L'odontologie de Gomphotherium angustidens (Cuvier, 1817) (Proboscidea, Mammalia) : données issues du gisement d'En Péjouan (Miocène moyen du Gers, France)

Geodiversitas

Pascal Tassy¹ 

关键词: Mammalia;    Proboscidea;    gomphotheres;    anatomy;    teeth;    Simorre;    Miocene;    Mammalia;    Proboscidea;    gomphothères;    anatomie;    dents;    Simorre;    Miocène;   

DOI  :  10.5252/g2014n1a2

学科分类：生物科学（综合）

来源: Museum National d Histoire Naturelle

【 摘 要 】

OdontologyofGomphotheriumangustidens(Cuvier,1817)(Proboscidea,Mammalia):datafromtheMiddleMiocenelocalityofEnPéjouan(Gers,France).ThedentalanatomyofGomphotheriumangustidens(Cuvier,1817),thecommonestproboscideaninthefaunallistsoftheMiddleMioceneofEurope,isdetailedonthebasisofthenumerousfossilsfoundatEnPéjouaninthevicinityofSimorre(Gers,France),alocalityofAstaracianage(mammalzoneMN7orMN7/8).Theexaminationofthemorphologicandbiometricvariationoftusks,premolarsandmolars,basedonanunprecedentednumberofspecimensofonespeciesinonelocality,yieldsseveralresults.ThefirstisabetterunderstandingoftheevolutionarylevelofG.angustidensduringtheAstaracian.Asecondisabetterknowledgeofdentalcharactersandtheirvariation.Thepresenceorabsenceofbothupperandlowerincisors(tusks),duetosexualdimorphism,isdocumented.Infemales,whentheyexist,uppertusksaremuchreduced.Therelationbetweenshapeandgrowthstagesofevergrowingtusksisillustratedinrelationtoadental-agescalebasedoncraniaandmandibleswithdifferentpremolarsand/ormolarsanddifferentwearstages.Variationoftheorientationoftheenamelbandofuppertusksandofthepiriformtransversesectionsoflowertusksisdirectlyobservedbasedonnumerousspecimens.Theuppertusksareslightlycurvedoutwards;theyarenotmuchcurvedventrallyalthoughtheydisplayatwistevenonratherstraighttusks:inlateralviewtheyareslightlyconcavedorsallyatthepremaxilla,thenratherstraight,andfinallyventrallycurvedatthetip.ThefabricationofenamelisstoppedonmaleuppertusksatthedentalagesXVII–XVIII(thatis,withwornM2/m2andfirstloph[id]ofM3/m3worn).Nolowertuskshowscircularorflattransversesections,evenifthelongitudinalsulcus(onbothdorsalandventralsides)ismoreorlessmarked;thedorsalwearfacetisstrongbutshort.Thebunolophodontpatternofdecidualpremolars,premolarsandmolarsisextremelyvariable.Therangeofthisvariationisunderstoodindetailsthroughseveralparameters:1)thesubdivisionoflophs(ids);2)theshapeofcentralconulesandtheresultingtrefoiledwearfigure;3)thedegreeofsubdivisionofthecentralconules;4)theposttriteconules;5)thelabialcristaeofuppermolars;6)thedevelopmentofthecingulum;7)thedegreeofdevelopmentofthefourthlophofM3sandofthefifthlophidofm3;and8)theamountofcement.Itappearsthat:1)thebinarysubdivisionoflophs(ids)istherule,posttritehalflophswiththreecuspsarerareandvariableonrightandleftsides;onuppermolars,themesoconeletisoftenfusedwiththeanteriorcentralconule;2)thepretritetrefoiledwearfigureofuppermolarsshowsanenlargementoftheposteriorcentralconulecomparedtotheanterior,andthisasymetryisseenevenonwornteeth—thistraitseemstobesignificantattheAstaracian,especiallyforM2andM3;3)pretritecentralconulesaresubdivided(“serridentine”pattern),ornot,andvariationcanbeseenofrightandleftmolarsofsameindividuals;4)posttriteconulesareweakorabsent;5)thelabialcristaeofuppermolars(mainlypostparacristaandpremetacrista)havetheshapeofsmoothridgesandareneverstronglydeveloped;6)thecingulumismostlyweakonthepretriteside,thedevelopmentofthepostingulumisextremelyvariable,asthatofthelastloph(id);7)allM3saretetralophodont,eventheteethwhichbearweakfourthlophandonlyonepostcingulumcuspshowanentoflexusbetweenthirdandfourthlophs;m3shavefourorfivelophids;and8)thecementisusuallypresentintheinterlophs(ids)andaroundthecusps.Thebiometricvariationofmolars,especiallyM3sandm3s,iscompatiblewiththeexplanationofsexualdimorphism.Asaconsequence,thevariationofthedentaltraitsdescribedatEnPéjouanbringsarationaleforunderstandinganyinterspecificvariationamongtheElephantida.Thestudyofwearfacetsofassociatedupperandlowerpremolarsandmolarsleadtothedescriptionofthemodeofocclusioninrespecttohorizontaltoothdisplacementandanatomicalcharactersoftheskull(craniumandmandible).TheocclusiondepictedforG.angustidensisprobablyvalidforanyspeciesthatbelongstothetrilophodontgradeofElephantidaandeven,basedoncomparisonswiththemammutidZygolophodonturicensis(Schinz,1824),ofElephantimorpha.Occlusalmovementsaremainlyduetoanupwardcomponent,asfordeinotheres,withasecondarycomponent,transversal(inward).Thelattercomponentisillustratedbytheanglemadebythelongitudinalaxisofoccludingmolars(=shearangleofelephantinemolars).Yet,theextensionofthependulousmovement(bothverticalandpostero-anterior)ofthemandibletowardthemaxillaislimitedinG.angustidenscomparedtoelephantsduetothedifferentorientationoftemporalis,digastricusandmassetermuscles.Theodontologyofthe“population”ofG.angustidensatEnPéjouan,andcomparisonswithMioceneEuropeantaxa,bringsomereliabletaxonomicstatements.Nosubtapiroidorannectens-likemorphotypes(ratherfrequentinMN4–MN5zones)isseenatEnPéjouan.MolarandincisorcharacteristicsofthecontemporaneousspeciesG.steinheimensearealsoabsentinthevariationdescribedatEnPéjouan.Fromtheviewpointofdentalmorphology,G.angustidensisclosertoArchaeobelodonfilholi(Frick,1933).Yet,anassociationofcharactersseenonlyinG.angustidensismadeofthedoubletwistofuppertusksandenamelband,thepiriformtransversesectionoflowertusks,theenlargementoftheposteriorcentralconuleswithresultingasymmetricaltrefoilandreducedposttriteconulesofuppermolars,thealternatepatternofthelophs(ids)ofdP3/dp3.Asaconsequence,derivedtraitsseenonthedentitionofGomphotheriumangustidensinAstaracianlocalitiesleadtotheconclusionthatmostmolarsofbunolophodonttrilophodontgradeofOrleanianlocalitiesratherbelongtoGomphotheriumsubtapiroideum(Schlesinger,1917).Identificationofisolatedteeth,especiallylowermolars,remainssometimestentativeifnothazardous.

【 授权许可】

Unknown