AMERICAN MUSEUM Novitates

PUBLISHED BY THE AMERICAN MUSEUM OF NATURAL HISTORY CENTRAL PARK WEST AT 79TH STREET, NEW YORK, NY 10024

Number 3597, 29 pp., 15 figures, 6 tables December 12, 2007

NEW MATERIAL OF ALAGOMYIDAE (MAMMALIA, GLIRES) FROM THE LATE PALEOCENE SUBENG LOCALITY, INNER MONGOLIA

JIN MENG,! XIJUN NI,27 CHUANKUI LI? K. CHRISTOPHER BEARD,’ DANIEL L. GEBO,* YUANQING WANG,? AND HONGJIANG WANG?

ABSTRACT

Newly discovered specimens of alagomyids, mostly isolated teeth collected by screenwashing at the Gashatan (Late Paleocene) Subeng locality in Inner Mongolia, document considerable intraspecific variation in Tribosphenomys minutus that has not been appreciated previously becatise of limited sample sizes, P4s of Tribosphenomys are described for the first time, which helps to clarify the posterior premolar identification of alagomyids. Some of the alagomyid specimens are referred to Tribosphenomys cf. T. secundus and Neimengomys gii gen. and sp. noy, Based on the new data, Tribosphenomys borealis from the Bumban Member of the Naran Bulak Formation, Mongolia, is considered to be a junior synonym of Alagomys inopinatus, and T, tertius from the Zhigden Member of the Naran Bulak Formation is regarded as a junior synonym of T, minutus. Alagomyidae, consisting of Tribosphenomys, Alagomys and Neimengomys, 1s maintained as a valid family. The presence of a diversity of alagomyids and other recently obtained fossils and stratigraphic evidence from the Erlian Basin suggest that the Gashatan and Bumbanian of Asia are probably correlative to the late Tiffanian— early Wasachian of North America. The faunal turnover during the Gashatan and Bumbanian in Asia is probably related to the Late Paleocene—Early Eocene global warming, but current evidence is insufficient to link any specific event with the Paleocene-Eocene Thermal Maximum.

' Division of Paleontology, American Museum of Natural History (jmeng@amnh.org).

* Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, P.O. Box 643, Beijing 100044, China (lichuankui@ivpp.ac.cn), (nixijun@ivpp.ac.cn).

* Section of Vertebrate Paleontology, Carnegie Museum of Natural History, 4400 Forbes Ave., Pittsburgh, PA 15213 (beardc@CarnegieM NH.org),

* Department of Anthropology, Northern Illinois University, DeKalb, IL 60115, USA (T20DLG1@wpo.cso.niu.edu).

° Administration Station of Cultural Relics of Xilinguole League, Xilinhaote, Neimenggu 026000, China.

Copyright © American Museum of Natural History 2007 ISSN 0003-0082

to

INTRODUCTION

Basal Glires, including mimotonids, eury- mylids, stem lagomorphs, and stem rodents, were abundant and diverse during the Paleocene and Eocene in Asia, suggesting that Asia was the center of origin for this group (Dashzeveg and Russell, 1988; Meng et al., 2003; Asher et al., 2005). Among these early Glires, the family Alagomyidae is particularly pertinent to the origin of rodents. The family contains the genera Tribosphenomys and Alagomys, with a total of seven named species (Dashzeveg, 1990b, 2003; Meng et al,, 1994; Meng and Wyss, 1994, 2001; Tong and Dawson, 1995: Dawson and Beard, 1996; Lopatin and Averianov, 2004a, 2004b). These species are minuscule mammals with gliriform, ever-growing upper and lower in- cisors typical of rodents. Their lower cheek teeth are rodentlike, but their upper cheek teeth are highly distinctive and less reminis- cent of those of other rodents or Glires. Isolated upper cheek teeth of Tribosphenomys collected at the Subeng locality during the late 1970s proved to be enigmatic for several decades. In an unpublished manuscript writ- ten by Zhai (see Russell and Zhai, 1987: 71) these teeth were considered to pertain to a dermopteran (flying lemur) because the paracone and metacone are widely separated by a transverse valley on the upper molars. More nearly complete material of alagomyids collected from the Bumban Member of the Naran Bulak Formation, Mongolia (Dash- zeveg, 1990b) and the Gashatan Bayan Ulan locality in Inner Mongolia (Meng et al., 1994) demonstrated that these tiny mammals are actually Glires.

To date, three species of Alagomys have been described: Alagomys inopinatus (Dashzeveg, 1990a), A. oriensis (Tong and Dawson, 1995) and A, russelli (Dawson and Beard, 1996). Four species of Tribosphenomys have been proposed: 7. minutus (Meng et al., 1994), T. borealis (Dashzeveg, 2003), T. secundus, and T. tertius (Lopatin and Averianov, 2004a, 2004b). These species range in age from the Gashatan to Bumbanian and all occur in Asia, except for A. russelli from the Clarkforkian (latest Paleocene) of North America (Dawson and Beard, 1996).

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Here we describe specimens of alagomyids collected by screenwashing at the Subeng locality in the Erlian Basin of Inner Mon- golia. Some of these specimens were collected during the 1970s, but most were recovered by renewed fieldwork at Subeng over the past few years.

MATERIAL AND METHODS

All specimens described here were collected by screenwashing sediment from the Subeng locality. A few teeth were collected in 1978, and most were obtained during the field seasons of 2003-2004, Based on the known morphology of alagomyids, these isolated teeth can be identified with a high level of confidence. The specimens are housed in the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP), Chinese Acad- emy of Sciences, Beijing. The SEM photo- graphs were taken from uncoated specimens using a Hitachi SEM machine at the American Museum of Natural History. We follow Meng and Wyss (2001) for terminology of dental features in the description. Because the dental morphology of Tribosphenomys and Alagomys has been described in detail (Dashzeveg, 1990b, 2003; Meng et al., 1994; Tong and Dawson, 1995; Meng and Wyss, 1998, 2001; Dawson and Beard, 1995; Lopatin and Averianov, 2004a, 2004b; Tong and Wang, 2006), our description will be brief and comparative, with a particular focus on in- traspecific variation and structures that were previously unknown. Measurements of cheek teeth are taken for the maximum length and width.

