSanMigueldeAllende,Guanajuato,DalquestandMooser(1980)assignedabundantlargeteethfromranchoElOcotetoAntilocapridaeindet.,indicatingthattheyarecloseinsizetothoseofH. fricki.Also,severalpapersmen-tionHexobelomeryxsp.,orHexobelomeryxcf.H. frickiintheHemphillianandBlancanlocalfaunasofSanMigueldeAllende(Carranza-CastañedaandFerrusquía-Villafranca,1978;MillerandCarranza-Castañeda,1984;Carranza-Castañeda,1989;Carranza-CastañedaandWalton,1992;MillerandCarranza-Castañeda,1998;Jiménez-Hidalgoet al.,2004;Carranza-Castañeda,2006),butuptodate,nospecimenhasbeenformallydescribed.

IntheUnitedStatesofAmericathereisarecordofcf.Hexobelomeryxsp.fromthelatestHemphillianofTexasandofHexobelomeryx?sp.fromthelateHemphillianofNevada(JanisandManning,1998).

OurstudyoftheantilocapridspecimensrecoveredfromHemphillianandBlancanlocalitiesoftheSanMigueldeAllendearea,allowustorecognizeseveralsize-groupsamongthediverseskeletalelementspresentinthecol-lection.Thesesize-groupsarenotrelatedtotheageofindividuals,butrepresentdifferentpronghornspecies(Jiménez-HidalgoandCarranza-Castañeda,2004).Oneofthesespecies–Capromeryxtauntonensis–waspreviouslydescribedfromtheBlancanlocalitiesofthestudyarea(Jiménez-Hidalgoet al.,2004).

Thepurposeofthispaperistodescribethelargestsize-groupofupperandlowerteethandmandiblesrecov-eredfromseveralHemphillianandBlancanlocalitiesofSanMigueldeAllendeandtodiscusssomepaleobiologi-calconsiderationsabouttheHexobelomeryxrecordfromGuanajuato,centralMexico.

GEOLOGIC SETTING AND GEOCHRONOLOGY

TheSanMigueldeAllendebasinislocatedinthenortheasternpartofGuanajuatostate,between20º55’-21º06’NorthLatitudeand100º50’-100º41’WestLongitudeW(Figure1),intheCentralMexicoHighlands.

Thestudiedspecimenswerecollectedfromclayandsiltytosandyclaywithvariablesand,gravelandtuffaceous

components, which were deposited as floodplain and point barsequencesoftheinformallithostratigraphicunitnamedRanchoViejobeds(Carranza-Castañedaet al.,1994;Flynnet al.,2005).

InthispaperweusedtheregisterednumbersoftheCatálogodeLocalidadesoftheColecciónNacionaldePaleontología,MuseoMa.CarmenPerrilliatM.,InstitutodeGeología,UniversidadNacionalAutónomadeMéxico.

ThirteenplaceswithHexobelomeryxfossilsarelo-catedbetween12and20kmnorthofSanMigueldeAllendecity (Figure 1). On five of those places, radiometric dates ormagnetostratigraphicanalysiswereobtainedbyotherauthors(Table1),whilstfortherest,thefaunalassociation

51

51

Blancan

Hemphillian

Tertiary volcanicrocks

Village

Fossillocality

Scale

2 km

San Miguel deAllende 20º 55’

21º 00’

100º 45’

354435453074

100º 50’

21º 05’

Studyarea

Guanajuato

RanchoViejo

Los Galván

3009

El Ocote

3521

N

3556

3566

3487

3510

3524

3546

3403

3564

Figure1.IndexmapandgeologicmapoftheSanMigueldeAllendeareashowingthelocalitieswheretheHexobelomeryxfrickispecimenswerecollected. Geologic map modified from Adams et al.(2006).


teetharerepresentedbyupperandlowercase:I/i(incisor),C/c(canine),P/p(premolar),M/m(molar),D/d(decidu-oustooth).Measurementabbreviationsare:L,length;W,width;kg,kilogram.Additionalabbreviationsinclude:Gto,Guanajuato;Ma,millionyears;n,samplesize;OR,observedrange; FLML, first lower molar length; LMRL, lower molar rowlength;Pad,Probableabsoluteageatdeath;SLMA,secondlowermolararea;UCMP,UniversityofCaliforniaMuseumofPaleontology;USA,UnitedStatesofAmerica;X,samplemean;yrs,years.

Giventhatthestageofwearstronglycorrelateswiththecharactersthatcanbeobservedonteethandthatalsoisrelatedwiththeobservedtoothcrownheight,itisveryimportanttocompareteethwiththesamestageofwear(Breyer,1977).

