Glires (Mammalia) from the Late Paleocene Bayan Ulan Locality of Inner Mongolia

2005·0.83 MB·English

by  Jin Meng

PUBLISHED BY THE AMERICAN MUSEUM OF NATURAL HISTORY
CENTRAL PARK WEST AT 79TH STREET, NEW YORK, NY 10024
Number 3473, 25 pp., 11 figures, 2 tables May 11, 2005
Glires (Mammalia) from the Late Paleocene
Bayan Ulan Locality of Inner Mongolia
JIN MENG,1 ANDRE´ R. WYSS,2 YAOMING HU,3 YUANQING WANG,4
GABRIEL J. BOWEN,5,6 AND PAUL L. KOCH5
ABSTRACT
Two new early diverging members of Glires, Eomylus bayanulanensis, n.sp. and Palaeo-
mylus lii, n.gen. and n.sp., are described from the late Paleocene Bayan Ulan Fauna, Inner
Mongolia, China. These species add significantly to the diversity of Glires known from the
early Paleogene of Asia. E. bayanulanensis and Palaeomylus sp. come from the lowest level
of the Bayan Ulan section, from which the classic Bayan Ulan Fauna was collected. Palaeo-
mylusliiandspecimensbelongingtotwoothergeneraofmammals,PseudictopsandPalaeos-
tylops,arefoundfromahorizonabout8mabovestratayieldingtheBayanUlanFauna.These
taxa are tentatively regarded as constituting a distinct faunal assemblage, which mayproveto
be of biostratigraphic utility within the region. Two calcanea tentativelyassignedtoGomphos
from a stratigraphic horizon slightly above the bed producing P. lii suggest the presence of
strata of Early Eocene age in the Bayan Ulan section.
INTRODUCTION field party of the Central AsiaticExpeditions
Paleontological investigation in the Bayan (CAE) of the American Museum of Natural
Ulan area traces back to the 1920s, when a History visitedtheNomKhongShirehofIn-
1DivisionofPaleontology,AmericanMuseumofNaturalHistory([email protected]).
2DepartmentofGeologicalSciences,UniversityofCalifornia,SantaBarbara,CA,93106 ([email protected]).
3Division of Paleontology, American Museum of Natural History ([email protected]); Institute of Vertebrate Pale-
ontologyandPaleoanthropology,ChineseAcademyofSciences;BiologyProgram(EEB),GraduateSchoolandCity
College,CityUniversityofNewYork.
4InstituteofVertebratePaleontologyandPaleoanthropology,ChineseAcademyofSciences,P.O.Box643,Beijing
100044, P.R.China ([email protected]).
5DepartmentofEarthSciences,UniversityofCalifornia,SantaCruz,CA 95064 ([email protected]).
6BiologyDepartment,UniversityofUtah,SaltLakeCity,UT84112 ([email protected]).
CopyrightqAmericanMuseumofNaturalHistory2005 ISSN0003-0082
2 AMERICAN MUSEUM NOVITATES NO. 3473
nerMongoliawhilemakingitswaynortheast which have been published (Meng et al.,
from Shara Murun to Erlian. Nom Khong 1998). Three taxa of Glires have been pre-
Shireh (also known as Nomogen today, viously reported from Bayan Ulan: Eomylus
meaning ‘‘holy’’) is an isolated mesa in the borealis, ?Khaychina elongata, and Tribos-
middleofavastbasin(seeMengetal.,1998, phenomys minutus, of which only T. minutus
for a review of the research history). The has been described in detail (Meng et al.,
Bayan Ulan Fauna was discovered at the 1994; Meng and Wyss, 1994, 2001). Efforts
northernfootofthemesaseveraldecadeslat- to understand the fauna and the stratigraphic
er. First reported by Qi (1979), the fauna has sequence hosting it have been continuously
recently been described (Meng et al., 1998). made,particularlyinthecontextofexploring
Fossils from Bayan Ulan have figured in nu- biotic and environmental changes across the
merous other recent studies (Miao, 1988; Paleocene–Eocene boundary (Bowen et al.,
Meng, 1992; Meng et al., 1994, 1998; Meng 2003, 2005). This interval coincides with a
and Wyss, 1994, 1995, 1997, 2001; Meng criticalperiodofglobalwarmingandthefirst
and McKenna, 1998). Current consensus appearances of several major modern clades
places the Bayan Ulan Fauna in the late Pa- of eutherian mammals in the evolutionary
leocene Gashatan Asian Land MammalAge, historyofmammals(Kochetal.,1992;Bow-
making it one of the earliest Tertiary mam- en et al., 2001, 2002). New specimens were
mal faunas known from the Mongolian Pla- collected from the Bayan Ulan locality in
teau (Meng et al., 1998; Meng and McKen- 2002–2004.Inadditiontothosepertainingto
na, 1998; Ting, 1998). theclassicBayanUlanFauna,asmallsample
Early diverging members of Glires are of fossils was collected from a stratigraphic
known primarilyfromPaleoceneandEocene horizon 8 m above the one containing the
localities of Asia, Bayan Ulan included. typical Bayan Ulan Fauna. As these speci-
Many of these forms are commonly termed mens are associated with precise stratigraph-
eurymylids (or eurymyloids) and mimoton- ic and paleomagnetic data, they enhance un-
ids. Eurymylids are regarded as those mem- derstanding of the biostratigraphy at Bayan
bers of Glires, exclusive of rodents, charac- Ulan considerably (Bowen et al., 2005).
