The earliest known pig from the Upper Eocene of Thailand

10 Pages·1998·4.4 MB·English

by  S Ducrocq

THE EARLIEST KNOWN PIG FROM THE UPPER
EOCENE OF THAILAND
by S. DUCROCQ, Y. CHAIMANEE, V. SUTEETHORN andJ.-J.JAEGER
Abstract. Several dental remains ofa new suid, Siamochoerus banmarkensis gen. et sp. nov., have been
collectedintheLateEoceneKrabibasininsouthernThailand.Thisspeciesismorphologicallyclosetobut
moreprimitivethanDubiotheriumwaterhousi(formerlyPalaeochoeruswaterhousi),andrepresentsoneofthe
oldestknownsuids.ThedateoforiginationofsuidscanthereforebeplacedbacktotheLateEoceneoreven
earlier,andtheearlyevolutionanddiversificationofthefamilymighthaveoccurredlargelyintheOligocene
ofAsia.
Pigs areartiodactylsbelongingto thefamily Suidae, characterizedbytheirubiquityandability
to radiate rapidly in new territories. For these reasons, suids are considered to be useful for
intercontinentalaswellaslocalandregionalbiostratigraphy. Thepalaeontologicalhistoryofthe
family Suidae iswell documented in the Miocene and afterwards (Pickford 1988, 1993), but the
fossilrecordandtheevolutionaryhistoryofsuidsandtheircloserelativesthetayassuidsisalmost
unknown during the Paleogene ofthe Old and New worlds. According to the recent work of
Pickford(1993),onlytwotayassuids(thepeccariesPalaeochoerusandDoliochoerus)andonesuid
(.Hyotherium) occur in Paleogene deposits of the Old World. The earliest known true suid is
Hyotherium from the Upper Oligocene ofEurope (Pickford 1993). Hyotherium belongs to the
subfamily Hyotheriinae which is considered to be the ancestral group that gave rise to further
subfamiliesofpigs. However,themajorevolutionanddiversificationofsuidsoccurredduringthe
NeogeneintheOldWorld,withover30generarecognized.Suidaeevolvedinaratherdifferentway
thanTayassuidaeandaredistinguishedbylimbstructure(fourtoesonbothfrontandhindlimbs
andunfusedthirdandfourthmetatarsals)anddentition(forexample,suidspossessoutwardlyand
upwardlycurveduppercanines; seeforexampleNowakandParadiso 1984).
TheLateEoceneKrabi BasininsouthernThailand(Text-fig. 1)hasyieldedarichanddiverse
mammalianfaunaincludingadermopteran,amegabat,primates,carnivores,rodents,atayassuid,
anthracotheres,ruminants,atapirandarhino(Ducrocqetal. 1995, 1996;Chaimaneeetal. 1997).
HolroydandCiochon(1994)havedatedtheKrabibasinaslateMidEocene,andofsimilarageto
thelocalityofPondaung,Burma,becauseofsimilaritiesbetweentheanthracothereassociationsat
bothsites.However,theybasedtheirworkoninformationfromDucrocqetal.(1992)thathassince
been updated. The anthracotheres from Krabi are somewhat more derived than those from
Pondaung, and morphologically closer to forms described from the basal Oligocene ofEurope
(Monteviale, Italy) and China (Ducrocq 1993, 1994a). In addition, the recently described
anthropoid primate Siamopithecus eocaenus Chaimanee, Jaeger, Suteethorn and Ducrocq, 1997,
displays strong similarities to Pondaungia cotteri from Pondaung, but shows somewhat more
deriveddentalfeaturescomparedtothoseoftheBurmesespecies.Wethereforeconcludethatthe
Krabifaunaismoreprobably LateEoceneinageratherthanolder.
DentalremainsofasmallartiodactylhavebeencollectedfromtheLateEoceneBangMarkpit
attheKrabimineinsouthernThailand.Themorphologyandstructureofthesefossilsallowthem
to be attributed to a suid and they are placed in a new genus and species. This new form from
ThailandfurtherillustratestheearlydifferentiationofSuidaeinSouth-eastAsia.
[Palaeontology,Vol.41,Part1,1998,pp.147-1561 ©ThePalaeontologicalAssociation
148 PALAEONTOLOGY,VOLUME41
text-fig.1.MapofThailandshowinglocationofthe
Krabi Basin fossil site. The mine that yielded the
specimens of S. banmarkensis reported here, is
representedbytheletter F.
