Table Of Content第42卷 第4期 古 脊 椎 动 物 学 报 pp.261～275
2004年10月 VERTEBRATA PALASIATICA figs.1～6
内蒙古依和苏布晚始新世兔形类1)
孟 津1,2 胡耀明1,2,3
(1中国科学院古脊椎动物与古人类研究所 北京 100044)
(2美国自然历史博物馆古生物学部 纽约 10024)
(3纽约市立大学研究生院及城市学院生物学系 纽约 10016)
摘要: 记述了内蒙古自治区脑木根乡附近依和苏布地点产出的4种兔形类上下颊齿化石。
依和苏布位于脑木根乡东南约18 km处,乌兰希热平台北缘,地理坐标为N42°36.114′,E111°
34.596′。依据化石组合,该点地层年代初步定为晚始新世。4种兔形类分别是:Gobiolagus
major、Hypsimylus yihesubuensis sp. nov.、Desmatolagus cf. D. vetustus以及Desmatolagus sp.。
新发现的 Gobiolagus major下颊齿和该种产自沙拉木伦地区乌兰戈楚层的正型标本
(AMNH26098)在大小和形态上很相似:齿冠较低,侧沟不伸入齿槽,三角座唇侧较舌侧短;m2
跟座明显大于m1跟座;舌侧有釉质脊连接三角座和跟座。根据个体大小和相对低冠的颊
齿,我们将一块带P3～M2的上颌骨归入 Gobiolagus major,成为该种上臼齿的首次记录。上颌
骨腭面门齿孔后缘与P3前缘齐,硬腭后缘止于M1与M2间位置,无前臼齿孔。P3～M2均具
三齿根,唇侧两齿根小。P4非臼齿化,前后脊完全,封闭一V形新月谷。M1嚼面椭圆状,舌
侧内沟短浅。后脊与舌侧的磨蚀面相连,其前方的横向谷应为三角座凹。前、后脊唇侧端应
分别相当于前、后尖。
Hypsimylus yihesubuensis sp. nov.是继北京长辛店北京种后该属的第2个记录。新种较 H.
beijingensis大,齿冠较低,m1较宽。下颊齿高冠,p4臼齿化,稍高于臼齿。下颊齿具有明显的
三叶结构,由下次小尖形成的第三叶强。外侧沟伸入齿槽。依据个体大小,将一带有 P2～4
的上颌骨归入该种。上颊齿相对低冠,P2单根,齿冠两叶,P3～4具三根,P4显著大于P3。与
北美的 Megalagus标本比较表明这类具高冠齿的动物是一种兔形类,而不是建属时认为的宽
臼齿兽类。其下颊齿明显的第三叶结构是年轻个体的属性。
归入 Desmatolagus cf. D. vetustus的标本与Desmatolagus vetustus的正型相似,但其个体稍
小,下跟座后缘直与后者不同。Desmatolagusvetustus仍被认为是链兔属最为原始的成员,它与
其他种的差别是具有较多的原始特征,如颊齿相对低冠、跟座无后褶、无前臼齿孔等。
关键词: 内蒙古依和苏布,晚始新世,兔形类
中图法分类号: Q915. 873 文献标识码: A 文章编号: 1000- 3118(2004)04- 0261- 15
LAGOMORPHS FROM THEYIHESUBULATE EOCENE OF NEI
MONGOL ( INNERMONGOLIA)
MENGJin1,2 HU Yao Ming1,2,3
(1 InstituteofVertebrate Paleontology and Paleoanthropology, ChineseAcademyof SciencesBeijing100044)
(2 Divisionof Paleontology, AmericanMuseumof Natural HistoryNewYork,NY10024 Email:[email protected]
or [email protected])
(3 Biology Program [EEB], GraduateSchool andCityCollege, CityUniversityof New YorkNewYorkNY10016)
Abstract Fourlagomorphs, Gobiolagus major, Hypsimylus yihesubuensis sp. nov. , Desmatolagus cf.