In this paper we propose the use of five tooth wear stages:I.Deciduouspremolarspresent,molartwoerupting.II.Permanentpremolarspresent,M1/m1slighttomoder-

atelyworn.III.Wornpremolars,M1/m1heavyworn,M3metastyleor

m3entostilidwithoutwear.IV.PermanentpremolarsandM2/m2moderatelyworn,M3

metastyleorm3entostilidslightlyworn.V.Heavywornmolars,M3metastyleorm3entostilidworn,

bothcrescentstendtofuse.Inordertoestimatetheprobableabsoluteageatdeath

oftheHexobelomeryxspecimensfromGuanajuato,weusedthetooth-wearscoringsystemproposedbyLubinski(2001)forAntilocapraamericana (Ord,1815),inwhichmandibu-larcheekteethareevaluatedforpresenceorabsenceoffossettids,dentinewearonridgesandcontinuousdentinelinks (see fig. 5 of Lubinski, 2001). The resulting scores werethencomparedwithtable7ofLubinski(2001)oftoothwear,knownageandestablished-agespecimens.

Ontheotherhand, thebodymassestimationsofstudiedspecimenswerebasedintheruminants’predic-tiveequationsofJanis(1990)forlowermolarrowlength(LMRL)andsecondlowermolararea(SLMA)aswellasthemultivariatebodymassdeterminationalgorithms7.2and7.3ofMendozaet al.(2006).

andstratigraphiccorrelationsareindicativeoftheageoftheselocalities(Flynnet al.,2005;Adamset al.,2006).

TheagesofalloftheselocalitiesspanfromthelateearlyHemphillian(Hh2)totheBlancanIII,fromaround7.0to3.0Ma(Kowalliset al.,1998;Flynnet al.,2005;Adamset al.,2006;Carranza-Castañeda,2006).

TheassociatedfaunarecoveredfromthesamebedwhereHexobelomeryxspecimenswerecollected,atlocality3545LaPresa,isassignedtothelateearlyHemphillian(Hh2),whiletheassociatedfaunaatlocality3521Coecillos,belongstolate(Hh3)andlatestHemphillian(Hh4).Theassociatedfaunaof locality3544RanchoSanMartín,locality3556Earthwatchandlocality3566Candado,areassignedtothelate(Hh3)andlatestHemphillian(Hh4).Forthesamereason,theassociatedfaunaoflocality3487Ferrocarril2,locality3510PerrosBravosandlocality3546CaraCara,isassignedtotheBlancanIII(Jiménez-Hidalgoet al.,2004;Bellet al.,2004;Carranza.Castañeda,2006andreferencestherein;Jiménez-HidalgoandCarranza-Castañeda2010).

TheHemphillianandBlancanassociatedfaunafromthesamebedswheretheHexobelomeryxmaterialwasrecoveredconsistsofdiversespeciesofxenarthrans,lago-morphs,rodents,ursids,canids,mustelidsandfelids;also,ghomphotherids,rhinocerosesequids,tayassuids,aprotoceratid,camelidsandsomeotherantilocapridspecieshavebeencollected(Carranza-CastañedaandMiller,2000;Carranza-Castañeda,2006;Jiménez-HidalgoandCarranza-Castañeda,2010).ThemammalianindexgeneraandspeciesofthebearinglocalitiesfromSanMigueldeAllendewerepreviouslypublishedbyCarranza-Castañeda(2006)andreferencestherein;so,theyarenotrepeatedhere.

MATERIALS AND METHODS

Thefossilmaterialdescribedinthispaperconsistsofisolatedupperandlowercheekteeth,somemaxillaryfragmentswithteethandramifragmentswithteeth.Theywerecollectedfromthesurface.

ThefossilswerepreparedatInstitutodeGeología,UniversidadNacionalAutónomadeMéxico(UNAM),MéxicoCity,usingstandardtechniques.ThespecimensarehousedattheColecciónNacionaldePaleontología,MuseoMa.CarmenPerrilliatM.,InstitutodeGeología,UniversidadNacionalAutónomadeMéxicowiththeac-ronymIGM.

Comparisonsof specimensweremadewith theparatypesanddrawnteethspecimensofFurlong(1941)ofHexobelomeryxfrickihousedattheNaturalHistoryMuseumofLosAngelesCounty(LACM[CIT]).

TheuseddentalnomenclatureisthatofGentryandHooker(1988).Themeasurementsofupperandlowerdentitionweretakenwithacaliperasmaximumlengthsandwidthsatocclusallevel(Janis,1990).Allmeasure-mentsareexpressedinmillimetres(mm).Upperandlower

Locality Radiometric date (Ma) Chron

3009ArroyolaCarreta 5.7±0.4to4.8±0.2 C3n.4n?-C3n.3n

3403LaRinconada 4.4±0.3 C3n.3n

3564ArroyoTepalcates C3n.2r–C3n.2norC3n.3r??-C3n.3n??

3074Arrastracaballos 3.3±0.13.6

C2An.3n

3524Porkchop MiddleC2An.3n

Table1.GeochronologyoftheSanMigueldeAllendelocalitiesthatbearHexobelomeryxfrickispecimens(DatafromFlynnet al.,2005;Adamset al.,2006).


