Table Of ContentPaleontologica] Research, vol. 6, no. 4. pp. 363-384, December 31, 2002
©
by the Palaeontological Society ofJapan
The Anthracotheriidae (Mammalia; Artiodactyla)
from the Eocene Pondaung Formation (Myanmar)
and comments on some other anthracotheres
from the Eocene of Asia
TAKEHISA TSUBAMOTO1 MASANARU TAKAI', NAOKO EGI1 NOBUO SHIGEHARA',
, ,
SOE THURA TUN\ AYE KO AUNG3 AUNG NAING SOE2 AND TIN THEIN4
,
Primate Research Institute. Kyoto University, Inuyama, Aichi, 484-8506, Japan (e-mail: [email protected])
'Department ofGeology, University ofPathein, Pathein, Myanmar
^Department ofGeology, Dagon University, Yangon, Myanmar
'Department ofGeology, University ofYangon. Yangon, Myanmar
Received May 9. 2002: Revised manuscript accepted October 7, 2002
Abstract. We reevaluate the classifications ofthe anthracotheres (Mammalia; Artiodactyla) from the latest
middle Eocene Pondaung Formation (central Myanmar), mentioning other anthracotheres from the Eocene
of Asia. The three anthracotheriid genera previously known from the Pondaung Formation, Anthra-
cothema, Anthracokeryx, and Anthracohyus, are synonymized intoAnthracotherium. As many as 13 species
had been recognized in the Pondaung anthracotheres, but they are summarized into four species
(Anthracotherium pangan, Anthracotherium crassum, Anthracotherium birmanicum, and Anthracotherium
—
tenuis), based on the difference of Mi size ( body size). Dental morphology in each species indicates high
variation, and the four species are not separable based on their dental morphology. The dental morphology
ofthe PondaungAnthracotherium species is distinct from that ofother species and is the most primitive. In
addition, the Pondaung Anthracotherium species are the oldest of the genus. The genus Anthracotherium
might have originated and rapidly radiated around the Pondaung area during the latest middle Eocene.
Siamotherium pondaungensis described from the Pondaung Formation as an anthracotheriid is synonymized
to Pakkokuhyus lahirii (Artiodactyla; Helohyidae).
Key words: Anthracotheriidae,Anthracotherium, Eocene, Myanmar, Pondaung Formation, systematics
Introduction The fossil record of anthracotheres is abundant and di-
verse throughout the world. In East Asia, they appeared
The Anthracotheriidae is an extinct group of browsing from the middle Eocene and survived until the Plio-
suiform artiodactyls that achieved wide distribution across Pleistocene (Colbert, 1938; Ducrocq, 1997). In Europe,
Eurasia, parts of Africa, and North America from the they appeared during the late Eocene and became extinct in
Eocene to Plio-Pleistocene periods (Black, 1978; Ducrocq, the Miocene. In Africa, they evolved from the late Eocene
1997; Kron and Manning, 1998). Their body size ranges to the Plio-Pleistocene (Black, 1978; Ducrocq, 1994a,
from small, terrier-sized animals to beasts approaching the 1997). In North America, they are recorded from the late
size of a hippopotamus (Black, 1978). Typical early middle Eocene to the early Miocene, but the fossil record
anthracotheres have complete dentition and bunodont or of North American anthracotheres is neither particularly
bunoselenodont molars, five cusped upper molars without abundant nor very diverse (Kron and Manning, 1998).