SUBENG LOCALITY AND FAUNA

The Upper Paleocene of central Asia was first described from the Gashato (Khashat) Formation in the Ulan-Nur Basin of Mongolia during the 1920s by the Central Asiatic Expedition. Morris first studied the beds in 1923 (see Matthew and Granger, 1925; Russell and Zhai, 1987: 57), and Matthew and Granger (1925) employed the term Gashato in describing the Gashato fauna. The Gashato Formation was formally proposed in 1927 by

2007

Berkey and Morris. The Gashato fauna derives from Member | of the formation, and is overlain by members II and III, which produced only one mammal taxon, Gomphos elkema (Dashzeveg, 1988).

Berkey and Morris (1927: 379) proposed the Gashato time as an interval in the Paleocene (their “Eocene’’). The Gashatan as an Asian land-mammal age was proposed by Romer (1966) without definition. Szalay and McKenna (1971) characterized Romer’s Gashatan as the joint overlapping time ranges of Palaeostylops, Pseudictops, Prionessus, and Eurymylus. Russell and Zhai (1987: 412) attributed the Gashato, Nomogen, Zhigden, and Naran faunas, along with several other faunas from other parts of China, to the late Paleocene Nongshanian (= Nungshanian; Li and Ting, 1983) Land Mammal Age of Asia, thereby abandoning the Gashatan. In a tenta- tive proposal of Chinese provincial land- mammal ages, Li and Ting (1983) considered the Nongshanian as Late Paleocene and correlated it to the North American Tiffanian, These authors also proposed ‘Bayanulanian” as a Paleocene-Eocene tran- sitional age, correlating with the North Amerian Clarkforkian. Tong et al. (1995) further expanded the Nongshanian to corre- late with both the Clarkforkian and Tiffanian. The Gashatan is now frequently used as the Latest Paleocene Asian Land Mammal Age (Meng and McKenna, 1998; Luterbacher et al,, 2004). Local faunas typically attributed to the Gashatan interval include those from the Gashato and Naran localities of Mongolia, and the Nomogen, Bayan Ulan, Urdyn Obo, and Subeng localities in China (Russell and Zhai, 1987; Wang et al., 1998; Meng et al., 1998, 2004, 2005), However, intercontinental correlation of the Gashatan remains an open issue. It is usually correlated with the North American Clarkforkian (Ting, 1998: Luterbacher et al., 2004), the lowest part of the Wasatchian plus the Clarkforkian (McKenna and Bell, 1997), or the upper part of the Tiffanian plus the Clarkforkian (Wang et al., 1998; Beard, 1998; Beard and Dawson, 1999).

The Subeng locality was first discovered in the 1970s by the geological mapping team from the Geological Survey of Inner Mongolia

MENG ET AL.: NEW MATERIAL OF ALAGOMYIDS 3

and the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences (Jiang, 1983; Qi, 1987; Russell and Zhai, 1987; BGBMRNMAR, 1991; Meng et al., 1998). Primary fieldwork at this locality was conducted by Qi Tao from the IVPP during the late 1970s and early 1980s, and the authors of this paper have worked in the area since 1987. Additional investigation was also con- ducted by the Belgian-Inner Mongolian Dinosaur project during the late 1990s. Numerous fossils have been collected by surface collecting, and over 40 tons of matrix from Subeng have been screenwashed by personnel from the [VPP and its collaborators since the 1980s. Magnetostratigraphic and paleoisotopic works have also been carried out during the last few years.

A short faunal list for Subeng (provided by Zhai Renjie) was published by Jiang (1983), which included Mongolotherium _ sp., Prionessus lucifer, Lambdopsalis _ bulla, Pastoralodon lacustris, Prodinoceras sp., and Palaeostylops iturus. Mongolotherium is now regarded as a junior synonym of Prodinoceras (Dashzeveg, 1982). In an unpublished manu- script written by Zhai (see Russell and Zhai, 1987: 71) dermopteran teeth were identified among the specimens collected at Subeng by screenwashing. Similar teeth were subsequent- ly found at Bayan Ulan and recognized as Tribosphenomys minutus (Meng et al., 1994; Meng and Wyss, 1994, 2001), a basal Glires, not a dermopteran. Other small mammals collected by screenwashing at Subeng include the carpolestid Subengius mengi (Smith et al., 2004) and Asionyctia guoi (Missiaen and Smith, 2005). As a result of the increased collecting efforts during the last three decades, the Subeng fauna now includes about 30 species (table 1).

SYSTEMATIC PALEONTOLOGY

Glires Linnaeus, 1758 Alagomyidae Dashzeveg, 1990b

Tribosphenomys minutus Meng, Wyss, Dawson and Zhai, 1994

Hototype: IVPP V10775, left maxilla with P3. DP4 and MI1-2 and associated mandible

4 AMERICAN MUSEUM NOVITATES NO. 3597

TABLE 1

Taxon list of the Late Paleocene Subeng fauna.

MULTITUBERCULATA

Ptilodontidae gen. et sp. nov. (2) Taeniolabididae

Lambdopsalis bulla

Prionesus sp.

ANAGALIDA

Pseudictopidae Pseudictops lophiodon

GLIRES

Eurymylidae Eomylus bayanulanensis Palaeomylus lii

Alagomyidae Tribosphenomys minutus Tribosphenomys cf. T. secundus Neimengomys qii gen. et sp. nov.

CIMOLESTA

Pastoralodontidae

Pastoralodon lacustris (from Jiang, 1983) Palaeoryctidae

Palaeoryctes sp. Cimolestidae

Gen. et sp. indet.

Paleotomus sp.

Tsaganius sp.

CREODONTA

Hyaenodontidae Prolimnocyon chowi

CARNIVORA

Viverravidae Viverravus sp.

ERINACEOMORPHA

Incertae familiae Litocherus sp.