SYSTEMATIC PALEONTOLOGY

SuborderRuminantiaScopoli,1777FamilyAntilocapridaeGray,1866

SubfamilyAntilocaprinaeGray,1866Genus HexobelomeryxFurlong,1941

Hexobelomeryx frickiFurlong, 1941Figures2and3;Tables2-4

Diagnosis.Three-tinedhornswithunequal,taperingtines;anteriortineshortestwithothertwomorenearlyequal;angleofdivergencebetweentineseithernearlyequalormiddleandposteriortinelessdivergentthananteriorandmiddle;teethhypsodont;superiorcheektoothseriesshort;m3withthreemajorlobesandasmallbutvariablefourthlobe;ramirelativelyshort,diastemashort,mandibledorsoventrallydeepatm3;P4withstrongstyles(Furlong,1941;JanisandManning,1998),andaP4medialribthatdisappearsastoothwearincreases.Description. Mandible. Thediastemalcrestissharpanditsventralborderisalmoststraight(Figure2).Thedepthofthe horizontal ramus increases significantly below the m2 andm3anditsdepthdecreasesrapidlytowardthediastema(Figure2).Themassetericridgeiswelldevelopedandtheangularprocessisconvex.

Lower dentition.Thecompletecheekteethseriesisp2-m3,butinlessthan9%oftheSanMigueldeAllendesamplethereisp2alveolus;inmostofmandiblesthereisnotsignofp2oritsalveolus.

Thep3-p4lengthpercentregardingthem1-m3lengthisaround26%to30%dependingontheteethwearstage(Tables2and3).

InthespecimenswithtoothwearstageI,thedp3isabout1/3largerthanitspermanentcounterpart.Inthelin-gualportionofthedp3therearetwovalleysthatdelimitateawidthanteriorenamelfold,amiddleenamelfoldandawell-developedposteriorenamelfold.Thedp4isworn,ithasthreecrescentsofwhichthecaudalisthelarger;aposteriorstylidismoderatelydeveloped(Figures2.1and2.2).

Them1,inastageofwearI,havelittlewornfosset-tids,themetaconidandentoconidareacute,theparastylidandentostylidarewelldevelopedbutthemetastylidisnotdeveloped.Them2isunworn,thefossettidsaredeepandtheparastylidandentostylidarewelldeveloped(Figures2.1and2.2).

TheteethofmandibleIGM9383showawearstageII.Itsp4isbrokenanteriorly,so,itisonlypossibletoobservethatissomewhatworn;them1hasawell-developedento-stylid.Them2islittleworn,theparastylidandentostylidarewelldeveloped.Them3iserupting,hasdeepfossettids,themetaconidandentoconidareacuteandtheparastylidisslightlyworn(Figures2.3and2.4).

Thep3,inwearstageIII,hastwovalleysthatseparatethewornstylids,theanterioroneiswiderthantheposterior,andpersistwithadditionalwear.Intheposteriolabialpart

thereisawidevalleydelimitinganenamelfold,whichdis-appearswithadditionalwear.Thep4iscompressed,short,theparaconidisverywelldevelopedandonitscaudalpartthereisadeepfossetid(Figures2.5and2.6).

Them1inwearstageIII,hasverywornfossettids,themetaconidandentoconidhavelittlewear,theparastylidandentostylidareslightlydeveloped.Theanteriorcrescentisslightlysmallerthantheposteriorone.Them2hasmoder-atelywornfossettids,themetaconidandentoconidareacuteandtheparastylidandentostylidaremoderatelydeveloped.Them3haveslightlywornfossettids,themetaconidandentoconidareacuteandtheparastylidiswelldeveloped(Figures2.5and2.6).

TheteethinwearstageIVaresimilartothoseinwearstageIII,buttheparastylidofp4iswideranditsposteriorfossettidisshallower.Inthem1theparastylidstillispres-entbuttheotherstylidsarelost,andbothfossettidshavedisappeared.Inthem2thefossettidsareseverelywornandtheentostylidisslightlydeveloped.Thefossettidsofm3aremoderatelyworn,beingthatofthehypoconulidthemost-wornonewhencomparedwiththeothertwo;theparastylidismoderatelytowelldevelopedandtheotherstylidsarelost(Figures2.7and2.8).

Finally,inwearstageV,thepremolarshaveatrian-gular-to-ovoidoutline;themolarsareseverelyworn,theyhavelostthefossetidsandthestylidsareslightlyornotdeveloped(Figures2.9-2.12).

Upper dentition.LargepartoftheupperteethsamplefromSanMigueldeAllendeshowsawearstageIVorV(Figure 3), and a few can be classified in stage of wear II or III.Also,therearenoupperteethwithawearstageI.

InwearstageIItheanteriorfossetteofM1iswellwornandtheposterioroneismoderatelyworn;theparaconeandmetaconearefairlyacute;theparastyleislittledevelopedbutthemesostyleisprominent;theanteriorcrescentissmallerthantheposteriorcrescent.

IntheM1withawearstageIIItheanteriorcrescentissmallerthantheposterioroneandthemesostyleisstillpresent.TheM2withthiswearstagehavemoderatelywornfossettes,theparaconeandmetaconeareacute,themesostyleandparastyleareprominentwhilethemetastyleispoorlydevelopedandthereisalittledevelopedanteriorrib.TheM3havelittlewornanddeepfossettes,thepara-coneandmetaconeareacute,theparaconeandmetaconeareacute,thestylesaremoderatelydeveloped;thereisamoderatelydevelopedanteriorrib.