SYSTEMATIC PALAEONTOLOGY
OrderartiodactylaOwen, 1848
FamilysuidaeGray, 1821
Genussiamochoerusgen. nov.
Derivationofname.FromSiam(formernameofThailand),andfrom‘choerus',theGreekforpig.
Typespecies.Siamochoerusbanmarkensissp.nov.
Diagnosis. Asforspecies.
Siamochoerusbanmarkensisgen. etsp. nov.
Text-figures2-4
Derivationofname.FromBangBanMark,thetypelocality.
Holotype.AleftlowerjawwithP4-M3;SpecimenNo.TF2905(Text-figs2-3),CollectionsoftheDepartment
ofMineralResources,Bangkok.
A2d9d0i6t)i;oananldmfatreargimaeln.tAalrlyflrefotmmBaxainlglaBraynwiMtahrkd:amiasogleadtedMlJe-ftMP33((TTFF22990673;;TTeexxtt--ffiigg..44ac)-.b);isolatedleftM3(TF
NTypaendloc8a°li1t2y'.1L6i"gNni;telomnigniet.udBea;nbgeBtwaeneMna9r8k°1pi1t',3K5r"aEbiabnadsi9n9,°s8o'ut3h5"erEn).Thailand(latitude:between7°54'49"
Horizon. UpperlevelofthemainligniteseamofBangBanMarkpit(FormationB2,seeBristow1991).The
mammalianfaunaassociatedwiththesuidremainsisidenticaltothatfromWaiLekpit(whichyieldedmost
ofthetaxaknowninKrabi)andBangMarkpit,andfaunalevidenceindicatesaLateEoceneage(seeabove
andDucrocqetal. 1995).
DUCROCQETAL.\EOCENEPIG 149
Table1.DentaldimensionsofSiamochoerusbanmarkensisgen.etsp.nov.(inmm).
Length Width(trig.) Width(tal.)
TTFF22996035 LLeeffttPPM43 109--34 65--34 —
LLeeffttM1 1103--98 107--92 79--64
M2
TTFF22990077 LLLeeeffftttMM332 111828---756 1—09--58 88--34
M
Left 3 13-7 14-6
text-fig.2.SMiaarmkoc,hosoeurtuhsbTahanimlaarnkde,nsai,slgaebina.l,etasnpd.nbo,vl.i;ngTuFal2v9i0e5w;s.lefStcalloewebrarjarwepprreesseenrtvsin1g0Pm4m-.M3;BangBan
DPi4agwniotshiss.maPlrlimmiettiavceosnuiidd,acnldosheytpooPcroonpiadlaaenodchloaecrkuisngpusailplaursaicnonsiizde..PLo3wseimrplmeollaarcksinsghoawmeatamcaornkiedd,
increaseinsizefromfronttoback,withthemesiallobewiderthandistallobe,asmallhypoconulid
andextremelyweakaccessorycusps. M elongatedwithtwo-cusped hypoconulid. Uppermolars
3
;
150 PALAEONTOLOGY,VOLUME41
C
tBeaxntg-fiBga.n3M.aSrika,moscohuotehrTuhsabialnanmda;rkLeantseisEogcene.neeta,spl.abnioavl,.;b,hoolccoltuyspael,,TanFd2c9,05li;ngluefatllvoiwewesr.dSecnatlaelbraorwsr(ePp4-reMse3n)t
1mm.
M
simple, lacking accessory cusps. 3 without distally salient talon. Enamel very finely wrinkled.
MeasurementsgiveninTable 1.
Description. Thelowerjaw is laterallycrushed, so that itis not possible to knowwithcertainty
ifthehorizontalramuswaswide,asisgenerallythecaseinsuids.However,thebaseofthemandible
is broken and itcanbe supposedthatitwas relativelydeep (Text-fig. 2). Threedental foramina
occuronthemandible oneundertheposteriorrootofP oneundertheposteriorrootofP and
athteThihpreods,tiwesrholiiaoctrhedeisnPda3lsoaofntdthhee:thssemyaflmrlpeahsgytms,eiunsntdarreeyractlhhoeewseaPrnt2je.ariworverroyotproof2,bMab4l.yOnbetlhoenlgintgouatlhefascaemoeftihnedimvaindduia3blloen,
thebasisoftheirstateofpreservation. P3displaysa sharptriangularshape, andismoredistally
curvedthanP4.Thereisnoaccessorycusp,andonlyasmalldepressionrunslinguallyfromtheapex
ofthecusptothebaseofthecrown. Thetalonidismadeofaweakelongationofthedistalpart
ofthetooth. Aweakcrestispresentonthemesialanddistaledgesofthetooth(Text-fig. 4a-b).