1) 国家重点基础科学研究发展规划项目(编号:G2000077700)和美国国家科学基金项目(编号:EAR- 0120727)资助。
收稿日期:2004- 04- 12
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262 古 脊 椎 动 物 学 报 42卷
D. vetustus,and Desmatolagus sp. from a new late Eocene locality, Yihesubu, Nei Mongol, are de
scribed. Upper dental morphologyof Gobiolagus major is knownfor the first time. Hypsimylus yihesu
buensis sp. nov. helpstodemonstratethisgenusalagomorph,notaeurymylid. Thetaxonomicpositionof
D. vetustus,the mostprimitive speciesof Desmatolagus,isreconsidered.
Key words Yihesubu,NeiMongol,Late Eocene,Lagomorph
1 Introduction
Early Tertiarylagomorphs, such as Lushilagus, Shamolagus, Gobiolagus, Desmatolagus and
Ordolagus , are commoninAsia and represent a main bodyof evidencefororiginandearlyradiation
of the group. Since the report of the Paleogene lagomorphs (Matthew and Granger, 1923) , many
studies have documented the rich fossil records from the region (Teilhard, 1926; Burke, 1941;
Bohlin, 1942; Li , 1965; Sych, 1975; de Muizon, 1977; Huang, 1986, 1987; Tong and Lei ,
1987; Qi , 1988; Tong, 1997; Erbajeva, 1999; Menget al. , 1999; Zhanget al. , 2001). These
taxa are usually considered the basal lagomorphs and are important in understanding the origin and
systematicsof Lagomorpha (McKenna, 1982). These species are also useful in early Tertiary bios
tratigraphic divisionsandcorrelationsbecauseof their distinctive morphologiesandrestrictedgeologi
cal distributions (Huang,1987; Tonget al. ,1995; Wang,1997; Erbajeva and Tyutkova,1997).
Herewe report new material of Gobiolagus, Hypsimylus and Desmatolagus from the locality
Yihesubu, Nei Mongol , China. Fossil mammalsfromthe localitywerefirstfound in 1992 byMeng
Jin. Additional specimenswere collected byMengJin, Hu Yaomingand YeJie in2000. These new
specimens cast newlighton the morphologyof Gobiolagus and Desmatolagus and help to illuminate
the taxonomic position of Hypsimylus beijingensis (Zhai , 1977) , a genus and species previously
considered a eurymylid.
All specimensreported here are collectedfromthelocality Yihesubu (N42°36.114′; E111°34.
596′) , about 18 kilometers southeast to theNomogenvillage. Thislocalityisat the north rimof the
Ulan Shirehplatformand is the east extensionof the ErdenObo cliff. Other taxa collectedfromthe
same locality are preliminarily identified as Ardynomys sp. , Pseudocylindrodon sp. , Lophiomeryx
sp. , and ? Ardynia sp. The fauna is probably late Eocene in age. The specimens are housed in
the Instituteof Vertebrate Paleontology and Paleoanthropology (IVPP) , Chinese Academyof Scien
ces, Beijing. Specimenscomparedare mainlyfromthe collectionsof theAmericanMuseumof Natu
ral History (AMNH) , New York. Wefollow Wood (1940) for most of the terminologyof the lago
morph dental morphology, and note the inconsistency in terminology used for cheek teeth of lago
morphs. Taxonomyof lagomorphsfollowsMcKenna andBell (1997).
2 Systematics
Lagomorpha Brandt, 1855
Leporidae Fischer de Waldheim, 1817
Gobiolagus Burke, 1941
Gobiolagus major Burke, 1941
Referred specimens V 14134, afragmentary right maxilla with P3～M2; V 14135, afrag
mentary right mandible withp4～m2 (Fig. 1).