SORICOMORPH

Micropternodontidae Hyracolestes sp. nov. Hyracolestes cf. H. ermineus Sarcodon minor

Nyctitheriidae Plagioctenodon sp. Leptacodon sp. Bayanulanius sp. nov. Bumbanius atf. B. rarus Asionyctia guoi

PLESIADAPIFORMES

Carpolestidae Subengius mengi

TABLE 1. (Continued )

DINOCERATA Uintatheriidae Prodinoceras cf. xinjiangensis ARCTOSTYLOPIDA Arctostylopidae Palaeostylops iturus MESONYCHIA Mesonychidae Pachyaena sp. Hapalodectidae Hapalodectes sp. nov.

with dp4—m3 from Bayan Ulan, Inner Mon- golia.

NEWLY REFERRED SPECIMENS: IVPP V14707.1— 2, 2 right upper incisors; V14707.3—-10, 8 isolated DP4s; IVPP V14707.11, a fragmentary maxilla with P4-M1; V14707.12-19, 8 isolated P4s; V14707.20-39, 20 isolated M1; V14707.40— 54, 15 M2s; V14707.55—63, 12 M3s; V14708.1, fragmentary right mandible with p4; V14708.2, right p4; V14708.3-9, 7 dp4s; V14708.10—20, 11 mls; V14708.21-44, 24 m2s; V14708.45—55, 11 ms. See tables 2 and 3 for measurements.

LocALity AND AGE: Subeng, about 25 km west of Erlian; Gashatan, Late Paleocene.

COMPARATIVE DESCRIPTION: Several iso- lated incisors can be referred to Tribo- sphenomys because of their similar size and morphology to those described from Bayan Ulan (Meng et al., 1994; Meng and Wyss, 2001). Two upper left incisors (V14707.1-2) are illustrated (fig. 1), which measure 0.68/ 1.12 mm and 0.62/1.3 mm (width/depth), re- spectively. The tip of the tooth is rounded buccally and has a straight medial edge of contact, and an elongated wear facet. The wear striations on the facet are parallel to its medial edge.

In Tribosphenomys minutus DP4 is molari- form and triangular in occlusal view, with distinctive protocone, paracone, metacone, and metaconule (fig. 2). Most of the isolated DP4s in our sample from Subeng are generally similar to that of the holotype from Bayan Ulan. They differ from that of 7. secundus in being smaller and having slimmer cusps and weaker crests. There are no published mea- surements for the cheek teeth in the holotype

MENG ET AL.: NEW MATERIAL OF ALAGOMYIDS 5

TABLE 2 Upper Tooth Measurements (mm) of Tribosphenomys minutus (Length/Width)

2007 Specimens L/W

DP4 (V14707.3) 0.67/1.08 DP4 (V14707.4) 0.85/1.12 DP4 (V14707.5) 0.77/1.12 DP4 (V14707.6) 0.73/1.10 DP4 (V14707.7) 0.73/1.15 DP4 (V14707.8) 0.71/1.06 DP4 (V14707.9) 0.69/0,93 DP4 (V14707.10) 0.65/1.00 MI (V14707.20) 0.77/1.38 M1 (V14707.21) 0.79/1.38 M1 (V14707.22) 0).75/1,48 M1 (V14707.23) 0.77/1.31 MI (V14707.24) 0).83/1.29 MI (V14707.25) 0.78/1.44 M1 (V14707.26) 0.83/1.44 M1 (V14707.27) 0.87/1.56 M1 (V14707.28) 0.87/1.48 MI (V14707.29) 0.77/1.56 M1 (V14707.30) 0.88/1.44 M1 (V14707.31) 0).85/1,44 M1 (V14707.32) 0.81/1.58 MI (V14707.33) 0.83/1.58 MI (V14707.34) 0.83/1.40 M1 (V14707.35) 0.85/1.59 M1 (V14707.36) 0.81/1.48 M1 (V14707.37) 0.88/1.52 MI (V14707.38) 0).87/1.48 M1 (V14707,39) 0.83/1.56 M1 (V14707.11) 0.92/1.58

of T. borealis (Dashzeveg, 2003), but the DP4 described here appears to be proportionally larger relative to Ml, particularly in the transverse dimension, and have a more prom- inent buccal shelf than that of 7. borealis. One specimen (fig. 2h) is smaller, shorter, and simpler, which may indicate that it pertains to a different species.

The fragmentary maxilla (V14707.11; fig. 3a) is similar to that of the holotype of Tribosphenomys minutus in the position of the anterior zygomatic arch, the ridge on the ventral surface of the zygomatic root, the size and position of the infraorbital foramen and the smooth floor of the orbit dorsal to the cheek teeth. A partial alveolus anterior to P4 indicates that P3 was present, although

Specimens L/W P4 (V14707.11) 0,46/1.17 P4 (V14707.12) 0,50/1.19 P4 (V14707.13) 0.41/1.13 P4 (V14707.14) 0.46/1.08 P4 (V14707.15) 0).42/1.06 P4 (V14707.16) 0).52/1.35 P4 (V14707.17) 0.42/1.27 P4 (V14707.18) 0.48/1.25 P4 (V14707.19) 0,52/1.25

M2 (V14707.40) 0.77/1,23 M2 (V14707.41) 0.78/1.31 M2 (V14707.42) 0.75/1.35 M2? (V14707.43) 0.83/1.40 M2 (V14707.44) 0,79/1.31 M2 (V14707.45) 0.79/1,21 M2 (V14707.46) 0.83/1.40 M2 (V14707.47) 0.85/1.38 M2 (V14707.48) 0,83/1.33 M2 (V14707.49) 0,85/1.40 M2 (V14707.50) 0.79/1.23 M2 (V14707.51) 0.88/1.40 M2 (V14707.52) 0.90/1.37 M2 (V14707.53) 0.90/1.50 M2 (V14707.54) 0,88/1.58 M3 (V14707.55) 0,65/1.02 M3 (V14707.56) 0,65/1.00 M3 (V14707.57) 0.69/1.00 M3 (V14707.58) 0,65/1.02 M3 (V14707.59) 0.63/0.98 M3 (V14707.60) 0.67/0.92 M3 (V14707.61) 0.73/0.94 M3 (V14707.62) 0.66/1.06 M3 (V14707.63) 0.75/1.10

examples of this tooth locus have not been recovered by screenwashing at Subeng. P3 was described in the holotype of Tribosphenomys minutus (Meng et al., 1994; Meng and Wyss, 2001). It is a small, unicuspate tooth with two closely spaced roots arranged anteroposter- iorly. A dP3 has been described for T. secundus (Lopatin and Averianov, 2004a), which has two roots that are more widely separated and a more complex crown mor- phology than P3 of T. minutus.

