A New Species of Gomphos (Glires, Mammalia) from the Eocene of the Erlian Basin, Nei Mongol, China

2009·2.2 MB·English

by  Jin Meng

PUBLISHED BY THE AMERICAN MUSEUM OF NATURAL HISTORY
CENTRAL PARK WEST AT 79TH STREET, NEW YORK, NY 10024
Number 3670, 11 pp., 4 figures, 1 table November 30, 2009
A New Species of Gomphos (Glires, Mammalia) from
the Eocene of the Erlian Basin, Nei Mongol, China
JIN MENG,1 BRIAN P. KRAATZ,1 YUANQING WANG,2 XIJUN NI,2
DANIEL L. GEBO,3 AND K. CHRISTOPHER BEARD4
ABSTRACT
Dental and postcranial specimens of Gomphos shevyrevae, sp. nov., from the lower part of the
IrdinManhaFormationattheHuheboerhelocality,ErlianBasin,NeiMongol(InnerMongolia),
aredescribed.ThenewspeciesdiffersfromG.elkemaandG.ellaeinhavingmorerobustteethwith
inflated cusps and stronger lophs and a calcaneus with extra articulation for the astragalus and
navicular. The new species is stratigraphically well constrained and probably represents the
youngest known species of the genus, extending its geological record into the Middle Eocene. It
alsoshowsthatmimotonidscoexistedformillionsofyearsasasidebranchofduplicidentateswith
the earlieststem lagomorphs,including Dawsonolagus.
INTRODUCTION 2007).WithinGlires,Simplicidentataisdefined
as all Glires sharing a more recent common
Glires consists of two modern groups: ancestorwithRodentiathanwithLagomorpha.
rodents and lagomorphs.Although the mono- It is the stem-based counterpart to Rodentia.
phyly of Glires has been a matter of debate, Duplicidentata is defined as all members of
recent morphological and molecular evidence Glires sharing a more recent common ancestor
stronglysupportstherecognitionofGliresasa with Lagomorpha than with Rodentia (Meng
clade (Meng and Wyss, 2001; Huchon et al., andWyss,2005).
2002;Mengetal.,2003;DouzeryandHuchon, Mimotonidae has been recognized as a
2004;Asheretal.,2005;Bininda-Emondsetal., paraphyletic group, within which a monophy-
1DivisionofPaleontology,AmericanMuseumofNaturalHistory([email protected]).
2Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, P.O. Box 643, Beijing
100044,P.R.China.
3DepartmentofAnthropology,NorthernIllinoisUniversity,DeKalb,IL60115.
4SectionofVertebratePaleontology,CarnegieMuseumofNaturalHistory,4400ForbesAvenue,Pittsburgh,PA15213.
CopyrightEAmericanMuseumofNaturalHistory2009 ISSN0003-0082
2 AMERICAN MUSEUMNOVITATES NO. 3670
leticLagomorphaisnested.Gomphoshasbeen tooth pattern, wherever it can be established,
shown to be a late-branching mimotonid and will aid in understanding the phylogenetic
the sister group of Lagomorpha based on relationships of Glires within Mammalia.
morphological evidence (Meng et al., 2003, However, because there is no consistent
2004; Asher et al., 2005). Mimotonids are terminology for nonmolariform premolars,
typical as gliriform mammals in having a pair we use descriptive, noncommittal terms, such
ofenlargedandever-growingupperandlower as a labial or lingual cusp, instead of terms
incisors.Inaddition,theybearaminorpairof that imply homology, such as the protocone
incisors in their upper and lower jaws, easily and paracone, in describing these teeth.
distinguishing them from most other Glires Measurements were taken using digital cali-
and mammals. pers. SEM photographs of teeth were taken
Gomphoselkema,thetypespeciesofthegenus, from uncoated specimens using a Hitachi
hasbeendescribedfromMongolia(Shevyrevaet SEM at the American Museum of Natural
al.,1975;Asheretal.,2005)andChina(Menget History.
al., 2004, 2005) and is considered to be a Institutionalabbreviation:IVPP,Instituteof
characteristic fossil for the earliest Eocene Vertebrate Paleontology and Paleoanthropo-
Bumbanian faunas in central Asia (Dashzeveg, logy,Beijing,wheretheoriginalspecimenswill
1988;DashzevegandRussell,1988).Theinitial reside.