hypocone and four cusped lower molars without paraconid In regard to the anthracotheres' phyletic relationships,
(Ducrocq et al, 1996). Their low-crowned teeth and fre- traditionally, most researchers have considered that
quent occurrence in paleochannel deposits suggest habits anthracotheres might have originated from a helohyid stock
and habitat similar to those of modern hippos (Kron and (Pilgrim, 1928, 1940, Coombs and Coombs, 1977; Ducrocq
Manning, 1998). et ai, 1997) or from diacodexoid forms (Ducrocq, 1994b),
364 Takehisa Tsubamoto et al.
and that they might have been the ancestors of extant hip- India, Kolkata, India; IVPP = Institute of Vertebrate
pos because some types ofanthracotheres are considered to Paleontology and Paleoanthropology, Beijing, China; NSM
have had a hippopotamid mode oflife and a body structure = National Science Museum, Tokyo, Japan; UCMP =
similar to hippos (Black, 1978; Colbert, 1935; Gentry and Museum of Paleontology, University of California,
Hooker, 1988; Thewissen etal, 2001). According to mo- Berkeley, USA.
lecular data (e.g., Nikaido et al, 1999), hippopotamids
comprise a monophyletic clade with cetaceans, so that Materials
anthracotheres might have originated from a stock of the
[Cetacea + Hippopotamidae] clade (Rose, 2001). On the The new collections of Pondaung anthracotheres used
other hand, a few researchers (Pickford, 1983; but see here were discovered in 1997 by Myanmar researchers
Ducrocq, 1994b for discussion) suggested that hippopot- (Pondaung Fossil Expedition Team, 1997; Takai et al,
amids could have originated not from an anthracothere 1999), and in 1998 (November) and 1999 (November) by
stock but from a peccary one (Ducrocq, 1997). Myanmar-Japan joint team (Takai et al, 2000, 2001; Egi
In regard to regional origin, many researchers have con- and Tsubamoto, 2000; Tsubamoto et al, 2000a, b, 2001,
sidered that anthracotheres might have originated in East 2002; Shigehara etal, 2002; Gebo etal, in press). These
Asia during the Eocene (e.g., Pilgrim, 1928; Suteethorn new fossil materials are stored in the National Museum of
et al, 1988; Ducrocq, 1994a, 1999), because Eocene the Union ofMyanmar (Yangon, Myanmar). They are se-
NMMP-KU
anthracotheres of East Asia are abundant and well diversi- rially catalogued under specimen numbers.
NMMP
fied and because they show a primitive bunodont condition stands for National Museum, Myanmar,
(Ducrocq, 1999). Paleontology; and KU for Kyoto University (Japan). The
The anthracotheres from the Eocene Pondaung dental measurements used here are listed in the Appendix.
Formation (Myanmar) are the first mammalian taxa in this
formation to have been described (Pilgrim and Cotter, Geologic setting
1916). They are among the oldest anthracotheres in East
Asia and consist ofthree genera and as many as 13 species The Pondaung Formation is distributed in the western
(Pilgrim and Cotter, 1916; Pilgrim, 1928; Colbert, 1938). part of central Myanmar (Figure 1). The Pondaung
Therefore, many studies have viewed the Pondaung Formation overlies and partially interfingers with the mid-
anthracotheres in relation to the origin and early radiation dle Eocene Tabyin Formation, and is conformably overlain
ofthis group (e.g., Pilgrim and Cotter, 1916; Pilgrim, 1928; by the late Eocene Yaw Formation (Stamp, 1922; Bender,
Colbert, 1938; Coombs and Coombs, 1977; Ducrocq, 1983; Aye Ko Aung, 1999). The Pondaung Formation
1999). consists of alternating mudstone, sandstone, and conglom-
Despite the richness ofthe fossil collections, the classifi- erate, and is subdivided into the "Lower" and "Upper"
cation of the Pondaung anthracotheres has been problem- Members (Aye Ko Aung, 1999). The "LowerMember" is
atic (Pilgrim and Cotter, 1916; Pilgrim, 1928; Colbert, dominated by greenish pebbly sandstone and mudstone and
1938; Holroyd andCiochon, 1991). Thetaxonomic confu- contains only a few fossil leaf fragments in its upper part
sion on the Pondaung anthracotheres is likely to be due to (Aye Ko Aung, 1999). The "Upper Member" is domi-
their highly varied and primitive dental morphology. nated by fine- to medium-grained sandstone and variegated
In this paper, we reevaluate the classification of the mudstone and contains many terrestrial mammalian and
Pondaung anthracotheres based on previously described other vertebrate fossils that indicate a freshwater environ-
fossil materials (Pilgrim and Cotter, 1916; Pilgrim, 1928; ment (Colbert, 1938; Bender, 1983; Aye Ko Aung, 1999;
Colbert, 1938) and new collections. We then also discuss Aung Naing Soe, 1999; Aung Naing Soe etal, 2002). Its
classifications ofsome other Eocene anthracotheres ofEast mammalian fauna and the fission-track age of the "Upper
Asia in relation to the revision of the classification of the Member" (37.2 ± 1.3 Ma) indicate a latest middle Eocene
Pondaung anthracotheres. age (Tsubamoto et al, 2002).