Description The p4～m2arepreservedinthe right mandible (V14135) andincreasein size
posteriorly (Fig. 1; tooth measurements are in Table 1～2). The morphologiesof the lower cheek
teeth are similarto thoseof thetype specimen (AMNH26098). Thelingual surfacesof theteethare
worn inpreservation but the toothmorphologyiswellpreserved. The cheekteethare semi hypsodont
(or unilaterally hypsodontyof Burke [1941]) and rooted. The hypostriid does not extend into the
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4期 孟 津等:内蒙古依和苏布晚始新世兔形类 263
Fig.1 Upperandlower dentitionof Gobiolagus majorfromlate Eocene Yihesubulocalityof NeiMongol, China
A～C. occlusal, medial andlateral viewsof rightp4～m2 (V14135) ; D. occlusal viewof right P3～M2
(V14134) ; E. the maxillawithcheekteeth (V14134)
alveolus. The trigonidof eachcheektoothis shorterlabiallythanlingually. Theposteriorwallof the
trigonid isformed bythickenamel and does not have aposteriorprojection. The talonidof p4isthe
smallest. The talonidof m2is significantlylargerthanthatof m1. At the lingual sideof eachtooth,
a nearlyvertical enamel ridge connectstheposterolingual cornerof thetrigonidwiththe anterolingual
cornerof the talonid. This ridge isa retainedportionof theworntalonid. The lateral contourof the
talonid is rounded and the enamel thins toward the hypotriid.
The maxilla (V14134) isconsideredfromthe same speciesbecauseof the unilateral hypsodont
teeth that matchthe lowerteethin size (Fig. 1). All teethare damagedat various degrees. Thezy
gomatic processis strong,withitsposterior margin beinglateral to M1. Aventral processispresent
at the anterior baseof the zygoma. Anteromedial to the process is a shallow anteorbital fossa. The
ventralportionof the infraorbitalforamenislateral to the alveolusof P2and hasanestimated diame
terof 2 mm. Fromtheorbit to thefacial region, the infraorbital canal extends anteroventrally. The
floorof theorbit is domed and its surface is uneven; endsof cheek teeth roots are exposed. On the
ventral sideof thepalate,theposteriormarginof theincisiveforamenlevelswiththe anterioredgeof
P3, andtheposterior borderof thepalateisbetweenM1andM2. Adistinctpalatineforamenisme
dial to the anteriorportionof M1andislocatedcompletelywithinthepalatine. There is nopremolar
foramen.
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264 古 脊 椎 动 物 学 报 42卷
P2 is not preserved, but the alveolus showsthat it hasa single root andis not so reducedasin
Desmatolagus. P3～M2are rooted,withtwo small labial rootsanda majorlingualone. These teeth
are unilaterallyhypsodont thelingual crownismuch higherthanthelabialone. Thelingual cusp is
the largest of three cuspson P3. It hasaflat lingual surface andlacksthe hypostria. Therefore,the
lingual cusp is either a large protocone or a combination of protocone and hypocone, according to
Wood (1940). The anterolophislowandextendsto the anterolabial sideof the tooth. Theposterior
crest is strong and extends to the posterior side of the central cusp , but its distal portion was bro
ken. The crescentic valleyis shallow, narrow, and in anopenV shape. The central cusp islarge;
itsposteriorpart was broken. It hasaninclinedlateral surface andis separatedfromthe labial cusp
by a shallowfold that opens anteriorly. The labial cusp is the smallest and lowest oneof the tooth.
The lingual surfaceof P4 is concave, suggesting a shallow hypostria that marks the divisionof
the lingual cusp. The anteroloph andtheposterior crest are complete, reachingthe anterolabial and
posterolabial sideof the tooth, respectively. Theyconfine a deep V shaped crescentic valley. Asin
the molars, the enamel formingthe anteroloph is much thicker than that of the posterior crest. The
central cusp is triangular. Its anterolingual and posterolingual surfaces are steep and covered with
thin enamel. Its lingual surface is a flat , slope area. Anterolabially, the central cusp sends a low
ridge to a small cusp at the anterolabial corner of the tooth. Posterolabialy, the central cusp con
nectswith a largeposterolabial cusp bya short crest. The lateral surfaceof theposterolabial cusp is
rounded.