ThemorphologyofP4istypicaloftheancestralconditionin suids. Itistriangular, somewhat
posteriorlycurved.Itdisplaysamaincusp(protoconid)andaverysmalllingualcusp(metaconid
or ‘InnenhugeT according to Stehlin [1899, 1900]) situated on the distolingual edge ofthe main
cusp,butslightlylowerthanit. Theapicesofthesetwocuspsarejoinedbyaveryweakcrest. A
talonidiswelldevelopedandconsistsofasinglelabialcusp(hypoconid)connectedwiththeapex
oftheprotoconidbyastraightcrest.Averyslightcingularspurofenameloccursonthemesialside
ofthecrownandmight beinterpreted asanincipientparaconid. Averyweakcrest runsup the
mesialfaceofthepremolar. Theenamelissmoothandthereisnocingulid(Text-fig. 3).
DUCROCQETAL.\EOCENEPIG
text-fig. 4. Siamochoerus banmarkensis gen. et sp.
nlionvMg.ua1a-l-3,bv,ioecTwcslF,us2ac,l96Tv3i;Fewi.2s9oS0lc7aat;leedlebflteafrtmsaPxri3e.lplraa,eswleainbttiha1lcmramuns.dhebd,
The Mj is an elongated tooth with four main cusps (protoconid, metaconid, hypoconid and
entoconid),themesialcuspsbeingslightlyhigherthanthedistalones.Asmallhypoconulidoccurs
onthedistalsideinamedianposition,andthereareslightswellingsoftheenamelinthetransverse
valleyandinthetrigonidbasinthatmightcorrespondtoincipientaccessorycusps.Weaktransverse
crestsjointhemetaconidtotheprotoconidandtheentoconidtothehypoconid.Themiddleofthe
ldiisntgiunaclt.aAndtilnaybicailnfgaucleisdooMfnltyheocmcoulrasroinstwhaeismteesdi,alsoedtghaetotfhMeal5ntaenrdiotrheanadntpeorsitoerrlioobreliosbselsigahrtelyclweiadrelry
thaMn3tdhiesppolsaytsertihoersloabmee. str2uicstusriemialsarMtox andeMxc2e,pbtuitniittsswliadrtgehrdseiczreea(Tseexst-ffrigo.m3f).ronttorear. This
toothalsopossessesahypoconulidconsistingofalargeposterolabialcuspassociatedwithasmaller
lingualone.ThethirdlobeisjoinedtothemiddleofthesecondlobebyaverylowlongitudinalcreMst.
dTtaohleeosntinwdootpdcaiurftsf,pesraftnrhdaotmtfhtoehresmMyts3hteoefmhtyhopefochgoornloouotlvyiepdse.aroOerns‘eaFplularrmcaohtleeandrpsblyatnhae’stohrfiaglovlnooiwndgiHrsousonoveme.rewmThahanetnihs(io1gl9ha6et8re)dthliesafntwtehlel3
exprMessed.Theenamelofallthemolarsisveryfinelywrinkled,especiallyontheapicesofcuspsand
on 3(Text-fig. 3).
Althoughtheyarebadlycrushed, theuppermolarsdisplayarathersimplestructurewithfour
maincusps(paracone, metacone, protoconeandhypocone) and nowell-definedaccessorycusps.
The‘Furchenplan’ispoorlyexpressed.Thecreststhatjointhedifferentcuspsareweakandlow,
andacingulumoccursonthemesialanddistalfacesandbetweentheparaconeandthemetacone
onthelabialface.Theenamelisfinelywrinkledandtherootsofthemolarsareunfused(Text-fig.
4c).