In the original description of Tribo- sphenomys minutus (Meng et al., 1994), the posterior premolars of the holotype (V10775) and referred specimen (V10776) were regarded as P4/p4. Additional material referred to T. minutus (V10778) clearly displays a nonmolari-

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TABLE 3 Lower Tooth Measurements (mm) of Tribosphenomys minutus (Length/Width)

Specimens L/W p4 (V14708.1) 0.61/0.62 p4 (V14708.2) 0.75/0.67 dp4 (V14708.3) 0.73/0.73 dp4 (V14708.4) 0.76/0.76 dp4 (V14708.5) 0.78/0.78 dp4 (V14708.6) 0.78/0.71 dp4 (V14708.7) 0.88/0.82 dp4 (V14708.8) 0.86/0.69 dp4 (V14708.9) 0.86/0.78 m2 (V14708.21) 0,92/0.88 m2 (V14708.22) 0.96/0.88 m2 (V14708.23) 0.92/0.94 m2 (V14708.24) 1.01/0.94 m2 (V14708.25) 0.88/0.90 m2 (V14708.26) 1.02/0.94 m2 (V14708.27) 0.96/0.92 m2 (V14708.28) 1.02/0.88 m2 (V14708.29) 0.88/0.83 m2 (V14708.30) 0.96/0.83 m2 (V14708.31) 0.96/0.83 m2 (V14708.32) 0.98/0.92 m2 (V14708.33) 1.00/0.96 m2 (V14708.34) 0.96/0.98 m2 (V14708.35) 0.98/0.98 m2 (V14708.36) 1.00/0.83 m2 (V14708.37) 1,04/0.98 m2 (V14708.38) 1.00/0.92

Fig. 1. minutus from late Paleocene Subeng locality. a. Buccal view (V14707.1). b. Wear facet of the incisor tip (V14707.1). c. Lingual view (V14707.2). Arrow indicates the direction of striations on the wear facet.

Right upper incisors of Tribosphenomys

NO. 3597 Specimens L/W ml (V14708.10) 1.00/0.81 ml (V14708.11) 0.96/0.88 ml (V14708.12) 1.02/0.87 ml (V14708.13) 0.94/0.81 ml (V14708.14) 0.81/0.83 ml (V14708.15) 0.92/0.81 ml (V14708.16) 0.88/0.77 ml (V14708.17) 0.90/0.85 ml (V14708.18) 0.96/0.85 ml (V14708.19) 0.96/0.88 ml (V14708.20) 0.92/0.69 m2 (V14708.39) 0.98/0.90 m2 (V14708.40) 1.02/1.00 m2 (V14708.41) 1.00/0.98 m2 (V14708.42) 1.01/0.98 m2 (V14708.43) 0.94/0.85 m2 (V14708.44) 0.94/0.92 m3 (V14708.45) 1.17/0.75 m3 (V14708.46) 1.10/0.87 m3 (V14708.47) 1.10/0.85 m3 (V14708.48) 1.09/0.88 m3 (V14708.49) 1.12/0.80 m3 (V14708.50) 1.04/0.71 m3 (V14708.51) 1.15/0.69 m3 (V14708.52) 1.13/0.87 m3 (V14708.53) 1.19/0.87 m3 (V14708.54) 1.14/0.86 m3 (V14708.55) 1.27/0.88

form p4, which led Meng and Wyss (2001) to regard the posterior premolars in V10775-—6 as DP4/dp4. According to this interpretation, the permanent replacements for these deciduous teeth are delayed. Meng and Wyss (2001) also extended this interpretation to Alagomys. P4 of Tribosphenomys has not been described previously. Lopatin and Averianov (2004a) reported additional DP4 from TJ. secundus. Dashzeveg (2003) named T. borealis based on a fragmentary maxilla bearing two cheek teeth that he interpreted as P4 and M1. In our view, however, the more anterior tooth locus in the holotype of T. borealis is actually a DP4. P4 is only known in A. inopinatus (Dashzeveg, 1990b), although interpretation of this tooth in the species was confused (see below).

The fragmentary maxilla bearing P4 and M1 and the isolated P4s illustrated in fig. 3 are the first record of P4 in Tribosphenomys. As

2007 MENG ET AL.: NEW MATERIAL OF ALAGOMYIDS 7

Fig. 2. (V14707.3-10). All images of specimens in figs. 2-13 are on the same scale.

predicted by Meng and Wyss (2001), the P4 is non-molariform, similar to but proportionally larger than that of Alagomys inopinatus. The P4 of JT. minutus is relatively larger in comparison with M1, more lingually posi- tioned, and has a wider buccal shelf than that of A. inopinatus. P4 is a short, transverse tooth bearing one lingual and one buccal cusp. It 1s double-rooted, with the lingual root being much stronger than its buccal counterpart (fig. 4a). The lingual cusp is lower than the buccal cusp and has a rounded lingual surface but a flat or concave buccal surface. In most of the specimens at hand, anterior and posterior ridges form the edges of the tooth. These ridges extend from the lingual cusp to the buccal shelf. In some cases, the posterior ridge is weak or absent. The buccal cusp is cresentic. In unworn specimens (fig. 3e), the buccal cusp forms an isolated island; in most cases, however, its anterior and posterior arms extend buccally to join the anterior and posterior edges of the tooth, respectively. The buccal shelf is broad, varying from one- fifth to one-third of the tooth width. As shown (fig. 3a), there is little if any interstitial contact

a-h, Crown views of DP4 of Tribosphenomys minutus from late Paleocene Subeng locality

between P4 and M1; therefore, interstitial facets are typically absent on the posterior surface of P4 and the anterior surface of M1.

