Extinct ground sloth dermal bones and their role in the ... · Extinct ground sloth dermal bones,...

Extinct ground sloth dermal bones, Central Patagonia, Chile 519

ABSTRACT

The results of the taphonomic analysis of extinct ground sloth dermal bones from the Baño Nuevo-1 archaeological site (Andean Central Patagonia, Chile) are presented. These pieces were recovered on all but one of the stratigraphic layers, including both the deepest levels (dated between 13,500 and 10,500 BP) and the most recent one (dated to the last century) and thus reveal complex formation processes. Since it is important to take into account this all too common depositional complexity, particularly when dealing with cave and rock shelter environments and with extinct fauna, we study the abundant ground sloth dermal bones found at the Baño Nuevo-1 cave. This effort is oriented to “make the most” (extract as much information as possible) from a single material, that is particularly abundant at this site and we believe it is especially suited to answer taphonomic questions.

Key words: Dermal bones, Extinct Ground Sloth, Taphonomy, Central Patagonia, Baño Nuevo-1 cave, Chile.

RESUMEN

Se presentan los resultados del análisis tafonómico de huesos dérmicos de perezosos terrestres extintos del sitio arqueológico y paleontológico Baño Nuevo-1 (Patagonia Central, Chile). Estos restos fueron recuperados tanto en los niveles más profundos (datados entre 13,500 y 10,500 AP) como en el más reciente (datado en el siglo recién pasado), lo que revela complejos procesos de formación. Debido a la relevancia de los procesos de formación de los depósitos estratigráficos, especialmente en refugios rocosos con restos de fauna extinta, el presente trabajo estudia los huesos dérmicos de perezosos extintos del sitio cueva Baño Nuevo-1. Este esfuerzo está orientado a extraer la mayor cantidad de información posible de estos restos óseos que son abundantes en el sitio y que creemos son ideales para responder a preguntas tafonómicas.

Palabras claves: Huesos dérmicos, Perezoso extinto, Tafonomía, Patagonia Central, Baño Nuevo-1, Chile.

Extinctgroundslothdermalbonesandtheirroleinthetaphonomicresearchofcaves:thecaseofBañoNuevo-1

(AndeanCentralPatagonia,Chile)

Patricio López-Mendoza1,* and Francisco Mena-Larraín2

1 Universidad Católica del Norte, Instituto de Investigaciones Arqueológicas y Museo Gustavo Le Paige (IIAM), Calle Gustavo Le Paige Nº 380, San Pedro de Atacama, Chile.

2 Centro de Investigación en Ecosistemas de la Patagonia (CIEP), Calle Francisco Bilbao Nº 449, Coyhaique, Chile.

* [email protected]

Revista Mexicana de Ciencias Geológicas, v. 28, núm. 3, 2011, p. 519-532

López-Mendoza,P.,Mena-Larraín,F.,2011,Extinctgroundslothdermalbonesandtheirroleinthetaphonomicresearchofcaves:thecaseofBañoNuevo-1(AndeanCentralPatagonia,Chile):RevistaMexicanadeCienciasGeológicas,v.28,núm.3,p.519-532.

López-Mendoza and Mena-Larraín520

INTRODUCTION

DermalbonesareratherpeculiarstructuresfoundonsomespecieswithintheXenarthrasuperorder(ordersPilosaandCingulata).Dermalboneshaveparticularmorphologi-calandhistologicaltraitsthathavebeendescribedbothforextinctandextantspecies(Carliniet al.,1997;VickaryousandHall,2006;Hill,2006;ChávezAponteet al.,2008;Krmpoticet al.,2009).AmongCingulata(armadillos,pam-patheresandglyptodons)thesebonesarefoundarticulatedintoastrongcarapacewithpolygonalshapesthateithermeetsidetosideorareimbricatedtooneanother(Hill,2006;ChávezAponteet al.,2008).AmongthePilosadermalbonesareisolatedspecimenswhichdonotarticulatewitheachother,andthereforelackarticularfaces(Hill,2006).

DermalbonesarecommoninLatePleistocenepale-ontologicalandarchaeologicalsitesinPatagoniaandTierradelFuego(Borrero,1986;Massone,1987;Borreroet al.,1991,1997;Latorre,1998;MenegazandNami,1999;Menaet al.,2000;SanRománet al.2000;BorreroandMartin,2008;Martin,2008).ThissituationisduetotheabundantremainsofanimalsoftheMylodon genusinseveralrocksheltersthroughouttheregion.InthecaseofMilodonCaveatÚltimaEsperanza,Chileevensofttissuessuchasdungandhidehavepreservedexceptionallywell(Hauthal1899;EmperaireandLaming1954,amongothers)ThepresenceofMylodonremainsincavesandrocksheltersisattributablebothtotheredundantuseofthesesitesbytheanimalandtotheirbeingcarriedinbypredatorsofseveraldifferentsizes,aswellasbyhuntingand/orscavengingbyancienthumanpopulations(Martin,2008).Thesedifferentpro-cesses produce specific taphonomic and zooarchaeological modifications that are best seen on certain fossils remains (e.g.,dermalbones)duebothtotheirsize,structureand/orsurfaceandtopredator’spreferenceofcertaintaxaandanatomicalunits,amongotherfactors(MartinandBorrero,1997;Martin,2008).

TheaimofthisarticleistopresenttheresultsofthetaphonomicalanalysisofXenarthradermalbonesfromBañoNuevo-1(AndeanCentralPatagonia,Chile).Wechosethismaterialtodiscusstaphonomicalandzooarchaeologicalissuessincedermalbonesaretemporallydiagnostic,theyareoftenfoundinfairlylargenumbersonshelteredspaceswithorwithouthumanoccupationsandtheirshapeandsizemakethemadequatetargetsofformationprocessestraces.We find these specimens particularly useful to identify severalkindsofalterations,althoughtheyaregenerallyignored,beingmerelycounted,giventheirpoorvaluefortaxonomic quantification of assemblages. We herein pro-vide not only a general quantification, but also record their maindimensions(length,width,thicknessandweight)andtheirspatialandchronologicaldistributionaswellastheirmicroscopicfeaturesinordertoaddressproblemssuchasverticalmigration,theroleofcarnivoresinthespeciesbonesaccumulationwithinshelteredspacesandthepossiblehu-man impact on this animal in the final Pleistocene.

