Journal of Vertebrate Paleontology

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Two new Pliocene hamsters (, Rodentia) from southwestern Tibet (China), and their implications for dispersal ‘into Tibet’

Qiang Li, Thomas A. Stidham, Xijun Ni & Lüzhou Li

To cite this article: Qiang Li, Thomas A. Stidham, Xijun Ni & Lüzhou Li (2017) Two new Pliocene hamsters (Cricetidae, Rodentia) from southwestern Tibet (China), and their implications for rodent dispersal ‘into Tibet’, Journal of Vertebrate Paleontology, 37:6, e1403443, DOI: 10.1080/02724634.2017.1403443 To link to this article: https://doi.org/10.1080/02724634.2017.1403443

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Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=ujvp20 Journal of Vertebrate Paleontology e1403443 (10 pages) Ó by the Society of Vertebrate Paleontology DOI: 10.1080/02724634.2017.1403443

ARTICLE

TWO NEW PLIOCENE HAMSTERS (CRICETIDAE, RODENTIA) FROM SOUTHWESTERN TIBET (CHINA), AND THEIR IMPLICATIONS FOR RODENT DISPERSAL ‘INTO TIBET’

QIANG LI, *,1,2,3 THOMAS A. STIDHAM,1,3 XIJUN NI, 1,2,3 and LUZHOU€ LI 1,3 1Key Laboratory of Vertebrate Evolution and Human Origins of Chinese Academy of Sciences, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing 100044, China, [email protected]; 2Chinese Academy of Sciences Center for Excellence in Tibetan Plateau Earth Sciences, Beijing 100101, China, [email protected]; 3University of Chinese Academy of Sciences, Beijing 100049, China, [email protected]; [email protected]

ABSTRACT—Two new of fossil hamsters (Cricetinae, Cricetidae) collected from early Pliocene sediments (»4.4 Ma) in the Zanda Basin, southwestern Tibet (China), demonstrate greater past diversity among cricetines in the hinterland of the Tibetan Plateau within the Himalayan Range (beyond the previously known ‘Plesiodipus’ thibetensis from the late Miocene of the Gyirong Basin). The occurrence of Nannocricetus qiui, sp. nov., in the Zanda Basin indicates a dispersal of Nannocricetus from its center of origin in northern and northwestern China and the Mongolian Plateau, into the hinterland of the high-elevation Tibetan Plateau and subsequently into the Himalayan Range. The new taxon Aepyocricetus liuae, gen. et sp. nov., possibly represents a specialized (and endemic) Neogene hamster from the Tibetan Plateau. The dispersal of these hamsters into the high-elevation portions of Tibet during the early Pliocene contrasts with the hypothesized biogeographic shift of several large lineages out of Tibet. The absence of Aepyocricetus and Nannocricetus from adjacent portions of the south slope of the Himalayans (and the Siwalik Hills in India and Pakistan) further implies that the Himalayan range functioned as a dispersal barrier for these small by the early Pliocene.

http://zoobank.org/urn:lsid:zoobank.org:pub:9EE286AA-0C00-4041-8F0C-877B35283181.

SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVP

Citation for this article: Li, Q., T. A. Stidham, X. Ni, and L. Li. 2018. Two new Pliocene hamsters (Cricetidae, Rodentia) from southwestern Tibet (China), and their implications for rodent dispersal ‘into Tibet’. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2017.1403443.

INTRODUCTION Cricetine fossils are distributed widely in the low-elevation regions of northern and southern China during the late Neogene, Hamsters are a diverse group of small and comprise but they are extremely rare in the high-altitude Tibetan (Qing- seven extant genera and 18 species placed in the subfamily Crice- hai-Xizang) Plateau. Currently, the only known record of fossil tinae within the family Cricetidae (superfamily Muriodea). They cricetines in the hinterland of the Tibetan Plateau are specimens are allied with Arvicolinae, Lophiomyinae, Neotominae, Sigmo- referred to the late Miocene ‘Plesiodipus’ thibetensis (including dontinae, and Tylomyinae (Musser and Carleton, 2005). Living ‘Himalayactaga liui’) from the Gyirong ( D Guizhong or Chi- hamsters form a monophyletic group, subfamily Cricetinae, orig- long) Basin (Li and Chi, 1981). Even to this day, Cricetulus is the inating in the Neogene, whose monophyly is supported by both sole extant of hamsters living on the Tibetan Plateau (Luo morphological traits and phylogenetic analysis of nuclear gene et al., 2000). Fossils of two new extinct cricetine species from Pli- sequences (Carleton and Musser, 1984; Michaux and Catzeflis, ocene sediments in the Zanda ( D Zhada) Basin, southwestern 2000; Michaux et al., 2001). If the fossil record and extinct taxa Tibetan Plateau (China), expand the known past diversity of the are included, Cricetinae comprises at least 22 genera, and all of group and demonstrate the longevity of the group in the region. its members were once considered as part of family The Zanda Basin is in Ngari District in southwestern Xizang (McKenna and Bell, 1997). Hamsters are particularly diverse in (Tibet) Autonomous Region, China. It is a late Neogene pull- China, with a fossil record extending into the late Miocene and apart sag basin located just north of the high Himalaya ridge including 10 genera (Sinocricetus, Nannocricetus, Kowalskia crest, and has a mean altitude of about 4000 m (Fig. S1, Supple- [including Neocricetodon and Chuanocricetus], Amblycricetus, mentary Data 1). Tectonically, it is mainly controlled by the Cricetinus, Allocricetus, Bahomys, Cricetus, Cricetulus [including Oligo–Miocene Great Counter Thrust (GCT) to the north and Tscherskia and Allocricetulus], and Phodopus) (Zdansky, 1928; the South Tibetan Detachment System (STDS) to the south Schaub, 1930, 1934; Chow and Li, 1965; Zheng, 1984, 1993; Wu, (Murphy and Yin, 2003; Wang et al., 2008b). The Zanda Basin 1991; Zheng and Cai, 1991; Qiu, 1995; Luo et al., 2000; Qiu and has a vast current outcrop extent of basin fill (>9000 km2), with Storch, 2000; Qiu and Li, 2003; Zhang et al., 2008; Li, 2010). about 800 m of maximum thickness (Wang et al., 2008a; Saylor et al., 2010). These sediments have yielded surprisingly abundant and diverse mammalian fossils (Deng et al., 2011, 2012; Wang *Corresponding author. et al., 2013a, 2013b, 2014, 2015a, 2015b, 2016; Tseng et al., Color versions of one or more of the figures in the article can be found 2013a, 2013b, 2016; Li and Wang, 2015). The discoveries of the online at www.tandfonline.com/ujvp. earliest woolly rhinoceros (Coelodonta thibetana) initiated the

