Gomphos elkema (Glires, Mammalia) from the Erlian Basin ...pkoch/pdfs/Koch... · MAMMALIA LINNAEUS,...

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Number 3425, 24 pp., 10 figures, 1 table February 27, 2004

Gomphos elkema (Glires, Mammalia) from theErlian Basin: Evidence for the Early Tertiary

Bumbanian Land Mammal Age inNei-Mongol, China

JIN MENG,1 GABRIEL J. BOWEN,2 JIE YE, 3 PAUL L. KOCH,2 SUYIN TING,4

QIAN LI, 3 AND XUN JIN3

ABSTRACT

Dental and postcranial specimens of Gomphos elkema, including lower and upper dentitionand pedal elements, from the Huheboerhe locality, Erlian Basin, Nei-Mongol (Inner Mongolia),are described. Postcranial elements of Gomphos are similar to those of Mimolagus, suggestingaffinity with lagomorphs. Gomphos elkema is a typical Bumbanian taxon, previously knownonly from Mongolia. Gomphos elkema specimens at Huheboerhe indicate occurrence of Bum-banian-equivalent beds and fauna in the region and suggest potential presence of the Paleo-cene–Eocene boundary in the Huheboerhe section.

INTRODUCTION

The Mongolian Plateau has been a majorsource of data for studies of Asian Tertiaryvertebrate paleontology and stratigraphy

1 Division of Paleontology, American Museum of Natural History. e-mail: [email protected] Department of Earth Sciences, University of California, Santa Cruz, CA 95064. e-mail: [email protected] Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, P.O. Box 643, Beijing

100044, P.R. China.4 LSU Museum of Natural Science, Louisiana State University, Baton Rouge, LA 70803. Institute of Vertebrate

Paleontology and Paleoanthropology, Chinese Academy of Sciences, P.O. Box 643, Beijing 100044, P.R. China.e-mail: [email protected]

since the Central Asiatic Expeditions (CAE)of the American Museum of Natural Historyin the 1920s. This region is important in itsabundance of fossil mammals from many lo-calities where strata ranging from the late Pa-

2 NO. 3425AMERICAN MUSEUM NOVITATES

leocene to Miocene are exposed. Most typelocalities for biostratigraphic and chronostra-tigraphic units of the terrestrial Tertiary ofAsia are in this region. These units form theframework for much of the Asian Paleogene.Initially much of this framework rested onresearch by the CAE, but numerous studiesconducted over the last few decades havenow expanded our knowledge of this criticalregion (e.g., Radinsky, 1964; Mellett, 1968;Szalay and McKenna, 1971; Rose, 1981;Jiang, 1983; Li and Ting, 1983; Qi, 1987;Russell and Zhai, 1987; Dashzeveg, 1982;1988; Meng, 1990; Meng et al., 1998). Onekey result of the more recent studies was thediscovery of a new fauna, the type assem-blage for the Bumbanian Asian Land Mam-mal Age (ALMA), and the suggestion thatthe Paleocene–Eocene boundary lies belowthat new faunal level, at the transition be-tween the Naran Member and Bumban Mem-ber of the Naran-Bulak Formation in the Ne-megt region, Mongolia (Dashzeveg, 1988).The fauna from the Bumban Member re-mains the richest early Eocene vertebrate as-semblage in central Asia.

Here we report a new occurrence of fossilmammals that we argue is correlative to theBumban fauna. The new fauna was discov-ered during the field season of 2002 in astratigraphic section along Huheboerhe cliff,an area near the Camp Margetts area of theCAE (Radinsky, 1964; Meng, 1990). Al-though a section in the vicinity was previ-ously reported by the local geological map-ping team (Jiang, 1983; Qi, 1987), only theprobable late Paleocene Nomogen Formationand probable middle Eocene Arshanto andIrdin Manha beds were recognized in thatsection; a disconformity was described be-tween the Nomogen and Arshanto Forma-tions. Our new section shows no significantsedimentological hiatus within the intervalencompassed by the Nomogen and ArshantoFormations. In addition, fossils, dominatedby the gliroid Gomphos elkema, were foundwithin this sequence. Gomphos is a commonelement in Bumbanian faunas of Mongolia,and recent work has shown that some (if notall) of the Bumbanian ALMA is earliest Eo-cene in age (Bowen et al., 2002). Below thislevel in the section, Prodinoceras, a fossiltypical of the Late Paleocene Gashatan

ALMA, was found. Above this level, typicalArshanto and Irdin Manha fossils, predomi-nantly perissodactyls, are abundant. Thesebiostratigraphic observations indicate the po-tential presence of earliest Eocene strata andthe Paleocene–Eocene (P-E) boundary in thissequence, the first evidence of this boundaryin Nei-Mongol (Inner Mongolia). The dis-covery of the new faunal level in the Huhe-boerhe section could lead to enhanced cor-relation of faunas across the P-E boundaryin central Asia.

The P-E boundary is marked by a dra-matic, short-term global warming event andby carbon cycle perturbations that produceda distinctive carbon isotope excursion record-ed in both marine and terrestrial sediments(Burchardt, 1978; Hubbard and Boulter,1983; Rea et al., 1990; Koch et al., 1992;Bown and Kraus, 1993; Gunnell et al., 1993;Wing and Greenwood, 1993; Zachos et al.,1993; Meng and McKenna, 1998; Bowen etal., 2002). These events are roughly coinci-dent with the first appearances of severalmodern orders of placental mammals inNorth America and Europe (Koch et al.,1992; Hooker 1998; Cojan et al., 2000; Bow-en et al., 2001), and several of these sametaxa have been associated with the P-Eboundary carbon isotope excursion in south-ern China (Bowen et al., 2002). Therefore,the discovery of the new fossil level and rec-ognition of the P-E boundary in Nei-Mongol(Inner Mongolia) should shed new light onfaunal evolution coincident with environ-mental change during this critical interval incentral Asia.

Thus far there is only one named speciesof Gomphos, G. elkema Shevyreva, 1975,from the Lower Eocene of Mongolia. Thisspecies is based on a fragmentary lower jaw,which was first named by Shevyreva (1975,in Shevyreva et al., 1975) and redescribed byZhegallo and Shevyreva (1976) and by Dash-zeveg and Russell (1988). Gomphos speci-mens, consisting of teeth and jaws, have pre-viously been reported from the BumbanMember of the Naran-Bulak Formation inTsagan-Khushu, Nemegt Basin, and fromBumban equivalent beds in the Gashato For-mation, Ulan-Nur Basin of Mongolia (Dash-zeveg and Russell, 1988; Dashzeveg, 1988).The specimens described here are the first

2004 3MENG ET AL.: GOMPHOS ELKEMA IN NEI-MONGOL, CHINA

Fig. 1. Locality map of the Erlian Basin (after Meng et al., 1998).

record of this taxon from Nei-Mongol inChina. Several postcranial elements are as-signed to this taxon, and additional dentalmorphology of the species is described. Withthe identification of this typical Bumbanianfossil, we present a preliminary discussionon the correlation and probable location ofthe P-E boundary in the Huheboerhe section.