THE BAÑO NUEVO-1 SITE: STRATIGRAPHY AND DATES

BañoNuevo-1islocatedsome80kmtothenortheastofthemoderncityofCoyhaique(45º17`S-71º32`W),withinanareathatischaracterizedbylargeplainsofmo-derateslopestraversedbyseveralravinesincludingArroyoGoichelandArroyoBañoNuevothatdraintotheÑirehuaoriver,oneofthemaintributariesoftheMañihualesriver,partofthelargebasinoftheAysénriver(Figure1).Thecavehasadepthof20mandanaveragewidthof4m,witharestrictedaccessduetotheentranceinthelatePleistoceneofcolluvialdebrisfromalateralgully(Figure2a).Thisma-terialprovidesanaturalbarrier,thusshelteringtheinteriorfrom abrupt thermic fluctuations as it has been reported for othercavesintheworld(Legge1972).

Thestratigraphicdepositofthecavehasadepthof150 cm (Figure 2b). From top to bottom, we can first rec-ognizealayer1thathasveryfewculturalitems,mostofthemduetoperturbationprocessessuchasthosethat-aswe will see- have acted upon bones. Below it, we find sev-erallayersthatarelargelyanthropogenicinorigin.Onthelowerpartoflayer3,thefragmentedremainsoftenhumanskeletonsarefound(Menaet al.,2003).Belowthisculturallayer and in a discordant fashion, we find layers 4 and 5 that areformedbyrockydebrisandcollapse,andbyorganicclayandsand,datingto10,500–9,000BPand10,500–13,500BP,respectively.Layer4correspondstoacomplexandnotyetwellunderstooddeposit(itencompassesa1,500yearshiatusandatleastonerockfallandactivedebrisentry,whichcouldhaveimpingedonverticalmigrationofdermalbones,butnotatthesite’sscale).Layers5(andperhapsthelowersectionof4or4B)arecharacterizedbyabundantbonesofextinctspeciesandbywhatseemstobedegradedextinctgroundslothfeces.Finally,Layer6corresponds to stratified sands belonging to a Pleistocene proglaciallakewhoseoriginaldateisunknown(itcontactstherockyfoundationsofthecave).Thislayerdoesn`thaveanyfaunalremains(notevendiatomsorothermicroscopicorganisms)andweonlyknowitwasdepositedbefore13,500BP(Núñezet al.,2005).

DIVERSITY AND TAPHONOMY OF LATE PLEISTOCENE FAUNAL REMAINS

The first mention of extinct fauna at Baño Nuevo-1 wasdonebyFelipeBateinthe70´s,associatinginprincipleculturalevidencesuchashearthsandlithicartifactstothebonesofAmericannativehorseandXenarhradermalbones(Bate,1979).Recentexcavationsandanalyseshaverecog-nizedalargerscopeofextinctfauna,althoughtheyhavefailedtoshowanyassociationwithearlyhumans(TrejoandJackson,1998;Menaet al.,2000;VelásquezyMena,2006;López,2009).Withintheassemblage,werecognizefossil remainsfromthefamiliesEquidae,Camelidae,


LamamorphotypeL.owenii(MenegazandNami,1999).The scarce Macraucheniidae remains have been identified asMacraucheniasp.Ontheotherhand,Canidaeremainsareabundantparticularlyonlayers4and3,anditisworthmentioningthattheextinctDusicyon avushasbeendi-rectlydatedto7,070±25AP(Menaet al.,2000.Table1)andisnowknowntohavesubsisteduntilevenlaterinCentralPatagonia(ValentinaTrejo,personalobservation).ComparedtosouthernPatagonianregions,thefaunalre-cordfromBañoNuevo-1issimilarintermsofdiversityandtaxonomiccomposition.InfactsixoutofnineextinctmammalsrecordedfromsouthernPatagoniahavebeenidentified at Baño Nuevo-1. Xenarthra remains from Baño Nuevo-1(Table2)havebeenradiocarbondated(AMS)between11,255±30and12,510±30BP(Menaet al.,2000;VelásquezandMena,2006),whichfallinthemiddleoftherangeofdatesonMylodonremainsfromÚltimaEsperanza(Borreroet al.,1997).

Amongthemanyzooarchaeologicalandtaphonomic

Macraucheniidae,Canidae,Ursidae,Megalonychidae,FelidaeandtheXenarthrasuperorder.Megalonychidaeremainscorrespondtoasyetunpublishedmandibleremainsandonephalanx,currentlyunderstudybyE.Bostelmann.He has formerly identified the remains as belonging to Diabolotheriumcf.nordenskioldi(Mammalia,Xenarthra,Tardigrada,Megalonychidae),aformuptonowrecognizedoutsideofChile,onlatePleistocenesedimentsfromnorthandsouthPeruandoncentralChubut,southernArgentina(Bostelmannpersonalcommunication).Thisspecieshasbeenrecognized–Albeitnotyetpublished–onthreelo-calitiesinChile(Bostelmannet al.,inpreparation).Equidremainsarerare,althoughmorphometricanalysessuggestthatthematerialfromBañoNuevo-1canbeattributedtoHippidion saldiasi,abundantinlatePleistocenePatagoniandeposits(López,2009).ThecamelidrecordconsistsbothofLama guanicoeandanundeterminedCamelidae,largerthanpresentdayguanacoandcloserinsizetogeneralikePalaeolamaandHemiaucheniaortospeciesnamedas

Figure1.MapshowingthelocationofBañoNuevo-1site.