Published online 16 Feb 2018 Li et al.—Pliocene hamsters from Tibet (e1403443-2)

‘out of Tibet’ hypothesis that proposes that earlier and more morphology on a Zeiss MA EVO25 scanning electron micro- primitive large mammals from the Pliocene of Tibet gave rise to scope without gold-plating and with 3.0 kV voltage. The teeth part of the Ice Age megafauna that extended across much of were measured with an Olympus SZX7 microscope to the near- what has been called the Mammoth Steppe (Deng et al., 2011; est 0.01 mm. Wang et al., 2015b). In contrast, the small mammals seem to have followed an ‘into Tibet’ pattern. This biogeographic pattern SYSTEMATIC PALEONTOLOGY is the case for (family Spalacidae) and hamsters. dispersal into Tibet is recorded in Kunlun Pass and Zanda Basins Class MAMMALIA Linnaeus, 1758 in the form of Prosiphneus (Li and Wang, 2015). The hamster Order RODENTIA Bowdich, 1821 fossils discussed below were collected from Institute of Verte- Family CRICETIDAE Fischer von Waldheim, 1817 brate Paleontology and Paleoanthropology (IVPP) field locality Subfamily CRICETINAE Fischer von Waldheim, 1817 0 0 ZD1001 (31 40 N, 79 45 E, »4114 m above sea level; Fig. S1), NANNOCRICETUS Schaub, 1934 where the hamster material is associated with specimens of ancient snow leopard (Panthera blytheae), arctic fox (Vulpes Type Species—Nannocricetus mongolicus Schaub, 1934. zhudingi), and zokor (Prosiphneus eriksoni) (Tseng et al., 2013b; Referred Species—N. primitivus Zhang, Zheng, and Liu, 2008; Wang et al., 2014; Li and Wang, 2015). Locality ZD1001 bearing N. qiui, sp. nov. (this paper). fossil hamsters is correlated to the geomagnetic polarity time Distribution—Early late Miocene to late Pliocene in northern, scale (GPTS) magnetochron C3n.1 r, with an age of »4.40 Ma northwestern, and southwestern China; Late Miocene, (Wang et al., 2013b). Mongolia. Institutional Abbreviations—CAS, Chinese Academy of Sci- ences, Beijing, China; IVPP, Institute of Vertebrate Paleontol- NANNOCRICETUS QIUI, sp. nov. ogy and Paleoanthropology of CAS, Beijing, China; IZ, Institute of Zoology of CAS, Beijing, China; NGS, National Geographic Nannocricetus sp.: Deng et al., 2011:table S1. Society, Washington, D.C., U.S.A.; NSFC, National Nature and Nannocricetus sp.: Wang et al., 2013a:282. Science Foundation of China, Beijing, China; V, prefix to verte- Nannocricetus sp.: Wang et al., 2013b:93, table 3. brate fossils of IVPP; ZD, Zanda Basin, prefix to IVPP field numbers Holotype—IVPP V 23220, a left M1 (Fig. 1A). Paratypes—IVPP V 23221.1–16, 16 isolated molars including four left M1s (one with only the posterior portion preserved), MATERIALS AND METHODS two M2s (one left and one right), one left M3, four m1s (one left, The new fossil material includes 43 isolated molars collected three right), four m2s (two left, two right), and one right m3. from the ZD1001 locality in the Zanda Basin by screen-washing Type Locality—IVPP field locality ZD1001, Zanda County, during the 2010 field season. All specimens are housed in the Tibetan Autonomous Region, China. Early Pliocene (»4.4 Ma). Institute of Vertebrate Paleontology and Paleoanthropology, Etymology—Named in honor of Prof. Zhuding Qiu from the Chinese Academy of Sciences, Beijing, and are available for IVPP of CAS, China, for his great contributions to research on examination by qualified researchers. The dental terminology the Neogene small mammals of China. (Fig. S2) is modified from Mein and Freudenthal (1971:fig. 1) Diagnosis—Large species of Nannocricetus. Size similar to N. and Qiu (1996:fig. 51). We took photographs of the dental mongolicus from Ertemte 2 of Inner Mongolia and N. primitivus