Institutional abbreviations Previous studies on Pondaung anthracotheres
AMNH = American Museum of Natural History, New Pilgrim and Cotter (1916) first described three genera
York, USA; CM = Carnegie Museum of Natural History, {Anthracohyus, Anthracotherium, and Anthracokeryx) and
Pittsburgh, USA; BMNH = The Natural History Museum seven species of anthracotheres from the Pondaung
(formerly British Museum of Natural History), London, Formation. Pilgrim (1928) revised the Pondaung anthra-
United Kingdom; DMR = Department of Mineral Re- cotheres into three genera {Anthracohyus, Anthracothema,
sources, Bangkok, Thailand; GSI = Geological Survey of and Anthracokeryx) and 13 species, describing new
Anthracotheriidae from Myanmar 365
h
Pondaung
MYANMAR
\angon
Figure 1. A. Map ofMyanmarshowingthe location ofthe Pondaungarea. B. Mapofthe Pondaung area showingthe location ofthethree
main regions offossil localities.
materials. Colbert (1938) reviewed the Pondaung anthra- these species were renamed by Pilgrim (1928) as
cotheres. and recognized the same three genera as Pilgrim Anthracothema rubricae, Anthracothema palustre, Anthra-
(1928) and seven to nine species, also describing new ma- cothema pangan, and Anthracothema crassum, respec-
terials. Thus, in the Pondaung Formation, the three tively. Afterwards, A. palustre and (questionably) A.
anthracothere genera Anthracohyus, Anthracothema, and crassum were synonymized to A. pangan by Colbert
Anthracokeryx have been traditionally recognized. All (1938). Therefore, two (or three) species ofthe Pondaung
these three genera were established based on the material Anthracothema were still recognized by him. The genus
from the Pondaung Formation. Most of the anthracothere Anthracothema was characterized by its larger size, weaker
materials collected from the Pondaung Formation have molar styles, and its more conical molarcusps than those of
been assigned to Anthracothema or Anthracokeryx, Anthracokeryx from the Pondaung Formation (Pilgrim,
whereas remains ofAnthracohyus have been very rare. 1928; Colbert, 1938). Recently, Anthracothema was
Anthracohyus was established by Pilgrim and Cotter synonymized to Anthracotherium by Ducrocq (1999).
(1916) and was characterized particularly by the absence or Anthracokeryx was erected by Pilgrim and Cotter (1916).
very feeble development of the styles on the upper molars. They described two species of Anthracokeryx, Anthra-
Originally, this genus included three species, that is, cokeryx birmanicus and Anthracokeryx tenuis. Pilgrim
Anthracohyus choeroides, Anthracohyus rubricae, and (1928) then described six more species of this genus,
Anthracohyus palustris. Subsequently, the latter two spe- namely Anthracokeryx hospes, Anthracokeryx bambusae,
cies were moved to a new genus Anthracothema as deter- Anthracokeryx myaingensis, Anthracokeryx ulnifer,
mined by Pilgrim (1928). This classification is followed Anthracokeryx moriturus, and Anthracokeryx? lahirii.
