New Dipodid Rodents from the Late Eocene of Erden Obo (Nei Mongol, China)

Historical Biology, 2016 http://dx.doi.org/10.1080/08912963.2016.1232406

New dipodid rodents from the Late Eocene of Erden Obo (Nei Mongol, China)

Qian Lia, Yan-Xin Gonga,b and Yuan-Qing Wanga aKey Laboratory of Vertebrate Evolution and Human Origins of Chinese Academy of Sciences, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing, China; bUniversity of the Chinese Academy of Sciences, Beijing, China

ABSTRACT ARTICLE HISTORY New dipodid occurrences (Heosminthus nomogenesis sp. nov., Sinosminthus sp., Allosminthus cf. A. Received 23 July 2016 majusculus, Allosminthus ernos and Allosminthus cf. A. diconjugatus) are reported from the ‘Upper Red’ Accepted 31 August 2016 beds of the Erden Obo section in Nei Mongol, China. Heosminthus nomogenesis is similar to Heosminthus KEYWORDS primiveris from the Caijiachong Formation, and it is more primitive than Heosminthus chimidae of the Mammalia; dipodid; Eocene; Mongolian biozone A. Allosminthus cf. A. majusculus has a more variable mesolophid and metalophid in Nei Mongol; Erden Obo m1-2. Based on the dipodid assemblage, the age of the ‘Upper Red’ of the Erden Obo section is late Eocene and correlative to the Ergilian. Based on the comparison of their morphological characters, we recognize some differences between the stem dipodoids and muroids.

Introduction ‘Upper Red’ bed is reddish muddy fine sandstone with some The earliest fossil record of the Dipodoidea is the Late Early calcareous nodules; the middle part is primarily red mudstone, Eocene or Middle Eocene in Asia and North American with bands of white fine sandstone and nodules; and the upper (Shevyreva 1984; Emry & Korth 1989; Dawson et al. 1990; Wang part is brownish red mudstone. All dipodid specimens reported & Dawson 1994; Tong 1997; Emry, Tyutkova et al. 1998; Emry here were collected via surface prospection and screenwashing

2007). Rich deposits of Dipodidae have been discovered from the from the ‘Upper Red’ beds, but at slightly different spots. Oligocene and Miocene in Asia (Wang 1985; Huang 1992; Emry, There are four dipodid genera known from the Middle and Lucas et al. 1998; Wang & Qiu 2000; Wang et al. 2003; Ye et al. Late Eocene of China. These are Allosminthus, Heosminthus, 2003; Bendukidze et al. 2009). In Europe and North American, Sinosminthus (Tong 1997; Wang 1985, 2008; Daxner-Höck the dipodid appeared later, and the earliest representatives were et al. 2014),and Priminsminthus (Tong 1997). Among these, collected in Late Oligocene deposits (Daxner-Höck & Wu 2003; Priminsminthus exhibits the most primitive character array for Flynn 2008; Zhang et al. 2013). the group. Allosminthus and Heosminthus are still earlier dipo- The dipodid specimens reported here came from the dids appearing in the Late Eocene of Central Asia. ‘Upper Red’ beds of Erden Obo (Urtyn Obo), Nomogen Sumu, The new material described here adds important data on Siziwangqi, Nei Mongol, China. Osborn (1929) first reported the dental morphology of Allosminthus and Heosminthus. The the Erden Obo section based on Granger’s and Spock’s field discovery not only expands the geographic distribution of the notes. He subdivided the deposits in the section into 8 units, late Eocene dipodids in Asia and increases the assemblage of termed in descending order as the ‘Upper White’, the ‘Upper fauna from the ‘Upper Red’ of Erden Obo, but also helps for Red’, the ‘Middle White or Gray’, the ‘Middle Red’, the ‘Lower stratigraphic correlation and determining the ages of fossilif- White’, the ‘Lower Red’, the ‘Basal White’, and the ‘Basal Red’. erous beds. These units were later referred to as the ‘Baron Sog Formation’, the ‘Ulan Gochu Formation’, the ‘Shara Murun Formation’ and Material and methods the ‘Arshanto? Formation’. Later, Chang (1931), Pei et al. (1963) and other researchers (Jiang 1983; Qi 1990; Qiu & Wang 2007) All newly described specimens were collected in several field divided the strata into different stratigraphic formations and expeditions during 2007–2012 by a joint team from the Institute ages. Because of the complicated research history and potential of Vertebrate Paleontology and Paleoanthropology (IVPP), sedimentary hiatuses in the sequence, the formal division and American Museum of Natural History, Carnegie Museum of correlation, including naming of the stratigraphic units, has yet Natural History and Northern Illinois University. Dental termi- to be completed (Wang et al. 2012). For the present, we continue nology in the description illustrated in Figure 1, was modified to use the descriptive term ‘Upper Red’ of Osborn to denote the from Wang (1985) and Daxner-Höck et al. (2014). Measurements beds yielding the fossils reported here. The lower part of the of teeth were taken using a reticle with an accuracy of 0.1 mm