FIGURE 1. Occlusal view of Nannocricetus qiui, sp. nov., from IVPP locality ZD1001 in the Zanda Basin, southwestern Tibetan Plateau. A, holo- type, left M1, IVPP V 23220; B–O, paratypes: B, left M1, IVPP V 23221.1; C, left M1, IVPP V 23221.2; D, left M2, IVPP V 23221.5; E, right M2, IVPP V 23221.6, reversed; F, left M3, IVPP V 23221.7; G, left m1, IVPP V 23221.8, reversed; H, right m1, IVPP V 23221.9; I, right m1, IVPP V 23221.10; J, right m1, IVPP V 23221.11; K, left m2, IVPP V 23221.12, reversed; L, left m2, IVPP V 23221.13, reversed; M, right m2, IVPP V 23221.14; N, right m2, IVPP V 23221.15; O, right m3, IVPP V 23221.16. Scale bar equals 1 mm. Li et al.—Pliocene hamsters from Tibet (e1403443-3) from Lantian of Shaanxi Province, but larger than N. mongolicus The anterolophule is short and connects the anteroloph and the from Bilike of Inner Mongolia. Differing from N. primitivus in junction of the anterior arm of the protocone and protolophule having a wider anterocone on M1, an elongated anteroconid por- I. The protoloph is double. Protolophule I is complete and strong tion on m1, and much reduced mesoloph(id)s on molars. Distinct on one specimen (Fig. 1D), but incomplete and weak on another from N. mongolicus by its complete lack of mesoloph(id)s on all (Fig. 1E), and both are anterolingually connected to the anterior molars except M2, complete absence of protolophule I and met- arm of the protocone. Protolophule II is thicker than protolo- alophule I on M1, presence of a posterosinus on M1–2, and pres- phule I and is posterolingually connected to the posterior arm of ence a cusped posterolophid on m1–2. Different from the protocone. Bounded by protolophules I and II, a distinct ‘Nannocricetus’ wuae by presence of much reduced mesoloph on oval fossa is present between the protocone and paracone. The M1–2 and mesolophid on m1–2, having strongly bifid anterocone mesocone and entomesoloph are both entirely absent. A rudi- on M1 and anteroconid on m1, and presence of a cuspidate post- mentary mesoloph is present at the entoloph. It divides the mes- erolophid on m1–2. osinus into a small, high, and shallow portion between the Age—At an elevation of 4114 m above sea level, IVPP field hypocone and metacone, and a large, low, and deep portion locality ZD1001 was correlated to the 335 m level of the South between the paracone and metacone. The metaloph is single, Zanda section by Saylor et al. (2010), which was correlated to with only an inconspicuous metalophule II, posterolingually con- magnetochron 3n.1 r, with a calibrated age of »4.40 Ma (Hilgen nected to the posteroloph. The hypoconule is absent. The poster- et al., 2012; Wang et al., 2013b). oloph is strong and curved. A small but distinct posterosinus is Measurement—See Table S1. present on one specimen (Fig. 1D). Cingula are not developed Description—The M1 is reniform with slightly concave buccal on the margins of all valleys, and tooth roots are not preserved. and convex lingual margins. Three pairs of main cusps are situ- The M3 is remarkably smaller than M2 and has a rounded tri- ated opposite one another and gradually widen posteriorly. The angular outline. Its lingual and posterior portions are strikingly anterocone is wide and anteriorly bifid, forming an anterior reduced. The anteroloph only has a buccal branch that is short transverse cusp row. The lingual and buccal anteroconules are but thick, and encloses a small anterosinus in front of the para- equal in size and have a slightly concave anterior wall. Derived cone. The lingual branch of the anteroloph and protosinus are from posterior walls of both anteroconules, the anterolophules both completely missing. The paracone is distinctly smaller than are always double, and converge at the junction with the anterior the protocone. The protoloph is single with only protolophule I arm of the protocone, forming a ‘Y’-type connection. The junc- (which is short but thick) and is anterolingually connected to the tion of the posterior arms of the anteroconules and the anterior anterior arm of the protocone. The mesocone, mesoloph, and arm of the protocone is high, above two-thirds of the crown entomesoloph are all entirely absent. The metacone is tiny and height from the base of the tooth. A narrow and shallow valley is completely merges with the posterobuccal margin of the tooth. present between the lingual and buccal anteroconules. The para- The metaloph is single with only metalophule I (which is short cone is slightly more posteriorly positioned than the protocone, but thick) and is anterolingually connected to the anterior arm and they form the middle cusp row on the tooth. The middle of the hypocone (or entoloph). The mesosinus and posterosinus cusp row is clearly separated from the anterior cusp row by the are well separated by metalophule I. The sinus is the widest and lingual protosinus and buccal anterosinus. In these two sinuses, largest of the tooth valleys. A crest-like protuberance is present neither the lingual nor the buccal spur of the anterolophule is on the anterior wall of the hypocone in the sinus (Fig. 1F). This present. The protoloph is single with only protolophule II pres- tooth has three roots of the same thickness (two anterior and ent. Protolophule I is entirely absent, causing the anterosinus to one posterior). extend posteriorly and terminate by protolophule II. Protolo- The m1 has a long, narrowly triangular outline with the ante- phule II is short and posterolingually connected to the posterior rior end being the narrowest, and it gradually widens posteriorly. arm of the protocone, and together with the entoloph (and ante- There are five alternately arranged main cusps on the tooth, con- rior arm of the hypocone), they form a second ‘Y’-type connec- sisting of the anterior anteroconid and a posterior set of four. tion. There is no trace of a mesocone. The metacone is The anteroconid is slightly bifid with shallow anterior and poste- positioned slightly more posteriorly than the hypocone, and they rior grooves. The buccal anteroconulid is equal to or slightly form the posterior transverse cusp row. The posterior row is well larger than the lingual one. The anteroconid is far from the pos- delimited from the middle row by a wide sinus and mesosinus. In terior main cusps due to the elongated anterolophulid. The ante- these two valleys, neither the entomesoloph nor the mesoloph is rolophulid is single and buccally situated on three of the four present. The metaloph is single with only metalophule II. Metal- specimens and connects the buccal anteroconulid and the junc- ophule I is completely absent, resulting in a conspicuous posteri- tion of the anterior arm of the protoconid and metalophid. How- orly extending mesosinus. Metalophule II is inconspicuous, ever, it is double on the other specimens (Fig. 1G). In the case of separating the posterosinus and mesosinus. The hypoconule is the double anterolophid, the lingual anterolophulid connects the absent or inconspicuous. The posteroloph is short and connects lingual anteroconulid and anterior wall of the metaconid, and the hypocone and metacone. The posterosinus is very small and the buccal anterolophulid links the buccal anteroconulid and the shallow. All four main valleys slope towards the base of the junction of the anterior arm of the protoconid and the metalo- tooth. An inconspicuous parastyle is present on lateral margin of phid. There is no buccal or lingual spur of the anterolophulid. the anterosinus of the holotype (Fig. 1A). Cingula are not devel- The metalophid is short and anterobuccally connected to the oped or are inconspicuous on the lateral margins of all valleys. anterior arm of the protoconid. The mesoconid, mesolophid, and Only the heavily worn and the broken teeth preserve partial ectomesolophid are all absent. The hypolophid is short but thick roots. The number of roots is estimated as three (a strong ante- and is anterobuccally connected to the posterior arm of the pro- rior and two slender posterior roots). toconid. The ectolophid is oblique and long. The posterolophid The M2 is more quadratic and lacks the anterior cusp row of is well cusped and much lower than the hypoconid. There are the M1. Its posterobuccal portion is projecting. The anteroloph five open valleys on the tooth. The buccal protosinusid and has two branches. The buccal is strong, long, and high, whereas sinusid are wide and deep, and the lingual mesosinusid and post- the lingual is weak, short, and descends lingually towards the erosinusid are narrow and strongly anteriorly curved. The ante- base of the tooth. The buccal branch reaches the anterior wall rosinusid is small on one specimen (Fig. 1G), but wide and deep base of the paracone and encloses a transverse anterosinus. The on the other three. A small mesostylid is present at the lingual space between the lingual branch of the anteroloph and proto- margin of the mesosinusid (Fig. 1I). Only one specimen pre- cone is limited and forms a shallow or inconspicuous protosinus. serves two broken roots. Li et al.—Pliocene hamsters from Tibet (e1403443-4)