InwearstageIV,theP3hasahalfmoonoutline,lackthefossette,theparastyleiswelldevelopedandthemetastyleisstronglydeveloped;betweenthemthereisarib.TheP4alsohasahalfmoonshape,itlacksthefossette;theparastyleisslightlydeveloped,themetastyleiswellde-veloped;betweenthestylethereisamoderatelydevelopedrib(Figures3.1-3.4).

TheM1inwearstageIVdoesnothavefossettes;theparastyleandmetastyleareabsent,themesostyleispromi-nentandtheocclusalsurfaceisacutewiththemesostyleas


thehigherstructure.TheanteriorcrescentofM1issmallerthantheposteriorone.TheanteriorfossetteofM2inwearstageIVisheavilywornorabsentandtheposterioroneisheavilyworn,theparastyleismoderatelyorslightly

developed,themesostyleisprominent,themetastyleismoderatelyorslightlydevelopedandthelittledevelopedanteriorribstillispresent.ThefossettesoftheM3aremoderatelyworn,theparaconeandmetaconearefairly

Figure2.MandiblesandlowerdentitionofHexobelomeryxfrickishowingthetoothstageofweardisplayedbytheSanMigueldeAllendespecimens.IGM9409,leftmandiblefragmentwithdp3-m3intoothwearstageI,1.Lingualview,2.Occlusalview.IGM9383,leftmandiblefragmentwithpartialp4-m3intoothwearstageII,3.Lingualview,4.Occlusalview.IGM9433,rightmandiblefragmentwithp3-m3intoothwearstageIII,5.Lingualview,6.Occlusalview.IGM9376,rightmandiblefragmentwithp3-m3intoothwearstageIV,7.Lingualview,8.Occlusalview.IGM9434,rightmandiblefragmentwithp3-m3intoothwearstageV.9.Lingualview,10.Occlusalview.IGM9427,rightmandiblefragmentwithverywornp3-m3intoothwearstageV.11.Lingualview,12.Occlusalview.Scalebarsequal30mm.


acute,theparastyleisprominentwhilethemesostyleandmetastylearemoderatelydeveloped;theanteriorribislittledeveloped(Figures3.5–3.10).

Finally,inwearstageV,theP2hasanovoidoutline(L=4.7mm,W=4.0mm),theP3hasamoderatelyde-velopedmesostyleandrib;itsparastyleisabsent.TheP4lacksthestyleandrib.WithadditionalweartheP3andP4showatriangular-to-ovoidoutline.IntheM1andM2themesostylepersistsbuttheotherstylesareabsent,theanteriorandposteriorcrescentstendtofuse.TheM3lostthefossettes,themetastyleiswidercomparedwithpreviousstagesofwear,formingawelldevelopedheelthattendstofusewiththeposteriorcrescentwithadditionalwear(Figures3.11-3.16).Material examined.Locality3009arroyoLaCarreta:IGM9363,rightM2;IGM9364,rightp4;IGM9365,rightM1;IGM9366andIGM9367,leftM2;IGM9368rightM2;IGM9369,rightm2;IGM9370,leftm3;IGM9371,M2;IGM9372,leftM2;IGM9373,rightm2;IGM9374,leftm2;IGM9375,leftM1.Locality3074Arrastracaballos:IGM9376,rightmandiblefragmentwithp3-m3andthep2alveolus;IGM9377,rightm3;IGM9378andIGM9379leftM3.Locality3521Coecillos:IGM9380rightM1;IGM9381,leftM3;IGM9382-1,rightmandiblefragmentwithp3,dp4andm1andIGM9382-2,leftmandiblefragmentwithp3,dp4,m1anderuptingm2ofoneindividual;IGM9383,leftmandiblefragmentwiththeposteriorpartofp4-m3(erupting);IGM9384,leftmandiblefragmentwithm1-m3;IGM9385-1,leftmandiblefragmentwithm2-m3,IGM9385-2,rightmandiblewithi3,c-m3;IGM9385-3,P2-P4series;IGM9385-4,leftM2andIGM9385-5,leftM3;IGM9385-6,rightserieswithP4-M2;IGM9385-7,rightM3,allfromasingleindividual;IGM9386,rightmandiblewithp4-m3andthealveoliforp2andp3;IGM9387,IGM9388,IGM9389,IGM9390,leftm1;IGM9391,