analysesfromthedifferentlayersatBañoNuevo-1(TrejoandJackson,1998;Núñezet al.,2005;VelásquezandMena,2006;LabarcaandLucero,2007;Mena,2009),thoseconductedbyLópez(2009)onthelatePleistocenefossilbonesaddressedinageneralfashiontheformationprocessesthatdeterminedtheirpresenceatthecave.López(2009:127)speculatedthatalltheseremainswererelatedtocarnivoresthatbroughttheirkillsorscavengedcarcassestothesite.Thatzooarchaeologicalstudyprovidedthebasichypotheses and motivation for the more specific taphonomic researchnowreported.Thefossilrecordfromthoselayerswithalargertaxonomicdiversitydisplaysahomogeneousdistributionofjuvenileandadultindividualsamongherbi-vorespecies(Table3).Amongspeciesthatoccupiedopenspaces,suchasequids,thepresenceofjuveniles(atleasttwoindividualsyoungerthan5months)isparticularlyimportant.Onthesespecimens,aswellasonCamelidaebones,tracesofcarnivoredamagewererecorded,oftenontheepiphysiaryextremesoflongbones.Onthebasisoftheirsize,shapeandlocalization,thesetraceswereattributedtocanidactivity(López,2009),althoughwecannotruleout

thepossibilityofalargepredator,aswediscussbelow.Thefinding of extinct ground sloth feces on the oldest layers of thesiterevealstheintermittentuseofthecavebythisanimal(MenaandStafford,2006).Asithasbeendocumentedthatsomemylodontinesusedtodigburrows(Vizcaínoet al.,2001;Dondaset al.,2010)itmaywellbethattheyoccupiedthe cave on a seasonal basis. However the finding of only twoteethandnoskeletalremainsatallindicatesthatnonaturaldeathsoccurredinBañoNuevo-1.Aswehavesaid,thesampleisoverwhelminglycomprisedbydermalbones,afewofwhichareevenfoundonthelateLayer1.

Oneofthemanyproblemsstemmingfromtheseob-servations is the identification of the agent or agents that accumulatedthedermalbonesinthecaveandhowthisrelatestootherextinctfauna(Figures3and4).Asecondproblemistheverticalandhorizontalmobilityofdermalboneswithinthecaveasthisrelatestotheirweight,theirsizeandsphericity(Menaet al.,2000).Thesevariablesdon`thavevalueinthemselvesandmustnecessarilybeinterpreted in light of their specific archaeological context. TheBañoNuevo-1siteisideallysuitedforsuchanevalu-ation,sinceitcomprisesabasicallyuninterruptedchrono-logicalsequencefromthelatePleistoceneuptothepresent,includinglayerswith(Layers4and3)andwithout(Layers5and1)culturalremains.Herecomplexformationprocesseshavebeenidentifiedinvolvingtheinterplayofseveralvariablessuchashumanoccupations,changingsurfacetopographiesandthesiteusebydifferentanimalspecies,amongothers(Menaet al.,2000;VelásquezandMena,2006;López,2009).

MATERIALS AND METHOD

Alltheanalyzedremains(579)comefromexcava-tionsconductedinearly2006.Althoughallfaunalremainswereanalyzed,456ofthem(nearly79%)correspondtodermalbonesfromLayers1,3,4and5onsquaresfromthebackendofthecave(3D,4D,5C,6C,5By6B),theareawewouldexpectforadenorcarnivorefeedingandbone-chewingspot.ThosesamplesfromLayer3arereportedbythefollowingculturalcomponents:Early(9,500–8,000BP,Middle(8,000–5,000BP)andLate(5,000–3,000BP),as defined on the basis of discrete grass “floors” and other datedfeaturesoftheunitsfromwheretheanalyzedmaterial

Figure 2. a: Planar view of the Baño Nuevo-1 site; b: Stratigraphic profile oftheBañoNuevo-1site.

Years BP Level Description Site location Material Lab. Code

7070 ± 25 Layer3 Dusicyon avus mandible Square2a Collagen UCIAMS-1949011,255 ± 30 Layer5 XenarthradermalboneBN-13 Profile W-Square 6D Collagen UCIAMS1010511,265 ± 35 Layer4A XenarthradermalboneBN-11 Profile W-Square 6D Collagen UCIAMS1010611,480 ± 50 ¿Layer4? Xenarthradermalbone Profile SW-Square 8D Collagen CAMS-3268512,510 ± 30 Layer5 XenarthradermalboneBN-14 Profile W-Square 6D Collagen UCIAMS10107

Table1.Radiocarbondates(14CAMS)takenfromextinctfaunalremainsoftheBañoNuevo-1site.


comes(VelásquezandMena,2006).Thesampleisgenerallywell-preservedandonlyone

specimen(Layer4)presentsexfoliationtraces,althoughthediaganeticprocessesresponsibleforthisareunknown.Theosteometricanalysiswasbasedoncategoriesrelatedtoeachbone’ssizeandsphericity.Eachcompletedermalbonewasmeasuredtoitsmaximumlength(l),width(w)andthickness(t),asshowninFigure5.ThesevalueswereusedtocalculateaSphericityIndex(SI),ψp,accordingtotheformulaprovidedinFigure5.Thisparametermeasureshowcloseacertainshapeistoaperfectsphere(SneedandFolk,1958),anditsvaluesgofrom0(lessspherical)to1(morespherical).Thereareothersphericityanalysesparticularlycommoninsedimentologythatuseformclasses(i.e.,platy,bladed,elongated)andusethemasabasistocomparethethickness-lenghtratio((l-w)/(l-t)ofanspheroid(SneedandFolk,1958).Thismethod,however,requiresapreviousformclassification, that in our case is nonexistent.