The m2 has a rectangular outline with the length greater than (Wu, 1991), N. primitivus from Lantian in Shaanxi Province the width. The lingual branch of the anterolophid is weak or (Zhang et al., 2008), and N. aff. N. primitivus from Builstyn Khu- completely missing, whereas the buccal one is always strong and dag, Mongolia (Maridet et al., 2014) (see Fig. S3). The Zanda long, descending posterobuccally and reaching the anterobuccal specimens are slightly larger than N. mongolicus from Gaotege base of the protoconid. The anterosinusid is tiny or completely in Inner Mongolia (Li, 2010) and Kunlun Pass Basin in Qinghai absent, whereas the protosinusid slopes towards the base of the (Li et al., 2014), and they are distinctly larger than N. primitivus tooth. The metalophid is very short and converges with the ante- from Shengou in the Qaidam Basin, Qinghai (Qiu and Li, 2008), rolophulid and anterior arm of the protoconid. The mesoconid, and N. mongolicus from Bilike (Qiu and Storch, 2000) (see Fig. ectomesolophid, and mesolophid are all absent. The hypolophid S3). Morphologically, the Zanda specimens have a wider antero- is short and in an oblique line with the posterior arm of the pro- cone on M1, an elongated anteroconid portion on m1, and toconid. The ectolophid is long, lower than the hypolophid and reduced mesoloph(id)s on its molars. The specimens resemble posterior arm of the protoconid, and extends posterobuccally. more those of N. mongolicus rather than specimens of N. primiti- The posterolophid is distinctly cuspidate. Of four main open val- vus (from Lantian of Shaanxi Province and Shengou of Qinghai leys, the sinusid is the widest, and the mesosinusid and posterosi- Province, China) and N. aff. N. primitivus from Builstyn Khudag, nusid both extend strongly anteriorly. No root is preserved. Mongolia (Qiu and Li, 2008; Zhang et al., 2008; Maridet et al., The m3 is the narrowest of the lower molars and similar to m2 2014). Furthermore, the Zanda sample differs from N. mongoli- in length (see Table S1). It has a long triangular outline with a cus of Ertemte 2, Harr Obo 2, Bilike, Gaotege, and Kunlun Pass much reduced entoconid portion. The anterolophid possesses Basin (Wu, 1991; Qiu and Storch, 2000; Li, 2010; Li et al., 2014) only a buccal branch. The metaconid is positioned more anteri- in completely lacking mesoloph(id)s on all molars except M2, orly than the protoconid. The metalophid merges with the buccal and the loss of protolophule I and metalophule I on M1, the pres- branch of the anterolophid and the anterior arm of the protoco- ence of a posterosinus on M1–2, and a cusped posterolophid on nid. The mesolophid is absent. The ectolophid is very thick and m1–2. Thus, the Zanda sample represents a new member of merges with the extremely reduced entoconid. The hypoconid Nannocricetus. also is reduced and more lingually positioned. The entoconid, hypoconid, posterolophid, and posterior portion of the ectolo- AEPYOCRICETUS, gen. nov. phid fuse into a small basin in the posterior portion of the tooth. Only three open valleys are present on the tooth. The protosinu- Type and Only Species—Aepyocricetus liuae, new genus and sid and mesosinusid are small and shallow, whereas the sinusid is species. large and deep. This tooth is double rooted. Etymology—Aepys, Greek prefix for ‘high,’ combined with Comparisons—The Zanda sample is easily referred to Nan- cricetus for living hamsters, indicating its high-crowned molars, nocricetus Schaub, 1934, based on a set of dental characters. and a reference to the high elevation of the fossil locality where Most of the molars lack mesoloph(id)s. The anterocone is dis- the specimens were collected. tinctly anteriorly bilobed, and protolophule I is absent on M1. Diagnosis—Medium-sized cricetine, slightly smaller than The m1 is slender with a narrow and bifid anteroconid, and the extant Cricetulus (Tscherskia) triton and similar to Sinocricetus metalophid and hypolophid on the lower molars are short and zdanskyi from Ertemte 2 of Inner Mongolia, China. Distinct nearly transverse. The valleys of the molars slope significantly from the Cricetodontinae, Gobicricetodontinae, Megacriceto- and are well opened. dontinae, and Copemyinae by its widened and well-bifid antero- The genus Nannocricetus was established by Schaub (1934) cone on M1, and elongated anteroconid portion on m1, and lack based on materials from the late Miocene site Ertemte, Inner of mesocone(id)s, style(id)s, an entomesoloph/ectomesolophid, Mongolia (China). Prior to our study, only three species have and lateral spurs of the anterolophule(id) on the molars. Distin- been referred to Nannocricetus: N. mongolicus, N. primitivus, guishable from ‘microtoid cricetids’ in having bunodont-lopho- and ‘N.’ wuae (Schaub, 1934; Zhang et al., 2008, 2011). The type dont molars. Differing from the known late Neogene, species, N. mongolicus, also is found in the late Miocene to late Quaternary, and extant cricetines by having combined dental Pliocene Chinese localities at Ertemte 2 and Harr Obo 2 (differ- features, such as molars high-crowned and bunodont-lophodont. ing from Ertemte and Harr Obo in Schaub, 1934) in Inner Mon- Upper molar main cusps closely and oppositely situated. Meso- golia, Bilike and Gaotege in Inner Mongolia, Lingtai in Gansu cones, styles, mesolophs, entomesolophs, ectolophs, and lateral Province, the Nihewan Basin in Hebei Province, and the Kunlun spurs of anterolophule all absent. Protolophs and metalophs Pass Basin in Qinghai Province (Cai, 1987; Wu, 1991; Qiu and double. Posterolophs reduced. M1 anterocone anteriorly bifid. Storch, 2000; Zheng and Zhang, 2001; Li, 2010; Li et al., 2014). After wear, occlusal surface of M1 flat and gerbillid Meriones- The second species, N. primitivus from the early Late Miocene, like. Lower molar main cusps alternately arranged. Mesoconids, typically is associated with the Bahe Formation in the Lantian stylids, mesolophids, ectomesolophids, and lateral spurs of ante- area of Shaanxi Province (China) but also is found in the Shen- rolophulid all absent. Posterolophids cuspidate. The m1 antero- gou Locality in the Qaidam Basin of Qinghai Province, China, conid well bifid. Anterobuccal parts of mesosinusid and and Builstyn Khudag, Mongolia (Qiu and Li, 2008; Zhang et al., posterosinusid enclosed, shallow, and much higher than other 2008; Maridet et al., 2014). The third published species, ‘N.’ valleys. wuae, was erected based on materials from the early late Mio- cene site Siziwang Qi, Inner Mongolia, and was treated as a tran- AEPYOCRICETUS LIUAE, gen. et sp. nov. sitional form between Democricetodon and Nannocricetus (Zhang et al., 2011). ‘Nannocricetus’ wuae possesses many primi- Cricetidae, gen. et. sp. nov.: Deng et al., 2011:table S1. tive dental characters, such as a narrow, less bifid, and more buc- Cricetidae, gen. et. sp. nov.: Wang et al., 2013a:282. cally positioned anterocone on M1, a consistently present Cricetidae, gen. et. sp. nov.: Wang et al., 2013b:93, table 3. metalophule I on M1–2, and a single anteroconid on all m1s and a long mesolophid on m1–2. Those characters overall are more Holotype—IVPP V 23222, a left M1 (Fig. 2A). consistent with Democricetodon Fahlbusch, 1964, rather than Paratypes—IVPP V 23223.1–25, 25 isolated molars including with Nannocricetus. Thus, we treat ‘N.’ wuae as a derived form six M1s (one left, five right), two M2s (one left, one right ante- of Democricetodon, instead of a member of Nannocricetus. rior), three M3s (one left, two right), four m1s (three left, one In size, the Zanda specimens are similar to Nannocricetus right), seven m2s (five left, two right), and three m3s (two left, mongolicus from Ertemte 2 and Harr Obo 2 in Inner Mongolia one right). Li et al.—Pliocene hamsters from Tibet (e1403443-5)