rightm2;IGM9392,IGM9393,leftm2;IGM9394,IGM9395,IGM9396,leftm3;IGM9397,IGM9398,rightm3;IGM9399,rightM1;IGM9400,leftM1;IGM9401,leftP4;IGM9402,leftM1;IGM9403andIGM9404leftM2;IGM9405,IGM9406,IGM9407,leftM3;IGM9408,rightm3;IGM9409,leftmandiblefragmentwithdp3-m2(erupting).Locality3487Ferrocarril2:IGM9410leftm2;IGM9411,leftm3;IGM9412,leftM1;IGM9413,rightM2;IGM9414leftM2.Locality3510PerrosBravos:IGM9415,rightp4;IGM9416,leftP3andIGM9417leftP4.Locality3544ranchoSanMartín:IGM9418,rightmandiblefragmentwithp3-m2;IGM9419,rightM3.Locality3403LaRinconada: IGM9420,rightm2;IGM9421,leftM1;IGM9422,rightM1;IGM9423,rightM2;IGM9424andIGM9425,leftM2.Locality3545LaPresa:IGM9426,leftm3.Locality3524Porkchop:IGM9427,rightmandiblefragmentwithp3-m3.Locality3546CaraCara:IGM9428,leftm2;IGM9429-1,leftP3andIGM9429-2,leftP4(bothofasingleindividual);IGM9430,leftM1;IGM9431,right

Figure3.UpperdentitionofHexobelomeryxfrickifromSanMigueldeAllende,Guanajuato.IGM9429-1,leftP3instageofwearIV,1.Occlusalview,2.Labialview.IGM9429-2,leftP4instageofwearIV,3.Occlusalview,4.Labialview.IGM9430,leftM1instageofwearIV,5.Occlusalview,6.Labialview.IGM9413,rigthM2instageofwearIV,7.Occlusalview,8.Labialview.IGM9405,leftM3instageofwearIV,9.Occlusalview,10.Labialview.IGM9385-3,leftP2-P4instageofwearV,11.Occlusalview,12.Labialview.IGM9385-6,P4-M2instageofwearV,13.Occlusalview,14.Labialview.IGM9385-5andIGM9385-4leftM2andM3instageofwearV,15.Occlusalview,16.Labialview.Scalebarsequal10mm.

Tooth Measurement n OR X

p3 LW

66

5.0–6.32.8–3.9

5.723.1

p4 LW

1010

5.6–8.33.0–5.6

6.923.97

m1 LW

1616

8.1–11.74.8–6.6

10.135.72

m2 LW

2525

10–14.53.7–7.7

12.266.51

m3 LW

2323

13.7–23.84.3–7.3

20.46.54

Table2.MeasurementsoflowerteethofHexobelomeryxfrickifromSanMigueldeAllende,Guanajuato,centralMexico.


entostylidwiththesecondcrescent.Also,inthespecimensfromGuanajuatoandChihuahuathemandibleisdeepandthediastemalcrestissharp.

Regardingtheupperdentition,thepremolarsfromSanMigueldeAllendeinwearstageIVhaveabetterdevel-opedmetastyleandribthanthoseofCIT2781(paratype),amaxillaryfragmentwithP3-M3inwearstageV;butinIGM9385-3,apremolarseriesinwearstageV,thestyleshavedisappearedandthepremolarshaveanovoidoutline,asisobservedinCIT2781.

TheM1andM2fromSanMigueldeAllendeinwearstageVandtheChihuahuaspecimenlackfossettes,haveamesostylebuttheylacktheothertwostyles;theanteriorcrescentofthesemolarsissmallerthantheposteriorone,especiallyintheM1.TheM3inwearstagesIVandVsharewiththeYepómeraspecimenawelldevelopedparastyleandamoderatelydevelopedmesostyle;alsointheM3withVwearstagethemetastyleformsawelldevelopedheelthatfuseswiththesecondcrescent,asseenintheM3ofCIT2781.

Additionally, thedepthof thestudiedmandiblesbelowthem3iswithintheobservedrange(30–37mm)inHexobelomeryxfricki(Table3)asisthediastemalength(42–51mm)(Furlong,1941).Also,theteethdimensionsarewithintherangereportedforthespecies(Table2and4),asisthep3-p4meanlengthpercent(26%to30%)regardingthem1-m3meanlength(Tables2and3).

InhisdescriptionofHexoblomeryx fricki,Furlong(1941)mentionsvariousfeaturesthatarepresentintheSanMigueldeAllendespecimens,suchasawelldevelopedmassetericridgeinrami,ashortdiastema,hypsodontteeth,shortupperteethseries,anabsenceofmetastylidinlowermolars,anentostylidthatformsatalonidinwellwornm3’s,smallupperpremolarswhencomparedtouppermolars,apersistentmesostyleonuppermolarsandawelldevelopedheelthatisformedbythemetastyleinwellwornM3’s.

Alloftheabovementionedcharacters,aswellastheclosedimensionsobservedbetweentheGuanajuatospecimens and those of Chihuahua, allow the confident classification of specimens from San Miguel de Allende, Guanajuato,asHexobelomeryxfricki.

M1;IGM9432,leftM2.Locality3556Earthwatch:IGM9433,rightmandiblefragmentwithp3-m3andthealveolusforp2.Locality3564arroyoTepalcates:IGM9434,rightmandiblewithp3-m2andthealveolusforp2;IGM9435,leftm2.Locality3566Candado:IGM9436,leftmandiblefragmentwithm1-m3.Distribution.LatestHemphillian(Hh4)ofChihuahua(Furlong,1941;Lindsayet al.,2006);lateearlyHemphilliantolatestHemphillian(Hh2toHh4),BlancanIandBlancanIIIofSanMigueldeAllende,Guanajuato.Discussion.ThestudiedspecimensfromtheHemphillianandBlancanofSanMigueldeAllendeshowaclosemor-phologyanddimensionswhencomparedtothefossilma-terialofHexobelomeryxfrickifromtheYepómera/Rincónfauna.