species identification was based on a fragmen-
tary lower jaw, which was first named by GEOLOGICAL SETTING
Shevyreva (Shevyreva et al., 1975) and rede-
scribed by Zhegallo and Shevyreva (1976) and The Tertiary beds in the Erlian Basin, Nei
byDashzevegandRussell(1988).Thisspecimen Mongol (Inner Mongolia), China, were first
was from members II and III of the Gashato explored by the Central Asiatic Expeditions
(Khashat) Formation at the Ulan-Nur Basin (CAE) in the 1920s (Granger and Berkey,
(Dashzeveg, 1988). More complete G. elkema 1922; Berkey and Granger, 1923; Berkey and
materialwasreportedbyAsheretal.(2005)from Morris, 1924, 1927; Matthew and Granger,
the Bumban Member of the Naran-Bulak 1926). The stratigraphy and vertebrate fossils
Formation in Tsagan-Khushu, Nemegt Basin. from this area have formed the bases for
Recently,asecondspeciesofthegenus,G.ellae, several Eocene Asian Land Mammal ages,
was reported from Tsagaan Khutel, Mongolia. such as the Arshantan and Irdinmanhan.
TheageofG.ellaeispoorlyconstrained,butthis However, the stratigraphy of this region
species probably dates to the late Early Eocene remained unclear(Radinsky,1964) untilmore
(Kraatzetal.,2009),youngerthanG.elkema. recent investigations revealed lithological and
biostratigraphic relationships for several key
sections inthisarea(Meng etal.,2004,2007c;
MATERIALS AND METHODS
Bowen et al., 2005).
All specimens reported here were collected The new Gomphos specimens were collected
fromtheHuheboerhelocalityeitherbysurface from the Huheboerhe locality, Erlian Basin.
collecting or by screen washing. Each speci- At this locality the section consists of the
menhasitsownGPSlogfromthespecificsite Nomogen, Arshanto, and Irdin Manha for-
collected. A large number of mammals have mations, from oldest to youngest. The new
been collected from the same level, though Gomphos fossils came from level 10 of the
only a handful of Gomphos specimens were section, equivalent to the lower portion of the
recovered. Similarities in foot bones to those IrdinManhaFormation(fig. 1).Specimensof
of G. elkema (Meng et al., 2004; Asher et al., Gomphos elkema have been collected from
2005) help to associate two tarsal elements leveloneofthesamesection,intheupperpart
with the teeth. oftheNomogenFormation.Thestratigraphic
Indescribingmolarstructureswefollowthe occurrences for these two species of Gomphos
dental terminology used for Tribosphenomys are clear, thus providing a solid superposi-
(Meng and Wyss, 2001), considering that a tional basis for discussing the evolutionary
consistent terminology with tribosphenic relationship of the two species.
2009 MENG ETAL.: ANEW SPECIESOFGOMPHOS 3
Fig. 1. Section at Huheboerhe, Erlian (modified from Meng et al., 2007c), showing stratigraphic
occurencesofGomphoselkema,G.shevyrevae,andsomeassociatedspecies.Asterisksshowspeciescurrently
knownfrom Nuhetingboerhe.
SYSTEMATIC PALEONTOLOGY INCLUDED SPECIES: Gomphos ellae and G.
shevyrevae, new species.
Mammalia Linnaeus, 1758 REVISED DIAGNOSIS: Similar to other mi-
motonids but differing from other Glires in
Glires Linnaeus, 1758
having two pairs of lower incisors. Differs
from Mimotona (Li, 1977) in its larger size,
Mimotonidae Li, 1977
less transversely extended and more unilater-
Gomphos Shevyreva, 1975 ally hypsodont upper teeth, stronger lophs, a
mesostyleusuallypresent,lowermolarswitha
TYPESPECIES: GomphoselkemaShevyreva, longer trigonid, and a mesoconid on lower
1975. molars.Differsfrom Anatolmylus (Averianov,
4 AMERICAN MUSEUMNOVITATES NO. 3670
1994;AverianovandGodinot,1998)inhaving TABLE1
a shallow horizontal ramus. Differs from ToothmeasurementsofGomphosshevyrevae(inmm)
Mimolagus (Bolin, 1951) in its smaller size
and higher-crowned cheek teeth with distinc- Length Width Fieldsite
tive cusps and ridges.