FIGURE 2. Occlusal view of Aepyocricetus liuae, gen. et. sp. nov., from IVPP locality ZD1001 in the Zanda Basin, southwestern Tibetan Plateau. A, holotype, left M1, IVPP V 23222; B–Y, paratypes: B, left M1, IVPP V 23223.1; C, right M1, IVPP V 23223.2, reversed; D, right M1, IVPP V 23223.3, reversed; E, right M1, IVPP V 23223.4, reversed; F, right M1, IVPP V 23223.5, reversed; G, right M1, IVPP V 23223.6, reversed; H, left M2, IVPP V 23223.7; I, left M3, IVPP V 23223.9; J, right M3, IVPP V 23223.10, reversed; K, right M3, IVPP V 23223.11, reversed; L, left m1, IVPP V 23223.12, reversed; M, left m1, IVPP V 23223.13, reversed; N, left m1, IVPP V 23223.14, reversed; O, right m1, IVPP V 23223.15; P, left m2, IVPP V 23223.16, reversed; Q, left m2, IVPP V 23223.17, reversed; R, left m2, IVPP V 23223.18, reversed; S, left m2, IVPP V 23223.19, reversed; T, left m2, IVPP V 23223.20, reversed; U, right m2, IVPP V 23223.21; V, right m2, IVPP V 23223.22; W, left m3, IVPP V 23223.23, reversed; X, left m3, IVPP V 23223.24, reversed; Y, right m3, IVPP V 23223.25. Scale bar equals 1 mm.