MATERIALS AND METHODS

All material reported here was collected atthe Huheboerhe section except for two as-tragali that came from Bayan Ulan, approx-imately 25 km to the southwest. The faunais dominated by specimens of Gomphos, astem taxon of lagomorphs. The specimensare fragmentary jaws and isolated postcranialelements. By the frequency of occurrence,

size, and morphology we are able to associ-ate the upper and lower dentition as well aspostcranial elements of Gomphos. Similari-ties to the foot bones of Mimolagus (Bohlin,1951) and to those of articulated mimotonidspecimens from the Bumban member ofMongolia (unpublished material) also help toassociate the teeth and postcranial elements.

INSTITUTIONAL ABBREVIATIONS: IVPP, Insti-tute of Vertebrate Paleontology and Paleo-anthropology, Beijing; AMNH, AmericanMuseum of Natural History.

STRATIGRAPHY

The locality is indicated in figure 1. Fossil-bearing strata in this area were first discov-ered by the CAE (Radinsky, 1964), and were


further investigated by workers from the lo-cal geological mapping team (Jiang, 1983)and the IVPP (Qi, 1987; Meng, 1990). Thelocality is in an area that is now an active oilfield. A section with a total thickness of 57.8meters is described below.

Huheboerhe section (starting point coor-dinate: 43821.8959N, 110844.1869E):

1. Fine-grained sandstone with light gray-green bands; some microbands cross-bed-ded; locally containing light brown muddysiltstone. 4.2 m thick.

2. Grayish white calcareous fine-grainedsandstone; fresh rock hard and darklybanded, but friable after weathering. 0.6 mthick.

3. Fine-grained sandstone with grayish greenbanding, locally containing light brownsiltstone. 13.5 m thick.

4. Interbedded light green-brown sandstoneand medium-coarse grained sandstone con-taining pebble conglomerates; pebbles areprimarily hollow, columnar, calcareousnodules. 10.5 m thick.

5. Light brown, thinly layered fine grainedsandstone. 3 m thick.

6. Grayish green mudstone and claystone. 2.5m thick.

7. Light brown mudstone and claystone, freshsurface showing black manganese nodules,weathered surface light red. 5.5 m thick.

8. Light brownish yellow fine-grained sand-stone, weathered surface pale white. 1 mthick.

9. Brown claystone. 1.5 m thick.10. Gray mudstone and claystone, fresh sur-

face with black manganese nodules, upperbeds richly fossiliferous and dominated byGomphos. 10 m thick.

11. Reddish brown sandy claystone. The baseof the unit is at 48.3 m (43821.7209N;111845.0639E) and contains white calcare-ous nodules. (A small perissodactyl and apossible palaeoryctoid were found in thisbed.). 7.5 m thick.

12. Grayish white, medium-coarse grainedsandstone with pebble conglomerates andcrossbedding. Bottom layers characterizedby fine pebble conglomerates comprisedpredominantly of quartz grains, which istypical of basal Irdin Manha Formation.Rich in fossil fragments of perissodactyls,including tapiroids. 2 m thick.

SYSTEMATIC PALEONTOLOGY

MAMMALIA LINNAEUS, 1758

GLIRES LINNAEUS, 1758

MIMOTONIDAE LI, 1977

Gomphos elkema Shevyreva, 1975

REVISED DIAGNOSIS: Similar to other mim-otonids but differing from other Glires inhaving two pairs of lower incisors. Differingfrom Mimotona in having less transverse up-per teeth, separated paracone and metacone,a longer trigonid, and presence of a meso-style on upper molars and a mesoconid onlower molars. Differing from Anatolmylus inhaving a shallow horizontal ramus (see alsoAverianov, 1994). Differing from Mimolagusin being smaller and having higher crownsof cheekteeth with distinctive cusps and ridg-es.

REFERRED SPECIMENS: IVPP V13509.1, aright mandible with m1–m3; V13509.2, a leftmandible with m2–m3; V13509.3, a rightmandible with m2–m3; V13509.4, a rightmaxilla with P4–M2; V13509.5, a left max-illa with P3–M1; V13509.6, right P4–M1;V13509.7, a right mandible with partial in-cisor; V13509.8, anterior portion of a rightmandible with the major incisor and the al-veolus of the minor incisor; V13510.1, aright calcaneus; V13510.2, a left astragalus;V13510.3, a right cuboid; and V13510.4, aright navicular. In addition to the numberedspecimens, many unnumbered fragmentaryjaws without teeth, isolated incisors, four ad-ditional naviculars, seven cuboids, 23 com-plete and partial astragali, 140 complete andpartial calcanea from Huheboerhe, and twocalcanea from Bayan Ulan were referred tothe species.

LOCALITIES AND AGE: Upper beds of the‘‘Nomogen Formation’’ in the Huheboerhesection, Erlian Basin of Nei-Mongol (InnerMongolia), China; the Bumban Member ofthe Naran-Bulak Formation in Tsagan-Khu-shu, Nemegt Basin; Members II and III inthe Gashato Formation, Ulan-Nur Basin ofMongolia (Dashzeveg and Russell, 1988;Dashzeveg, 1988); and probably the upperpart of the Nomogen Formation at BayanUlan. The fauna contained in the BumbanMember has been conventionally consideredas early Eocene (Dashzeveg, 1988; Ting,


Fig. 2. Lower jaw fragments of Gomphos elkema. A. lateral view of the anterior portion of rightmandible (IVPP V13509.8); arrow indicates alveolus of the second lower incisor. B. Medial view ofleft partial mandible, showing the fenestrae on the ventral portion of the bone (V13509.2). C. Medialview of partial right mandible, with arrow pointing to the posterior end of the lower incisor (V13509.7).D. Lateral view of partial left mandible, with arrow pointing to the anterior edge of the masseteric fossa(V13509.2). Panels A, B, and D, are at the same scale.

1998), although an alternative hypothesisthat the Bumbanian fauna is of Late Paleo-cene age exists (Beard, 1998). Gomphos isknown only from Bumbanian faunas, bywhich the beds containing the taxon in Hu-heboerhe are biostratigraphically correlativeto rocks bearing Bumbanian fossils else-where on the Mongolian Plateau (see below).

DESCRIPTION

MANDIBLE AND MAXILLA: No cranial ele-ments have been discovered except partialmaxillae. The roots of the cheekteeth are ex-posed on the maxillary floor of the orbit, andthe bony floor bears numerous tiny fenestrae.The posterior edge of the anterior root of thezygomatic arch aligns with the anterior halfof M2. The infraorbital foramen is probablyabove the anterior edge of P3. Fragmentarymandibles are preserved (fig. 2). The bodyof the mandible is thick. The angular processaligns in roughly the same plane as the in-cisor. The ventral margin of the horizontalramus is gently curved in lateral view; thedeepest portion of the ramus occurs below

the m1. There are probably two mental fo-ramina: one lateral to p3 and the other lateralto the posterior half of the p4. Along the ven-tral part of the medial surface of the dentarythere are numerous tiny fenestrae (fig. 2B).The masseteric fossa extends anteriorly to apoint even with the anterior edge of m3, end-ing at a blunt, rounded knot (fig. 2D). Thefossa is shallow and broad, lacking a distinctupper crest. The symphyseal area is narrow,roughly surfaced, and unfused. The positionof the mandibular foramen cannot be deter-mined.