COMPARISONS
Dental and mandibular features ofsuids include the transversely thick horizontal ramus ofthe
mandible; a dental row that crosses over the body of the mandible from anterolabial to
posterolingual atendencyforthelowercaninetosplayoutlaterally;lingualandlabialflaringof
;
152 PALAEONTOLOGY,VOLUME41
the molars; molars with four main cusps and anterior, median and posterior accessory cusps;
scoring of the main cusps of the molars by three distinct grooves (=‘Furchenplan’ of von
Hunermann(1968);anteriorandposteriorcingulaontheuppermolars; strongelongationofM3
with a complex and polybunous talonid; and unfused molar roots. Other cranial features that
characterize suids, although lacking in the Thai fossil material, are a basicranium in which the
convexglenoidcavityishigherthantheleveloftheocclusalplaneofthecheekteeth,butlacking
apostglenoid ‘stop’ andthusallowinglateralmovementofthejaws(Pickford 1993).
SeveraldentalfeaturesallowustosuggestthatSiamochoerusbelongsnotinTayassuidaebutin
Suidae. Siamochoerus displays the association oflaterally flared lower and upper molars, with
M
pianrcti,piaenntdaucncfeusssoerdymcoulsaprsaronodtsa.dMisotrienoctve‘rF,uraclhtehnopulgahn’i,tadinsepllaoynsgaatepdroto3c,owniitdhaancdomapltiicnyatmeedttaacloonniidd
theP4trigonidoftheThaispecimenisnotfullydevelopedandthetwotrigonidcuspsaredissimilar
in size. In addition, the P3 is about the same size as the P4 (whereas it is generally smaller in
Twaeyraesspuriodbaaeb)l,yaenvdenisltehsessmiomlpalreirzeodf.tIhteitswnoo,tipnodsisciabtliengttohkatntohwewanhteetrhieorrlthoewehrorpirzeomnotlaalrrsa(mPu4sanodftPh2e)
mandible was thick laterally, because the bone was crushed during fossilization however, the
;
unfusedmolarrootstogetherwiththelateralflareofthemolarssuggestthatthemandiblewasthick.
AmajordifficultyinthefamilialattributionofthisspecimenisthattheclassificationofSuoidea
differs from one author to another. Pickford (1988, 1993), for example, considered the genus
PalaeochoerustobelonginSuidae,whilstHellmund(1992)attributedittoTayassuidae.Moreover,
severaltaxaformerlyreferredtoTayassuidaehave,afterrevisions,beenshiftedintoSuidae.This
isthecase, forexample, forPalaeochoeruswaterhousii, nowconsideredby Hellmund(1992)asa
suid (=Dubiotherium waterhousii) and for Odoichoerus uniconus from the Lower Oligocene of
China(TongandZhao1986)whichDucrocq(19946)referredtoSuidae.Asthesepointsofvieware
not shared by different authors, we think it is better to begin by comparing Siamochoeruswith
differentearlysuoidtaxa,includingmembersofTayassuidaeandSuidae.
Tayassuidae
AsthebasictopologicalmorphologyofthemolarsisratherconstantthroughoutthefamilySuidae
(Pickford 1988),thelowerpremolarsofSiamochoerusareavaluableelementthatallowtheThai
speciestobecomparedwithotherknownsuoidtaxa.ThefourthpremolarofSiamochoerusdiffers
markedly from that of members of the Doliochoerinae (a tayassuid subfamily), described by
Ginsburg (1974, p.60), in its distolingually situated with respectto themaincusp (protoconid),
accessory cusp or ‘Innenhugel’ (metaconid), its labially displaced and unique posterior cusp
(hypoconid),itsmuchlessdevelopedparaconid,andinitslowercrestconnectingthehypoconidand
theprotoconid.Also,theP3ofSiamochoerusdisplaysaposteriorcrestrunningfromtheprotoconid
downtothedistalbaseofthecrownwhich,unliketheconditionintayassuids,isnotdividedinto
smallcusplets.
TheoldestknownEuropeansuoidsarethegeneraPropalaeochoerusandDoliochoerusfromthe
Lower Oligocene of Europe (Ginsburg 1974; Hellmund 1992). Both these genera differ from
SiamochoerusinthatP3hasahigherandbetterdevelopeddistalcrest,P4ismuchmoremolarized
and the upper and lowermolars have betterdeveloped accessorycusps andMcrests. Doliochoerus
cqiunegruclyaiodinfftehresufprpoemrSmioalmaorcshaonedrusstrfounrgtehrertailnonitosnshMor3t.ePralaanedocmhooreerusma(sastiavyeassu3i,dbketntoerwndefvreolmoptehde
beginningoftheOligocene, accordingtoGinsburg(1974)andPickford(1993),butconsideredas
asuidbyHellmund(1992))alsodiffersfromtheThaispeciesinitsdistolinguallytomesiolingually
elongateduppermolars,itsmuchstrongercingula,itsbetterdevelopedpreprotocrista,anditsless
bulbouslowermolarsandmoremolarizedP
4.