terized by having one pair of enlarged inci- Herein we describe the specimens of Glires
sors in the lower and upper jaws. This con- from Bayan Ulan section, except Tribos-
cept changed recently with the discovery of phenomys, and briefly comment on current
aeurymylidtaxon,Sinomylus,whichhastwo controversies regarding their phylogenetic
pairs of lower and one pair of upper incisors relationships and taxonomy as well as their
(KondrashovandLopatin,2003;Lopatinand biochronologic implications.
Kondrashow, 2003). Mimotonids are gener-
allyregardedasthosemembersofGlires,ex- MATERIAL AND METHODS
clusive of Lagomorpha, having two pairs of
incisors in both the upper and lower jaws. Specimens referred to Eomylus borealis,
Eurymylids and mimotonids are quite di- ?Khaychina elongata, and Tribosphenomys
verse in the Paleogene of Asia and are piv- minutus (Meng et al., 1998) were either sur-
otal to understanding the origin of rodents face collected or acid prepared from copro-
andlagomorphsonthecontinent.Inaddition, lites originating from the beds, which have
these fossils are useful in biostratigraphic produced the other elements of the Bayan
correlations across the region because of Ulan Fauna. Treatment in dilute formic acid
their restricted stratigraphic distribution and revealed the fragmentary bones, isolated
distinctive morphology. teeth, and jaws of small mammals contained
Since Russell and Zhai’s (1987) compila- inthesenodules.Theproducersofthesenod-
tionoftheBayanUlanFauna,additionalcol- ules remain uncertain, but they were likely
lecting has been carried out at the locality, carnivorous mammals, birds, or both (Meng
primarily by Meng and Wyss during field et al., 1994, 1998). Several jaw fragmentsof
seasons in 1987 and 1992–1995. This work members of Glires bear tooth marks and ap-
yielded several new taxa and additional ma- pear etched by stomach acid. Other taxa re-
terial of previously known taxa, most of covered from these nodules includePalaeos-
2005 MENG ET AL.: GLIRES FROM BAYAN ULAN 3
tylops sp., Lambdopsalis bulla, and at least ble on fresh surfaces (thickness: 3 m). Fossil
two ‘‘insectivorans’’ (Meng et al., 1998). mammals including Palaeostylops iturus,
Pseudictops lophiodon, and Palaeomylus, lii
In describing molars and molariform pre-
n.gen.and n.sp.
molars we follow the dental terminology 6. Thinlayeredgrayishgreensiltstone(thickness:
used for Tribosphenomys (Meng and Wyss, 1 m). Fragments of mammals including cal-
2001). Because there is no consistent termi- caneaofGomphos.
7. Lightreddishbrownsiltstone(thickness:1m).
nology for nonmolariform premolars,weuse
8. Deeply weathered, poorly exposed rock in an
general, descriptive terms, such as labial or
areaoflowrelief(thickness:4m).Lateralvar-
lingual cusp, instead of terms that imply ho- iation in the color and texture of weathered
mology, such as protocone and paracone, in rock suggests that this interval may include
describing these teeth. someyoungerrocksfillingerosionalchannels.
Fragmentsoffossiltapirs.
Photographs of most teeth are taken from
9. Light grayish green siltstone (thickness: 0.5
casts.Measurementsweretakenusingdigital m).
calipers. IVPP is the Institute of Vertebrate 10. Reddish brown mudstone and light brownish
Paleontology and Paleoanthropology, Bei- clay (thickness:7.5m).