Figure 1. Terminology used in this paper to described molars, modified from Wang (1985) and Daxner-Höck et al. (2014). Notes: (a) upper molar: 1. paracone; 2. mesostyle; 3. mesoloph; 4. metacone; 5. metaloph; 6. posteroloph; 7. hypocone; 8. mesocone; 9. entoloph; 10. sinus; 11. protocone; 12. anterior cingulum; 13. protoloph II; 14. anterior arm of protocone; 15. lingual anteroloph; 16. anterolophule; 17. protoloph I; 18. labial anteroloph. (b) lower molar: 1. metaconid; 2. posterior arm of protoconid; 3. mesolophid; 4. entoconid; 5. hypolophid; 6. hypoconulid; 7. posterolophid; 8. hypoconid; 9. ectolophid; 10. mesoconid; 11. protoconid; 12. anteroconid; 13. mesostylid; 14. ectomesolophid; 15. ectostylid; 16. labial anterolophid; 17. anterior arm of protoconid; 18. metalophid; 19. lingual anterolophid. mounted in an Olympus SZX7 microscope. SEM photographs mesoloph of M1-3 long or of medium length; anteroconid of m1 of coated teeth were taken using a JSM-6100 SEM machine present; posterior arm of the protoconid extended to metaconid; at the Key Laboratory of Vertebrate Evolution and Human mesolophid of m1-2 long. Origins of Chinese Academy of Sciences, Institute of Vertebrate Differential diagnosis: Heosminthus nomogenesis differs from Paleontology and Paleoanthropology, Chinese Academy of H. primiveris in being larger and having a complete protoloph II Sciences. of M2, shorter mesoloph on M3, weaker mesostylid and hypoconulid on m1-2. It differs from H. borrae in being larger, having Systematic Paleontology more robust cones, lower crests and wider sinus, having a protoloph II on M2 and a strong posterior arm of the protoconid of Rodentia Bowdich, 1821 m2. It differs from H. chimidae in having shorter anterior arm of Dipodoidea Fischer de Waldheim, 1817 the protocone on M1, and weaker anteroconid, mesoconid and Dipodidae Fischer de Waldheim, 1817 hypoconulid on lower molars. Sicistinae Allen, 1901 Etymology: The specific name is derived from Nomogen, the Heosminthus Wang, 1985 type locality of the species. Heosminthus nomogenesis sp. nov. Holotype: IVPP V 17810, right maxilla with M1-3. Description Referred specimens: IVPP V 17811.1-2, right M1; V 17811.3-4, left M2; V 17811.5-7, left m1; V 17811.8-9, right m1; V 17811.10, M1 is trapezoidal in outline, and is slightly wider posteriorly than left m2; V 17811.11-12, right m2. anteriorly. The contact facet on the anterior surface is consistent Locality and Horizon: Erden Obo, Nomogen, Siziwangqi, Nei with presence of a small P4. The four main cones are robust Mongol; lower and middle part of the ‘Upper Red’ beds. in opposed position. The short anterior arm of the protocone Diagnosis: small-sized species; four main robust cusps; lophs neither reaches to the anterolabial edge of the tooth, nor joins short and lower than cusps; short anterior arm of protocone the paracone, from which it is separated by a shallow groove of M1 and paracone separated by a shallow groove; M1-2 with (Figure 2(a) and (b)). The protoloph II is complete and connects protoloph II; metaloph of M1-3 continuous with hypocone; with the posterior arm of the protocone. The metaloph joins the Historical Biology 3

Figure 2. Cheek teeth of Heosminthus nomogenesis sp. nov. in occlusal view. Notes: (a) IVPP V 17810, right maxillary fragment with M1-3; (b) V 17811.1, right M1; (c) V 17811.3, right M2; (d) V 17811.5, left m1; (e) V 17811.9, right m1; (f) V 17811.10, left m2.