Type Locality—IVPP field locality ZD1001, Zanda County, forming two ‘X’-type connections. Three narrow and shallow fos- Tibetan Autonomous Region, China. sae are present from anterior to posterior: between the lingual and Etymology—Named in honor of Dr. Juan Liu from the IVPP buccal anteroconules, between the protocone and paracone, and of CAS (China) and the University of Alberta (Canada), for her between the hypocone and metacone. A short posteroloph is pres- fortuitous discovery of ZD1001 in 2010 that resulted in the mate- ent. On moderately and heavily worn specimens (Fig. 2B, E, and rial basis of this study. F), the occlusal surface becomes flat, with the main cusps and Diagnosis—Same as for the genus. lophs confluent into a modern gerbillid Meriones-like dental pat- Age—Early Pliocene, »4.40 Ma (see Wang et al., 2013b). tern. The main cusps form three pairs of salient structures, and the Measurement—See Table S2. anterolophule, protoloph, and metaloph merge into robust Description—The M1 has a reniform occlusal outline. The lin- midsagittal lophs. The posteroloph is nearly completely absent. A gual and buccal main cusps are closely opposing and form three small and narrow fossa or groove at the anterior wall of the ante- nearly parallel, transverse cusp rows. The lingual and buccal spurs rocone is only present on slightly worn specimens (Fig. 2A, C). of the anterolophule, mesocone, mesoloph, and entomesoloph are The protosinus, sinus, anterosinus, and mesosinus slope towards all absent. The space between the anterocone and posterior proto- the base of the tooth and are well open. Neither cingulum nor cone-paracone row is wide. On slightly worn specimens, the main style is developed at the margins of these four valleys. The tooth cusps are more separated and pointed, forming a jagged occlusal has four separate roots (an anterior, two lingual, and a posterobuc- surface (Fig. 2A). The anterocone is wide and clearly bifid anteri- cal root). orly, and the lingual and buccal anteroconules are nearly equal in The M2 has a rectangular outline with a projecting posterior size. The paracone and metacone are posterolingually directed, wall. The lingual and buccal main cusps are opposing and are con- and the anterolophule, protoloph, and metaloph are all double, nected to each other by shortened lophs, forming a figure ‘8’ type Li et al.—Pliocene hamsters from Tibet (e1403443-6) of dental pattern. The anteroloph has two branches, with the lin- (Fig. 2S, T). The protoconid and metaconid are more confluent. gual being short and weak and the buccal being strong and buccally Of the five valleys, the posterosinusid is the smallest and shallow- ending in a bulge. The protoloph and metaloph are both double. est. The protosinusid also is small, but the mesosinusid and The mesocone, mesoloph, and entomesoloph are all absent. Two sinusid are wider and lower. The tooth has two roots. shallow midsagittal fossae are present and comprise an anterior The m3 has a rounded rectangular occlusal outline. It is slightly one (between the protocone and paracone) and a posterior one shorter and narrower than the m2. The lingual main cusps are (between the hypocone and metacone). The posteroloph is cuspi- placed distinctly anteriorly relative to the buccal ones. The antero- date and located at the posterior limit of the tooth. Of the open lophid has a weak lingual branch, a strong buccal branch that valleys on the tooth, the protosinus and posterosinus are nearly forms a tiny and shallow anterosinusid, and a rather large and absent, the anterosinus is small, and the mesosinus and sinus are deep protosinusid. The metalophid is short and anterobuccally wide and slope towards the tooth base. No root is preserved. joins the anterior arm of the protoconid. The space between the The M3 is similar to the M2 in dental pattern, but is conspicu- protoconid and metaconid is narrow, forming a shallow fossa ously smaller. It has a rounded triangular outline with a narrow (Fig. 2Y) that is distinctly higher than the posterior mesosinusid, posterior portion. The lingual and buccal main cusps are oppos- and completely disappears after heavy wear. There is no meso- ing and form a figure ‘8’ dental pattern. The lingual branch of conid, mesolophid, or ectomesolophid. The ectolophid is short, the anteroloph is almost absent, but the buccal branch is well wide, and longitudinally extended. The entoconid is less reduced developed and bulged. The protoloph and metaloph both are and merges with the hypoconid through the anterior hypolophid double. The mesoloph, entomesoloph, and posteroloph are all and posterior posterolophid. On a slightly worn specimen absent. Two midsagittal fossae are present and include an ante- (Fig. 2W), a round posterosinusid is clearly present between the rior one (between the protocone and paracone) and a posterior entoconid and hypoconid, and is shallower or deeper than the one (between the hypocone and metacone). The posterior junc- anterior mesosinusid. The sinusid is the deepest of the valleys, and tion of the hypocone and metacone is incomplete on the slightly far deeper than the others. The tooth has two roots. worn specimen (Fig. 2I). The anterosinus is distinctive, and the Comparison—Aepyocricetus liuae differs from ‘Plesiodipus’ mesosinus and sinus both are wide and slope towards the tooth thibetensis (including ‘Himalayactaga liui’) from the late Mio- base. The tooth has two roots (an anterior and a posterior one). cene Gyirong Basin of Tibet (Li