11. Lightbrownishmudstone(thickness:2.5m).
jing.
12. Lightgrayishgreensiltstone(thickness:4 m).
13. Lightgrayishbrownsandysiltstone(thickness:
STRATIGRAPHY AND FAUNAL LIST 3 m).
14. Reddishbrownblockymudstone(thickness:7
The stratigraphic section measured at the m).
BayanUlansiteispresentedbelow.Thissec- 15. Lightgrayishbrownsandysiltstone(thickness:
tion represents only the lower portion of the 1 m).
16. Reddishbrownmudstone(thickness:3 m).
entire Bayan Ulan sequence. The upper part
17. Lightgrayishgreenmuddysiltstone,withfos-
ofthestratigraphicsequenceinthisareacon- siltapirs(thickness:2.5 m).
sistsofsedimentaryrocksoftheIrdinManha
and Sharamurum formations which make up Endofthesection(top):N43807.8689,E111835.3389.
the Nomogen mesa about 3 km south of the
Bayan Ulan site. The total thickness of the SYSTEMATIC PALEONTOLOGY
measured section is 48 m. An updated list of
SIMPLICIDENTATA WEBER, 1904
the Bayan Ulan Fauna is incorporated in the
description of the section. Additional litho- Eomylus Dashzeveg and Russell, 1988
logical description of the sequenceisprovid-
Eomylus bayanulanensis, new species
ed in Bowen et al. (2005).
Starting point (bottom): N43808.5279, E111835.2559. HOLOTYPE: V14126.1, a right M2 (fig. 1).
REFERRED SPECIMENS: V14126.2, a left
1. Reddish muddy siltstone (thickness: 3 m; the maxilla with M1–3 in which the labial edges
base of the sequence is covered by flat grass- of the teeth are incomplete; V14127.1, a
lands of the Mongolian steppe); fossil mam-
fragmentary right mandible with m1–3;
mals (updated) include: Bayanulanius tenuis,
Dissacusserratus,Pachyaenasp.,Tribosphen- V14127.2, a fragmentary left mandible with
omys minutus,Eomylusbayanulanensis,n.sp., partial m2–3; V14127.3, a fragmentary right
Palaeomylus, n.gen., Pseudictops lophiodon, mandible with broken m2–3 and a separated
Gashatostylops macrodon, Palaeostylops itu-
anterior portion of a left mandible with in-
rus,Hyracolestesermineus,Lambdopsalisbul-
cisor (from the same nodule containing
la,Prionessuscf.,P.lucifer,Pastoralodonla-
custris, Prodinoceras xinjiangensis, Prolim- V14126.2); V14127.4, a left m3 with little
nocyonchowi,Sarcodonminor,Sarcodonpyg- wear.
maeus, Viverravus sp., Perissodactyla indet., LOCALITY AND AGE: Upper Nomogen for-
Leptictidaindet.,and Cimolestaindet.
mation at Bayan Ulan (within the lower 3 m
2. Light grayish green, blocky clay (mudstone)
(thickness:0.5m). of the section), late Paleocene.
3. Reddish brown mudstone, dense and blocky ETYMOLOGY: The trivial name is after that
withoutbeddingstructures(thickness:2.5m). of the locality, Bayan Ulan.
4. Sandy grayish green siltstone. Fragments of
DIAGNOSIS: Eomylus differs from other
fossils including Prodinoceras (thickness: 2
generaofbasalGliresinhavingalargemeta-
m).
5. Reddish brown mudstone, green nodules visi- conule, a distinct hypoconulid on lower mo-
4 AMERICAN MUSEUM NOVITATES NO. 3473
Fig. 1. Upper cheek teeth of Eomylus bayanulanensis, n.sp. a–d, occlusal, lingual, labial, and an-
terior views of a right M2 (V14126.1, holotype); e, f, occlusal and lingual views of left M1–3
(V14126.2).