WhencomparingthemandiblefragmentIGM9433andtheisolatedlowermolarsIGM9410,IGM9411andIGM9373,allinwearstageIIIwithCIT2787(paratype),amandiblefragmentwithp3-m3,alsoinwearstageIII,itisevidentthattheyshareasharpdiastemalcrestandaverydeepmandiblebelowm2andm3;theyhaveveryhypsodontteeth;thep3sharesawide,anteriorvalley.Theyalsoshareap4withawelldevelopedparaconid;theyshowashortp3andp4comparedtothemolars;theparastylidandentostylidofthem1arelittledeveloped;theparastylidandentostylidofthem2aremoderatelydeveloped,andthem3haveawelldevelopedparastylidandacutemetaconidandentoconid.

ThemandiblefragmentswithteethandisolatedlowermolarsinwearstageIandIIofGuanajuato,havemoderatelytowelldevelopedstylids,asarethosewithslightwearm2andm3molarsofCIT2787.

ThespecimenCIT2793,amandiblefragmentwithatoothwearstageVandtheSanMigueldeAllendespeci-mensIGM9384,IGM9386IGM9434andIGM9436,shareatriangularoutlineofpremolarswithoutanytraceoffossettids, a lack of stylids in the molars, a flat lingual wall onm1,aslightlydevelopedentostylidinm2,andm3withroundedwornentostylid;themandiblesfromChihuahuaandGuanajuatoarealsodeepbelowm2andm3.TheisolatedlowermolarsinwearstageValsoshowasimilardegreeofstylidsdevelopmenttothoseofteethseeninCIT2793.

ThemandiblefragmentsIGM9427,IGM9385-2andCIT2799(paratype)aremorewornthanCIT2793orIGM9434,theyshowaratherovoidoutlineofthemolarsbe-causeofthefusionofanteriorandposteriorcrescents,theysharetheabsenceofmolarstylidsandafusionofthem3

Length alveolusp2–m3

Lengthp3–m3

Lengthm1–m3

Depth of mandible below m3

n 4 5 9 8OR 52.8–63.8 51.3–56.0 31.6–44.2 30.3–37

X 58.2 54.7 40.48 34.91

Table3.MeasurementsofmandiblesofHexobelomeryxfrickifromSanMigueldeAllende,Guanajuato,centralMexico.

Tooth Measurement n OR X

P3 LW

33

5.0-5.44.5-5.0

5.154.67

P4 LW

44

6.4-7.96.0-6.9

6.866.28

M1 LW

1212

7.1-13.07.6-9.4

10.08.7

M2 LW

1515

10.9-169.2-11.0

13.7610.26

M3 LW

99

15.0-18.48.3-9.9

16.689.12

Table4.MeasurementsofupperteethofHexobelomeryxfrickifromSanMigueldeAllende,Guanajuato,centralMexico.


IntheirdiagnosisofHexobelomeryx fricki,JanisandManning(1998)mentiontheabsenceofamiddleribintheP4;nevertheless,intheSanMigueldeAllendespecimensthisribispresentinthepremolarswithawearstageIV,disappearinginawearstageV.So,thepresenceorabsenceofthemiddleribdependsontheP4wearstage.

AftercomparingtheSanMigueldeAllendespecimenswithUCMP3978,aleftmandibleofTetrameryxwithp2-m3,themorphologicaldifferencesareevident,sincethemandiblefromCaliforniaisshallow,itsdiastemaislong,theirpremolarsarelargewhencomparedwiththemolars,itsp2isalmostaslargeasp3,andthewornp4isbilobatedwithawelldevelopedanteriorfossettid.

ThestudiedspecimensdifferfromtheHemphillianTexocerosintheirlargerteethdimensions,higherhypso-donty,largerp3-p4meanlengthpercentwhencomparedwiththemeanm1-m3length,andshallowermandibles(Hesse,1935); theyalsodiffer fromtheHemphillianSubantilocapraandtheBlancanCapromeryxintheirmuchlargerteethdimensions,hypsodontyandthelessdevelopedstyles(Webb,1973;MorganandMorgan,1995;Jiménez-Hidalgoet al.,2004).

PALEOBIOLOGICAL CONSIDERATIONS

Age at death distribution in the Hexobelomeryx fricki sample

We estimated the probable absolute age ofHexobelomeryxfrickifromSanMigueldeAllende,atthethetimeoftheirdeath,inordertoobtainanapproximationof their mortality profile. We used the wear-scoring system proposedbyLubinski(2001)forAntilocapraamericana.

ThepronghornantelopeA.americanaistheonlysur-vivingspeciesoftheFamilyAntilocapridae,soweconsideritasagoodproxyofthelifehistoryofH. frickisinceitisitsclosestlivingrelative.