TEETH: The dental specimens are fragmen-tary. Still, we recognize the dental formulaof Gomphos as 2?-0–3–3/2-0-2-3 based onjaw specimens that preserve teeth, alveoli,and contact facets on teeth.

Numerous isolated upper and lower inci-sors are available. The enlarged upper inci-sors are typical of gliroid mammals in beingtransversely compressed with enamel cover-ing their anterior (labial) surfaces, wrappingslightly onto the lateral and medial sides, andextending longitudinally the entire length of


the tooth. The labial surface is gently round-ed and the enamel band forms the widest partof the tooth in cross-section. The incisor tipis chisel-shaped, with the lingual surfacebearing a curved wear facet. The incisor pulpcavity is narrow in cross-section. No upperincisors were found in situ. The major lowerincisor is presumably the di2 (Luckett, 1985;Meng and Wyss, 2001; Meng et al., 2003),which lies in the same plane as the cheek-teeth and is ventral to the roots of the cheek-teeth; its posterior end extends posterior tothe m3 (fig. 2C). The exposed section of thelower incisor above the alveolus is short, andin life the tip of the lower incisor was lowerthan the occlusal surface of the cheekteeth.A minor lower incisor (presumably i2) ispresent. Although the tooth crown was bro-ken, the alveolus of this tooth in V13509.8indicates that it is small, procumbent, andposterolateral to the enlarged major incisor(fig. 2A).

P2 is not preserved, but a partial alveolusin IVPP V13509.5 suggests its presence. P3is oval in occlusal view (fig. 3A). The lingualside of the tooth is significantly higher thanthe labial side. A single centrolabial cusp isconical, strong, and lateral to the protocone;it is the highest cusp of the tooth. Its lingualbase has a weak ridge connecting to the pro-tocone. The protocone is crestlike, sendingtwo strong lophs labially. The anterior lophforms the anterior edge of the tooth and ex-tends to the anterolabial corner of the centercusp. The posterior loph is longer than theanterior one and extends to the posterolabialcorner of the tooth. Between the posteriorloph and the center cusp, a broad, concaveshelf basin is formed. No hypocone is pres-ent. A cuspule is present posterolabial to thecenter cusp. No cingulum is present on lin-gual and labial sides of the tooth.

P4 is similar to P3 but is more unilaterallyhypsodont and more transversely elongate(figs. 3, 4; table 1). The unilateral hypsodon-ty decreases from P4 posteriorly. As in P3, alow ridge connects the single, transverselyextended centrolabial cusp to the protocone.The anterior and posterior lophs extend far-ther labially and surround the centrolabialcusp more completely than in P3.

M1 differs from the premolars in beingmore angular and in having a hypocone on

the lingual side and the paracone and meta-cone on the labial side (figs. 3, 4). The hy-pocone is strong and is lower than the pro-tocone. It extends labially as a strong poste-rior loph (or postcingulum). After wear, alarge facet is formed on the protocone andhypocone. The enamel is thick around theprotocone and paracone, both along the edgeand on the occlusal surface of the tooth, andgradually becomes thinner labially. The an-terior edge of the tooth is only slightly higherthan the posterior edge of the precedingtooth. These structures suggest that grindingis the main function of the cheekteeth inGomphos. The lingual surface of the tooth isrounded, with a flat area between the proto-cone and hypocone; a groove is not presentbetween the two cusps. The paracone andmetacone are labially positioned. The para-cone is teardrop shaped and at the anterola-bial corner of the tooth; its tip connects tothe broad loph that comes from the proto-cone. The metacone is slightly more lingualthan the paracone. The metaconule is not dis-tinct and is confluent with the metacone afterwear, such that a broad ridge is formed anddirected toward the protocone. The ridgeconnecting the metaconule and protocone isnarrow. The three main cusps and crests be-tween them form a V shape. A triangular tri-gon basin is present. A distinct mesostyle ispresent on the labial margin and between theparacone and metacone. The tip of thewedge-shaped mesostyle extends lingually tothe trigon basin between the bases of theparacone and metacone. There is no lingualor labial cingulum. M2 is nearly identical toM1 except being slightly narrower (fig. 4).M3 is not preserved, but its presence is in-dicated by a contact facet on the posteriorsurface of M2 in V13509.4.

Lower premolars are not preserved. Frag-mentary mandibles bearing roots and alveoliof cheekteeth (not illustrated) show that thereare two premolars. The p3 is a single-rooted,small, tooth. The p4 is double-rooted and isas long as, but narrower than, the m1. Them1 and m2 are similar, with the former beingslightly larger; both are somewhat squareshaped (figs. 5, 6). The paraconid is absent.The metaconid aligns with the protoconid atthe same level. The metaconid is as thick as,and is higher than, the protoconid. The tri-


Fig. 3. Upper teeth of Gomphos elkema. A. Occlusal view of left P3–M1 (IVPP V13509.5). B.Occlusal view of right P4–M1 (V13509.6).

gonid is anteroposteriorly short. Anterior(paralophid) and posterior (protolophid) ridg-es extend from the protoconid to join the an-terior and posterior side of the metaconid,respectively. A small trigonid basin is pres-ent between the two ridges. The talonid islower than the trigonid and about twice aslong. Immediately posterior to the trigonid,at the longitudinal axis of the tooth, is aninflated mesoconid, which occupies much the

position of the cristid obliqua. The hypocon-id is large and is separated from the proto-conid by a deep, narrow hypoflexid. Similarto the protoconid, the hypoconid is some-what laterally extended. On the lingual sideof the talonid, the mesostylid and entoconidare close to each other, with their bases beingconfluent. The mesostylid is slightly smallerthan the entoconid. The hypoconulid is at thelongitudinal axis of the tooth and protrudes


Fig. 4. Upper teeth of Gomphos elkema (IVPP V). A–C. Occlusal, labial, and lingual views of rightP4–M2 (IVPP V13509.4).

backward as the most posterior point of thetooth. Wear facets are present on the occlusalsurfaces of all teeth, particularly on the pro-toconid and hypoconid. There is no shear

facet along the posterior surface of the tri-gonid, which again suggests that grinding isthe main chewing function of Gomphos.

The m3 differs from m1 and m2 in having


TABLE 1Measurements (in mm) of Teeth of

Gomphos elkema

a more inflated entoconid and hypoconulidsuch that a posterior lobe is formed (figs. 5,6).