M1 is oval in occlusal view (Fig. 1). A shallow hypstria is between the protocone and
hypoconeon the lingual surface. The lingual outlinesof the protocone and hypocone are roughly e
qual in size, with the hypocone beingmore linguallypositioned. Ontheocclusal surface, however,
theprotocone is larger than the hypocone. The lingual half of the crown is a large wear surface,
which consistsof alarge anteriorportionanda smallposteriorportion, separated bya comma shaped
fossetteof which the tail points posterolabially. The anterior portionof the wear surface is concave
and continueslaterallyasthe anterolophthatformsa strong shearingedgewalledwiththickenamel.
The comma shapefossette ispresumablythe labial portionof a groove that continueswiththe hypos
tria. Withwear the lingual portion is gone. The inner surface of the fossette is covered with thin
enamel. Lateral to thefossette, awideningof the metaloph indicates a transverselyelongated meta
conule; the metaloph connects lingually with the lingual occlusal surface. Because of the connec
tion, a V shaped crescentic vally is not formed; instead, two valleys are on the labial half of the
tooth. The anterior valleyis in reversedL shape and coveredwith enamel ,whichwe believe is ho
mologouswith the trigon basinof a tribosphenic tooth. The posterior valleyis between the metaloph
andposteroloph and is narrower and lower than the anteriorone. On the labial margin, the endsof
the anteroloph and metaloph are homologous with the paracone and metacone, respectively. These
cusps are intheformof transverse ridgesandfusedlabiallytoformthe rounded surfaceof the tooth.
The posteroloph that extendsfromthe hypocone ends labially as a small cuspule. M2 was broken,
but its general shape is similar to that of M1. M3 is not preserved, but the anterior portionof the
alveolus suggests that the reductionof the tooth is moderate.
Hypsimylus Zhai, 1977
Hypsimylus yihesubuensis sp. nov.
Holotype Afragmentary right lower jawwith eruptingp4 and m1～2 (V 14136) (Fig. 2).
Referred specimens V 14137, an isolated right m2; V 14138, a fragmentary right maxilla
with P2～4 (Fig. 2).
Etymology Specific name after the locality name.
Diagnosis Differs from H. beijingensis in being larger and having lower crown and wider
m1.
Description At the talonid of m1 the mandible measures 9.8 mm deep and 5.6 mm thick
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4期 孟 津等:内蒙古依和苏布晚始新世兔形类 265
Fig.2 Teethof Hypsimylus yihesubuensis sp. nov. fromlate Eocene Yihesubulocalityof NeiMongol, China
A～C. occlusal,lateral andmedial viewsof rightp4～m2 (V14136, holotype) ; D～F. occlusal,lateral and
medial viewsof a right m2 (V14137) ; G. occlusal viewof right P2～P4 (V14138)
(toothmeasurementsare in Table 1～2). Amentalforamenis belowthe talonidof p4onthe lateral
sideof the mandible. One small hole anteriorto theforamenandtwo similaronesonthe medial sur
faceof the mandible areprobablytoothmarks. The massetericfossa terminatesasa blunt knot at the
level of the m2 trigonid. The incisor is not preserved, but the posterior endof the alveolus exposed
on the lingual surfaceof the mandible revealsthat the incisor extendsposteriorlymedial to the roots
of p4 and m1 and ends at the level of the m1 trigonid.
The lower jawisfroma youngindividual (Fig. 2). Afragmentof the dp4posterior root ispre
served on thelateral sideof the eruptingp4. Thep4crownismostlywithinthe mandible, butowing
to breakageof the bone the morphologyof the tooth is visible. The p4 is molariformand appears to
have a higher crown than those of m1 and m2. Nonetheless, the morphologies of these teeth are
quite uniform; thus, our description is focused on m1. The tooth is highcrowned and has strong
roots thisis uncertainforp4that is notfullydeveloped. The lingual sideof the toothcrownislow
er thanthe labialone. The toothis broadat crown base andtapers dorsally. Afterwear, the lingual
© 1995-2005 Tsinghua Tongfang Optical Disc Co., Ltd. All rights reserved.