by Colbert (1938). The only remaining species in the Colbert (1938) later on recognized four to six species ofthe
genus Anthracohyus, A. choeroides, was characterized by Pondaung Anthracokeryx. The genus Anthracokeryx was
the conical cusps on its molars, by the absence or very fee- characterized by its smaller size, better marked molar
ble development ofthe molar styles, and by the mesiodistal styles, and its more crescentic (selenodont) molar cusps
diameter of the upper molar being shorter on the buccal than Anthracothema and Anthracohyus from the Pondaung
side than on the lingual side (Colbert, 1938). Formation (Pilgrim, 1928; Colbert, 1938). On the other
Anthracothema was established by Pilgrim (1928). hand, the taxonomic validity of keeping Anthracokeryx?
Four species of the Pondaung anthracotheres described by lahirii in the Anthracotheriidae was discussed by both
Pilgrim and Cotter (1916) were referred to this genus: Pilgrim (1928) and Colbert (1938). Recently, this species
Anthracohyus rubricae, Anthracohyus palustris, Anthra- was referred to the Helohyidae (Artiodactyla) and renamed
cotherium pangan, and Anthracotherium crassum. All Pakkokuhyus lahirii by Holroyd and Ciochon (1995).
366 Takehisa Tsubamoto et al.
lars, like the lower molars of P. lahirii; (2) the sizes and
M M
cusp configurations of 2 and 3 ofS.pondaungensis well
M M
match those of 2 and M3 ofthe type ofMP. lahirii (GSI B-
766), respectively (e.g., 2 protocone, 3 protocone, and
M M M
3 mMetaconule match 2 talonid basin, 3 talonid basin,
and 3 hypoconulid basin, respectively) (Figure 2); (3) the
upper dental morphology ofS. pondaungensis is similar to
that ofhelohyids, such as Helohyus, in having similardent-
al size, bunodont and conical cusps with enlarged
metaconule, and no or vestigial styles; and additionally, (4)
both S.pondaungensis and P. lahirii have been found only
in the Pondaung Formation. Further discoveries of better
materials are necessary to settle the classification, but fol-
lowing our observations on the dental materials, we treat
Siamotherium pondaungensis as a junior synonym of
Pakkokuhyus lahirii (Helohyidae) in this paper.
cm Dental morphology and size variation
1
of Pondaung anthracotheres and their classification
Figure2. Comparison of Siamotherium pondaungensis and
Pakkokuhyuslahirii. A. M2-3 ofthe typeofSiamotheriumpondaun- Generic status ofAnthracothema and Anthracokeryx
gensis [NMMP-KU 0039 (Kdw 6): a right maxillary fragment with As mentioned above, after the review ofColbert (1938),
M2"3] inocclusal view(reversed). B.M2.jofthetypeofPakkokuhyus the Pondaung anthracotheres have beenclassified intothree
lahirii(GSIB-766:arightmandibularfragmentwithMi_3)inocclusal genera, Anthracohyus, Anthracothema, and Anthracokeryx,
view.
and into as many as 13 species. This is because Colbert
(1938) and earlier researchers (Pilgrim and Cotter, 1916;
On Siamotheriumpondaungensis Pilgrim, 1928) recognized various dental morphologies
among the Pondaung anthracotheres.