POSTCRANIAL ELEMENTS: The astragalus isventrodorsally (or anteroposteriorly) flat (fig.7). The astragalar head is transversely ex-tended. In distal view, the long axis of theconvex astragalonavicular facet has a 30–408angle to the axis of rotation of the bone (fig.7F). The same facet extends posteriorly onthe medial surface to the midpoint of thebone. Breakage is present on the facet in theillustrated specimen, but other specimensshow a continuous, elongated articular sur-face, which is broader laterally than medial-ly. There is no distinct boundary indicatingthe facet for articulation with the cuboid, buta narrow area at the ventrolateral side of thehead may contact the cuboid. This area andthe astragalonavicular facet make a blunt an-gle. In dorsal and ventral views, the head islarge compared to the rest of the bone. Theastragalonavicular facet does not extend tothe ventral side of the bone to contact thesustentacular facet, but extends somewhat tothe dorsal side of the bone. The neck of theastragalus is short and broad. The trochlea istransversely broad and shallow; it is exten-sive on the dorsal side but weak on the ven-tral side. Therefore, the distal astragalotibialfacet is almost absent on the ventral side. Thelateral rim of the trochlea is sharper and

much longer than the medial one, and it ismore posteriorly extended. Anterior to themedial rim of the trochlea, the bone has alarge concave area, so that the bone is thinin this region.

On the ventral astragalar surface, the sus-tentacular facet is large, convex, and roughlyoval in outline. This facet is separated fromthe astragalonavicular facet by a broad, shal-low valley that forms the neck of the bone.The sulcus astragali is well defined as a nar-row, deep trough between the sustentacularfacet and the calcaneoastragalar facet. Thesustentacular hinge is short and shallow.There is no astragalar foramen. The calca-neoastragalar facet is extensive and concaveand is oriented diagonal to the long axis ofthe astragalus.

In lateral view, the astragalar head appearsthicker than the rest of the bone. The astra-galofibular facet is a narrow, semilunarshaped area, and is nearly perpendicular tothe calcaneoastragalar facet.

The calcaneus is dominated by a long tu-ber, which is about half of the total length ofthe bone (measured from the posterior edgeof the calcaneoastragalar facet) (fig. 8). Indorsal (cranial) view, it gradually thickensdistally. The distal end of the tuber is rough-surfaced, suggesting attachment of tendonsin life. In dorsal view, the calcaneoastragalarfacet and the sustentacular facet are alignedat the same level. The calcaneoastragalar fac-et is a narrow convex band in a proximodis-tal orientation, nearly parallel to the long axisof the bone. The sustentacular facet is round-ed and concave. A narrow sulcus calcaneiseparates the two facets.

In plantar view, the anterior plantar tuber-cle is low and blunt. The peroneal process isstrong. Between the process and the anteriorplantar tubercle is a broad, concave area. Adistinct pit is present on the plantar side ofthe peroneal process. On the dorsal side ofthe peroneal process is the calcaneoastragalarfacet. In distal view, the calcaneocuboid facetis large, concave, and oblique, with the lat-eral edge extending more distally than themedial one. Therefore, the calcaneocuboidfacet faces anteromedially. The medial edgeof the facet is notched. Between the notchand the broad groove for the tendon of theflexor fibularis, which is on the plantar side


Fig. 5. Lower teeth of Gomphos elkema. A–C. Occlusal, lingual, and labial views of right m1–m3(IVPP V13509.1).


Fig. 6. Lower teeth of Gomphos elkema. A–B. Occlusal views of left and right m1–m3, respectively(IVPP V13509.2, V13509.3).

of the process bearing the sustentacular facet,is a concave area with a rough surface, inwhich there is a small foramen of unknownfunction. There is no calcaneal canal.

The cuboid is shorter than it is wide (fig.9). The proximal articular surface (or calca-neal facet) is flat and rounded except that themedial side is irregular. This surface is in-clined, with the dorsal edge being more dis-tally located than the ventral edge. The distal

articular surface (facet for metatarsals IV andV) is roughly triangular and concave; its ven-trolateral edge is ridge-shaped. In ventralview, the plantar tubercle is prominent, pro-jecting ventrally from the midportion of thecuboid body. This process is better illustratedin medial and lateral views. Ventral (or dis-tal) to the tubercle is a broad peronealgroove. On the medial side, a central processbears two articular facets, the proximal one


Fig. 7. A–F. Ventral, dorsal, posterior, medial, lateral, and distal views of left astragalus of Gomphoselkema (IVPP V13510.2). Abbreviations: ACu, astagalocuboid facet; AFi, astragalofibular facet; AN,astragalonavicular facet; ATim, medial astragalotibial facet; CaA, calcaneoastragalar facet; sa, sulcusastragali; su, sustentacular facet; suh, sustentacular hinge; trlr, lateral rim of astragalar trochlea; trmr,medial rim of astragalar trochlea.

for the navicular and the distal one for theectocuneiform; both facets are small. Thereis no indication of a contact between the cu-boid and the astragalus. The dorsal side ofthe cuboid is smooth and featureless.

The body of the navicular is proximodis-tally short (fig. 10). Its proximal surface,which receives the astragalar head, is a semi-lunar, smoothly concave fossa that is dorso-ventrally elongated. The lateral edge of thisfossa is notched. The distal surface is flat andpresumably contacts the ectocuneiform and

the mesocuneiform, but there is no boundaryto show the two facets. On the lateral side ofthe body, the facet for the cuboid is smalland concave. The dorsal and the lateral sur-faces of the body are confluent and convex.The plantar process of the navicular is robustand short.

Fragmentary distal ends of tibiae are pre-served (not illustrated). The tibia bears amoderate medial malleolus and a weak pos-terior process; the fibula would not be fusedto the tibia.


Fig. 8. A–E, Dorsal, ventral, lateral, distal, and medial views of the right calcaneus of Gomphoselkema (IVPP V13510.1). Abbreviations: at, anterior plantar tubercle; CaA, calcaneoastragalar facet;CaCu, calcaneocuboid facet; gtff, groove for the tendon of M. flexor fibularis; pp, peroneal process;ptca, calcaneal protuberance; su, sustentacular facet; sus, sustentaculum talus; and tub, tuber of thecalcaneus.


Fig. 9. A–F, Distal, proximal, dorsal, ventral, medial, and lateral views of the right cuboid ofGomphos elkema (IVPP V13510.3). Abbreviations: CaCu, calcaneocuboid facet; CuEc, facet for ecto-cuneiform; CuM45, facet for metatarsals IV and V; CuN, cuboidonavicular facet; and pp, plantar process.