lars, and a diagonal wear trough on the tal- DESCRIPTION: Association of the upperand
onid. Further differs from Rhombomylus, lower teeth is based on their comparablesiz-
Matutinia, Mimotona, Heomys, Eurymylus, es (tables 1, 2) and morphology. This asso-
Gomphos, and Amar by its anteroposteriorly ciation is supported by the fact that the left
shorteneduppercheekteethandlessexpand- maxilla with M1–3 (V14126.2) and the frag-
ed hypocone shelf. Further differs from Sin- mentary right mandible with broken m2–3
omylus in having a stronger hypocone, more (V14127.3)werepreparedfromasinglecop-
marginally positioned paracone and meta- rolitic nodule. The fragmentary maxilla
cone and more lingually positioned proto- shows that the posterior border of the hard
cone. Further differs from Tribosphenomys palateisevenwithM2.Theposteriormargin
in having more lophatecheekteeth.Thenew of the palate bears a blunt, rounded lip, the
species differs from Eomylus zhigdenensisin postpalatine torus. This lip continues poste-
having upper cheek teeth less anteroposteri- riorly as a ridge that bounds the choanal or-
orly shortened and less unilaterally hyspo- ifice. Lateral to the ridge is a broad, shallow
dont; the hypocone and metaconule more troughinwhichasmallforamenofunknown
prominent; M2 with the paracone and meta- function resides. Two small palatine forami-
coneseparatedbyalabialnotch;lowercheek na occur within the palatine opposite M1.
teeth less transversely extended; the mandi- Upper premolars are unknown. The alve-
ble proportionally shallower. Differs from E. olus of P4 indicates a large (as wide as M1)
borealis in being smaller and molars less double-rooted tooth (fig. 1). The external
transverselyextended,andinhavingapartial edges of M1–3 (V14126.2), prepared from a
anterior cingulid on the lower molars and a coprolitic nodule in which V14126.2 is also
relatively smaller hypoconulid. contained, are seemingly etched by acid (fig.
2005 MENG ET AL.: GLIRES FROM BAYAN ULAN 5
TABLE1 TABLE2
Measurements of Lower Teeth (in mm) Measurements of Upper Teeth (in mm)
L 5 length; W 5 width; tri 5 trigonid; tal 5
talonid; * 5 nonavailable measurement. Length Width
Palaeomyluslii
L L/W (tri) L/W (tal)
V14128
Palaeomyluslii P4 1.43 3.32
V14132 M1 1.84 3.75
p3 1.71 1.55
Eomylusbayanulanensis
m1 2.27 0.87/1.86 1.35/2.06
V14126.1
m2 2.26 0.85/1.97 1.35/2.38
m3 3.05 0.82/1.99 2.15/2.07 M2 1.53 2.51
V14129.1 V14126.2
m1 1.87 0.91/1.87 0.93/2.11 M1 1.44 2.70 (est)
m2 2.04 0.89/2.03 1.14/2.41 M2 1.45 2.80 (est)
m3 2.85 0.74/1.93 2.10/2.23 M3 1.43 2.40 (est)
V14129.2
m1 1.98 0.91/* 0.96/1.98
m2 1.93 0.82/* 1.11/2.27
m3 2.92 0.69/1.96 2.28/2.08 1). The first upper molar resembles M2 ex-
V14130.1 cept in being slightly shorter lingually. The
p3 1.30 */1.17 description of the upper molar is based pri-
p4 1.93 marily on V14126.1 (the holotype; fig. 1).
m1 2.12 1.04/1.77 1.16/1.96
The tooth is low crowned, wider anteriorly
m2 2.44 1.03/2.07 1.37/2.17
than posteriorly, and has one large lingual
V14130.2
and two small labial roots. The protocone is
p4 2.05 1.01/1.79 1.03/1.66
the largest cusp of the tooth and is more lin-
m1 1.89 1.12/1.83 0.78/*
gually extended than the hypocone, although
m3 3.06 0.98/2.21 2.15/2.26
the tips of the two cusps are aligned antero-
V14129.3
posteriorly. Lingually, the protocone and hy-
m3 3.11 1.02/2.08 2.06/2.32
poconeareseparatedbyashallowvalleythat
V14129.5
deepens toward the hypocone. In occlusal
m3 3.08 0.90/2.11 2.16/2.26
view,theprotoconehasalargewearfaceton
V14129.4
its posterior surface; this aligns with the ori-
m2 2.14 0.85/1.94 1.34/2.24
entation of a wear facet on the posterior sur-
V14131
face of the metaconule. The protocone con-
m1 2.27 0.98/1.88 1.20/2.02
nects with the paracone by alow,strongpre-
m2 2.19 0.98/1.95 1.18/2.21
protocrista that forms a curved anterior edge
Palaeomylussp.