Inordertoidentifythedermalbonessurfacealterationsandstructure,weusedaScanningElectronicMicroscopeJeol5410©enhancedbytheAnamaker©softwaretogener-atespectroscopicimages.Thisworkwasconductedattheelectronicmicroscopylab(LaboratoriodeMicroscopiaElectrónica)oftheMetallurgicalEngineeringDepartmentattheUniversidad de Santiago de Chile.ThesurfaceoftheanalyzedboneswassubjectedtomoldmakingthroughhighprecisionSiliconetypeCandreplicatedwithepoxycresin.Eachsamplewascoatedwithgold-palladiuminordertogenerateimagesfromthemicroscopeexamination.

RESULTS

Size, sphericity and mobility

WehavealreadymentionedthatoneoftheproblemstoaddressatBañoNuevo-1isthemigrationofdermalbonesfromLatePleistocenestratauptotherecent,uppermostlayer, an observation that is confirmed by direct radiocar-

bondates.OnLayer4,forinstance,aextinctgroundslothdermalbonedatedto11,480±50BP(CAMS-32685)wasfoundaboveahumanmadehearthdatedto9,200±80BP(Beta90888)(Menaet al.,2000).

Verticalmigrationcouldberelatedtofuneraryactivi-ties,althoughthoserecordedatthesitedonotinvolvedeepdigging.Itcouldalsobeduetosedimentrelocationduringthecleaningofhearths,preparationofgrassbeddingsorothersoildisturbingactivitiesthatwerenotclearlyrecog-nized.Butitseemsmorelikelythatlargescaledisplacementoftinydermalossicleswasnotcausedbyanthropicactivi-ties,butbydiggingand/orburrowinganimals,extinctand/orextant. We must also consider the floor relief and the cave’s topography,althoughnostratigraphicreversionswerenotedanywherewithinthedepositsandthefewincongruenciesbetweenabsolutedepthofthesamplesandtheradiocarbondatesobtainedfromthemareminimal(nevercontradictingthemainstratigraphicsequence)andcaneasilybeexplainedby irregularities on the surface of the occupation floors. Finally,wemustconsidercollectingofoldbonesbymorerecenthumanoccupantsofthecave,aphenomenonthathasbeenobservedatotherPatagoniansitessuchasCuevadeLosChinguesatPaliAike(SanRománet al.,2000;Martin,2008)andArroyoFeoattheRíoPinturaslocality,Argentina(Mengoni,personalcommunication).

Layer5andthebottomofLayer4aretheonesthatcontainmostdermalbones.Layer4canbedividedintosub-layers–4Aand4B–basedonsomesedimentologicalchangesand,aboveall,differencesobservedonthefossilassemblages:onlymylodontineremainswerefoundon4A,whileon4Bfaunaldiversityismuchlarger,includ-ingunidentifiedCamelidae,Lama guanicoe,Equidaeaff.Hippidion sp.,Macrauchenia sp.,Felidae,XenarthraandDiabolotherium cf. nordenskioldi(Table3).On4B,273dermalossicleswererecovered,whileon4Aonly37wererecorded.ThisdifferencemaysuggestthatdermalbonesfoundatLayer4areduetomigrationfromLayer5,immediatelybelow,consideringabsolutedatesandthegradualdiminutionoftheseremainsalongLayers3and,obviously,inLayer1.

AsTable4shows,nodifferencewasobservedwithinthemeasuresandsphericityofdermalbonesondifferentlayers.BetweenLayer1andLayer3analysisbyStudent`st test indicatesnodifferencebetweentheSImean(t=0.78625,P<0.5),whichalsowasnoobservedbetweentheLayer3andLayer4(t=0.40036,P>0.5),andbetweenthislayerandLayer5(t=-0.85656,P<0.5).Astheyalltendtobesmallandsemi-spherical,thisvariabledoesnotseemtobearelevantpredictorofmobilitynordoesitseemtobeimportantwithothermaterials.Itisinterestingtonote,though,thatthosefoundontheuppermostlayer–althoughtoofew–aretheheaviestones,thusindicatingthatgravityisnotarelevantvariableeitherandthattherewasaforceactingfrombelow.Sincetheanalyzeddepositsareshelteredfromsunlightandthecavedoesnotsustainanykindofvegetation, floralturbation mechanisms such as root and

Layer Feature/Component Element NISP

1 Nodata Dermalbones 33 Nodata Dermalbones 113 Feature2/tMiddleComponent* Dermalbones 43 Feature11/MiddleComponent* Dermalbones 73 Feature13/EarlyComponent** Dermalbones 24A Nodata Dermalbones 374B Nodata Dermalbones 2735 Nodata Dermalbones 100Collapse Nodata Xiphoid 1TOTAL 457

Table2.XenarthraspecimensbystratigraphiclayeratBañoNuevo-1.

*MiddleComponent:8,000–5,000BP;**EarlyComponent:9,500–8,000yearsBP)


Layer Taxa MNI (Juv/Adult) % Juv NISPtotal (Teeth/Bones)

Layer 1 Xenarthra ? ? 3(0/3)

Layer 3 Xenarthra 1(?/?) ? 43(0/43)Lama guanicoe 1(0/1) 0 1(0/1)Mammalia ? ? 3(0/3)

Layer 4A Xenarthra ? ? 37(0/37)

Layer 4B Xenarthra ? ? 273(0/273)Diabolotheriumcf.nordenskioldi 1(0/1) 0 2(1/1)Felidae 1(0/1) 0 1(0/1)Equidaeaff.Hippidion sp. 2(1/1) 50 4(1/3)Macraucheniasp.* 1(?/?) ? 1(1/0)Camelidae ? 0 3(0/3)Lama guanicoe 2(1/1) 50 6(0/6)Mammalia ? ? 27(0/27)

Layer 5 Xenarthra ? ? 100(0/100)Felidae 1(0/1) 0 1(0/1)Arctotherium sp. 1(0/1) 0 1(1/0)Macraucheniasp. 1(0/1) 0 3(2/1)Equidaeaff.Hippidion sp. 1(1/1) 50 7(1/6)Lama guanicoe 2(0/1) 0 2(0/2)Artiodactyla ? ? 1(0/1)Camelidae ? ? 1(0/1)Xenarthra ? ? 16(1/15)

Table3. Summarybystratigraphiclayer ofthenumberofindividualsbytaxa,showingjuvenilesandadults,NISPandtheTeeth/Bonesratio.*Lostsample.