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266 古 脊 椎 动 物 学 报 42卷
part of the trigonid (the metaconid) becomespointed andis higher thanthe labial portion (thepro
toconid). The trigonid is higher but much shorter than the talonid; the latter is characterized by
presenceof a distinct third lobe,formed bythe hypoconulid. Thus, the tooth is trilobate in crown
view. On thelateral sideof thetooth,the hypostriid,the vertical groove betweentheprotoconidand
hypoconid, iswide. The hypostriidextendsinto the alveolus. Withinthe hypostriid, several minute
enamel ridges runparallel to the columnsformed by the protoconid and hypoconid. On the lingual
side of the tooth, the internal groove betweenthe columnsformed bythe metaconidandentoconidis
narrow and short; it does not extendinto the alveolus. As showninthe lateralor medial viewof the
tooth, the thirdlobeflaresposterodorsally. A narrowand short grooveonthe lateral andmedial side
separatesthe third lobe from the rest of the talonid, respectively. In crown view, the talonid is
about twice the length of the trigonid on V 14136, but this proportion will be changed when the
tooth crown getsworn. The talonid is still longer than the trigonid in deeper worn lower tooth (V
14137). The m2 differs from m1 in being more anteriorly inclined and having a slightly shorter
crown.
The m2 (V 14137) is associated with the species because of the similar size and comparable
morphology. It is larger than Gobiolagus major. Asin the type specimen, it has a broad hypostriid
withinwhich small enamel ridges are present. The talonid of the m2 is rectangular and, although
considerably worn, the trigonid and talonid are separated. The talonid is rectangular, wider than
long, with the thickest enamel at the posterolateral cornerof the tooth.
The maxilla with P2～4 (V 14138) are associated with V 14136 because of its large size,
whichisconsiderablylargerthanthose assignedto Gobiolagus major. Theposterioredgeof the ante
rior zygomatic root levelswith the anterior part of M1. Although the specimen is poorlypreserved,
the structureof P2 showsthat these teeth have not been deeplyworn duringlife; thusthe large size
of these teeth is not resulted from wear. In addition, the upper teeth are relatively lowcrowned,
whichis consistent with the lower teeth assigned to the same species. The P2 has one root. The
crown is bud like, divided bya narrowgroove that extendsanteroposteriorly. Thus, the P2 hasonly
two lobes,withthe labial lobe largerthanlingualone (Fig. 2). A bilobate P2 differsthis specimen
fromthe trilobate P2 of North American and Asian early Tertiary lagomorph species where P2 is
known (Wood, 1940; Burke, 1941;Li , 1965; Sych, 1975; Huang, 1986, 1987; Tong, 1997).
The P3 has one large lingual and two small labial roots. The protocone is anteroposteriorly com
pressed and lingually extended, and shows no hypostria. The central cusp is relativelylateral com
pared to that inother early Tertiary lagomorphs. The posterolateral cornerof P3 was broken. P4 is
much largerthan P3 and hasthree roots. Similarto P3, theprotoconeof P4isalso anteroposteriorly
short but linguallyextended. The central cusp is distinct at itslingual side andfadesawaylabially.
The labial valleyis broad. Theoutlineof either P3or P4,particularly P3,ismore similarto thatof
the typical tribosphenic therian toothpattern. In contrast , the P3 and P4 inother early lagomorphs
usually have anoval contour. The hypostria is absent on P3 and P4. The lingual root of P4 is so
curvedthat the tipof the root pointslabially. A similar conditionisalso truefor P3. TheM1 alveo
lus showsthatM1is slightlywiderthan P4and hastwo reducedlabial rootsanda large lingualone.