All upper molars have a strong lingual root and two small labial ones (fig. 4b, cc). Compared to the holotype of T. minutus, M1 in the maxilla (fig. 3a) is heavily worn, in- dicating that it derives from an older in- dividual. This observation supports the in- terpretation that the eruption of P4 was ontogenetically delayed in alagomyids. Among the 20 isolated M1 illustrated (fig. 5; some poorly preserved specimens are not figured), only one (fig. Ss) is in a similar wear stage as that of V14707.11. This perhaps indicates a minority of older individuals in the population.

M1 differs from DP4 mainly in having a more rectangular occlusal outline, with the lingual side being slightly shorter than its buccal counterpart, and by its greater width (fig. 5). Reflecting this difference in width, all cusps and crests on M1 are transversely more extended than the corresponding structures on DP4, All the MIs have a distinct buccal shelf, which differs from that of Alagomys.

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Fig. 3. (V14707.11). b-i, Crown views of P4s of Tribosphenomys minutus (V14707.12-19) from late Paleocene Subeng locality.

However, these teeth vary slightly in size and their width/length proportions. Minor mor- phological variation is also evident within the M1 collection. Examples include rounded ys. squared anterobuccal and posterobuccal cor- ners, straight vs. concave buccal margins, and variable development of the buccal shelf, the hypocone, the postcingulum, and conules. In general, M1 cusps of Tribosphenomys are more transverse than those of Alagomys.

M2 is less symmetrical than M1 in that it reduces in width posteriorly (fig. 6). It is narrower and has a less pronounced buccal shelf, particularly the part buccal to the metacone, than is the case for Ml. M2 generally lacks any trace of the hypocone. In tooth width, they range from 0.75 to 0.9 mm (table 2). However, because of the continuous variation within the sample in terms of both size and morphology, we regard these differ- ences as intraspecific variation. In most cases

a. Ventral view of a fragmentary left maxilla with P4—M1 of Tribosphenomys minutus

the anterior and posterior surfaces of M2 do not have interstitial facets.

M3 is the smallest, simplest, and least transverse of the upper molars (fig. 7). The posterior half of M3 is further reduced over the condition seen in M2, and the buccal shelf is narrower. The metacone is reduced to a tiny accessory cuspule or a crest on the postero- buccal side of the metaconule, or else it disappears completely. The protoconule is absent.

A fragmentary right mandible bearing a p4 is preserved (fig. 8a). The diastema between the posterior edge of the incisor alveolus and p4 root is 1.8 mm. A large mental foramen occurs below p4. Several dp4s are present in the sample from Subeng (fig. 8c-i). These teeth lack interstitial facets on their anterior surfaces. Some of the teeth are severely worn, so that their original morphologies are ob- scured. Two dp4 are unworn and have

2007 MENG ET AL.: NEW MATERIAL OF ALAGOMYIDS 9

Fig. 4. Root conditions of Tribosphenomys minutus from Subeng. a. Anterior view of P4. b. Posterior view of M1. c. Dorsal view of M2. d. Buccal view of m2. e. Buccal view of m3.

a rudimentary cusp at the anterior end of the trigonid, which is called the paraconid for convenience of description (fig. 8c, d). The dp4 is similar to ml except being smaller and having the paraconid.

Two p4 are in the collection; one is an isolated tooth and the other is preserved in the fragmentary mandible (fig. 8a, b). These two p4s are similar in morphology but differ slightly in size. The p4 has a wider trigonid that consists of two subequal cusps that may be interpreted as protoconid and metaconid. The talonid is a transverse ridge that is more lingually positioned, posterior to the metaco- nid. These p4s are similar to that of T. minutus from Bayan Ulan (Meng and Wyss, 2001: fig. 7b—d).

The ml bears an interstitial contact facet on its anterior surface and lacks the paraconid. The protoconid and metaconid are closely appressed, with the metaconid being more anteriorly extended (fig. 9). The m2 differs from ml in having a wider trigonid and the metaconid less anteriorly extended (fig. 10). The protocristid and mesoconid are usually present, but weak. The paraconid is absent. Unlike m3, the hypoconulid is transverse.

The m3 differs from ml—2 in having the metaconid more anteriorly extended and a prominent hypoconulid that forms the third lobe of the tooth (fig. 11). The size of the hypoconulid varies considerably and _ the ridges connecting it to the hypoconid and entoconid also take different shapes. The

10 AMERICAN MUSEUM NOVITATES NO. 3597

Fig. 5. a-t. Crown views of Mls of Tribosphenomys minutus (V14707.20—-39) from late Paleocene Subeng locality.

2007 MENG ET AL.: NEW MATERIAL OF ALAGOMYIDS 1]

Fig. 6.

protocristid (posterior arm of the protoconid) varies from being completely absent to a weak- ly developed crest. As is the case for the upper molars, deeply worn lower cheek teeth, such as the one shown in fig. 11k, are rare.

Tribosphenomys cf. T. secundus Lopatin and Averianov, 2004a

Ho.otype: PIN3104/435, A fragmentary max- illa with alveolus of DP2, Dp3, Dp4 and M1

a—o. Crown views of M2s of Tribosphenomys minutus (V14707.40—54) from late Paleocene Subeng locality.

from the Zhigden Member of the upper Naran- Bulak Formation, Tsagan-Khushu, Mongolia.

REFERRED SPECIMENS: V14709.1, left M1; V14709.2, left M2; V14709.3—5, three left M3s; V14710.1—2, two left dp4s; V14710.3, right ml or m2; V14710.4, right m2; V14710.5, left m3. See table 4 for measurements.

LOCALITY AND AGe: Subeng, about 25 km west of Erlian; Gashatan, Late Paleocene.

COMPARATIVE DESCRIPTION: The Mi (fig. 12a) is 20% larger than those referred to

12 AMERICAN MUSEUM NOVITATES NO. 3597

Fig. 7. a-i. Crown views of M3s of Tribosphenomys minutus (V14707.55-66) from late Paleocene Subeng locality.