P4(V14671.1) 2.22 3.38 (H2A)
LOCALITIES AND AGE: Gomphos elkema is M1(V14669) 2.87 4.57 (H2A,holotype)
known from Bumbanian faunas in Tsagan- M1(V14671.2) 2.74 4.30 (H2)
Khushu, Nemegt Basin, and Ulan-Nur Basin M2(V14671.3) 3.38 4.08 (H2A)
of Mongolia (Dashzeveg and Russell, 1988; M2(V14671.4) 3.56 4.03 (060722WYQ01)
Dashzeveg,1988);Huheboerhe,Wulanboerhe,
m1(V14670) 3.30 3.30 (H2)
and Nuhetingboerhe (Meng et al., 2004), and m1(V14672.1) 2.84 2.70 (060720BB01)
Bayan Ulan (Meng et al., 2005) in the Erlian m2(V14672.2) 3.31 3.24 (H2A)
Basin of Nei-Mongol (Inner Mongolia), m3(V14672.3) 3.46 3.32 (H2)
China. G. ellae is from Tsagaan Khutel,
Mongolia (Kraatz et al., 2009). G. shevyrevae
is from the Irdin Manha Formation at
as wide as the posterior and bears a tiny
Huheboerhe, Erlian. Early to Middle Eocene.
posterolingualcusp,suggestingthatitislikely
a right P4. The P4 is a recently erupted tooth
Gomphos shevyrevae, new species that bears no wear. As is typical of Gomphos,
the P4 is unilaterally hypsodont and consists
HOLOTYPE: A right M1 (IVPP V14669). of a lingual main cusp, presumably the
PARATYPE: A right m1 (IVPP V14670). protocone,andalabialcusp.Thelingualcusp
INCLUDED SPECIMENS: A right P4 (or P3) is crescent shaped, and its two lophs form the
(V14671.1),arightM1(V14671.2),arightM2 anterior and posterior edges of the tooth. The
(V14671.3), a left M2 (V14671.4), a right m1 labial cusp is conical, having a steeper labial
(V14672.1), a right m2 (V14672.2), a left m3 surface and a more rounded lingual surface.
(V14672.3), a left calcaneus (V14673), and a Unlike P4 of Gomphos elkema, in which P4 is
left astragalus (V14674). known (Meng et al., 2004; Asher et al., 2005),
ETYMOLOGY: The species name is in honor theridgeconnectingthetwocuspsisabsentin
ofN.S.ShevyrevawhonamedGomphoselkema. G. shevyrevae; instead, there is a weak ridge
DIAGNOSIS: Differs from Gomphos elkema extendingfromthelabial cuspanterolingually
and G. ellae in having more robust teeth with to join the midpoint of the anterior loph. The
higher crowns, inflated cusps; upper molars absence of a connection between the lingual
withmoreposteriorlyexpandedhypoconeand and labial cusps of P4 recalls the condition
shelf; absence of the ridge connecting the found in Mimotona (Li, 1977; Li and Ting,
lingual and labial cusps on P4; lower molars 1993).
withrelativelylongtrigonidandshorttalonid, Upper molars have one major lingual and
reduced mesoconid and hypoconulid, and two minor labial roots. The enamel is thick.