and Chi, 1981) in being smaller, The m1 has a long narrow triangular outline with the narrow- having main cusps on upper molars more opposing, and in the est point at the anterior end. There are three pairs of cusps on presence of protolophule I and metalophule I that fail to form the tooth crown. Of the posterior two pairs, the lingual cusps are strong oblique lophs (anterior arm of protocone and protolo- placed more anteriorly than the buccal ones. On unworn or phule II) on upper molars, the complete absence of mesolophids, slightly worn specimens (Fig. 2L, O), the occlusal surface is jag- and the presence of a lingual anterolophid and short metalophid ged. The anteroconid anteriorly splits into two equal-sized conul- on the lower molars. Aepyocricetus liuae is different from Nan- ids. The lingual anteroconulid is isolated from the buccal nocricetus qiui from the same locality of the Zanda Basin in anteroconulid, anterolophulid, and posterior cusps. The space being slightly larger and having a conspicuously higher tooth between the anteroconid and posterior protoconid-metaconid crown. Aepyocricetus liuae is remarkably smaller than the extant complex is conspicuously wide. The anterolophulid is single, and Cricetus cricetus, slightly smaller than the extant Cricetulus it connects the buccal anteroconulid and the junction of the ante- (Tscherskia) triton from China, and falls within the size range of rior arm of the protoconid and metalophid. Neither the lingual Sinocricetus zdanskyi from Ertemte 2 of Inner Mongolia (Wu, nor the buccal spur of the anterolophulid is present. The metalo- 1991) (Fig. S4). Aepyocricetus liuae possesses a very simple den- phid is short and slightly anterobuccally directed. A shallow tal pattern that lacks mesocone(id)s, style(id)s, an entomeso- fossa is present between the metaconid and protoconid and is loph/ectomesolophid, and lateral spurs of the anterolophule(id) higher than the posterolingual mesosinusid. The mesoconid, on the upper and lower molars. It also exhibits ectolophs on the mesolophid, and ectomesolophid are all absent. The hypolophid upper molars, and displays a widened and well-bifid anterocone is short and slightly anterobuccally connected to the middle of on M1 and an elongated anteroconid portion on m1. These fea- the oblique ectolophid. The hypoconulid is absent. The postero- tures exclude any possibility of attributing the teeth to the ances- lophid is a striking bulge of a cusp on the posterolingual limit of tral cricetids (e.g., the subfamilies Cricetodontinae Schaub, 1925, the tooth. On moderately or heavily worn specimens (Fig. 2M– Gobicricetodontinae Qiu, 1996 [Qiu, 1996; Rummel, 1999], Meg- N), all cusps and lophs are worn down considerably, resulting in acricetodontinae, and Copemyinae Jacobs and Lindsay, 1984). a confluent dentine space. The tooth occlusal surface becomes In having bunodont-lophodont molars, A. liuae also conspicu- flat and comprises three salients and two reentrants on the buccal ously differs from the late Neogene microtoid cricetids that have side, with four salient and three reentrants on the lingual side. Of highly lophodont molars and prismatic salient angles (e.g., the five main valleys, the anterosinusid, protosinusid, mesosinu- Microtoscoptinae Kretzoi, 1955, Baranomyinae Kretzoi, 1955, sid, and sinusid are wide and slope towards the base of the tooth. Trilophomyinae Kretzoi, 1969, and Pannonicola Kretzoi, 1965 The posterosinusid is the smallest and narrowest, and it is higher [D Ischymomys], and others [McKenna and Bell, 1997; Fejfar, than the anterior four valleys. The tooth has two roots. 1999; Fejfar et al., 2011]). Aepyocricetus liuae is characterized by The m2 has a rectangular occlusal outline that is longer than having high-crowned and bunodont-lophodont molars, lacking wide. The lingual main cusps are placed distinctly anteriorly rela- mesoloph(id)s and lateral spurs of anterolophule(id)s on the tive to the buccal ones. The anterolophid has a short and weak molars, and closely spaced main cusps. This combination of fea- lingual branch and a long, strong, and shelf-like buccal branch. tures contrasts with forms that have a peculiar gerbillid Mer- The anterolophulid is very short. The metalophid is short and iones-like pattern on M1 that are alternating with confluent nearly transversely connected to the anterior arm of the protoco- dentine spaces on the lower teeth after wear. In these characters, nid. The protoconid and metaconid are closely attached. The lin- A. liuae clearly differs from all known late Neogene, Quaternary, gual surface of the posterior arm of the protoconid is swollen, and extant cricetines (e.g., Cricetinus Zdansky, 1928, Sinocricetus limiting the shallow fossa between the protoconid and metaconid Schaub, 1930, Allocricetus, Schaub, 1930, Nannocricetus Schaub, posteriorly. This fossa is higher than the posterior mesosinusid. 1934, Bahomys, Chow and Li, 1965, Kowalskia Fahlbusch, 1969, The mesoconid, mesolophid, and ectomesolophid are all absent. and Amblycricetus Zheng, 1993, from China; Rotundomys Mein, The hypolophid is very short and anterobuccally connected to 1966, Microtocricetus Fahlbusch and Mayr, 1975, Collimys Dax- the curving ectolophid. The posterolophid is cusped. After heavy ner-Hock,€ 1972, Hattomys Freudenthal, 1985, Hypsocricetus wear, the dentine space of all the cusps and lophs is confluent Daxner-Hock,€ 1972, and Rhinocricetus Kretzoi, 1956, from Li et al.—Pliocene hamsters from Tibet (e1403443-7)