Thelowercheekteethrepresent51.8%oftheentiresample.Sincetheupperteethshowthesamestageofwear-patterndistribution(withfewspecimenswithastageofwearII-IVandagreatmajoritywithastageofwearV),weconsidered that the mortality profile displayed by the lower teeth, adequately reflects that of the entire sample.

Table5andFigure4showthatlessthan10%ofindividualsareyoungerthanoneyear,fewerarearoundoneyear;lessthan15%arearound3.3–4.3yearsandthebest-representedageclassisthatofmorethannineyears,representingmorethan60%ofthesample.ThispatternofseveralyoungindividualsandalargemajorityofoldindividualsisalsoobservedifwedividethesampleinaHemphilliansetandaBlancanset,soforthesakeofsim-plicitywetreatedthemtogether.

Two kinds of idealized mortality profiles characterize mammalianpopulations:catastrophicandattritional.Inthecatastrophicprofile theyoungindividualsare the

mostabundantandthesuccessiveagegroupscontainprogressivelyfewerindividuals,whereasintheattritionalprofiletheveryyoungandoldindividualsarethebestrepresented,whileprime-ageadultsarerare(Klein,1982;Steele,2003).

The San Miguel de Allende mortality profile of H.frickishowsa“J”shape,somewhatsimilartotheidealizedattritional profile, but with an impoverishment of very young individualsandamuchhigherproportionofoldindividuals(Figure5).

Thisunderrepresentationofveryyoungindividualsinthestudiedsamplecanbeexplainedtakingintoaccountthatjuvenilebonesanddeciduousteetharemoresusceptibletocarnivoreravaging,compaction,fragmentationandsoilleachingthanadultspecimens(Steele,2003,2004),so,theyhaveahigherprobabilityofbeingdestroyedthanthoseofadultspecimens.

Ontheotherhand, theoverrepresentationofoldindividuals(>9years)inthesamplewhencomparedwiththeidealizedattritionalmortalitymodel,suggeststhattheindividualsofH. frickifromGuanajuatowouldhavehad

Pad (yrs) tooth wear stage % individuals

0.7–0.8 I 9.31.3–1.4 II 2.32

3.3 III 9.33.5–4.3 IV 11.62

>9 V 65.1

Table5.ProbableabsoluteageatdeathoftheHexobelomeryxfrickiindividualsfromthePlioceneofSanMigueldeAllende,Guanajuato,centralMexico.

60

45

30

15

0

0 1.5 3.0 4.5 6.0 7.5 >9.0

%in

divi

dual

s

Age (yrs)Figure4.DistributionofprobableageatdeathoftheHexobelomeryxfrickisamplefromSanMigueldeAllende,Guanajuato,centralMexico.


a great expectancy of life after surviving the first years (therearenotindividualsrepresentedinthe4.5–7.5yearsrank)(Figure5),takingintoaccountthatthelifespanofA.americanareachesmorethannineyears(Howard,1995).Whentheindividualsbecomeveryold,probablytheynatu-rallydiedormaybepredatorshuntedthem,giventhattheybecomemorevulnerable;afterwardstheywereincorporatedinthefossilrecord.

Body mass of Hexobelomeryx fricki from Guanajuato

Weestimatedtheprobablebodymassofseveralspecimenswithineachstageoftoothweartohaveanap-proximationtobodymassbetweenthedifferentagesatdeathrepresentedinthesample.

TheestimatedvaluesinTable6donotshowanyten-dencytowardstheincrementordiminutionofbodymassastoothwearstageadvanced.Thissuggeststhatsinceearlyintheontogeny,theteethdimensionsofHexobelomeryx reflect thebodymassofadults.So,torecoverthebodymassofjuvenilesandyoungadultsitwillbenecessarytousecranialand mandible data, which reflect the change in dimensions duringtheontogeny,butatpresent,suchvariablesareun-availableintheSanMigueldeAllendesample.

ThelowestbodymassvaluesarefromtoothstageofwearIIandnottoothstageofwearI(Table6).IntoothwearstagesIIItoVthereissomeoverlapbetweentherangesoftheestimatedbodymasses.Thevaluesderivedfromthe

LMRLequationshowthewidestrange(around6kg).Thelowestbodymassvaluesobservedintoothstage

ofwearVfortheLMRLandSLMAequations(12.73kgand13.57kg),couldresultfromthesmalldimensionsoftheveryheavy-wornmolarsofsomespecimens.Giventhat both equations are univariate, the influence of these teethvaluesishigherthaninthemultivariatealgorithmsofMendozaet al.(2006),wheresomeoftheseteethvaluescanbebufferedwiththeothervariables,diminishingtheobservedbodymassvaluerange(Table6).

Ontheotherhand,somevaluesaresimilarindiffer-enttooth-wearstages(Table6)andconsistently,thehigherbodymassestimationswereobtainedfromthebodymassdeterminationalgorithmsofMendozaet al.(2006).