Ochotonidae Thomas, 1897
Desmatolagus Matthewet Granger, 1923
Desmatolagus cf. D. vetustus Burke, 1941
Referred specimens V 14139.1, a fragmentary right mandible with p3～m2 (Fig. 3) ; V
14139.2, a fragmentary right mandible with broken p4～m2; V 14139. 3, a fragmentary left
mandible with m1～m2; V14139.4, anisolated right m2; V14140.1, afragmentaryright maxilla
with P3～M2; V14140.2,afragmentaryright maxillawith P3～M2; V14141,afragmentaryright
maxilla with P4～M2; V 14142.1, afragmentary right maxillawith P3～M2; V14142.2, afrag
mentaryright maxilla with P3～M2; V 14142. 3, a fragmentary left maxilla with P3～M2; V
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4期 孟 津等:内蒙古依和苏布晚始新世兔形类 267
14142.4,anright P4 (orM1) (Figs. 3,
4).
Description The incisor alveolus
indicates that the incisor extends posteri
orlyto the trigonid of m2. The mandible
is 6mmdeep and3mmthickat theposi
tionof m1. Thep3is single rooted (Fig.
3; tooth measurements in Table 1～2).
The anterior enamel of the tooth crown
was worn in preservation, but it is still
recognizable that the tooth is only lightly
worn from mastication, suggesting that
the specimen is notfromanold individu
al. The low crown has a lateral fold that
does not enter the alveolus. The p4 is
molariform and the highest lower cheek
tooth. The occlusal dimensions of teeth
increasewhereasthe tooth height decreas
es posteriorly from p4 to m2. The mor
phology of p4 and m1～2 are similar.
The trigonidiswide and hasa thicklayer
of enamel on its posterior wall. A spike
at the midpoint of the posterior wall , Fig.3 A～C. occlusal,lateral andmedial viewsof right
which is most distinct on p4, projects p3～m2of Desmatolagus cf. D. vetustus (V14139.1)
posteriorly. The talonidis narrowandpo fromlate Eocene Yihesubu,NeiMongol
sitionedon the posterolingual sideof the trigonid. The posterior portion of the talonid is worn at a
greater degree than the anterior edge, so that a thin wall was left abut to the posterior base of the
trigonid. The baselineof thewall is straight ,indicatingtransverse movementof the lowerjawduring
mastication. The posterior border of the talonid is straight. The posterolabial corner of the talonid
projects laterally and bears the thickest enamel of the talonid. The hypostriid is broad and does not
enter the alveolus. The m3 is not preserved.
Table1 Measurementsof upper dentitions (mm)
Hypsimylus
Gobio Desmato
yihesubu
lagus lagus Desmatolaguscf. D. vetustus
ensis
major sp.
sp. nov.
V14134 V14138 V14143 V14140.1 V14140.2 V14141 V14142.1 V14142.2 V14142.3 V14142.4
P2L 2.10
P2W 2.40
P3L 2.47 2.78 2.12 1.61 1.37 1.61 1.57
P3W 4.52 5.38 4.56 3.39 2.69 3.81 4.62
P4L 2.62 2.85 1.60 1.37 1.54 1.62 1.53 1.58 1.53
P4W 4.92 8.28 3.48 3.22 4.50 4.56 4.58 5.42 5.45
M1L 2.69 1.53 1.32 1.36 1.53 1.45 1.61
M1W 5.65 3.66 3.26 4.52 4.10 4.48 4.82
M2L 2.56 1.37 1.25 1.38 1.32 1.33 1.23
M2W 5.41 3.35 3.05 4.25 3.47 3.82 4.03
Note:L. length; W. width.
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268 古 脊 椎 动 物 学 报 42卷
Table2 Measurementsof lower dentitions (mm)
Gobiolagus Hypsimylusyihesubuensis Desmato
Desmatolaguscf. D. vetustus
major sp. nov. lagus sp.