COMPARISON

Zhegallo and Shevyreva (1976) comparedGomphos primarily with Tertiary rodentssuch as Paramys, Reithroparamys, and Lep-totomus. However, because the type and onlyspecimen known to these authors was a frag-mentary lower jaw with m1–m2, distinctivefeatures, such as the second pair of lower in-cisors and upper dentition, were not avail-able. A more complete comparison was madebetween Gomphos and Rhombomylus byDashzeveg and Russell (1988) in order to fa-cilitate identification of specimens collectedfrom Tsagan-Khushu. They recognized thatG. elkema differs from R. turpanensis by thepresence of two lower incisors; by an elon-gate p3; by a molariform p4 with a muchwider talonid; by the lower cheekteeth tend-ing to be subquadrate and the m3 beingshorter with no enlarged third lobe formedby the hypoconulid; by the columns formed

by the protoconid and hypoconid being veryclose together; and by a greater degree ofunilateral hypsodonty. According to Dash-zeveg and Russell (1988: 151), the upperteeth of Gomphos differ from those of Rhom-bomylus by the subcircular contour of P4 inocclusal view; by the presence on P4 of asingle centrolabial cusp (four cusps in Rhom-bomylus); by P4–M2 being greatly inflatedlingually and more sloping with considerableunilateral hypsodonty; by the absence of avertical groove between the protocone andhypocone; by the paracone being circular andcuspate, becoming lophlike only with ad-vanced wear, and being well separated fromthe anterior and labial cingula; by the stron-ger metaconule; and by the presence in themolars of a possible mesostyle. These differ-ences appear constant except that the four-cusped P4 in Rhombomylus is not confirmedby a study involving many more complete


Fig. 10. A–D, Proximal, distal, medial, and lateral views of the right navicular of Gomphos elkema(IVPP V13510.4). Abbreviations: AN, astragalonavicular facet on navicular; napp, plantar process ofnavicular; NCs, articular facet for cuneiforms on the distal surface of navicular body; and NCu, navi-culocuboid facet on the lateral surface of navicular body.

specimens of Rhombomylus (Meng et al.,2003). Rhombomylus also differs from Gom-phos in having a broader hypocone shelf onupper cheek teeth and in lacking a mesoconidon the lower molars. In addition, the molarsof Rhombomylus have a pair of strong shear-ing facets between the anterior edge of anupper molar and the posterior surface of thecorresponding lower molar. The enamel dis-tribution is thicker along the cutting edges ofRhombomylus teeth (Meng et al., 2003). Thecheekteeth of Gomphos are more suitable forgrinding than for shearing, in which a pair ofshear facets is not present. The enamel onthe occlusal surface of the cheek teeth is alsothick in Gomphos.

In Dashzeveg and Russell (1988), Matu-tinia was considered to be a junior synonymof Rhombomylus. Following recent studies(Ting et al., 2002; Meng et al., 2003) Ma-tutinia is regarded as a valid taxon, whichdiffers from Rhombomylus in several aspects,such as cheekteeth crowns relatively lower,p3 single-rooted, a partial vertical groove onlingual side of upper cheekteeth, the para-cone and metacone more conical and moredistantly separated, and the hypocone shelfrelatively less well developed. The differenc-

es between Gomphos and Rhombomylus areapplicable to Matutinia.

The presence of two pairs of lower inci-sors readily distinguishes Gomphos elkemafrom species frequently referred to as eury-mylids, rodents, Mimolagus, and lago-morphs. Double lower incisors of Gomphosare shared with Mimotona and Anatolmylus.Mimotona (Li, 1977; Li and Ting, 1985,1993) differs from Gomphos in having small-er body size, less developed mesoconid onthe lower molars, shorter trigonid, a moreelongated talonid on m3, a less molariformp4, absence of the mesostyle on upper mo-lars, upper cheekteeth anteroposteriorlyshorter, and lower degree of hypsodonty.

Averianov (1994: 401) recognized thatAnatolmylus ‘‘Differs from Mimotona andGomphos by extremely deep and relativelyshort horizontal ramus of mandible with dis-tinctly more curved tooth row and incisor(i2). Diastema is short, shorter than in Mim-otona. The lower and upper cheek teeth withunilateral hypsodonty; p4 molariform. Para-conid on the lower molars virtually absent.On the upper molars paraconule and meta-conule absent, the mesostyl[e] small.’’ Ex-cept for depth of the mandible, these differ-


ences between Gomphos and Anatolmylusare not fully demonstrated because of thefragmentary specimens (Zhegallo and Shev-yreva, 1976; Dashzeveg and Russell, 1988;Averianov, 1994).

The astragalus and calcaneus of Gomphosare very similar to those of Mimolagus (Boh-lin, 1951; Bleefeld and McKenna, 1985; Sza-lay, 1985) in several aspects: the tuber of thecalcaneus gradually expanding distally; thecalcaneoastragalar and the sustentacular fac-ets being aligned at the same level; the cal-caneoastragalar facet being a narrow convexband in a distoproximal orientation, nearlyparallel to the long axis of the bone; a dis-tinctive pit being present on the plantar sideof the process bearing the calcaneoastragalarfacet; astragalonavicular facet on the head ofthe calcaneus with similar orientation; andthe astragalus being ventrodorsally (or an-teroposteriorly) narrow. They differ in that inMimolagus a longitudinal ridge is present onthe plantar surface of the calcaneus, the headof the astragalus is transversely narrower, theastragalonavicular facet is less medially ex-tended, and the trochlear rims are moresharply delineated. This combination of fea-tures makes the ankle bones distinctive fromother gliroids, such as Rhombomylus (Li andTing, 1993; Meng et al., 2003), Tribosphen-omys (Meng and Wyss, 2001), and early ro-dents (Wood, 1962). The astragalus and cal-caneus of Gomphos are also different fromthose that were referred to ‘‘Mixodontia sp.’’from the early Eocene of Kirgizia (Averi-anov, 1991). The astragalus and calcaneus of‘‘Mixodontia sp.’’ described by Averianov(1991) are more elongate. The general shapeand the condition of calcaneoastragalar facetand sustentacular facet aligned at the samelevel, with the calcaneoastragalar facet beinga narrow convex band in a distoproximal ori-entation, are similar to those of lagomorphs.However, in both Gomphos and Mimolagusa calcaneofibular facet is absent on the cal-caneus and the distal portion of the calcaneusis not elongated. In addition, the astragalusis transversely broader than that of lago-morphs (Dawson, 1958; Szalay, 1985).Nonetheless, the foot bones of Gomphos aresimilar to Mimolagus in being more lago-morphlike, not rodentlike.

The head of the astragalus articulates ex-

clusively with the navicular in most euthe-rians, but in anagalids (Simpson, 1931; Boh-lin, 1951) and macroscelideans (Szalay,1985) it also has a small contact with thecuboid. As in Gomphos, the contact is absentin lagomorphs (Szalay, 1985) but is presentin Mimolagus (Bleefeld and McKenna, 1985;Szalay, 1985).

In most eutherians, the calcaneoastragalarfacet is either somewhat rounded or is elon-gate but oriented diagonal to the long axis ofthe calcaneus body. In mimotonids (Averi-anov, 1991), Mimolagus (Bohlin, 1951; Sza-lay, 1985), and lagomorphs the calcaneoas-tragalar facet is oriented such that the majoraxis is nearly parallel to the long axis of thecalcanear body. A similar condition is pre-sent in Gomphos.

The sustentaculum on the calcaneus is an-teromedial to the calcaneoastragalar facetand in most eutherian taxa the two facets areseparated by a distinctive sulcus calcanei. Incontrast, the sustentaculum in lagomorphs isimmediately medial to the calcaneoastragalarfacet. Mimolagus and Gomphos are similarto each other and comparable to lagomorphsin this condition.