Fig. 8. a. A fragmentary right mandible with p4 of Tribosphenomys minutus (V14708.1). b. Crown view of a right p4 of 7. minutus (V14708.2). c-i. Crown views of dp4s of Tribosphenomys minutus (V14708.3—9) from late Paleocene Subeng locality.

2007 MENG ET AL.: NEW MATERIAL OF ALAGOMYIDS 13

Fig, 9. Subeng locality.

T. minutus (table 4; figs. 14, 15), being similar in size to that of T. secundus (Lopatin and Averianov, 2004a), Moreover, the buccal shelf is decorated with cuspules, a condition that also occurs in T. secundus. This tooth repre- sents an old individual, but the degree of wear is similar to that of V14707.38 (fig. 5s); therefore, its larger size cannot be attributed to its age.

The M2 (fig. 12b) is larger than any known M2 of alagomyids but closely matches the M1 of T. cf. T. secundus, M2 of T. secundus has not previously been documented. The M2 cusps appear slimmer than those of the M1 referred to T. cf. T. secundus, so that the central basin 1s relatively broader. The weath- ered surface of the M2 may have altered its morphology.

Like M2, M3 has not previously been described for T. secundus. Here we refer three M3s to Tribosphenomys cf. T. secundus be- cause of their relatively larger sizes. These teeth also differ from those of T. minutus in

a-k, Crown views of mls of Tribosphenomys minutus (V14708,10—-20) from late Paleocene

having a large metacone and accessory cus- pules on the buccal shelf, although these structures are heavily worn in one of the teeth (fig. 12e).

Two dp4 are assigned to Tribosphenomys cf. T. secundus because of their size (fig. 12f, g). Of the two, one is little worn and shows a distinct cusp anterior to the metaconid, the so-called paraconid, so that the trigonid 1s relatively complete; the other is heavily worn and the paraconid is absent.

Two lower teeth are identified as m2s of Tribosphenomys cf. T. secundus because of their relatively large size (fig. 12h, i), although one (fig. 12h) could be ml. In addition, these teeth have a small cusp (the so-called para- conid) at the lingual end of the paracristid and bear a small mesostylid on the lingual margin of the talonid basin between the metaconid and entoconid. These features are absent in Subeng lower cheek teeth referred to T. minutus. A vestige of the paraconid, however, is present in some less worn ml-2s of T.

14 AMERICAN MUSEUM NOVITATES NO. 3597

we

Fig. 10. a-x. Crown views of m2 of Tribosphenomys minutus (V14708.21—44) from late Paleocene Subeng locality.

2007 MENG ET AL.: NEW MATERIAL OF ALAGOMYIDS 15

Fig. 11. Subeng locality.

minutus from Bayan Ulan (Meng and Wyss, 2001). The paraconid is absent in previously described specimens of TJ. secundus, but because the only known lower teeth are from an old individual with teeth that are heavily worn and p4 fully erupted; the apparent absence of a paraconid in 7: secundus may be caused by wear, as in T. minutus. The lower m2s from Subeng are longer than wide, contrasting with those described previously for T. secundus, which are slightly wider than long (Lopatin and Averianov, 2004a).

The m3 is previously unknown for T. secundus. Here we refer an m3 (fig. 12j) to

TABLE 4 Tooth Measurements (mm) of Tribosphenomys cf. T. secundus (Length/Width)

Specimens L/W Specimens L/W M1 (V14709.1) 1.08/1.94 dp4 (V14710.1) 0.98/0.94 M2 (V14709.2) 1.02/1.60 dp4 (V14710.2) 0,.90/0.89 M3 (V14709.3) 0.79/1.27 =m2 (V14710.3) 1.06/1.00 M3 (V14709.4) 0.80/0.96 m2 (V14710.4) 1.19/1.00 M3 (V14709.5) 0.79/1.24 m3 (V14710.5) 1.43/0.98

a—k. Crown views of m3s of Tribosphenomys minutus (V14708.45—55) from late Paleocene

Tribosphenomys cf. T. secundus because it is distinctively larger than those of T. minutus. This m3 is 22% longer than that in the holotype of T. minutus.

Similarities and differences are present between the Subeng specimens and the type specimen of 7. secundus. Given the fragmen- tary nature of both samples, we tentatively recognize the Subeng specimens as Tribo- sphenomys cf. T. secundus.

Neimengomys, new genus

TYPE SPECIES: cles

EtrymMo.ocy: WNei-Meng is Chinese (in pin- yin) for “Inner Mongolia’; mys is Greek for “mouse”, in analogy with A/agomys and Tribosphenomys.

Diacnosis: Differs from Alagomys but resembles Tribosphenomys in having a buccal shelf and a hypocone on upper molars, a more transverse M3 with conical cusps, and a nar- rower talonid basin. Differs from Tribo- sphenomys in having a P4 with a weak buccal

Neimengomys qii, new spe-

16 AMERICAN MUSEUM NOVITATES

NO. 3597

Fig. 12. Crown views of cheek teeth of Tribosphenomys cf. T. secundus from late Paleocene Subeng locality. a. Left M1 (V14709.1). b. Left M2 (V14709.2). c-e. Left M3s (V14709.3-5); f-g. Left dp4 (V14710.1- 2). h. Right ml or m2 (V14710.3). i. Right m2 (V14710.4). j. Left m3 (V14710.5).

shelf, a more inflated protocone on upper molars, a more inflated and buccally posi- tioned hypocone, and a smaller hypoconulid on lower molars.

Neimengomys qii, new spesies

Ho.ortyre: IVPP V14711.1, a right M1 (or M2) (fig. 13b). INCLUDED SPECIMENS: V14711.2, left P4;

V14711.3-10, 8 Mls and/or M2s; V14711.11-14,

4 M3s (or M2s); V14712.1, right p4; V14712.2, right ml; V14712.3-5, 14712.3-5, 3 m3s (fig. 13). See table 5 for measurements.

EtyMo.Locy: The trivial name is after Qi Tao (IVPP), who made the first discovery of Tribosphenomys specimens by screenwashing at Subeng in the 1980s.