OdoichoerusuniconuswasdescribedasatayassuidfromtheLowerOligoceneofChinabyTong
andZhao(1986).Recently,Ducrocq(19946),suggestedthat,onthebasisofitsdentalmorphology,
DUCROCQETAL.\EOCENEPIG 153
thisChinesespeciesprobablybelongstoSuidaeratherthantoTayassuidae.Therefore,itseemsthat
Odoichoerusmight beoneoftheearliest knownrepresentatives ofthe family Suidae. This genus
chdriyefpsftoe.rcsoTnfournloigmdaSonindamMZohc3haaoonedr(u1ss9h8ai6rn)peifrtusratsnhmdearlslicemropmlspeizraer,Pe4idtwsiOstdhhoaoiluclthoowaeecrrcuessmsatonordyitbhcueuslptasar,yarbsausmtuuiwsdis,thaTaamuhucicagnhhaemrsomdailasltneadrl
Albanohyus(thelatterbeingconsideredasynonymofTaucanamobyPickford(1993))inwhichP4
issimplerthaninothertayassuidtaxa. TaucanamoisknownfromtheMioceneofEurope(Thenius
1956) and Turkey (Pickford 1979, 1993). The P4 ofSiamochoerus is similar to that figured by
Pickford (1979, fig. 6) and attributed to cf. Taucanamo from Turkey. However, the Turkish
premolardiffersfromthatoftheThaispeciesinitslargersize,thegreaterlingualdevelopmentof
the metaconid, which is more mesially situated, and in its better developed mesial and distal
cingulids. In addition, the lower molars associated with cf. Taucanamo are less elongated, less
cwsaaiaacnnnicdgsesuasttlnsehiiodeder,nylusaMpebc,piu4easblrpiulssytm.moaduSlincfiadfarhemsrmossoacmfrrahreeloolesemmrqrauuisstatsrhiiienasvdnei,atlsMtnsmhoo2aot,dnrtieehsltetoheinolsngsoygaenstugteioa,sefthmaeeSddonifadMfmgrohr3oca,ohmvoioetvesTer.vssuempsra(y.yl‘glFwTemuehrraaechekhyulepamantocptcclbeoearynsnius’isolt)sriayidbssohcapouunortstodperstril.hnyePies3tisxzwpweireteoahsfkoseueTrt.d
The tayassuid Egatochoerusjaegerifrom the Krabi Basin, southThailand (Ducrocq 19946) is
very different from Siamochoerus. Both taxa differ in their size and in their upper and lower
premolarandmolarstructureandmorphology.Giventheirdistincttoothmorphology,Tayassuidae
andSuidaeseemthereforetohavedivergedbeforetheLateEocene.
Suidae
InhisrevisionoftheOligocenesuoidsfromwesternEurope,Hellmund(1992)erectedthenewgenus
Dubiotherium for the Late Oligocene Palaeochoerus waterhousi, and placed it in Suidae.
Dubiotherium and Siamochoerus are similar in overall morphology, but can be distinguished by
asevmeorralefreeatduurceesd:Smieatmaoccohnoiedruasndisasosmheawllhoawtersmaalnlderw;etahkeemrescireasltfcacoennoefcittisnPg3itshceopnrvoetxo;ciotnsMiPd4eaxnhdibtihtes
hypoconid;thelowermolarslacktheenamelcuspulesontheirdistalcingulid,andthe 3ismore
slenderdistally.
According to Pickford (1993, p.242), it is not clear whether early suid-like fossils from Asia
attributedto the genus Propalaeochoerusare suids ortayassuids. Pickford (1993) further argued
thattheEuropeanformsofthisgenus,consideredtobelonginHyotheriinae,mightbetheancestral
groupforothersubfamiliesofpigs.TheonlyhyotheriidknowninthePaleogene(UpperOligocene
inEurope andtheIndiansubcontinent)is Hyotherium. However, Siamochoerusdiffersmarkedly
fromHyotheriuminitsnarrowerP3_4withmuchlessdevelopedtrigonidandtalonid,withoutmesial
and distalcingulid, its slenderlowermolars that lack well developed accessorycusps and labial
cingulid,itsshorteruppermolarswithastrongerincreaseinsizefromfronttoback,theMabsence
ofmesialandmedialdistinctaccessorycusps,andtheabsenceofadistalexpansionofthe 3talon.