Primitively, the calcaneus does not extendfarther distally than does the astragalus;therefore, the astragalar-navicular articulationand the calcaneus-cuboid articulation areroughly at the same level and the ankle ismore flexible in rotation. In some lago-morphs, the calcaneus is distally extended tocontact the navicular. The foot bones aremore firmly interlocked such that sidewardmovement of the foot is limited but fore-and-aft movement of the foot is emphasized(Dawson, 1958). In some early lagomorphs,such as Palaeolagus haydeni and Megalagusturgidus, the astragalus reaches approximate-ly the same distance distally as does the cal-caneum; in others, such as Hypolagus andthe recent leporids, the calcaneus extends far-ther distally than does the astragalus, makingpossible a calcaneonavicular contact (Daw-son, 1958). The calcaneonavicular contact isnot present in ochotonids (McKenna, 1982).The calcaneonavicular facet also occurs onthe calcaneus of Mimolagus (Bohlin, 1951;Szalay, 1985), suggesting that the calcaneusextended farther distally than did the astrag-alus.


The calcaneal canal is a perforation pres-ent in the mammalian calcaneus to providechannels for small blood vessels. In lago-morphs, the canal starts from the lateral sideof the calcaneal tuber as a circular foramen,traverses the calcaneal body diagonally, andexits the calcaneus via an aperture situatedbetween the sustentaculum and the cuboidfacet (Bleefeld and Bock, 2002). Accordingto Bleefeld and Bock (2002), the calcanealcanal is a feature that appeared early in thehistory of this order, and it occurs in all rec-ognized Recent and fossil lagomorph calca-nea. The ancientness of the calcaneal canaland its ubiquity among lagomorphs empha-size the monophyly of the order. The canalis greatly reduced, or lost, in extant leporids,which is regarded as a derived lagomorphfeature (Bleefeld and Bock, 2002). Becausethe calcaneal canal is unknown in the cal-caneus of any rodent, or other suggested Re-cent or fossil lagomorph relatives (e.g., ma-croscelidids, anagalids), Bleefeld and Bock(2002) concluded that the morphological dis-tinctiveness of the earliest recognized pedesof lagomorphs from those of macroscelididsand anagalids may indicate a long evolution-ary separation of those mammalian orders.

Mimolagus and Gomphos are stem taxa toLagomorpha and share many derived simi-larities with lagomorphs (Meng and Wyss,2001; Meng et al., 2003). The calcaneus doesnot have the calcaneal canal in either taxa.This indicates that the calcaneal canal char-acterizes only Lagomorpha, but not Dupli-cidentata that includes Lagomorpha and itsstem taxa (Meng and Wyss, 2001). The old-est lagomorph calcanei observed by Bleefeldand Bock (2002) are from the early Oligo-cene Hsanda Gol Formation of Mongolia. Itis probable that in earlier lagomorphs the cal-caneus may have had the calcaneal canal.Nonetheless, because the age of Mimolagusis roughly early Oligocene (Bohlin, 1951), itseems that the morphological distinctivenessin the calcaneal structure may not adequatelyindicate the relative timing of the evolution-ary separation between Lagomorpha and itsstem taxa.

Because several recent phylogenetic stud-ies of Glires have shown that Gomphos is astem taxon to Lagomorpha (Meng and Wyss,2001; Meng et al., 2003; Meng, in press), we

will not provide additional phylogeneticanalyses concerning this taxon in the presentstudy. However, we note that the foot bonesof Gomphos are apparently similar to thoseof lagomorphs in several aspects, which un-doubtedly strengthens the position of Gom-phos as a stem taxon to lagomorphs. Our dis-cussion will focus on the implications forfaunal correlation of the presence of Gom-phos in Nei Mongol.

STRATIGRAPHY

LITHOSTRATIGRAPHY: Rock units relevantto the discussion include the Nomogen, Ar-shanto, and Irdin Manha Formations. TheNomogen Formation was first described byZhou et al. (1976), who noted the earlier, in-formal use of the name by the GeologicalSurvey of Inner Mongolia mapping team. Atthe type locality, Haliut, the formation con-sists of three levels of approximately 16 maggregate thickness (reported as 14 m inRussell and Zhai, 1987; see also Meng et al.,1998) and produced a mammalian fauna tra-ditionally considered to be late Paleocene(Zhou et al., 1976; Zhou and Qi, 1978). Theformation is mainly composed of red sandyclaystone of lacustrine orign (Russell andZhai, 1987). The bottom of the formation isnot exposed.

The second locality where the NomogenFormation is known is Bayan Ulan, at thenorthern foot of the Holy Mesa (Meng, 1990;Meng et al., 1998). Qi (1979) formally rec-ognized the Bayan Ulan Fauna for a fossilassemblage derived from this area and as-signed these fossiliferous deposits to the‘‘Bayan Ulan Formation’’, a name first usedby the geological mapping team (see alsoJiang, 1983; fig. 2). Qi’s listing of mamma-lian taxa from Bayan Ulan did not includedetailed stratigraphic information or descrip-tions of the fossils. In addressing the generalstratigraphic context, however, Qi suggestedthat at Bayan Ulan the ‘‘Bayan Ulan For-mation’’ is continuous with the underlyingNomogen Formation. Further, Qi observedthat most species from the Bayan Ulan andNomogen formations are the same, exceptingthe occurrence of ?Lambdotherium sp. and?Heptodon sp. at Bayan Ulan. The first de-scription of the ‘‘Bayan Ulan Formation’’ at


the Bayan Ulan locality occurs in the sum-mary of the 1:200,000 mapping study on theearly Tertiary in the Erlian Basin (Jiang,1983). Because the ‘‘Bayan Ulan Formation’’was based primarily on its fossil content, in-stead of its lithology, the designation of‘‘Bayan Ulan Formation’’ has been aban-doned (Qi, 1987; BGMRNMAR, 1991;Meng et al., 1998). Meng et al. (1998) sug-gested that the basal beds at Bayan Ulan arethe lateral extension of the Nomogen For-mation.

The type section of the Arshanto Forma-tion is about 30 km southeast of the ErlianBasin and was first studied in the 1920s (Ber-key and Morris, 1924; Mathew and Granger,1925). At the type locality the Arshanto For-mation is a sequence of dark reddish sandyclaystone and siltstone beds (Zhou et al.,1976; Russell and Zhai, 1987) and is overlainby the coarser rocks of the Irdin Manha For-mation. The base of the Arshanto Formationis not visible. The exposed portion is lessthan 10 m thick (Zhou et al., 1976). Red bedsof similar lithology assignable to the Arshan-to Formation are present throughout the ba-sin, but the thickness of the formation variessignificantly from one place to another.