IfasinAntilocapraamericana,Hexobelomeryxfrickireachesitsadulthoodbetween1.5–2.0years(Howard,1995),thebodymassofadultsintheSanMiguelsamplewasreachedintoothwearstageIIorIII(Table6).Forcom-parison,thebodymassofAntilocapraamericanainTexashasameanof40–41kgandof50kginAlberta,varyingseasonallyfrom47to70kg(O’Gara,1978).

Geographic distribution and biochronology

Hexobelomeryxseemstobeanendemicmexicanprong-horn(Figure5),withrecordsinthelateearlyHemphillianofHidalgoandthelateHemphillianofJalisco,aswellasthelatestHemphillianofChihuahua(Carranza-Castañeda,1994;Montellano-Ballesteros,1997;Carranza-CastañedaandMiller,2000;MillerandCarranza-Castañeda,2001,2002).InSanMigueldeAllende,HexobelomeryxfrickiispresentfromthelateearlyHemphillian(Hh2)toBlancanIII,from7.0to3.0Ma(Table1).ItsrecordinthelateearlyHemphillianandtheBlancanIIIofGuanajuatoistheoldestoneandtheyoungestocurrenceinNorthAmerica.

TherearetwodoubtfullateandlatestHemphillianrecordsofthegenusinTexasandNevada,southwesternNorth America, that if confirmed as Hexobelomeryxwouldextenditsgeographicrangetowardsthenorth(Figure5).

After thelatestHemphillianextinctionevent,H.

??

1

23 4

Figure5.GeographicdistributioninNorthAmericaofHexobelomeryxduringtheHemphillianlandmammalage.1.YepómeraandMatachiclocalitiesofChihuahua,2.Tecolotlánlocality,stateofJalisco,3.SanMigueldeAllende,stateofGuanajuato,4.Zietla-Tehuichilalocality,stateofHidalgo.?denotesthepossiblerecordofthisgenusinUSA.

Tooth wear stage

Janis (1990) Mendoza et al. (2006)LMRL SLMA Algorithm7.2 Algorithm7.3

I 19.27* 16.09 -- --II+ 10 10.75 14.89 30.1III 18.8 18.0–18.45 25.41 29.42IV 18.13 15.3–18.49 25.51 29.6V 12.73-19.29 13.57–17.1 24.3-27.64 26.8-29.51

Table6.EstimatedbodymassofHexobelomeryxfrickifromthePlioceneofSanMigueldeAllende,Guanajuato,centralMexico.Inthistoothstageofwearthebodyvalueswereobtainedfromoneindividual.*ValueobtainedwithequationforFLMLofJanis(1990).


frickionlysurvivedduringtheBlancanlandmammalageincentralMexico,intheSanMigueldeAllendearea.JanisandManning(1998)andDavis(2007)wronglyrecordH. frickifromtheBlancanofChihuahua,becausetheMatachiclocalityisofHemphillianage(Lindsayet al.,2006).

CONCLUSIONS

ThepresenceofHexobelomeryx fricki intheSanMigueldeAllendegrabenisestablishedbasedupondescrip-tionandcomparisonofteethandmandiblespecimenswiththeparatypesofthispronghornspeciesfromtheYepómera/RincónfaunaofChihuahua.

The mortality profile of the H. frickitoothsamplefromthestudyareaisanattritionalone,withfewjuvenilesandalargemajorityofoldindividuals.

ThebodymassestimationsbasedontheSanMigueldeAllendespecimensindicatethattheH. frickiindividualshadabodymassthatrangedfrom10to30kgaccordingtotheirageatdeath.

HexobelomeryxfrickiwaspresentintheSanMigueldeAllendegraben,stateofGuanajuatosincethelateearlyHemphilliantotheBlancanIII.ItsrecordinGuanajuatoextendsthegeographicdistributionofthespeciesfromnorthwesternMexicotocentralMexicoduringtheLateHemphillian.

ThepresenceofH. frickiinlateearlyHemphillianagesedimentsandintheBlancanagedepositsofSanMigueldeAllenderepresentsitsoldestandyoungestoccurrenceinNorthAmerica,respectively.

ACKNOWLEDGEMENTS

WethankX.WangandJ.TsengoftheNaturalHistoryMuseumofLosAngelesCountyforprovidingthecalibratedphotographsofH. frickifromYepómera,ChihuahuaandtoR.WhiteoftheInternationalWildlifeMuseumforthecalibratedphotographsofTetrameryx.WeacknowledgeV.Bravo-Cuevasandananonymousreviewerfortheirvalu-ablecomments,whichhelpedtoimprovethispaper.WearegratefultoG.Ruelas-InzunzaforreviewingtheEnglishversionofthetext.ThanktoPAPIITprojectIN106307andEarthwatchvolunteersoftheMexicanMegafaunaProjectforcollectingpartofthefossilmaterialdescribedinthispaper.EJHisindebtedtoCONACYTandDGEP,UNAMforthePh.D.scholarships.

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Manuscriptreceived:January7,2011Correctedmanuscriptreceived:June19,2011Manuscriptaccepted:June20,2011

the pliocene pronghorn hexobelomeryx fricki (mammalia ... · hexobelomeryx fricki from the pliocene...

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