V14135 V14136 V14137 V14144 V14139.1 V14139.3 V14139.4
p3L/W(tri) 1.42/ ?
p3L/W(tal) 1.34/ ?
p4L 2.80 3.33 1.45
p4L/W(tri) 1.81/2.91 1.5/3.71 0.76/2.00
p4L/W(tal) 1.00/2.10 1.86/2.34 0.75/1.10
m1L 2.89 3.87 1.68 1.85
m1L/W(tri) 1.77/3.32 1.67/4.00 0.82/2.19 0.92/2.07
m1L/W(tal) 1.23/2.10 2.22/2.91 0.89/1.21 0.97/1.17
m2L 3.12 3.60 3.70 2.95 1.62 1.80 2.46
m2L/W(tri) 1.64/3.52 1.46/3.96 2.05/3.96 1.36/3.48 0.82/2.10 0.89/2.14 1.12/2.41
m2L/W(tal) 1.50/2.45 2.15/2.60 1.68/2.61 1.62/2.18 0.85/1.35 0.93/1.35 1.36/1.36
Note:L. length; W. width;tri. trigonid;tal. talonid.
2
Fig. 4 A～F. occlusal viewsof uppercheekteethof Desmatolagus cf. D. vetustus (V14140.2,V14140.1,
V14142.2,V14141,V14142.1,V14142.3) fromlate Eocene Yihesubulocality,NeiMongol, ranging
roughlyfromthe youngest tooldest
4期 孟 津等:内蒙古依和苏布晚始新世兔形类 269
All the maxillasarefragmentary. Butfromwhat preservedit isclearthat apremolarforamenis
absent. Fig. 4A～Fshows upper cheekteeth roughlyrangingfromthe youngest tooldest. P2is not
preserved. Other cheek tooth has three roots. In unworn P3, the tooth tapers dorsally. The lingual
cusp , presumablytheprotocone,isthelargest. Thereisno hypostria. Ananterolophisabsent. The
crescentic valley opens anteriorly and extends posterolaterally as a curvature. In worn specimen
(Fig. 4F) the valleybecomes short and shallow. The central cusp hasa vertical anterior surface and
gradually becomes narrower posterolaterally. Thefold that separates the central cusp and the labial
cusp isopenand shallow. P4 does not have the hypostria inall specimens. The anterolophandpos
teroloph are complete. The central cusp is distinct. The crescentic valley between the cusp and the
anteroloph is shallowthantheposteriorportion separatingtheposterolophfromthe cusp , so that the
anteriorone is gone first with wear. The external valley is square shaped, with its posterior side
more lingually extended. The posterolabial corner of P4 is more labially extended than the antero
labialone. Withwear P4 andmolarswill become transverselyelongated. The elongationisprimarily
owing to constant growthof the lingual sideof the tooth. Therefore the labial structuresof the tooth,
such as the external valley and the distance from the crescentic valley to the labial margin of the
tooth, change verylittle proportionally.
M1 is similar to P4 in general morphology, but differsfromthe latter in havingits anterolabial
corner more labiallypositioned and in lacking the crescentic valley in anyontogenetic stage of the
tooth. In relativelyyoungspecimens,a shallowhypostriaispresent , but inolder specimensit isab
sent so is the enamel fossette left fromthe labial portionof the hypostria. Compared to the M1of
Gobiolagus major described above, the trigon of M1 in D. vetustus is proportionally more short
ened. The anteriorwallof thetoothisthe major shearingedge,equippedwiththethickest enamelof
the tooth. M2 is narrower thanM1,withitsposterior half beingreduced. M3 is not preserved, but
the alveolus indicates a small , single rooted tooth.
Desmatolagus sp.
Referred specimens V 14143, a fragmentary left maxilla with damaged P3～4; V 14144,
an isolated right m2 (Fig. 5).
Fig.5 A～B. P3 and broken P4of Desmatolagus sp. (V14143) from
late Eocene Yihesubu,NeiMongol
Description P3 hastwo labial roots. Thelingual cusp hasa squaredlingual contour. Alower
ridge is anterior to the central cusp and is separatedfromthe lingual cusp bya gap (Fig. 5). Be
causeof the existenceof the partial loph the tooth is considerablywider labiallythan lingually. The
central cusp is rounded, except at itsposterolabial cornerwhere it is connected to a strongpostero
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labial cusp by enamel crest. P4 is broken, of which the central portion indicates a complete cres
centic valley. The m2 matchesthe upperteethin size. Itslabial side is significantlyhigherthanthe
lingual side.