In a later study Qi (1987) considered in-stead that the CAE’s ‘‘Arshanto Formation’’can be divided into three units, the late Pa-leocene Nomogen beds, the early EoceneBayan Ulan beds, and the middle Eocene Ar-shanto beds. Qi grouped the Nomogen andBayan Ulan beds into the Nomogen Forma-tion, whereas the Arshanto beds and overly-ing Irdin Manha beds were associated in theIrdin Manha Formation. Although Qi (1987)suggested that the Nomogen Formation (hisdefinition) could be distinguished from theArshanto beds by the presence of celestitenodules therein, the features of these twounits are not otherwise distinctive (Meng,1990). Also, because the Arshanto and IrdinManha rocks are lithologically distinctive,Meng (1990) disagreed with Qi’s placementof the two sets of deposits into the IrdinManha Formation. The complexities and un-certainties associated with the definition anddivision of these units hamper efforts to cor-relate the early Tertiary red beds in Nei Mon-gol in general and at Huheboerhe in partic-ular.

At Huheboerhe, the rock sequence poten-tially contains the conventionally definedNomogen, Arshanto, and Irdin Manha For-mations. A stratigraphic section from the Hu-heboerhe region was previously published(Jiang, 1983; Qi, 1980), although the sectionwas subdivided differently in the two studies(Meng, 1990; Meng et al., 1998). For thepresent study, we choose not to assign thelower beds (beds 1 to 11) to any named rockunit, but recognize that the top of these bedsis truncated by an erosional surface markingthe base of the Irdin Manha Formation (bed12). In our opinion, this erosional surfaceand grain-size change represents the mostdistinctive and regionally traceable strati-graphic datum for this part of the early Pa-leogene sedimentary package of the ErlianBasin. Contrary to previous studies in thisregion (Jiang, 1983; Qi, 1987), we do notnote evidence for significant sedimentologi-cal hiatuses in the Huheboerhe section, withthe exception of that marking the base of theIrdin Manha Formation. We do, however,note distinctive stratigraphic changes in thelithologic characteristics of the rocks under-lying the Irdin Manha Formation at Huhe-boerhe. There is a significant color transitionbetween beds 10 and 11 in the above section,and a large number of white calcareous nod-ules in the bottom of bed 11 are indicativeof paleosol formation under conditions dif-ferent from those during the deposition of theunderlying beds. The presumably Bumbani-an fauna, dominated by Gomphos, occurs inthe upper part of bed 10.

BIOSTRATIGRAPHY: The faunas relevant toour discussion include those of the Gashatanand Bumbanian Asian Land Mammal Ages(ALMA). The Gashato fauna of Mongoliacomes from Member I of the Gashato(Khashat) Formation at the Ulan-Nur Basin,which is overlain by Members II and III thatproduced only one mammal taxon, Gomphoselkema (Dashzeveg, 1988). These beds werefirst studied by Morris in 1923 (Matthew andGranger, 1925; Russell and Zhai, 1987). Thename Gashato was first published in Mat-thew and Granger (1925) in describing theGashato fauna, but without a description ordefinition for the rock unit. The Gashato For-mation was formally proposed in 1927 byBerkey and Morris. The only datable bed at


the type locality, a basalt, was found abovethe Member II, but dates for the basalt aretoo contradictory to be of use: one is 51 62 Ma (Dashzeveg, 1988) and the other is 3761 Ma (Devyatkin 1981, 1994).

The Gashatan ALMA was based on theGashato and its equivalent faunas, and it wasdefined by Ting (1998) on the basis of thefirst appearance of Rodentia as representedby Tribosphenomys (but see Meng and Wyss,1994, 2001; Wyss and Meng, 1996). This def-inition was considered to be less than felic-itous (Beard, 1998) because Tribosphenomysis known only from the Bayan Ulan locality(Meng et al., 1994, 1998), although there arespecimens collected from Subeng, a localityabout 50 km northeast of Bayan Ulan (un-publ. data). Defining the Gashatan on the ba-sis of the first appearance of the more com-monly encountered Prodinoceras was con-sidered to be more stable and easily recog-nized. Indeed, among many typical Gashatanspecies (Dashzeveg, 1988; Meng and Mc-Kenna, 1998), Prodinoceras martyr is one ofthe most common taxa. It was found in theZhigden and Naran members of the NaranBulak Formation, Mongolia, as well as in theNomogen Formation at Nomogen (Chow andQi, 1978), Bayan Ulan (Meng et al., 1998),and Subeng (unpubl. data), Nei-Mongol ofChina.

Prodinoceras was one of the taxa com-monly used to establish the correlation be-tween the Gashatan ALMA and the Clark-forkian North American Land Mammal Age(Szalay and McKenna, 1971; Dashzeveg,1988; Krause and Maas, 1990), although thiscorrelation may be problematic. Prodinocer-as was proposed by Matthew et al. (1929)and was thought to be a senior subjectivesynonym of nine other junior names (Schochand Lucas, 1985), including Bathyopsoidesfrom North America. Although Lucas (1989)considered Prodinoceras an indicator of latePaleocene, lumping all these names into asingle genus decreases the precision of Pro-dinoceras as an age indicator because it in-creases the geological age distribution of thetaxon. Prodinoceras, as defined by Schochand Lucas (1985), has a distribution from thelate Paleocene to early Eocene in NorthAmerica; consequently, faunal correlationsusing the genus Prodinoceras are less precise

than previously thought. Nonetheless, thefaunas that contain the species Prodinocerasmartyr in central Asia are all considered tobe Gashatan equivalent, and the species hasnot been reported from the younger faunasof the Bumbanian ALMA (Meng et al.,1998; Ting, 1998). Isotopic and paleomag-netic correlation suggests that GashatanALMA is entirely restricted to the late Pa-leocene (Bowen et al., 2002).

The Bumbanian ALMA is based on theBumban fauna from the Bumban Member ofthe Naran Bulak Formation, Nemegt Basin(Dashzeveg, 1988). Gomphos elkema is oneof the typical Bumbanian species and is alsoknown from Members II and III of the Gash-ato Formation (Dashzeveg, 1988). This spe-cies was only known from Mongolia untilthis study and is not found in any of theGashatan faunas.

At Huheboerhe, we have found Prodino-ceras martyr in the lower part of the sectionand numerous specimens of Gomphos elke-ma in bed 10. Along with Gomphos elkema,fragmentary fossils possibly representingHomogalax and a creodont are also found inbed 10. Fossils representing later ALMAs(probably Arshantan and Irdinmanhan) arefound on the upper part of the section. Basedon the presence of fossils with Gashatan andBumbanian affinities and the traditionalviews regarding the ages of these faunas, itis possible that the stratigraphic section atHuheboerhe spans the Paleocene–Eoceneboundary.

THE PALEOCENE–EOCENE BOUNDARY: Theposition of the Paleocene–Eocene boundaryin Asia has usually been determined by bio-stratigraphic correlation to North America. Inrecent years the position of the boundary inNorth America has shifted relative to theboundary defined in the marine stratotype(Gunnell et al., 1993; Wing, 1984; Archibaldet al., 1987; Woodburne and Swisher, 1995),largely due to changes in the operational def-inition of the boundary. It seems likely thatthe marine boundary will be formally placedat the position equivalent to the short termwarming and carbon-cycle perturbations dis-cussed in the introduction, at about 55 Ma.This position is precisely coincident with theboundary between the Clarkforkian and Was-atchian North American Land Mammal


Ages, when Artiodatcyla, Perrisodactyla, andthe family Hyaenodontidae all make theirfirst appearance (Bowen et al., 2001). Thus,accepting this definition, the correlation ofthe Paleocene-Eocene boundary from NorthAmerica to Asia essentially involves the cor-relation of this major event in mammalianhistory between the two continents.