of the tooth. A small, transverse enamellake
V14125.1
p4 2.00 1.08/1.73 0.98/1.42 is present at the labial end of the preproto-
m1 2.25 0.94/* 1.27/* cristaandislingualtotheparacone;thissug-
V14125.2 gests the presence of a small paraconule on
m1 1.96 0.91/1.65 1.04/1.94 an unworn tooth. The paracone and meta-
Eomylusbayanulanensis cone are placed on the labial margin of the
V14127.1 tooth. The paracone is larger and more la-
m1 1.73 0.83/1.18 0.91/1.53 bially positioned than the metacone.Thetwo
m2 1.74 0.76/1.43 1.02/1.74 cusps are separated by a notch at the labial
m3 2.22 0.75/1.39 1.67/1.63 edge of the tooth. A small cuspule is at the
Eomylusbayanulanensis posterolabial base of the paracone, probably
V14127.2 representing the mesostyle. The metaconule
m2 1.92 0.82/1.57 1.10/1.76
is larger than the paracone or metacone. It is
m3 2.41 0.56/* 1.86/1.56(est)
connected lingually with the protocone by a
V14127.3
short postprotocrista and labially with the
m3 2.59 0.69/1.46(est) 1.88/165
metacone by the premetaconule crista. Be-
6 AMERICAN MUSEUM NOVITATES NO. 3473
tween the swollen metaconule and the meta- posterior wall of the trigonid near its mid-
cone, an enamel bay is formed. The anterior point. In littlewornspecimens,themesocon-
surface of the metaconule is bare dentine, id is distinct. Following wear the cristid ob-
formingpartoftheconcavetrigonbasin.The liqua becomes a sharp ridge; the lingual sur-
hypocone is distinct and separated from the face of the cristid obliqua forms a flat wear
protocone by a narrow valley. It merges la- facet that merges posteriorly with the facet
bially with the postcingulum, forming the on the posterolingual side of the hypoconid,
hypocone shelf. The hypocone shelf forms obliterating the mesoconid. The entoconid is
an oblique valley that parallels the plane anterior to the hypoconid and separatedfrom
aligning the wear facets on the posterior the hypoconulid by a narrow gap. The gap is
sides of the protocone and metaconule. This deeper on m2 than on m1. The wear facets
concave areaiscoveredwiththinenamelbe- on the anterolabial surfaces of the entoconid
tween the protocone and metaconule,where- and hypoconulid are extensive, forming the
as the rest of the shelf is bare dentine. This lingual wall of a V-shaped talonid basin that
obliqueorientationandthewearfacetsonthe slants from a point between the hypoconid
hypocone shelf fit well against the oblique and hypoconulid to the point between the
wear facets on the lower molars (seebelow). metaconid and entoconid. The valley is ori-
The last upper molar is narrower than M1 ented diagonally, as indicated by the arrow
and M2 and retains the basic molar pattern in figure 2c. Orientations of the valley and
(fig. 1). the wear facets indicate anterolingual move-
The mandible (V14127.2) is 4.82 mm ment of the tooth row during mastication.
deep at m1 and 2.42 mm thick at m2. Tooth The hypoconulid is widely and deeply sep-
marks occur on the medial surface of two arated from the hypoconid. The cusps are
mandibles (V14127.1–2). Breakages in the morerobustonm3thanonm2;nevertheless,
mandibles (V14127.1–2), both in cross sec- the basic pattern of the two teeth is compa-
tion and along the lingual surface, reveal no rable except that the hypoconulid forms a
incisor under the molars. The narrow space distinct third lobe on m3. The posterolingual
ventral to roots of molars also indicates that surfaceofthehypoconulidbearsalargewear
the incisor probably ends anterior to the first facet. Unworn m3 in V14127.4 exhibits a
molar. The incisor preserved in V14127.3 distinct mesoconid. A small enamelpeakris-
shows that it probably extends posteriorly to es between the hypoconid and hypoconulid
the level of p4. On the lateral surface of the in V14127.1 (fig. 2e).
mandible the anterior edge of the masseteric COMPARISON: Eomylus was named based
fossa is aligned with the talonid of m3. on specimens from the late Paleocene of
No lower premolar is preserved. The mo- Mongolia(DashzevegandRussell,1988),in-
lars are all double-rooted. The m1 and m2 cluding E. zhegdenensis and E. borealis. In
differ mainly in m2 being larger (fig.2).The the same study, another taxon, Khaychina
trigonid is short and has a steep posterior elongata, was proposed based on a fragmen-
wall. A partial anterior cingulid occurs labi- tary mandible with molars from the Naran
ally below the protoconid. The protoconid Member of the Naran-Bulak Formation.
