the pliocene pronghorn hexobelomeryx fricki (mammalia ...dec 14, 2008 · hexobelomeryx fricki from...
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Hexobelomeryx fricki from the Pliocene of Guanajuato 533
EN PRENSA
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.,OscarCarranza-Castañeda,O.,2011,ThePliocenepronghornHexobelomeryx fricki(mammalia:artiodactyla)fromSanMigueldeAllende,Guanajuato,centralMéxico:RevistaMexicanadeCienciasGeológicas,v.28,núm.3,p.533-543.
Publicado en línea el 26 de octubre de 2011Disponible en http://satori.geociencias.unam.mx/
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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).
Hexobelomeryx fricki from the Pliocene of Guanajuato 535
EN PRENSAC/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,atlocal-ity3545LaPresa,isassignedtothelateearlyHemphillian(Hh2),whiletheassociatedfaunaatlocality3521Coecillos,belongstolate(Hh3)andlatestHemphillian(Hh4).Theas-sociatedfaunaoflocality3544RanchoSanMartín,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).Upperandlowerteetharerepresentedbyupperandlowercase:I/i(incisor),
Locality Radiometricdate(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).
Jiménez-Hidalgo and Carranza-Castañeda536
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SYSTEMATIC PALEONTOLOGY
SuborderRuminantiaScopoli,1777FamilyAntilocapridaeGray,1866
SubfamilyAntilocaprinaeGray,1866
Hexobelomeryx Furlong, 1941Hexobelomeryx frickiFurlong, 1941
Figures2and3;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
Hexobelomeryx fricki from the Pliocene of Guanajuato 537
EN PRENSA
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.
Jiménez-Hidalgo and Carranza-Castañeda538
EN PRENSA
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.
Hexobelomeryx fricki from the Pliocene of Guanajuato 539
EN PRENSAentostylidwiththesecondcrescent.Also,inthespecimens
fromGuanajuatoandChihuahuathemandibleisdeepandthediastemalcrestissharp.
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
Lengthalveolusp2–m3
Lengthp3–m3
Lengthm1–m3
Depthofmandiblebelowm3
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.
Jiménez-Hidalgo and Carranza-Castañeda540
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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) toothwearstage %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.
Hexobelomeryx fricki from the Pliocene of Guanajuato 541
EN PRENSA
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.
Toothwearstage
Janis(1990) Mendozaet 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)
Jiménez-Hidalgo and Carranza-Castañeda542
EN PRENSA
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