Figure3.DifferentlyshapeddermalbonesrecoveredatBañoNuevo-1.


Mena,2006)meansthatiftherewereanysedimentsremovalitwouldhaveaffectedearlylayersandthus,cannotexplainthepresenceofdermalbonesonLayer1.Thissituationalsoappliestothehearthcleaningandotherhumanactivitieseveniftheywentundetected(e.g.,postholes)sincenodermalboneshavebeenreportedfromtheinterveningLayer2(3,500–100BP).Finally,wemustconsiderthecollectionofoldobjectsandtheirbeingcarriedandusedbythesiteoccupants(Schiffer,1987).However,wewouldexpecttheseactionstoberelatedtoartifactmaking(forpracticalordecorativeuse)whichisnotthecase.

Weareleft,therefore,withbioturbationorcryoturba-tionastheonlyagentstoconsiderfortheobservedupward

rootletscannotbeconsidered.Trampling,whichiscom-mon in restricted spaces with an intense traffic by humans andotherheavyanimals,suchastheBañoNuevo-1cave,tendstomoveartifactsandecofactsafewcentimetersdown(Barton,1982;VillaandCourtin,1983;Gifford-Gonzálezet al.,1985;VermeerschandBubel,1997),butnotintheoppositedirection.

Aswehavesaidbefore,thefunerarycontextsimplylimitedsedimentmovementsinceadultindividualswereplacedclosetotherockywallsandcoveredbyrockswhiletheinfantcorpsesseemtohavebeenlaidtorestonthesur-face.Ontheotherhand,theirdirectdatingbetween9,000and8,500BP(MenaandStafford,2006;Velásquezand

Figure4.DamagegeneratedbycarnivoresonbonespecimensfromBañoNuevo-1.a:DistaltibiaepiphysisofEquidaewithcrenulatedborders(Layer4);b:Longbonefragmentofanindeterminatemammalwithscoring(Layer4);c:DistaltibiaepiphysisofCamelidaewithcrenulatedborders(Layer5);d:Equidaeectocuneiformwithsurfacecorrosionduetodigestiveacids(Layer4);e:Indeterminatebonefragmentwithcrenulatedbordersandsurfacepitting(Layer4).


migration. There are several site-specific and actualistic studiesofanimalsasagentsofstratigraphicdisturbances(Langmaid,1964;Stein,1983;Erlandson,1984;Bocek,1986;McBrearty,1990;Balek,2002;AraujoandMarcelino,2003).AtBañoNuevo-1,themostimportantstratigraphicdisturbancerecordedconsistsofrodentburrows,clearlyobservedinthelowerlayers,althoughitislessclearwhethertheyoccurredontheHolocenelayers(Figure6).Thestudyofrodentremains(LabarcaandLucero,2007)recoveredintheexcavationssuggeststhatmostofthemcorrespondtosmallCricetidaethatwereintroducedbypredatorbirds,thusrulingouttheimportanceofin situhabitationortunneldiggingbyfossorialcaviomorphs,butthisanalysisreferstoonlytwoexcavationunits.Althoughtheirpresenceandburrowinginsideacavewithnograssycoverisnotwellunderstood,thepresencewithintheburrowsofpositivelyidentified skulls of Ctenomys sp.leadsustoconcludethatthisistheorganismthatdugtheobservedburrows.ThiskindofalterationhasbeenobservedbyBorrero(2003)atTresArroyos1(TierradelFuego,Chile)whererabbit(Oryctolagus cuniculus)boneswerecollectedonthelatePleistocenelevels,whilethisspecieswasjustintroducedtotheislandinthe1930’s.SinceatBañoNuevo-1wearedealingwithanupwardmigration,burrowingrodentsareagoodcandidatetobetheagentsresponsibleforthismove-ment,sinceitiswellknownthattheseanimalscancarrysedimentaryparticlesfromlowerlevels,particularlyiftheyareeasytotransportduetotheirsizeandshapeandhaveasmallerradiusthanthegalleries(Bocek,1986;Balek,2002).Thus,althoughrodentandleporidactionhasbeeninvokedtoexplainthedownwardmigrationofartifactsandecofactsin stratified Patagonian archaeological sites (Massone et al.,1993;Borrero,2003;Martin,2008),inBañoNuevo-1theinversephenomenonseemstobemostcommon,sinceonlatePleistocenelevelswithextinctfaunathepresenceofculturalmaterialsthatmayhavefallenfromaboveisminimal.AnothercaseofverticalmigrationisrepresentedatthesitebyremainsoftheFelidaethatmostlikelybelongtoasingleindividualdespitebeingrecordedinthetwolowermostcontiguouslayers.

Cryoturbationoralternatethawingandicing(freeze-thawweathering)isanothermechanismthatmayexplaintheupwardmigrationofgroundslothdermalbones.Weknowthattheseprocessescanindeedtransportparticlesupward(WoodandJohnson,1978;Schweger,1985;Schiffer,1987),

butwealsoknowthatpresentdaytemperatureconditionsatthecavearefairlystable(between7and12°C,asrecordeddaily during the January 2006 - southern summer- field cam-paign)andarenotlikelytohavepromotedthisphenomenon.Noneofthealterationstosediments,bonesandartifactscharacteristicofthesekindofdisturbance(Lavilleet al.,1980;Farrand,2001)havebeenrecorded.Itmaynevergetbelow0ºCinsidethecaveandevenifitdoes,thestabilityofthethermalconditionscouldnotpromotethealternatefreezeandthawingthatcauseverticalmigration.