absenceofthemesostylid;furtherdiffersfrom M1 is so assigned because its hypocone and
G. elkema in having extra facets on the shelf are less expanded posteriorly than those
calcaneus for articulation with the astragalus of M2, and M1 is usually proportionally
and navicular (unknown for G. ellae). shorter than M2 (fig. 2). M1 of G. shevyrevae
TYPE LOCALITY AND AGE: Huheboerhe es- is similar to that of G. elkema in general
carpment, Erlian Basin of Nei-Mongol, morphology. However, the M1 and other
Middle Eocene lower beds of the Irdin upper cheek teeth of G. shevyrevae are more
Manha Formation. robust with more inflated cusps and stronger
COMPARATIVE DESCRIPTION: Measurements lophs. The protocone and its anterior and
of all teeth are in table 1. The tooth we posterior lophs form a V-shaped structure
identified as a P4 could also be a P3 (fig. 2). with a slightly rounded apex (the lingual side
However, the anterior border of P3 is usually of the protocone). The angle created by the
narrower than the posterior border in lophs is about 45u. The lingual portion of the
Gomphos. This tooth has its anterior portion anterior loph may be homologized with the
2009 MENG ETAL.: ANEW SPECIESOFGOMPHOS 5
Fig. 2. Crown views of upper cheek teeth of Gomphos shevyrevae from the lower Irdin Manha
Formation, Huheboerhe at Erlian, Nei Mongol. A, right P4 (or P3) (V14671.1); B, right M1 (V14669,
holotype);C,rightM1(V14671.2);D,rightM2(V14671.3);E,leftM2(V14671.4).Scalebarsare200mm.
preprotocristaandthelabialportion,whichis postparaconule crista. The metacone is small-
much thinner, with the preparaconule crista. er and more lingually positioned than the
Aparaconule(protoconule)isnotdistinct,but paracone. It is connected with the protocone
is indicated by a slight inflation where the by the posterior loph of the protocone. The
postparaconule crista joins the anterior loph. latter loph is strong, almost as wide as the
Absenceoftheparaconuleiscommoninbasal metacone. Amesostyleoccursattheposterior
Glires, both in simplicidentates, such as base of the paracone, which blocks the labial
Matutinia and Rhombomylus (Ting et al., exit of the narrow trigon basin. The metaco-
2002; Meng et al., 2003), and duplicidentates, nuleispresent,butisdifficulttodistinguishin
such as Mimotona and Gomphos (Li, 1977; Li heavily worn specimens. In lightly worn
and Ting, 1993; Meng et al., 2004), but it is specimens (fig. 2E) a broader wear facet
usually present in taxa that are more closely indicates the metaconule that must be higher
related to rodents and in basal rodents, such andbroaderthanthecristainorderforsucha
as Tribosphenomys (Meng et al., 1994, 2007b; wear pattern to be created. In G. elkema the
Meng and Wyss, 2001), Neimengomys (Meng posterior loph of the protocone is relatively
et al., 2007b), Archetypomys (Meng et al., slimmerandthereisusuallyarestrictionlabial
2007a), and Cocomys (Li et al., 1989). to the protocone.
Thereisaweak ridge between theparacone Thehypoconeandthepostcingulumarewell
and the anterior loph, which is probably the developed, and both structures are expanded
6 AMERICAN MUSEUMNOVITATES NO. 3670
Fig. 3. Lower molars of Gomphos shevyrevae from the lower Irdin Manha Formation at Huheboerhe,
Erlian,NeiMongol.A1–2,crownandlingualviewsofrightm1(V14670,paratype);B,rightm1(V14672.1);
C1–2,crownand labialviews ofright m2 (V14672.2); D,leftm3 (V14672.3). Scalebarsare 200mm.
posteriorly. The hypocone is separated from in the upper molars, the enamel appears
the protocone by a transverse groove, and a thicker than that of G. elkema and G. ellae.
similarconditionoccursonM2.Thiscondition The m1 is identified as such because it has a
appearstodifferfromthemorphologyprevail- transversely oriented, ridgelike hypoconulid
ing in G. elkema and G. ellae. In well-worn (fig 3). In Gomphos, the lower molar hypo-
specimens (fig. 2C), however, the protocone conulid becomes more pronounced posterior-
and hypocone become confluent. On the ly.Them1andm2areverysimilarexceptthat
lingual surface of the tooth, a vertical groove m2 has a more posteriorly projecting hypo-
between the protocone and hypocone exists conulid. The lower molars are proportionally
neartheocclusalsurfaceofthetooth,andthis wider, and therefore relatively shorter, than
feature also seems more pronounced than on those of other species of the genus. Lower
uppermolarsofG.elkemaandG.ellae. molar trigonids are proportionally longer in
M2issimilartoM1ingeneralmorphology, relation to their corresponding talonid than is
but it is proportionally longer. The mesostyle the case in other species of the genus. As is
is reduced, either being incorporated into the typical of Gomphos, the protoconid and
posterior base of the paracone or entirely metaconid are transversely aligned, and the
absent. No M3 was collected. trigonid has nearly vertical anterior and
The lower molars have an anterior and a posterior walls. The protoconid and the
posterior root. The tooth crown is somewhat metaconid are subequal in size. The protoco-
higher thanthatofG.elkema andG.ellae.As nid is worn more extensively and is therefore
2009 MENG ETAL.: ANEW SPECIESOFGOMPHOS 7
Fig. 4. Tarsal bones of Gomphos shevyrevae from the lower Irdin Manha Formation at Huheboerhe,
Erlian, Nei Mongol. A, dorsal view of left astragalus (V14673); B, dorsal view of left calcaneus (V14674).