Based on mammalian faunal composition,the Paleocene–Eocene boundary of Mongo-lia has been suggested to be at the bottom ofthe Bumban Member within the Naran-Bu-luk Formation (Dashzeveg, 1988; Ting,1998). While acceptance of this boundary isperhaps the most obvious choice, existingproblems merit discussion. Correlationsbased on faunal composition implicitly as-sume the synchronicity of episodes of evo-lution, extinction, and dispersal at regional toglobal scales. For basin-scale correlations theuncertainty imparted by this assumption isperhaps small (see, however, Gunnell andBartels, 2001), but at the continental to glob-al scale geographic and climatic barriers todispersal and the potential for diachronousextinction of isolated populations representsignificant sources of uncertainty. Eventhough faunal changes occur across the Nar-an–Bumban boundary (Dashzeveg, 1988),these changes are not in themselves sufficientto indicate the Paleocene–Eocene boundary.Without a temporal framework developed in-dependently of faunal composition, the Nar-an–Bumban boundary can only be regardedas a boundary between lithological units andbetween biozones. The recognition of the P-E boundary is a different, unsettled issue.

Independent of the potential problems as-sociated with assuming the equivalence ofbiostratigraphic and chronostratigraphicboundaries, differences of opinion existamong workers as to the proper correlationof Asian and North American land mammalages. Although the Clarkforkian–Gashatanintercontinental correlation is commonly cit-ed (Szalay and McKenna, 1971; Dashzeveg,1988; Krause and Maas, 1990; Meng et al.,1998; Ting, 1998), other opinions exist.Wang et al. (1998) suggested that the Tiffan-ian–Clarkforkian boundary lies within theGashatan, so that the Gashatan is correlativein part with the latter part of the Tiffanian, aview supported by Beard (1998). Beard ar-

gued that Tribosphenomys, known only fromthe Gashatan of Asia, is more primitive thanits close relative, Alagomys, a genus foundin the Bumbanian of Asia (Dashzeveg, 1990)and in the Clarkforkian of North America(Dawson and Beard, 1996). He further sug-gested that this relationship indicates that Al-agomys likely originated on the Asian con-tinent and dispersed to North America priorto early Clarkforkian time, giving reason tothink that the Gashatan record of Tribos-phenomys in the Bayan Ulan fauna antedatesthe Clarkforkian record of Alagomys at BigMulti Quarry, Wyoming (Dawson and Beard,1996; Beard, 1998). In contrast, Lucas(1999) has proposed, based on the distribu-tion of coryphodontid pantodonts, that fau-nas considered here to be Gashatan correlatewith the latter part of the Clarkforkian andthe early Wasatchian.

Most workers have considered the Bum-banian to be correlative with the NorthAmerican Wasatchian and the EuropeanSparnacian (McKenna, 1975; Savage andRussell, 1983; Russell and Zhai, 1987; Dash-zeveg, 1982, 1988; Krause and Maas, 1990;Stucky, 1992; Meng and McKenna, 1998;Ting, 1998). However, Beard (1998) arguedthat this correlation likely underestimates theantiquity of at least some Bumbanian mam-mal faunas, which more likely correlate withthe North American Clarkforkian. The tra-ditional correlation of Bumbanian with Was-atchian is based primarily on the appearanceof Perissodactyla, Artiodactyla, Primates,and Hyaenodontidae in the Bumban fauna(Dashzeveg, 1988). The inverse relationshipof multituberculate and rodent diversity (i.e.,extinction of multituberculates and radiationof rodents) across the Naran–Bumbanboundary is also comparable to that at thePaleocene–Eocene boundary in North Amer-ica and Europe (Krause, 1986; Stucky, 1992;Legendre and Hartenberger, 1992). Beard(1998) pointed out that phylogenetic and bio-stratigraphic data support an Asian origin forPerissodactyla, Artiodactyla, euprimates, andHyaenodontidae (the East of Eden model),all of which share North American first ap-pearance datum (FADs) at the beginning ofthe Wasatchian and European FADs at thebeginning of the Sparnacian. Among thesetaxa, hyaenodontids and questionable peris-


sodactyls are known from the Gashatan Bay-an Ulan fauna (Meng et al., 1998). Beard(1998) argued that if these taxa did indeedoriginate in Asia, there is no reason to as-sume that their earliest records on that con-tinent are synchronous with their NorthAmerican and/or European FADs. Therefore,the Bumbanian may correlate, at least in part,with the North American Clarkforkian, andthe P-E boundary may well be within theBumbanian. A recent chronostratigraphicstudy shows that the Bumbanian ALMA can-not confidently be assigned a later Paleoceneage, but neither can it be confidently as-signed in its entirety to the early Eocene(Bowen et al., 2002).

Thus, the presence of Gashatan and Bum-banian taxa suggests that the Huheboerhesection may contain the Paleocene–Eoceneboundary and that it is also possible that theepoch boundary may lie above the Gomphoslevel. Assuming that the climatic changesthat affected the marine realm at the end ofthe Paleocene were manifested in the Asianinterior, these may have forced sedimento-logical changes in the Erlian Basin. Wetherefore suggest that likely locations for theplacement of the Paleocene–Eocene bound-ary in our section include the contact be-tween beds 10 and 11, which is above theGomphos assemblage, or at the unconformityat the base of the Irdin Manha Formation(also above the Gomphos level). Alternative-ly, the Paleocene–Eocene transition may notbe expressed as a significant lithologicalboundary at Huheboerhe, and it may occurwithin bed 10 or in the underlying strata, pre-dating the deposition of the Gomphos-bear-ing sediments. Our ongoing isotope and pa-leomagenetic studies may help to pinpointthe Paleocene–Eocene boundary in the Hu-heboerhe section, clarifying the relationshipbetween the Bumbanian Land mammal ageand the Paleocene–Eocene boundary.

ACKNOWLEDGMENTS

We thank Drs. Lawrence J. Flynn,Chuankui Li, and Kenneth D. Rose for com-ments on the manuscript. This project hasbeen supported by the IVPP, the AmericanMuseum of Natural History, and the Univer-sity of California. Most of the fieldwork was

supported by the Institute of Vertebrate Pa-leontology and Paleoanthropology, Beijing,the Chinese Academy of Sciences, the Na-tional Natural Science Foundation of China(special funds for major state basic researchprojects of China [G200007707] and anNNSFC grant [49928202] to Meng), and aU.S. National Science Foundation grant toMeng, Koch, and collaborators (EAR-0120727). Bowen was supported by the Na-tional Science Foundation Graduate Re-search Fellowship Program.

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