Based on a right maxillary fragment with M2"3 (Kdw 6 = However, the differences in dental morphologies
NMMP-KU 0039; Figure 2A) from the Pondaung between two of the genera, Anthracothema and Anthra-
Formation, Siamotheriumpondaungensis was described by cokeryx, in the Pondaung Formation are very subtle com-
Ducrocq et al. (2000) as a new species of Siamotherium pared to other anthracotheriid taxa. In addition, these two
(Anthracotheriidae). Siamotherium was known only from genera have variations in selenodonty (crista development)
the Krabi basin, the late Eocene of Thailand (Suteethorn and style development on the uppermolars, which were the
etal, 1988; Ducrocq, 1999). However, the dentition dis- diagnostic characters for distinguishing them (Figures 3-5;
played by the unique material ofS. pondaungensis matches Pilgrim and Cotter, 1916, plates 2-5; Pilgrim, 1928, plates
that of Pakkokuhyus lahirii (Helohyidae) (Figure 2B) de- 1-4; Colbert, 1938, figs. 41-52). Although Anthra-
scribed from the Pondaung Formation by Pilgrim (1928) cokeryx, the smaller anthracothere group, generally has
and Holroyd and Ciochon (1995) based on a right ratherselenodontmolars with betterdevelopedmolar styles
mandibular fragment with Mi.3. Ducrocq et al. (2000) did compared toAnthracothema, and althoughAnthracothema,
not compare S. pondaungensis with P. lahirii. Although the larger anthracothere group, generally has rather
the upper dentition of P. lahirii has never been described, bunodont molars with less-developed styles compared to
we believe that this upper dental material described as S. Anthracokeryx, the development of selenodonty and styles
pondaungensis shouldbereferredtoP. lahiriiratherthan to is variable. We examined all previously described materi-
AMNH
another taxon because (1) the upper molars of S. als of the Pondaung anthracotheres stored in and
pondaungensis are conical, bunodont, and brachyodontmo- GSI, and recently collected materials in the National
^ Figure3. New upperdental materialsofthe Pondaunganthracotheres (Anthracotherium) inocclusal view (1). A,A'. NMMP-KU0053, an
rrfmiiregganhhgtttmumewapnixpttiehlwrldijatPrah\ywMfwDr3i..atghNmGePM.n'Mt-NMPwM3i-M(tKshPtUeM-reK03o0'U"5p2a6'.i0,r0)a7.J0.r,iNBgah.MtrNMimgaMPhxtMi-lMPKla-3Ur.Ky0UfH3r.7a90g,4Nm5Mea5n,MlteaPfwt-riiMtKghh3Ut?M.m0wa3Kx8..i2l,lNEa.MaryNMlefMPfrtM-amgPKamx-UeinKltl0Ua3wr8iy04t4,hf0r4aPa,"rg.iamgehrntitCgM.hwtiNlM(t'xMh23,.M.MP"FS-.cK(aNolUMreMMb0'a3P~r2-2s7).,K=aU2I.r0ciNg4mh1Mt(1Mm,leaPafnt-ldemifKibtdUumdlala0xer3il2sfl6cra,aarlgeya-
corresponds to A, A', central upperscale corresponds to B-C, and right lowerscale corresponds to D-K).
Anthracotheriidae from Myanmar 367
368 Takehisa Tsubamoto et al.
1"<' ""' \^m.
ft
- vl* *A
' tffcv\S
'-iSl^KJiHTf^UC'*löSI5ffi
BR
j
Figure4. NewupperdentalmaterialsofthePondaunganthracotheres(Anthracotherium)inocclusalview(2). A. NMMP-KU0413,aright
maxillaryfragmentwithP'M'"2. B. NMMP-KU0216, arightmaxillary fragmentwithM". C. NMMP-KU0329,aleftmaxillaryfragmentwith
M'"3 Scale bars =2 cm.
.
Museum ofMyanmar. We did not find any critical differ- with M2"3 (Figure 3D), has large dental size suggesting that
ences in selenodonty and style development between the it is referable to Anthracothema. However, the molar
Pondaung Anthracothema and Anthracokeryx. Further- styles ofthis material aredeveloped as well as ormorethan
more, we did not recognize any dental characteristics sepa- the small molar materials in Figure 3G-K, which may be
rating these two Pondaung anthracotheriid genera. For referable to Anthracokeryx. Therefore, we conclude that
example, NMMP-KU 0056, a right maxillary fragment the two genera are identical to each other.