Scalebaris1 mm.
lower than the metaconid. The protoconid The astragalus measures 11.34 mm long
sends the paralophid (paracristid) to the and 8.82 mm wide (maximum dimensions)
anterolingual base of the metaconid and the and is partly broken at its head (fig. 4A).
protolophid (protocristid) to the posterior The morphology of the preserved portion is
base of the metaconid. A crescentic trigonid identical to that of Gomphos elkema (Meng et
basin is enclosed by the trigonid cusps and al., 2004). The bone is ventrodorsally (or
lophids. anteroposteriorly) flat. The trochlea is trans-
Lower molar talonids are as wide as the versely broad and shallow. In G. elkema, the
trigonidandarelongerthantheircorrespond- lateral and medial rims of the astragalar
ing trigonid. The hypoconid is as large as the trochlea appear to be more asymmetrical,
protoconid and is very close to the posterior with the lateral much larger than the medial
wall of the trigonid, so that the hypoflexid (Meng et al., 2004).
becomes a narrow groove.The cristid obliqua The calcaneus measures 20.8 mm long and
(ectolophid) is short and the mesoconid 8.93 mmwide(maximumdimensions;fig. 4B).
appears very weak relative to other mimoto- ItistypicalofGomphosinthatthecalcaneoas-
nids. The entoconid is low. Unlike the tragalarandsustentacularfacetsarealignedat
condition in G. elkema and G. ellae, in which the same level, similar to that of lagomorphs
a mesostylid is present (Meng et al., 2004; (Bleefeld and McKenna, 1985), but the bone
Kraatzetal.,2009),themesostylidisabsentin lacks a calcaneal canal that is characteristic of
the new species. Because of the reduction of lagomorphs (Bleefeld and Bock, 2002). The
themesoconidandcristidobliqua, thetalonid calcaneoastragalar facet is a narrow, long and
basin is a broad, featureless concavity. The convex surface in a proximodistal orientation,
hypoconulidisatransverseridgeattherearof nearly parallel to the long axis of the bone,
thetooth, being connected with the entoconid whereasthesustentacularfacetisroundedand
by a strong postcristid. concave. Although the general morphology of
The m3 is little worn and displays several thecalcaneusissimilartothatofG.elkema,it
additional minor cuspules on the occlusal differs from the latter in having a facet on the
surface; these structures would be quickly distal side of the bone, which probably
removed with wear (fig. 3). The m3 differs articulates with the navicular and astragalus.
from m1 and m2 in having a longer talonid Thisfeaturedoesnotoccurinknownastragali
and a more pronounced hypoconulid. ofG.elkema(Mengetal.,2004,2005;Asheret
However, compared to those of G. elkema al.,2005),andismostlikelyaderivedfeatureof
and G. ellae, the talonid of G. shevyrevae is G. shevyrevae. However, in Mimolagus, a
relatively shorter and the hypoconulid is similarfacetispresentonthecalcaneus,which
reduced; it does not form a projection at the is in articulation with the astragalus (Bohlin,
posterior end of the tooth. 1951).
8 AMERICAN MUSEUMNOVITATES NO. 3670
DISCUSSION closely related to Gomphos than to any other
Glires.