Europe; Gromovia Erbajeva, Alexeeva, and Khenzykhenova, Nannocricetus in other northern Asian regions could be the result 2003, from Siberia; and extant representatives, Cricetus Leske, of a collection bias due to the fact that the late Miocene record in 1779, Cricetulus Milne-Edwards, 1867 [including Tscherskia some countries is still poorly known. Early in the history of Nan- Ognev, 1914, Cansumys Allen, 1928, and Allocricetulus Argyro- nocricetus (N. primitivus), the group had already spread into the pulo, 1933], Mesocricetus Nehring, 1898, and Phodopus Miller, relatively low-altitude Qaidam Basin on the northern Tibetan Pla- 1910). Of these genera, Mesocricetus shares the most similar den- teau by the early late Miocene (Qiu and Li, 2008). Nannocricetus tal features with Aepyocricetus liuae, but the latter differs in the mongolicus appears to have replaced N. primitivus since the late absence of a posterosinus on the upper molars and a lingual ante- Miocene. The former species is common in the late Miocene roloph on M2–3, and in having a much reduced lingual anterolo- through Pliocene in northern andnorthwesternChina(Schaub, phid on m2–3 and a much elongated m3 relative to the m2. 1934; Cai, 1987; Wu, 1991; Qiu and Storch, 2000; Zheng and Zhang, 2001; Li, 2010), and it also reached the Kunlun Pass Basin DISCUSSION on the northern margin of the high-altitude platform of the Tibetan Plateau by the early Pliocene (Li et al., 2014). Nannocri- The Zanda hamsters are clearly divided into two taxa, the cetus qiui of the Zanda Basin (Tibet) is another early Pliocene smaller and low-crowned Nannocricetus qiui and the larger and representative of Nannocricetus.BothN. mongolicus from the high-crowned Apyocricetus liuae. They are both easily distin- Kunlun Pass Basin and N. qiui from the Zanda Basin of the guished from the late Miocene ‘Plesiodipus’ thibetensis (Fig. S5), Tibetan Plateau are from the early Pliocene. Therefore, the time which is the first (and previously only) formally reported fossil of divergence between those species should be older than the hamster from the hinterland of the Tibetan Plateau (Li and Chi, early Pliocene (>4.4 Ma). Similar to the fossil zokor Prosiphneus 1981). The Gyirong (Tibet) hamster was referred to the gobicri- eriksoni (Li and Wang, 2015) from Tibet, the occurrence of N. cetodontid Plesiodipus Young, 1927, by Li and Chi (1981:246) qiui in the Zanda Basin appears to indicate another dispersal and consists of a fragmentary left lower jaw with m1–3 (IVPP V event (of rodents) from their center of origin in northern or 5205.1), a right M1 (IVPP V 5205.2), and a right lower dentition northwestern China and the Mongolian Plateau into the remote, m2–3 (IVPP V 5205.3). In the same publication, they also high-elevation southwestern Tibetan hinterland (Fig. 3). described a questionable dipodid, ‘Himalayactaga liui’, based on The dental morphology of Aepyocricetus liuae is so unique ‘two lower molars’ (IVPP V 5204) from the locality of Gyirong that, for the moment, it is difficult to determine its phylogenetic (Li and Chi, 1981:251). However, these two teeth were misidenti- relationship to any known Neogene cricetines from either the fied and actually are right upper molars. These teeth compare high-altitude Tibetan Plateau or lower-altitude Eurasian regions, well with the right M1 (IVPP V 5205.2) of ‘Plesiodipus’ thibeten- suggesting that it may be a Tibetan endemic lineage. Similarly, sis in both size and morphology. The Gyirong specimens display ‘Plesiodipus’ thibetensis from the late Miocene of Gyirong Basin features that are common in gobicricetodontids such as the also lacks clear affinities to other taxa. They may possibly repre- absences of protolophule I and metalophule I and the formation sent two specialized (possibly endemic) faunal components of of strong oblique ridges by the anterior arm of the protocone the hinterland of the Neogene Tibetan Plateau. Moreover, the and protolophule II on the upper molars (Qiu, 1996). However, first upper molar of Aepyocricetus liuae displays a modern ger- the Gyirong hamster conspicuously differs from Plesiodipus leei billid Meriones-like dental pattern. That dental similarity may (the type species of Plesiodipus) in having a wider and more reflect the past presence of a rather dry environment in the early deeply antero-bifid anterocone and double anterolophule on the Pliocene of the Zanda Basin because the extant species of Mer- M1, more curved buccal valleys on the upper molars, a wider iones occupy habitats including deserts, bush country, arid and more elongate anteroconid, a metalophid not connected to steppes, and grasslands (Nowak and Paradiso, 1983; Luo et al., the anteroconid (but to the anterolophulid), a deep anterosinusid 2000). Whereas several of the large mammals present in the Plio- on the m1, a less quadratic protoconid and hypoconid, a longer cene of Tibet have been hypothesized to have dispersed out of ectolophid between the entoconid and hypoconid, a longer post- Tibet near the end of the Pliocene (and Pleistocene), these new erolophid on the lower molars, and other characters (see remarks rodent taxa indicate that Tibetan biogeographic affinities and in Qiu, 1996:107). Moreover, the Gyirong hamster shares some history are more complex. Nannocricetus appears to have dis- morphological similarities with the European Collimys and Pseu- persed into the Tibetan interior by the early Pliocene from the docollimys Daxner-Hock,€ 2004, but it differs from them in having north, indicating the absence of a significant barrier to dispersal a much higher tooth crown, completely lacking mesolophs on the in that direction. Along with Aepyocricetus, N. qiui appears to upper molars, the absence of a mesolophid on m1, the antero- represent a high-elevation endemic taxon. cone on M1 being clearly wide and anteriorly bifid, and the ante- On the south slope of the Himalayas and in the foothills of roconid portion of m1 being elongated. For the Gyirong hamster, the mountains (Pakistan and India), there are numerous late the attribution to Plesiodipus thus appears inappropriate, and a Cenozoic fossil sites yielding a handful of cricetids, such as new generic name should be erected in future work. Eucricetodon, Eumyarion, Primus, Spanocricetodon, Megacri- cetodon, Democricetodon,andPunjabemys (Hussain et al., ZOOGEOGRAPHY 1979; De Bruijn et al., 1981; De Bruijn and Hussain, 1984; Nannocricetus is a common member of the late Neogene crice- Lindsay, 1987, 1988, 1994; Downing et al., 1993; Patnaik, tines of northern and northwestern China, and is often associated 2016). However, these localities lack Pliocene hamster with two other hamsters, Sinocricetus and Kowalskia (Schaub, records. Their absence in the more extensively studied fossil- 1930, 1934; Wu, 1991; Qiu and Storch, 2000; Zheng and Zhang, iferous sediments of the Siwalik Hills in northern India and 2001; Li, 2010). Some researchers have proposed that Nannocrice- Pakistan points to the Himalayan crest as a potential mam- tus possibly was derived from a lineage of the cricetodontid malian (or possibly just hamster) dispersal barrier during the Democricetodon (Zhang et al., 2008, 2011), and we agree with late Neogene. The presence of such a barrier in the late Neo- that hypothesis. The oldest records of Nannocricetus are from the gene suggests that either the Himalayas were relatively high early late Miocene and include N. primitivus (from the Lantian of in elevation at that time (possibly near their current eleva- Shaanxi Province and Shengou of Qinghai Province) and N.aff. tion) or, possibly in combination, the elevational difference N. primitivus from Builstyn Khudag of Mongolia (Qiu and Li, between the Himalayas and the Siwalik Hills also coincided 2008; Zhang et al., 2008; Maridet et al., 2014). Given this fossil with a habitat or environmental shift that blocked or dis- record, it seems likely that Nannocricetus originated in northern suaded the dispersal of hamsters (and other mammals) across China or on the Mongolian Plateau; however, the absence of those mountains. The current data support the movement of Li et al.—Pliocene hamsters from Tibet (e1403443-8)

FIGURE 3. Map of Nannocricetus localities in China and Mongolia, and the possible dispersal route of this genus. The topographic map was gener- ated by GeoMapApp (version 3.6.0; Ryan et al., 2009). We hypothesize that Nannocricetus originated on the Mongolian Plateau and northern or northwestern China (colored ellipses) in the Miocene and later spread into the hinterland of the Tibetan Plateau prior to the early Pliocene. The brown dashed-and-arrowed line shows its possible dispersal route. Colored dots are known fossil localities with specimens of Nannocricetus.

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