These two specimensare largerthanthose referredto Desmatolagus cf. D. vetustus but small
er than thoseof Gobiolagus majorand Hypsimylus. The sizeof these specimensiscomparableto D.
robustus and the morphologyof P3 is similar to thatof D. robustus; they differfrom D. robustus ,
however,in havingrelativelylowercrown. Thefragmentarymaterial does notpermit more specific i
dentification.
3 Discussion
3.1 Gobiolagus
Gobiolagus isaleporidgenusproposed byBurke (1941) basedon specimenscollectedfromA
sia. A total of four specieswere named at the time the genuswas established: G. tolmachovi, G.
andrewsi, G. ( ?) major, and G. ( ?) teilhardi. All the type specimens are fragmentary lower
jawsand those of G. ( ?) major and G. ( ?) teilhardi are particularly poor. Thus, these two
species were namedwith uncertainty. Oneof the species, G. ( ?) teilhardi ,wasrelocatedto a dif
ferent genus Ordolagus becauseof its highcrowncheekteeth,inwhichthe toothcrownextendsinto
the alveolus (de Muizon, 1977; Huang, 1986). Compared to other early Tertiary lagomorphs, such
as Desmatolagus , specimensof Gobiolagus are relatively rare. SinceBurke (1941) ,possible Gobio
lagus specimens have been reported from several localities of Asia (Li , 1965; Qi , 1988; Tong ,
1997; Erbajeva, 1999;Menget al. ,1999; Zhanget al. ,2001) ,all represented bypoor specimens.
Possible upper teethof Gobiolagus (V 3012) were first describedfrom the Shara Murun beds
at UlaUsu,Nei Mongol (Li , 1965) , but these teethwere consideredto belongto theNorthAmeri
can Mytonolagus (Qi , 1988). Qi (1988) noted presence of the hypostria on the upper molars of
Li’s specimen,which does not seemconsistent with the relativelylowcrowned lower cheek teethof
Gobiolagus. However,the hypostriaonV3012isonlyhalfof the crown height (Li ,1965) andmay
be lost with further wear. Qi (1988) considered an upper jaw from the Shara Murun beds, Ula
Usu, to be from G. tolmachovi. Absence of the hypostria and transverse extension of the upper
cheek teeth are the mainfeatures bywhich Qi assigned the specimen to G. tolmachovi. However,
Ordolagus teilhardi has high crown lower teeth but its upper teeth lack the hypostria (Huang,
1986). In addition, the widthof the upper toothof earlylagomorphsis highly related to the degree
of wear (Huang, 1986, 1987). The specimen described by Qi (1988) is heavilyworn so that its
width isat least partlyage related. The associationof the upper andlowerteethweproposed here is
not certain either, but the coexistence of these specimens in the same locality and their matching
size andmorphologyfavorstheirassociation. Basedon similarmaxillaryfeatures but smaller size and
non molariform P4with the maxilla referred to G. tolmachovi byQi (1988) , Zhanget al. (2001)
named a species, G. lii , from the Yuli member of the Heti Formation in Yuanqu, Shanxi
Province. The type specimenof this species is a maxilla with P3～M2.
Gobiolagus major has been the largest lagomorph of early Tertiary in Asia and is now only
smaller thanthe specimenassignedto Hypsimylus (see below). The Yihesubulowerjaw (V14135)
described here iscomparable to the typeof Gobiolagus major (AMNH26098). The cheekteeth are
slightly smaller than thoseof AMNH26098, but the difference isprobably exaggerated bythe wear
of the lingual surfacesof the Yihesubu teeth. The lingual enamel ridge between the trigonid and ta
lonidandthe relativelywidetalonidonm2are sharedfeaturesof theAMNH26098andthe Yihesubu
lower jaw.
3.2 Hypsimylus
Hypsimylus beijingensis is known from the Changxindian Formation of Beijing (Zhai , 1977) ,
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