TAXONOMY: The specimens described here BIOSTRATIGRAPHICIMPLICATION: TwoAsian
clearly represent a new species of Gomphos
Land Mammal ages, the Arshantan and
and the youngest species of the genus, and
Irdinmanhan, have been widely accepted, but
perhaps, the youngest representative of their definition and recognition have not
Mimotonidae. It differs from G. elkema and always been clear (Meng et al., 2007c).
G. ellae in having more robust teeth that are Dashzeveg and Hooker (1997: 136) suggested
higher crowned with more inflated cusps, thattheArshantan‘‘predatestheIrdinmanhan
upper molars with more posteriorly expanded and belongs to the earliest Middle or latest
hypocones and posterolophs, and lower mo- Early Eocene.’’ Lucas and Emry (2001) con-
lars that are proportionally shorter, lack sidered the Irdinmanhan and Arshantan ages
mesostylids, and have reduced hypoconulids. as collectively correlative with the Bridgerian,
In addition, an extra articular facet is devel- although Luterbacher et al. (2004) correlated
oped on the calcaneus. the Arshantan with the entire Bridgerian and
G. shevyrevae is more similar to G. elkema theIrdinmanhanwiththeUintan.Lucas(2001)
than to G. ellae in general morphology. The alsoregardedtheArshantanlandmammalage
major difference from G. elkema is inflation to be older than the North American middle
of cusps and lophs in G. shevyrevae. G. ellae Bridgerian.
has shorter, therefore transversely longer Oneexampleofthedifficultyindistinguish-
upper molars, which may be partly attribut- ing these two ages is found in the fauna from
able to the advanced wear of the known the Kholboldchi Formation of Mongolia.
specimen. The M1 (fig. 2B, C) of G. shevyr- Localities that supposedly expose the same
evaealsoshowssignificantwear,butdoesnot formationhaveproduceddifferentfossils,and
display the tendency of transverse elongation most of the fossils were typical of both
present in upper molars of G. ellae. Although Arshantan and Irdinmanhan faunas. These
worn, which suggests an adult individual, localities have been considered either
the tooth enamel layer of G. ellae remains Irdinmanhan or Arshantan (Russell and
thinner than that of G.shevyrevae.Thickened Zhai, 1987; Dashzeveg and Hooker, 1997;
tooth enamel confers a greater resistance to Lucas,2001;Mengetal.,2007c; Kraatzetal.,
wear,suggesting anabilitytoprocesstougher 2009). Similarly, Averianov and Godinot
food. (2005) considered the Andarak fauna to be
G. shevyrevae indicates that Gomphos, the Irdinmanhan in age, following the definition
sister taxon of lagomorphs of modern aspect, of the Arshantan and Irdinmanhan (Lucas,
was more diverse and has a longer strati- 2001). However, because the Andarak mam-
graphic range than was previously thought. mals are found within a marine section that
The earliest lagomorphs of modern aspect are yields a selachian fauna, Averianov and
known from theEarlyEocene(Li etal., 2007; Godinot (1998, 2005) argued that the
Rose et al., 2008; Lopatin and Averianov, Andarak fauna, and thus the Irdinmanhan,
2006), as are the earliest representatives of becorrelatedwiththelatestEarlyEocene(late
Gomphos. The fossil record indicates that the Ypresian).
divergence between the Gomphos and modern Part of the current confusion may have
lagomorph lineages was no younger than the stemmed from the original collections of
Early Eocene. In addition, G. shevyrevae Arshantan and Irdinmanhan faunas, which
further suggests that a branch of mimotonids appear to have been mixed assemblages of
coexisted with the earliest lagomorphsat least fossils from different ages, as pointed out by
up to the Middle Eocene. This is further Meng et al. (2007c). With new stratigraphic
supported by the description of both mimo- and faunal data, clarification of both the
tonids and lagomorphs from the Andarak 2 Arshantan and Irdinmanhan faunas can be
locality in Kyrgyzstan (Averianov, 1994). The expected. Here, the stratigraphic relationships
Gomphos lineage may have extended into still of Gomphos elkema, G. shevyrevae, and many
younger age if Mimolagus proves to be more other associated fossils, are clear (fig. 1).
2009 MENG ETAL.: ANEW SPECIESOFGOMPHOS 9
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