Carnivore alterations

ThedermalbonesoftheMylodontidaearemorpho-logicallysimilar,astheytendtoberoundwhenviewedfromaboveandovoidinalateralperspective(Hill,2006).Their

Figure5.SphericityIndexformula.t=thickness;l=length;w=width.

3 2 / lwtΨLength Width Thickness Weight Sphericity Index

N 451 451 451 451 451Min 4.6 3.3 1.7 0.1 0.4Max 25.4 14.8 11.4 2.1 0.95Sum 4,994.2 3,507.3 2,504.8 231.1 323.9Mean 11.1 7.8 5.6 0.5 0.7Std.Error 0.2 0.1 0.07 0.02 0.005Variant 11.6 4.3 2.4 0.2 0.01Stand.Dev. 3.4 2.07 1.6 0.4 0.1Median 10.5 7.5 5.4 0.4 0.7

Table4. Univariatestatisticsforlength(mm),width(mm),thickness(mm),weight(g)andsphericityindex(SI)ofdermalbonesfromtheBañoNuevo-1site.

Figure6.Rodentburrowsonlayer5atBañoNuevo-1.


surfaceisroughtosmoothandinsomecasesitiscoveredbyseveralsmalldepressions.OsteologicalstudiesperformedonGlossotheriumchapadmalensedermalbonesrevealtheirinteriortobesmoothandformedbyanetworkofmineral-ized fibers with different orientations. Close to the surface, these fibers tend to be distributed on orthogonal layers, while a second group of fibers are oriented perpendicular to the bone’s main axis. Towards the center, mineralized fibers are less well defined and do not display a clear orientation. Thisareaisformedbysmallvascularchannelslimitedbyconcentricbonelamellae(primaryosteons)inwhichwecan

observe osteocite lagunae. On the bone’s periphery, fibers areorientatedinaperpendicularfashiontothedermalbonesurfacethatarecutbygrowthlinesparalleltothesurface(Hill,2006).Figures7a,7b,7cand7dshowanextinctgroundslothdermalbonefromBañoNuevo-1bothonitssurfaceanditsinterior,anditcanbeobservedthatthein-ternal fibers are very well-preserved. These characteristics pointtothedermalossiclesasverydenseunits,well-suitedto record taphonomic modifications both at the macroscopi-calandmicroscopicallevel.

Despitetheintegrityofthedermalbonessample

Figure7.a:AnundamageddermalbonefromBañoNuevo-1;b:SEMviewofthesurfaceoftheundamageddermalbone;c:SEMviewoftheinteriorfibers of the undamaged dermal bone; d: SEM of the subsuperficial structure of the undamaged dermal bone; e: A heavily damaged dermal bone from layer 4 (4B); f: SEM view of the surface of the damaged dermal bone; g: SEM view of the interior fibers of the damaged dermal bone; h: SEM view of the subsuperficial structure of the undamaged dermal bone.


fromBañoNuevo-1andthegenerallygoodpreservation,afewspecimensdisplayimportantexternalandinternalalterations,thatcanbemacroscopicallycharacterizedaserosionandperforationsofthebonytissueaffectingthecentralcoreofthesepiecesfromLayers3,4and5(Figure7e). At first, these damages were interpreted as resulting fromtheactionofdigestiveacidsduringthepassageofgroundslothremainsthroughthedigestivetractoflargecarnivores(López,2009).ThisphenomenonisnotunusualinFuego-Patagonianarchaeologicalsites,wherealternatingoccupationsbyhumansandanimalsofthesameshelteredspacesfacilitatedthepreservationoffecesfromseverallate-Pleistocenecarnivores(Borreroet al.,1997; SanRománet al.,2000; Martin,2008).Damageddermalbonesareusu-allyfoundisolated,althoughatMilodonCavecarnivore(Panthera onca mesembrina) feces wererecorded inperfectpreservationconditioncontainingMylodonhidefragmentsanddermalbones(Martin,2008).ThiskindofdamagebydigestiveacidshadalsobeenreportedonerodedsurfacesitesinthesemiaridregionsofChile,wheresmallscattersofbonesplintersanddermalboneswiththiskindofdamagewereexplainedbytheactivitiesofapredator(LópezandJackson,2004;López2007).

TheSEManalysisofaselecteddermalbone(NISP=1)fromBañoNuevo-1ascomparedwithanotherone(NISP=1)withnomacroscopicallyobserveddamagerevealserosionaffectingtheuppermostbonylayerthatexpressesitselfasirregularitiesandsmall,homogeneouslydistributedpits(Figure7f).Asmentionedbefore,theinternalstructureof dermal bones is shaped as a net of fibers with different orientationsthataredistributedonthesurfaceasorthogo-nallayers.Onthealteredspecimen,theinteriorispartiallyremoved and the fibers appear corroded and the original structurecannotbeobserved(Figure7g).However,thestructureofthetissuetowardsthesurfaceofthedamageddermaldoesnotpresentmajordifferenceswiththeunaf-fectedcontrol(Figure7h).Weconcludethatintenseacidcorrosioncanaffectthesurfaceandtheinteriorofdermalbones, while leaving the subsuperficial structure largely intact.Thesecharacteristicsaresimilartothoseproducedbycarnivoredigestiveacidsandtheirlowincidenceonthesamplecouldreveallittlehideconsumptionbyasmalltomediumsizedpredatorand/orbytheageofthepredator(HorwitzandGoldberg,1989;Andrews,1990;Tappen

andWrangham,2000).ThiskindofdamagehasalsobeenobservedonaLama guanicoe thirdphalanxfromthebot-tomofLayer4andonanEquidaecf.Hippidion sp.radialcarpalfromLayer5(Tables5and6).

Was there consumption of extinct ground sloth by humans at Baño Nuevo-1?