* Figure 5. Newlowerdentalmaterialsofthe Pondaunganthracotheres(Anthracotherium). A,A',B-C. NMMP-KU0052,arightmandibular
fragmentwith P,P4-M3: A,A',occlusal view(stereopair); B,lingual view; C,buccal view. D,D',E-F. NMMP-KU0086,aleftP4:E,E',occlusal
view (stereopair);F, lingual view; G,buccalview. G. NMMP-KU0330, aleft mandibularfragmentwithMu,inocclusal view. H. NMMP-KU
0419, atalonid partofleft M3, in occlusal view. I. NMMP-KU 0332, aright mandibularfragmentwithM,, inocclusal view. J,K. NMMP-KU
0433, a right P<; J, lingual view; K, occlusal view. Scalebars =2 cm (leftmiddle scale corresponds to A-C, A', and leftlowerscale corresponds
to D-K, D', and right lowerscale corresponds toJ-K).
Anthracotheriidae from Myanmar 369
370 Takehisa Tsubamoto et al.
Anthracotherium Anthracotherium Anthracotherium
from Pondaung chaimanei magnum
Upper left P3
occlusal view
Lower right P
4
occlusal view
med
lingual view
r*N
Figure6. Schematic drawings ofleft P3 and right P4 ofthe PondaungAnthracotherium,Anthracotherium chaimanei, andAnthracotherium
magnum. Abbreviations: pad, paraconid; med, metaconid.
Furthermore, these two genera, Anthracothema and more progressive Anthracotherium species, such as
Anthracokeryx, are also similar to the genus Anthra- Anthracotherium chaimanei from the late Eocene Krabi
cotherium in regard to dental morphology (Pilgrim and basin of Thailand and European Anthracotherium (e.g.,
Cotter, 1916; Pilgrim, 1928; Colbert, 1938). Describing a Anthracotherium magnum from the Oligocène). These
new species of Anthracotherium from the late Eocene premolar characteristics indicate that the Pondaung
Krabi basin of Thailand, Ducrocq (1999) synonymized Anthracotherium species resemble each other in their dent-
Anthracothema to Anthracotherium. He mentioned that al morphology more than they do any other species ofthis
the graduation observed in the style development of P4- genus (Figures 3, 5, 6). The P3 in all materials of the
M\ in therobustness andorientation ofP3 andin thedevel- Pondaung Anthracotherium has a mesiodistally elongated
,
opment ofthe lingual crests on the lower premolars among triangular outline in occlusal view with pre- and
Anthracothema pangan from Pondaung, Anthracotherium postprotocrista extending mesiodistally; whereas the P3 of
chaimanei from Krabi, and Anthracotherium monsvialense A. chaimanei has a more mesiodistally compressed
from Europe probably indicates adirectrelationship among trianglar outline with the pre- and postprotocrista running
these three taxa. We concur with Ducrocq's (1999) con- more diagonally, and that ofA. magnum has a trapezoidal
clusion. In addition, we also synonymizeAnthracokeryxto outline in occlusal view with pre- and postprotocrista run-
Anthracotherium in this paper because Anthracokeryx and ning more diagonally (Figure 6; Ducrocq, 1999). The P4
Anthracothema are not separable from each other, as men- in all materials of the Pondaung Anthracotherium has a
tioned above. All these three genera have bunodont denti- vestigial metaconid but does not have any trace of
tion, quite similar upper and lower molar morphologies to paraconid, whereasA. chaimanei andA. magnum haveboth
one another, and mesiodistally elongated simple P No tiny paraconid and metaconid (Figure 6). The P4 in all ma-
4.
distinctcharacteristics ofdental morphologydistinguish the terials ofthe PondaungAnthracotherium is less selenodont
three genera. and has much weaker styles than those in the P4 ofA. mag-
num and A. chaimanei, as mentioned by Ducrocq (1999).