DiAcnosis: Same as for the genus.

Type LOCALITY AND AGE: Subeng, about 25km west of Erlian, Gashatan, Late Paleocene.

2007 MENG ET AL.: NEW MATERIAL OF ALAGOMYIDS 17

Fig. 13. (V14711.2). b. Right M2 (or M1, the holotype, V14711.1). c-j. Mls and/or M2 (V14711.3-10). k-n. M3 (or M2s, V14711.11-14). o. Right p4 (V14712.1). p. Right m1 (V14712.2). q-s. m3s (14712.3-5).

COMPARATIVE DESCRIPTION: A P4 is identi- fied for this species (fig. 13a). This tooth is smaller than those of 7. minutus and trans- versely oval-shaped. The P4 buccal shelf is much narrower than that of 7. minutus and is similar to that of A. inopinatus (Dashzeveg, 1990b). It differs from both Tribosphenomys

Crown views of cheek teeth of Neimengomys qii from late Paleocene Subeng locality. a. Left P4

and Alagomys in bearing a very small cuspule on the buccal shelf.

Nine upper cheek teeth are considered to be either M1 or M2 (fig. 13b—j), which we cannot yet distinguish with confidence. These teeth have an oval shape in occlusal view. Compared to M1 of Tribosphenomys minutus

18 AMERICAN MUSEUM NOVITATES

NO. 3597

2.00 T .secundus (V14709.1} xX Paramys adamus (CM68765) x T. secundus (PIN3 104/435) Mongolia 1.80 x x Cocomys lingchaensis x Bandaomys zhonghuaensis (V10689)

1.60 s 1.40 =

A, oriensis (V10693) 1,20 ) Alagomys inopinatus (PSSN20-176) % Pappocricetodon antiquus x A. russelli (CM69771) 1.00 Fay Neimengomys qii © Tribosphenomys minutus Subeng x (] fribosphenomys minutus (BayanUlan) Archetypomys erlianensis (M1) X Others .80

.60 .70 .80 90 1.00

1.10 1.20 1.30 1.40 1.50 1.60

Length

Fig. 14. M1 size distributions of alagomyids and some early rodents. See tables 2, 4-6 for measurements. Data from non-Subeng specimens for this and fig. 15 are adopted from Meng and Wyss (2001, Tribosphenomys minutus); Lopatin and Averianov (2004a, 2004b 7. secundus, T. tertius); Dashzeveg (1990b, Alagomys inopinatus); Dawson and Beard (1996, A. russelli, Paramys adamus); Tong and Dawson (1995, A. oriensis); Meng et al. (2007, Archetypomys erlianensis); Li et al. (1989, Cocomys lingchaensis); Tong and Dawson (1995, Bandaomys zhonghuaensis); Averianov (1996, Petrokozlovia cf. P. notos), and Wang and

Dawson (1994, Pappocricetodon antiquus).

these teeth are narrower and less symmetrical along the transverse axis, the protocone is more inflated, the hypocone is more inflated and buccally positioned, and the buccal shelf of the tooth is distinctly narrower. The width/ length ratio of these teeth is similar to that of

M2 of 7. minutus, but M2 of T. minutus is more triangular in occlusal view, the anterior edge of the tooth is much wider than its posterior counterpart, the protocone is ante- roposteriorly more compressed, and the hy- pocone is usually absent. These teeth are also

2007 MENG ET AL.: NEW MATERIAL OF ALAGOMYIDS 19 1.50 4 Paramys adamus (CM68765) 1.40 Bandaomys zhonghuaensis (V 10689) 1.30 Cocomys lingchaensi x T. secundus (PIN3 104/435, Mongolia) 1.20 x A, oriensis (V 10693) 1.10 = Alagomys inopinatus (PSSN20-176) re = 1.00 T. tertius (PIN3 104/775, Mongolia) A. russelli (CM69771) X Pappocricetodon antiquus 90 Neimengomys qii (V14712.2) 80 Z\ Neimengomys qii (V14712.2) 9 © Tribosphenomys minutus Subeng [] Tribosphenomys minutus (BayanUlan) Archetypomys erlianensis (M1) x Others .60 .70 80 90 1.00 1.10 1.20 1.30 1.40 1.50 Length Fig. 15. The ml size distributions of alagomyids and some early rodents. See tables 3-6 for

measurements and fig. 14 for source of data.

distinctive in size and morphology from upper molars of 7. secundus (Lopatin and Averi- anov, 2004a).

The molars of Neimengomys gqii differ from M1 and/or M2 of Alagomys in having a hypocone and a more prominent buccal shelf. Among species of Alagomys, N. qii 1s more comparable with the North American A. russelli than the Asian A. inopinatus and A. oriensis. The Asian species are similar in have an anteroposteriorly compressed protocone,

while A. russel/li has an inflated protocone and a somewhat oval-shaped occlusal outline (Dawson and Beard, 1996: plate 1F). Subeng specimens differ from A. russelli in having a distinct buccal shelf on the upper molars, whereas the same structure is completely absent in A. russelli. In addition, the molar cusps of A. russelli are less isolated, the lophs are stronger, the protocone is more inflated, and the trigon basin is deeper. In addition, the basin and postcingulum between the proto-

1.60

20 AMERICAN MUSEUM NOVITATES

TABLE 5 Tooth Measurements (mm) of Neimengomys qii (Length/Width)

Specimens L/W P4 (V14711.2) a 0.45/0.84 M1 (V14711.1 b 0.77/1.27 M1 (V14711.3) ¢ 0.75/1.31 M1 (V14711.4) d 0.81/1.25 M1 (V14711.5e 0.87/1.35 M1 (V14711.6) f 0.71/1.21 M1 (V14711.7) g 0.79/1.27 M1 (V14711.8) h 0.77/1.25 M1 (V14711.9) i 0.79/1.25 M1 (V14711.10) j 0.77/1.23

cone and metaconule are more prominent in Subeng specimens.