Thehumanconsumptionofextinctgroundsloth hasbeendiscussedatlengthinPatagonianarchaeology(Hauthal,1899;Lehmann-Nitsche,1899;EmperaireandLaming,1954;Borrero,1986,1997,2001;Massone,1987;MengoniGoñalons,1987;Borreroet al.,1991;Prieto,1991;SanRománet al.,2000;Tonniet al.2003;Hajduket al.2004;BorreroandMartin,2008;Martin,2008),aswellasinotherpartsoftheAmericas(López,2007).Whendeal-ingwithBañoNuevo -1 this might appear a difficult topic toevaluate,sincemostofthepiecesthatcanbeattributedtothisspeciesaredermalbones(thereareinadditiontwoteethandthefragmentofaxyphoid).ThissituationdoesnotdifferfromtheinformationavailablefromotherPatagoniansites,wheresmallbonessuchasphalanges,carpals,tarsalsanddermalbonesaredominant(Martin,2008).Theexcep-tionsarerepresentedbyFellCave,whereMylodoncon-sumptionbyhumanshasbeendocumented(Martin2008).AtcavesitesinÚltimaEsperanza(i.e.,MilodonCave,LagoSofía-4)cranialandpostcranialremainsarefairlyabundant,buttheymaypertaintonaturaldeathsorin situkillingbynon-humanpredators(Martin2008).TheallegedcutmarksonmylodontinedermalbonesatElTrébol(RíoNegro, Argentina) could reflect some use of the animal by humans(Hajduket al.2004),althoughtheobservedgroovesaremostlikelytheresultsoftrampling(F.Mena,personalobservation).MacroscopicandmicroscopicobservationsontheBañoNuevo-1dermalboneswerealsoaimedatidentifyinganthropogenicmarksandonespecimenwasfoundtopresentthinparallelorsubparallelgroovesononeofitsfaces.TheanalysisthroughSEM,however,revealsthattheirmicrotopographydoesnotexhibittheexpectedtraitsfortracesofacuttinginstrument(Shipman,1981),withamoreirregularshapethanthatexpectedforthemandalessparalleldirection.Infact,thesubparallelgroovedistributionandtheirsize(Figures8aand8b),suggestthey

Bone Pits Scores Punctures Crenulated Furrowing Gastric acids

Vertebra,body - - - 1 1 -Px.rib - - 1 1 - -Px.tibia - - - 1 - -Px.tibia - - - 1 - -PosteriorIIIphalanx - - - - - 1TOTAL - - 1 4 1 1

Table5.CarnivoredamageonLama guanicoeremainsfromLayer4atBañoNuevo-1.


Bone Pits Scores Punctures Crenulated Furrowing Gastric acids

Maxilar - - - 1 - -Scaphoid - - - - - 1Pissiform 1 - - 1 - -TOTAL 1 - - 2 - 1

Table6. CarnivoredamageonEquidaeaff.Hippidion sp.remainsfromLayer5atBañoNuevo-1.

wereproducedbytramplingonabrasivesediments,ashasbeenobservedforextincthorseandundeterminedmammalremains from Layers 4 and 5. No evidences of fire that couldsuggestburningofhideorcookingofmeatdirectlyon the fire were recorded. Despite these relatively humble andnegativeresults,itisimportantthatresearchersusethistoolanddevotespecialattentiontotheanalysisofthesurfaceofallmaterialsrecoveredatanarchaeologicalsite,eventhoseoftenneglected,asaredermalbones.

DISCUSSION

Xenarthradermalbonestendtobeanabundantoc-currenceatLatePleistocene/EarlyHolocenedepositsinFuego-Patagonia.OnevalueoftheseremainsisthatthesepiecesarechronologicallydiagnosticastheextinctiondatesforanimalsoftheMylodon andGlossotherium gen-eraarewellestablishedfortheSouthernCone(Borrero,1997).Astheyarerelativelydenseparticlesthatprovideanadequatesurfaceforrecordingtheimprintofdifferenttaphonomicprocessestheyareparticularlywellsuitedforinvestigationsofformationprocesses,buttheyhavebeenlargelyignoredwhileinterpretingarchaeologicalandpa-leontologicalsites.

Asithasbeenpreviouslydiscussed,theintroductionofdermalbonestoshelteredspacescanbetheresultofdif-ferentagents(orcombinationsofagents),includingnaturalin situdeaths,transportbydifferentpredatorsandhuntingandconsumptionbyhumans.Itisparticularlyimportant,therefore,todisentanglethiscomplexmulticausalityandtoattempttointerpretthepresenceofdermalbonesintermsoftheagent(oragents)responsiblefortheirtransport,pres-enceandintersitemigration,sincetheycanthrowvaluablelightonthetaphonomichistoryofadeposit.Sincethesesmallandfairlyubiquitousparticlesareparticularlypronetoverticalmigration,theirstudycanalsoprovideinformationaboutstratigraphicintegrityintheabsenceofmoreevidentorlargescaledisturbances.

BañoNuevo-1providesandexampleoftheutilityofdermalossiclesinstratigraphicanalysis.Asifitwerenotenoughtoknowthatthebearersofthedermalboneswereextinctbytenthousandyearsago,manyofthemhavebeendirectlydatedtothe12thmillenniumBPbuttheyarenone-thelessfoundinminorquantitiesonlayersthatcorrespondtothelastcentury.Onedermalbonewasevendatedto11,480±50BPoverahearthdatedto9,200±80BP(Mena

et al.,2000).Wehaveattemptedtoexplainthismigrationbothintermsofthebone’sdimensionsandshapeandintermsofthetransportagents,mostlikelyburrowingrodents.Verticalmigrationcanalsooccurwithothersmallandcom-pactbonessuchascarpalsandtarsals,althoughthishasnotbeenobservedatBañoNuevo-1,giventheabsenceoftheseelementsfromotherextincttaxaonHolocenelevels.