and metaconid are subequalin size,thelatter Based on a larger sample of specimens in-
cusp being higher and more anteriorly cluding articulated dentitions from the late
placed. A wear facet is present on the anter- Paleocene Zhigden Member of the Naran
olabial side of the protoconid. A similar fac- Bulak Formation at Tsagan Khushu, Kon-
et,havingthesameorientation,occursonthe drashovandLopatin(2003)wereabletorec-
hypoconid. The paracristid and protocristid ognize that the holotype of E. zhigdenensis,
are complete and straight, enclosing the tri- a fragmentary maxilla with M1–2, and the
gonid basin anteriorly and posteriorly. The holotype of Khaychina elongata (PSS 30-3,
talonid is longer and wider than the trigonid. Dashzeveg and Russell, 1988) areconspecif-
All talonid cusps are high. The hypoconid, ic; thus, Khaychina and K. elongata are ju-
the largest talonid cusp,iselongatedlabially. nior synonyms of Eomylus and E. zhigde-
The cristid obliqua is strong and extends an- nensis, respectively.
terolingually from the hypoconid to join the In the previous study of the Bayan Ulan
2005 MENG ET AL.: GLIRES FROM BAYAN ULAN 7
Fig. 2. Lower cheek teeth of Eomylus bayanulanensis, n.sp. a, b, lateral and medial views of a
partial right mandible with m1–3 (V14127.1); c–e, occlusal, lingual, and labial views of m1–3. Arrow
indicates the direction of trough created by wear.
Fauna, Meng et al. (1998) tentatively re- to another notch between the hypoconid and
ferred some specimens that have the incisor hypoconulid; the hypoconulid is well sepa-
ventral to the cheek teeth and ending at the rated fromboththeentoconidandhypoconid
first molar to ?Khaychina elongata, although and on m3 forms a distinct third lobe. Given
the possibility that these specimensrepresent the observations of Kondrashov and Lopatin
a new taxon was also entertained. The dia- (2003), however, Khaychina should be a ju-
stema on the fragmentary mandible nior synonym of Eomylus. With additional
(V14127.3) appears long, which is the main specimens, particularly upper cheek teeth
reasonforthetentativereferral,butthelower collected since 1998, we here regard these
cheek teeth resemble those of Khaychina Bayan Ulan specimens as a new species.
elongata (DashzevegandRussell,1988).For Eomylus bayanulanensis differs from E.
instance, as recognized by Dashzeveg and zhigdenensisinhavinguppercheekteethless
Russell (1988: 143), an oblique groove anteroposteriorly shortened and less unilat-
crossesthetalonidbasinofEomylusfromthe erally hyspodont. In addition, the molar hy-
notch between the metaconid and entoconid poconeandmetoconulearemoredistinctand
8 AMERICAN MUSEUM NOVITATES NO. 3473
there is a labial notch between the paracone Eomylus bayanulanensis is the smallestof
and metacone on M2. The lower cheek teeth the threespecies,althoughthesizedifferenc-
of the two species are similar in size and es between these species are not significant.
shape. The m1 and m2 of Eomylus bayanu- Its m1 and m2 are less transverselyextended
lanensis are less transversely extended. In than thoseofE. borealis.Thepartialanterior
contrast, these teeth are wider than long in cingulid on the lower molars and relatively
E. zhigdenensis. The partial anteriorcingulid smaller hypoconulid of E. bayanulanensis
on the lower molars of E. bayanulanensis is also distinguish it from E. borealis.
not seen in E. zhigdenensis. In addition, the
mandibleofE.bayanulanensisisproportion- Palaeomylus, new genus
ally shallower than that of E. zhigdenensis.
TYPE SPECIES: Palaeomylus lii, n.sp., the
Eomylus borealis was originally referred
only species of the genus.
to Mimotona and distinguished from other
DIAGNOSIS: Differs from other early di-
species of Mimotona by the presence of a
verging members of Glires in having on up-
single incisorin themandible(ChowandQi,
per molars a weak centrocrista between the
1978). This difference was considered suffi-
paracone and metacone, distinct conules
cient to place the specimen in the eurymylid
close to the paracone and metacone but dis-
genus Eomylus (Dashzeveg and Russell,
tant from the protocone, and a distinct hy-
1988). Eomylus borealis differed from E.
polophid connecting the pointed entoconid
zhigdenensis ‘‘by its greater size and by the
and the hypoconid on m3. Further differs
presence of a deep groove in the lower mo-
from Mimotona, Gomphos, and Sinomylusin
lars between the hypoconid and the hypo-
possessing only a single, evergrowing lower
conulid’’ (Dashzeveg and Russell, 1988:
incisor (presumably di2) in each lower jaw.