Four specimens are identified as M3s (fig. 13k-n) of Neimengomys qii because of their small size and reduced metacone. Compared to M3 of T. minutus, the M3 of N. qii is also oval-shaped and has a more inflated, conical protocone. The M3 of N. gii is more transverse and cuspate than those of Alagomys (Dawson and Beard, 1996; Tong and Dawson, 1995). However, we cannot rule out the possibility that these teeth are M2.

One p4 is identified as N. gii because of its smaller size and unique morphology. Unlike those referred to T. minutus, the p4 (fig. 130)

NO. 3597 Specimens L/W M3 (V14711.11) k 0.65/1.22 M3 (V14711.13) m 0.63/1.00 M3 (V14711.14) n 0.63/1.08 p4 (V14712.1) o 0.57/0.53 ml (V14712.2) p 0.96/0.77 m3 (V14712.3) q 1.02/0.84 m3 (V14712.4) r 1.07/0.81 m3 (V14712.5) s 1.02/0.81

has a narrow trigonid consisting of two closely appressed cusps. In contrast, the talonid is wider than the trigonid and formed by two transverse, ridgelike cuspids. The p4 is similar in morphology to that of 7. secundus (Lopatin and Averianov, 2004a), but differs in being much smaller.

The ml of N. gii (fig. 13p) has a narrower trigonid than those of T. minutus and a hypo- conulid that is aligned with the entoconid as a ridge. Three m3s are referred to T. gii (fig. 13q-s). The hypoconulid on these m3s is more transverse than conical, differing from those referred to 7. minutus. The trigonid basin of the lower molar is somewhat re-

TABLE 6 M1 and m1 Length/width (mm) and L/W ratios (R) in Selected Glires Taxa

M1

Tribosphenomys minutus V10775 0.86/1.56 (Bayan Ulan) V10776 0.78/1.50

V10778 /

V10779 0.77/1.54

V10780a /

V10780b / T. secundus (PIN 3104/435) 1.00/1.80 T. tertius (PIN 3104/775) / Alagomys inopinatus (PSS N20-176) 0.70/1.10 A, russelli (CM 69771) 0.64/1.01 A. oriensis (V10693) 0.68/1.10* Archetypomys erlianensis 0.77/0.88(6) Cocomys lingchaensis 1.40/1.79(8) Bandaomys zhonghuaensis (V10689) 1.44/1.68 Paramys adamus (CM 68765) 1.56/1.90 Petrokozlovia cf. P. notos 3.17/3.33(3) New myodont rodent (V14615.1) 1.00/1.03

Pappocricetodon antiquus

1.24/1.10(62)

R ml R 0.55 0.90/0.99 0.91 0.52 0.91/1.01 0.90

0.91/0.98 0.93 0.50 /

0.87/0.96 0.91

0.89/1.00 0.89 0.56 1.05/1.20 0.88

0.85/0.90 0.94 0.63 0.81/0.84 0.96 0.63 0.84/0.87 0.97 0.62 0.71/0.81 0.88 0.88 0.84/0.76(7) 1.11 0.78 1.32/1.28(3) 1.03 0.86 1.60/1.29(2) 1.24 0.82 1,49/1.49(2) 1.00 0.95 3.33/3.33(4) 1.00 0.97 1.03/0.81(5) 1.27 1.12 1.26/0.94(54) 1.34

2007 MENG ET AL.: NEW MATERIAL OF ALAGOMYIDS 21

stricted, similar to that of 7. minutus. In contrast, the trigonid basin in Alagomys is more open.

Although the material is fragmentary, the morphologies of these teeth show that they must be from a previously unknown taxon. Based on these specimens, we propose a new genus and species of alagomyid.

DISCUSSION

ALAGOMYS: Three species have been re- ported for the genus: A. inopinatus (Dashzeveg, 1990b), A. oriensis (Tong and Dawson, 1995), and A. russelli (Dawson and Beard, 1996). The holotype of A. inopinatus is a fragmentary maxilla bearing three teeth. In the original description, these teeth were interpreted as P4-M2 (Dashzeveg, 1990b), but the upper tooth loci were identifed as P3—M1 in a more recent work (Dashzeveg, 2003), based on a personal communication from J.-L Hartenberger. In the original de- scription (Dashzeveg, 1990b: 38) it was noted that “P3 not preserved but remains of its posterior alveole[alveolus] present.”’ Presence of the P3 alveolus was also cited by Meng and Wyss (2001: 34) as a personal communication from Dashzeveg, although the alveolus in question could also pertain to DP3. Interpreting these teeth as P3-M1, as did Dashzeveg (2003), would make the anterior alveolus, if it is present, attributable to P2 (or DP2). However, the latter alveolus was not mentioned in the recent study (Dashzeveg, 2003), and it was never properly illustrated or described in the original study. The new interpretation of the holotype by Dashzeveg (2003) creates a large and fully molariform “P4” for A. inopinatus, which is unsupported by evidence from other alagomyids. We did not have a chance to examine the holotype of A, inopinatus, but judging from tooth mor- phology we think the teeth in question are P4— M2; this view 1s supported by P4 morphology of Tribosphenomys described here. We think the original tooth assignment of the holotype of A. inopinatus by Dashzeveg (1990b) is correct. It remains uncertain whether there is an alveolus for P3 or DP3 in the holotype of A. inopinatus. There is no convincing evidence that P3 or DP3 is present in any species of

Alagomys. Because P4 in the holotype of A. inopinatus 1s significantly reduced in compar- ison to that of 7. minutus from Subeng, it is probable that P3 has been lost in Alagomys, although a DP3 could occur.

The questionable assignment of the teeth in the holotype of A. inopinatus leads to the problematic identification of Tribosphenomys borealis (Dashzeveg, 2003). The holotype, the only known specimen of the species, is a fragmentary maxilla with two teeth, inter- preted as P4 and M1. This specimen came from the Lower Eocene Bumban member of the Naran Bulak Formation, the same bed and locality that yielded the holotype of A. inopinatus. This record, if correct, would extend the known stratigraphic range of Tribosphenomys from the Gashatan to the Bumbanian. Dashzeveg (2003: 54) diagnosed the species as “‘differs from 7. minutus in its absence of P3, lack of hypocone on P4 and M1 and