Assuspectedbyapreliminaryinspectionandcon-firmed by Electronic Scanning Microscopy, some of these boneswerepartofthefecesofacarnivore.ThisobservationraisesthequestionaboutthecarnivoreorcarnivoresthatpreyedonmylodontinesnearthecaveandthatoccupieditbeforethearrivalofhumansontheEarlyHolocene.Onthesite,twopotentialcandidatesformovingand/orchewingextinctgroundslothhidesarecanids,Arctotheriumandaspecies similartoPanthera onca mesembrina (TrejoandJackson,1998;López,2009).NoremainsofDusicyon avus havebeenfoundinthePleistocenelayersatthesite(ValentinaTrejo,personalobservation),butismostlikelythattheywerethere.FromtheboneevidenceofotherlatePleistocenespeciesatthesite(i.e.,Camelidae,Lama gua-nicoe,Equidae)weobservefairlyfrequentdamagemarks,includingpits,scores,puncturesandfurrowing(Tables5and6),ofasizeclosertothoseproducedbycanidsthanthoseleftbylargerpredatorssuchasPanthera onca me-sembrina or Arctotherium (Martin,2008).However,felineandbearsmarksarenotveryabundantandtendtobelo-calized on specific places of the bones (Martin, 2008). In thosecasesinwhichtheageoftheherbivoretaxafoundon the Pleistocene layers could be detected, we find a large proportionofjuvenilehorseandguanaco,including–aswesaidbefore–twoequidindividualsranginginagefrom1to5months(seeTable3).Biganimalssuchasextinctgroundsloth andMacrauchenia areonlyrepresentedbysmallnonskeletalelementssuchasdermalbonesandteeth.ThisrecordhencesuggeststhattheaccumulatingagentatBañoNuevo-1couldnothuntandtransporttothecavelargepreybutitwaseffectiveincollectinganddestroy-ingbonesofanimalsbetween100and300kg.TheroleofDusicyon avus and/orothercanids ontheformationofthepaleontologicalcontextmustbereevaluatedastheycouldhavehuntedsmallerspeciesand/orscavengedthosepreyhuntedandconsumedbylargerpredatorslikePanthera onca mesembrina andArctotherium.Thepresenceofgroundslothfecespointtotheeffectiveoccupationofthecavebythisextinctedentate,anditispossiblethatdifferentcarnivoresalsodwelledinthesite,althoughinalternatingfashion.As


ithasbeenobservedatdensiteslikeLagoSofìa4,wherealargerdiversityofMylodon elementsarefound,carnivoresselected specific bones with meat attached and the hide may havebeenabandonedatthekill,anargumentthatwouldalsoexplainthescarcityofdermalboneswithdigestiveacidtracesatCuevadeLosChingues(PaliAikeregion)andothersites(Martin,2008).UnlikemostPatagoniansites,however,BañoNuevo-1doesnotexhibitaclearpatternofdamagebylargefelinesorbears,andthesizeoftheobservedpuncturesappearmorelikethoseproducedbysmalltomiddle-sizedcarnivoreslikefoxes.

Anotherpossiblescenarioisrelatedtotheintroduc-tionbyhumansofgroundslothhidesadheredtomeatforconsumptioninsidethecave.Aswehaveseen,however,noevidencesuggestsanthropicactionontheseanimalsand/ortheirbones.Inthisrespect,theBañoNuevo-1siterevealsitselfasaparticularlystrongcaseagainstthecoexistenceandcontemporaneityofPleistocenespeciesandhumanbeings,sincetherearethewelldatespre-humansedimentswithagoodfossilrecordofmanyextinctspecies,andiftherewasanyhumanactivitythereitshouldhaveleftsomeevidence.Thismightbeastrictlylocalphenomenonanddoesnotgrantuspermissiontogeneralizeandtonegate,forinstance,theroleofhumanpressureontheextinctionofthesespecies.

Justasthepresenttaphonomicalresearchstemsfromlargely “intuitive” or “heuristic” observations and questions in the framework of a first zooarchaeological approach to thePleistoceneassemblagecollection,wearecertainthattheseobservationshavereducedthealternativehypothesesandhaveprovidedabetterfocustoquestionstobead-dressedbyfutureresearch.Dermalboneshadnotbeenthespecific focus of observation before and we have learned that in the future more specific data must be collected in the field and more analytical analyses are due (i.e., defini-tionofsedimentologicaland/ormicroscopicindicatorsofcryoturbationandargilliturbation).Sincedermalbones

are–aswehavesaid-particularlywellsuitedforforma-tionprocessesresearch(abundant,dense,goodimprintingsurfaces),itwasimportanttoinitiatetheirthoroughstudy,althoughamorecompleteinterpretationinthefuturemustconsider other contextual variables. Since no final defini-tiveconclusionsexistinscience,ifwewereonlytoreport“success stories” and/or sensational results we would fail to provide a feeling of the long enquiry that is any scientific research,whichalltoooftendoesn’tleadusdirectlytotheexpectedresults.Thechallengeforfutureresearchispre-ciselytotreadtheseunpredictablepathsandtobuildnewknowledgewhileapplyinginformationalreadyobtainedtotheanalysisofthosesiteswherehumanconsumptionofextinctgroundslothisclaimed.

ACKNOWLEDGEMENTS

OurthankstoEmilyLindseyforreadingandcommentonanearlierversionofthiswork.WealsothankGladysOlivares(LaboratoriodeMicroscopiaElectrónica,USACH)fortheirhelpandEnriqueBostelmann.WealsowanttothankDr.GustavoJuanScillato-Yanéforhiscommentstoapreviousversionofthismanuscript,andtoasecondanonymous reviewer. Research financed by FONDECYT 1090027Project,Ser humano y contexto de la ocupación inicial de la estepa del norte y centro de Aisén (transición Pleistoceno terminal /Holoceno): estrategias de búsqueda e interpretación del registro arqueológico temprano.

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Manuscriptreceived:April8,2011Correctedmanuscriptreceived:June5,2011Manuscriptaccepted:June5,2011

Extinct ground sloth dermal bones and their role in the ... · Extinct ground sloth dermal bones,...

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