138). However, the lower dentition referred
Further differs from Sinomylus in beinglarg-
to E. zhigdenensis is now considered to be
er, cheek teeth less unilaterally hypsodont,
from an unnamed species of Sinomylus and
the apex of the protocone more lingual and
the lower jaw originally referred to Khay-
confluent with the hypocone, weaker lophs
chinaelongata(PSS30-3)isnowtransferred
but more distinct conules, lack of a mesos-
to E. zhigdenensis (KondrashovandLopatin,
tyle, a simpler p4 (the p4 of Sinomylus illus-
2003).DashzevegandRussell(1988)didnot
trated in Dashzeveg and Russell, 1988, has a
make comparisons between the lower jaw of
better developed trigonid), and stronger hy-
E. borealis and PSS 30-3. Although E. bo-
polophid. Further differs from Tribospheno-
realis is larger than E. zhigdenensis (based
mys in being much larger, having a stronger
on PSS 30-3) in terms of lower cheek tooth
hypocone shelf, more labially positioned
dimensions, the ‘‘deep groove in the lower
conules, lack of an external cingulum, lower
molars between the hypoconid and the hy-
teeth more lophate than cuspate, and better
poconulid’’ is no longer diagnostic of E. bo-
developed posterior lobe on m3. Further dif-
realis. This condition is present in the three
fers from Eurymylus, Rhombomylus, and
species currently recognized in the genus.
Matutinia in being smaller and having more
The features that are unique to PSS 30-3 in-
cuspate cheek teeth that possess a less de-
clude a long diastema between the incisor
veloped hypocone shelf. Further differsfrom
andp3,theproclivityoftheincisor,andshal-
Eomylus in having a low, transverse hypo-
lowanteriorpartofthemandible(Dashzeveg
conulid on m1–2, in lacking a diagonal
and Russell, 1988). These features are not
shearing pattern, in having a broader third
preserved in the holotype of E. borealis.
lobe on m3. Further differs from Heomys in
Dashzeveg and Russell (1988: 141), howev-
having less inflated cusps.
er, recognized that PSS 30-3 further differs
‘‘from Eomylus by having lower crowned
Palaeomylus lii, new species
molars with a smallerhypoconulid’’.Thus,it
canberephrasedthatE.borealisdiffersfrom HOLOTYPE: IVPP V14128, a fragmentary
E. zhigdenensis in being larger and having right maxilla with P4 and M1 (fig. 3).
higher crowned molars with larger hypocon- REFERRED SPECIMENS: V14129.1, a frag-
ulids. mentaryleftmandiblewithm1–3;V14129.2,
2005 MENG ET AL.: GLIRES FROM BAYAN ULAN 9
Fig. 3. Occlusal, labial, posterior, and lingual views of P4–M1 of Palaeomylus lii, n.gen. and n.sp.
IVPP V14128, holotype.
a fragmentary left mandible with m1–3; LOCALITY AND AGE: Upper part of the
V14129.3,afragmentaryrightmandiblewith Nomogen Formation at Bayan Ulan, Inner
m3; V14129.4, a fragmentary left mandible Mongolia; late Paleocene.
with m2; V14129.5, a fragmentary right ETYMOLOGY: Palaeo (Greek), ancient; my-
mandible with m3; V14129.6, a fragmentary los (Greek), grinder or millstone, in analogy
left mandible with talonid of m2 (a young with Eurymylus, Eomylus, Rhombomylus,
individual); V14130.1, a fragmentary left and Sinomylus. The trivial name honors Pro-
mandible with p4, m3, and partial m1–2 fessor Chuankui Li for his landmark contri-
(from the same individual of V14130.2); butions to the study of Glires.
V14130.2,afragmentaryrightmandiblewith DESCRIPTION:Theposterioredgeofthean-
p3–m2 (p4 and m1 broken);V14130.3,aleft terior root of the zygoma lies lateral to the
partial femur and innominate (associated anterior half of M2. The tips of the P4–M1
with V14130.1–2); V14131, a fragmentary roots are exposed in the floor of the orbit.
right mandible with m1–2; V14132, a left The third upper premolar is not preserved;
mandible with erupting p4 and m1–3. Nu- its alveolus indicates a double-rooted tooth
merous fragmentary mandibles with broken that is narrower than P4. The nonmolariform
teeth are not numbered. See tables 1 and 2 P4 isalso double-rootedandhasanovalout-
for measurements. line in occlusal view (fig. 3). One cusp oc-
10 AMERICAN MUSEUM NOVITATES NO. 3473
curs at the anterolingual corner of the tooth; ly equal in size and are connected by a weak
itsanteriorsurfacebearsaflatwearfacet.No centrocrista. The paracone isthetransversely
contact facet for P3 occurs on the anterior wider of the two cusps and tapers lingually.