OtherEurasiansuidgenera(e.g.theMioceneAureliachoerusandListriodon)aretoospecialized
to berelated, atleast from amorphologicalpointofview, to theThai species. Theydiffer from
Siamochoerus in the same features that distinguish Hyotherium, and Listriodon also exhibits
lophodontcheekteeth.
Amongotherprimitiveartiodactyls, thegenus CebochoerusfromtheMiddle-UpperEoceneof
westernEuropehaspreviouslybeensuggestedasapossibleancestortotheSuidae(Pearson1927).
This hypothesis is now abandoned (Hellmund 1992). The lower molars ofthe European genus
displayfewsimilaritiestothoseofSiamochoerus(bunodontcusps,bulbouslabialtoothwalls,and
faintmesialcingulid).However, CebochoerusdiffersfromSiamochoerusinitsmoreelongaMtedand
massivelowerpremolarswithbetterdevelopedmesialanddistalcusps,itsshorter,lower 3with
moresimplethirdlobe, andindetailsoftheuppermolarswhichhaveexternalcuspsjoinedbya
crestandmoreselenodontinternalcusps.
154 PALAEONTOLOGY,VOLUME41
DISCUSSION
Initsoverallmorphology,thedentalmaterialofSiamochoerusbanmarkensisismoresimilartothat
ofDubiotherium than to other European species, although the ThaiMspecies is obviously more
primitive. The structurally simple lower molars and the elongated 3 with relatively simple
hypoconulid arefoundinboth EuropeanandThai genera. Indeed, thelowermolarsofthe suid
Dubiotheriumwaterhousi(fromtheMP26Europeanlevel(=lateEarlyOligocene))aresimilarto
thoseofSiamochoerusintherelativeheightofthemesialandMdistalcusps,thelackofwelldefined
accessory cuspules, the salient distal hypoconulid on 4_2 (called the enamel knob or
‘Schmelzknospe’ by HellmMund (1992, p.25)), andthe absence ofanaccessorycusp between the
secondandthirdlobeon 3,whichalsohasatwo-cuspedhyopoconulidlobe. Inaddition,P4in
Dubiotherium and Siamochoerus has a crest that connects the apex ofthe protoconid and the
imnesbiootlhinggeunaelrca,orwnietrhofatvheerytosohtho,rtantadlolnacikdsaanddisltaicnkcstpthaeraicnocnipiidenatndmeetnatcoocnoindi.d.NPo3iuspaplesorvmeorlyasrismpalree
attributedtoDubiotherium;thusacomparisonoftheseteethisnotpossible.However,theposterior
lowerpremolars aremore derivedinDubiotherium, reflectingthe separation ofTayassuidae and
SuidaebeforethesefamiliesinvadedEurope.
The Thai species might be related to the previously described Odoichoerus uniconus from the
LowerOligoceneofChina(TongandZhao1986).Indeed,theEarlyOligoceneageattributedtothe
Chinesespeciesdoesnotcontradictsuchanhypothesissinceithasrecentlybeensuggestedthatmost
Chinese fossiliferous localities might be older than currently considered (Ducrocq 1993). In
addition,thesparsemammalfaunaassociatedwithOdoichoerus(cf.Anthracokeryxsp.,Heothema
sp., cf. Indomeryx sp., and Guixial sp.) seems to indicate a Late Eocene rather than an Early
Oligocene age (it should also be noted that the genus Heothema is synonymized with
Anthracotherium by Ducrocq 1992, inpress). Odoichoerusdisplays lowermolars withfourmain
cusps and slight swellings of the enamel in the transverse valleys which might be regarded as
incipientaccessorycusps.ThesestructuresarebetterdMevelopedinSiamochoerusandDubiotherium,
andfullydevelopedinHyotherium. Inaddition, the 3 hypoconulidlobeofOdoichoerusisvery
small,butexhibitsthreedistinctcuspletsthatmightforeshadowfurthercomplexityofthisstructure,
aSsiasmeoecnhoienruSsiaamlsoocMhoocecruurssianndOdoDiucbhiooetrhuesr,iuamn.dTMhe1ilsabmiaarlkefldalrye somfaltlheerltohwaenrMmo2l(arMsxiosb2s0e-rv2e6dpeirn
cent,shorterthan 2)inboththesetaxaandDubiotherium.Inaddition,theP4ofOdoichoerusdoes
notdisplaythelingualcusp(metaconid)asinSiamochoerus,butexhibitsasharpandhighcrestthat
runsfromtheapexoftheprotoconiddowntothedistalendofthecrown.Thedistalhalfofthecrest
shows twoweakcusplets reminiscent ofthe P ofHyotheriumratherthanthatofSiamochoerus.