Specific identification Also, the development of the lingual crests on the lower
Amongthe species ofthe genusAnthracotherium,defini- premolars of the Pondaung Anthracotherium is weaker
tive characteristics in upper and lower posterior premolars (Ducrocq, 1999). In such premolar morphologies, there
distinguish the Pondaung Anthracotherium species from are no critical characteristics that distinguish any group
Anthracotheriidae from Myanmar 371
800
AMNH 20027
AMNH 2001
I
AMNH20011 (right) Xs 500
NMMP-KU0053
GSIB621
GS1B756(right)
GASM1NBH75250017(left) AAMMNNHH2200001177((lreifgth)t)
AMNH20017(right]
NMMP-KU0052
M| M; M3 M' M2 Mi
Figure 7. Size change (line chan) ofupperand lower molars ofthe Pondaung anthracotheres in each individual
among the Pondaung Anthracotherium. In addition, al- (Figures 8, 9).
though there are individual variations, the Pondaung Among the dental size distributions (Figures 8, 9), it is
Anthracotherium species are distinct from other noteworthy that the Mi size can be more readily divided
Anthracotherium species in having such molar morpholo- into four groups than the other tooth classes. In general,
gies as weaker selenodonty and weaker development of the first molars are the first of the adult dentition to erupt
styles (Figures 3-5). These characteristics indicate that and express less size variation among the adult dentition.
the Pondaung Anthracotherium species possess the most A number of extant herbivores, including both browsing
primitive dentition within the genus (Ducrocq, 1999). and grazing forms and certain species of hippos and suids,
Similar to the case of the dental morphology, the dental compensate for tooth wear by sequential or delayed tooth
sizes of the Pondaung anthracotheres are highly variable. eruption (Kron and Manning, 1998). As the anterior teeth
Figure 7 shows the line chart of the molar areas (width X (and/or teeth erupting earlier) wear out, the emerging last
length) in individuals of the Pondaung anthracotheres. molars (typically enlarged) take aprogressively greater role
M M
The size of M'A relative to :/: and :/; relative to M'A in in food comminution, resulting in no net loss offeeding ef-
a single individual is not constant among the Pondaung ficiency (Kron and Manning, 1998). Thus, the teeth erupt-
anthracotheres. For example, Mi in GSI B751 is much ing later (posterior molars and premolars) are considered to
smaller than in GSI B617, while M. in the former is rather express much wider dental size variations than do first mo-
larger than in the latter. This kind of variation shown in lars in each species. In particular, lower first molars (Mi)
Figures 7 can be explained by individual variation and can- have been considered to express less size variation com-
not be attributed to specific differences, as mentioned pared to upper first molars (M1), and to correlate very
below. closely to the body size of mammals compared to other
The dental sizes ofeach tooth class of all the Pondaung tooth classes (Gingerich, 1974; Gingerich and Schoeninger,
anthracothere materials are also highly variable (Figures 8, 1979; Legendre, 1986, 1989; Conroy, 1987; Legendre and
9). For example, the size of smallest M' is about 15 mm Roth, 1988; Dagosto and Terranova, 1992; Bown et al,
in width and 14 mm in length, while that of largest M' is 1994). —
about 45 mm in width and 39 mm in length (Figure 8). Therefore, the distributional pattern of Mi size ( body
Such size differences do not support the idea that the size) in the Pondaung anthracotheres (Figure 9) suggests
Pondaung anthracotheres consist of one species. How- that the Pondaung anthracotheres can be divided into four
ever, this distributional pattern of the dental size supports subgroups within a single taxonomic group, that is, four
the argument that these animals belong to the same taxo- species within a single genus, although a very high degree
nomic category (that is, genus) because the scatter plots of of size variation exists particularly in the posterior molars.
the mesiodistal length and buccolingual width of P'A In relation to the specific classification of the Pondaung
M'A are easily fitted to a straight-line by simple regression anthracotheres, we should mention here one dental charac-
372 Takehisa Tsubamoto et al.
M
Figure8. Size distribution ofP and uppermolars ofthe Pondaung anthracotheres.
Figure9. Size distribution ofPMand lowermolars ofthe Pondaung anthracotheres.