surfaceofP4.Threeridgesemanatefromthe A narrow cingulum is present at the labial
lingual cusp. The anterior ridge (anteroloph) baseoftheparacone.Themetaconeisrough-
extends labially,formingtheanterioredgeof ly conical, but its tip is broken. There is no
the tooth. The central one of the three ridges cingulum on the external side of the meta-
curves to meet the lingual base of the labial cone. The postcingulum is low, long, and
cusp.Asmalldepressionoccursbetweenthis convex posteriorly. The hypocone shelf is
ridge and the anteroloph. The third ridge sizable and concave. The thin enamel that
(posteroloph) runs posteriorly from the lin- covered the shelf has been eliminated by
gual cusp, and bends labially at the poster- wear; the area adjacent to the metaconule
olingual corner of the tooth, extending in a still bears enamel. A contact facet for M2 is
gentle curve to reach thelabialmarginofthe present on the posterior surface of M1.
tooth. The extended posterolingual corner Several fragmentary mandibles bear tooth
marks. The best preserved mandibles are
suggests a hypocone. An elongate basin is
V14132 and V14130.1–2 (figs. 4, 5).
formed between the posterior and the middle
V14132isfromarelativelyyoungindividual
ridges. The labial cusp is conical and a short
because its p4 is not erupted and the molars
ridge projects from it posterolabially to join
are little worn, whereas V14130.1–2 are
the posteroloph.
fromanoldindividual,ofwhichtheteethare
A large lingual root and two small labial
deeply worn. The symphysis of V14132 is
ones occur on M1. The tooth’s lingual sur-
partially preserved and is an inclined,simple
facebearsashallowdepressionborderingthe
articular surface. In V14130.1–2 the sym-
protocone and hypocone. The protocone
physis is much more extensive with uneven
forms a curved ridge that is continuous an-
articular surface, the morphology being pos-
teriorly with the preprotocrista and posteri-
sibly age related. The lower diastema is 4.6
orly with a crescentic hypocone (fig. 3). The
mm long in V14130.1 and 3.2 mm in
preprotocrista, made up of thick enamel, is
V14132; the mandible is 6.7 mm deep at m1
low and curved; it ends labially at the small
and 3.48 mm thick at m2 in V14130.1 and
paraconule. Because the paraconule is posi-
4.8 and 3.0 at the same positions in V14132.
tioned labially very near the paracone, the
There are two mental foramina on the lateral
preprotocrista is unusally long. A short,
surface of the mandible, a large one under
curved postparaconule crista connects the
p3 and a small one under p4. One or two
paraconuleandthebaseoftheparacone.The
very small foramina are also present lateral
preparaconulecristaextendsanterolabiallyto
to the diastema.Ventrallythemandiblebears
the anterolabial base of the paracone. The
numerous small fenestrations. The anterior
boundary between the protocone and hypo-
edge of the masseteric fossa is even with the
cone is indistinct in occlusal view. The latter
m3 trigonid and ends at a knob.
is slightly more lingually positioned. The
The lower dental formula is 1-0-2-3. The
metaconule is as large as the metacone and incisor is slender, measuring 1.39 mm wide
issituatedmuchnearerthemetaconethanthe and 1.75 mm thick in V14130.1, and 1.1 by
protocone. The long postprotocrista is faint 1.36 mm in V14132, again a difference due
near the protocone but strengthens toward to age. It extends posteriorly to the talonid
the metoconule. The metaconule’s posterior of m3 and is positioned ventromedial to the
surface is rounded and covered by enamel. roots of the cheek teeth. Therefore, a protu-
Light wear is reflected on the posterior sur- berance is created on the lingual surface be-
faces of the metaconule and metacone. In low the molars. In V14132, the incisor is
contrast, the anterior surface of the metaco- complete and is 15.7 mm long, of which the
nule is a heavily worn facet of bare dentine, exposed tip is a small portion (2.2 mm). The
which slopes into the large, transversely ori- entire lingual surface of this exposed tip
ented trigon basin. A short premetaconule bears a wear facet. The wear facet is oval
crista connects the metaconule and meta- and forms a step at its base. The tip of the
cone. The paracone and metaconearerough- incisoris ata positionslightlylowerthanthe