4
Nevertheless, OdoichoerusmightrepresentanancestortoSiamochoerusbecauseofitssmallersize
andthemoreprimitivemorphologyofP4andthelowermolars.Theseobservationsandtheknown
fossilrecordsuggestthatsuidshadappearedbytheLateEoceneinAsiaandthencolonizedEurope
intheEarlyOligocene. ThisiscontrarytotheopinionofPickford(1993)whosuggestedthatthe
earliestknownsuidsarefromtheUpperOligoceneofEurope.Moreover,ifsuidsoriginatedfrom
Tayassuidae,asPickfordsuggested(1993,p.242),thesplittingprobablyoccurredduringtheLate
Eoceneorevenearlier,becausetheoldestknowntayassuid(Egatochoerusjaegeri)sofardescribed
was found in the Upper Eocene of Thailand (Ducrocq 1994). This species already displays
characteristic tayassuid features, such as the vertical lower canine without labial symphyseal
splaying, the strongly developed trigonid ofP4 and the poorly expressed ‘Furchenplan’ on the
molars,andthussuggeststhatthetayassuidpatternwasachievedbytheearlyormidLateEocene.
Thedifficultyofdistinguishingbetweenearlytayassuidandsuiddentalmorphologies,together
withthepoorlydocumentedfossilrecordofAsianOligocenemammallocalities,haslongobscured
ourknowledgeoftheearliestrepresentativesofsuoids.Inaddition,ourincompleteunderstanding
ofsuoidsystematicsrenderstheiruseforbiostratigraphicalpurposesandstratigraphicalinferences
somewhat doubtful. These apparent deficiencies in the fossil record and related biostratigraphy
clearlydemandfurtherinvestigationofthePaleogenemammallocalitiesofAsia.
1 :
DUCROCQETAL.:EOCENEPIG 155
CONCLUSIONS
At present, the Krabi mammal fauna contains the most primitive known Eurasian tayassuid
{EgatochoerusjaegeriDucrocq,1994),but,itmustbestressedthat,becauseofthewell-differentiated
structureofitsteeth, thisformcannotrepresentthestem-taxonofbothtayassuidsand suids. In
thesameway, Siamochoerusbanmarkensisgen.etsp. nov. fromtheUpperEoceneofThailandis
atruesuidthatdisplaysaffinitieswithEuropeanDubiotheriumandtheChineseOdoichoerus.These
newfinds suggestthat the origin ofsuoidsmight have occurredearlierthanpreviouslythought
obviouslybytheLateEoceneandperhapsevenearlier.Theasyetunknowncommonancestorof
SuidaeandTTayassuidaeprobablyemergedinsouthernAsia,andshouldbesearchedforindeposits
olderthanthosethatyieldedEgatochoerusand Siamochoerus.
Acknowledgements. WorkinThailandwassupportedbytheMissionPaleontologiqueFranchiseenThailande
(MinisteredesAffairesEtrangeres).ThisstudywasconductedinpartduringanAlexandervonHumboldt
Fellowship(S.Ducrocq)attheStaatlichesMuseumfurNaturkunde,Stuttgart,Germany.DrawingsarebyL.
Meslin(Montpellier).ThisispublicationISEM97-083,CNRS.
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stephaneducrocq
JEAN-JACQUESJAEGER
ISEM,UMR5554,C.N.R.S.
Case064,UniversiteMontpellierII
PlaceE.Bataillon,F-34095
Montpelliercedex5,France
[email protected]
[email protected]
YAOWALAKCHAIMANEE
VARAVUDHSUTEETHORN
PaleontologicalSection
GeologicalSurvey
DepartmentofMineralResources
Typescriptreceived29April 1996 RamaVIRoad,Bangkok 10400
Revisedtypescriptreceived5April 1997 Thailand