hypocone. The posteroloph is thin and long, and runs from the Comparisons hypocone to the posterolabial corner of the tooth. The mesol- The new specimens from the Erden Obo possess characters of oph is usually long and reaches the labial border (present in 2 Heosminthus, including: upper molars are small and have three specimens of a total of 3 specimens), but it is of medium length roots; labial cusps are opposite to lingual cusps; anterior arm of in V 17810 (Figure 2(a)). The entoloph is continuous. A small protocone of M1 is separated from paracone by a shallow groove; mesostyle is present. The sinus is broad and shallow. M1 has complete protoloph II; mesoloph is medium in length The M2 is trapezoidal in occlusal outline, longer than wide or long; metaloph of M1-2 joins anterior arm of hypocone; and and narrower posteriorly than anteriorly (Figure 2(c)). The labial mesolophid of m1-2 is long. anteroloph is long, and the lingual anteroloph is short. The dis- Heosminthus includes three previously described species: tinct protoloph I is connected with the anterior arm of the pro- H. primiveris, H. borrae, H. chimidae, and H. sp.. H. primiveris tocone, the protoloph II is slightly weaker and lower than the was named and described by Wang (1985) based on specimens protoloph I, and connected with the entoloph. The metaloph is from the Qujing basin in Yunnan. H. nomogenesis is slightly transverse and extended to the hypocone (2/3) or the anterior larger than H. primiveris. In M2 of H. primiveris, the protoloph arm of the hypocone (1/3). The mesoloph is medium to long II is absent in most cases, but it is complete in H. nomogene- length. The sinus is broader and deeper than that in M1. sis. In addition, the mesostylid and the hypoconulid on m1 of The M3 is a rounded triangular in outline and broader ante- H. nomogenesis are weak, and the mesoloph in M3 is short. riorly than posteriorly (Figure 2(a)). The protocone is the larg- H. borrae, H. chimidae and H. sp. from the Taatsiin Gol est cusp in M3. The protoloph I is distinct, but the protoloph and Taatsiin Tsagaan Nuur area in Mongolia, were described II is absent. The metaloph is short and moves backwards to by Daxner-Höck et al. (2014). H. nomogenesis differs from join the hypocone. The mesoloph is shorter than the metal- H. borrae in several aspects of the cheek teeth, including lower oph. A short longitudinal crest from the protoloph I extends cones/conids and weaker lophs, shorter anterior arm of protocone to the metaloph. The sinus is very small and slightly oblique of M1, more distinct protoloph II of M2, more shallow sinus posterolingually. of M1-2, longer posterior arm of the protoconid on m2, and The m1 (Figure 2(d) and (e)) is smaller than m2 and tapers absent ectostylid at the base of the sinusid on m1-2; however, anteriorly. An isolated anteroconid is usually present. The proto- H. nomogenesis has a more primitive dental pattern than H. borrae. conid and metaconid are subequal, and the posterior arm of the In H. nomogenesis and H. chimidae both the protoloph I and protoconid is complete, U-shaped and connected to the metac- the protoloph II of M2 are present. In M1 of H. nomogenesis the onid. The longitudinal ectolophid is straight and connected with anterior arm of the protocone is short and the anterior cingulum the protoconid at its base. A small mesoconid is present on the is distinct, but they are variably developed in H. chimidae. In ectolophid. The mesolophid is long and slightly directed for- lower molars of H. nomogenesis, the anteroconid, the mesoconid ward. The hypolophid is complete and extends to the anterior and the hypoconulid are weaker than these in H. chimidae. The arm of the hypoconid. The well-developed posterolophid usually posterior arm of the protoconid in m2 of H. chimidae is con- reaches the base of the entoconid. The mesostylid and hypoco- nected with the mesolophid in some cases. H. sp. from the Valley nulid are weak. of Lakes in Mongolia is larger than H. nomogenesis (see Table 1). The m2 (Figure 2(f)) is similar to m1 except for its anterior H. nomogenesis is more similar to H. primiveris and H. chimi- part. The lingual anterolophid is longer than the labial one. The dae than to H. borrae. H. borrae ranges from the Early Oligocene metalophid and the anterior arm of the protoconid are con- (biozone B) to the Early Miocene (biozone D) (Daxner-Höck nected with the anteroconid, and the posterior arm of the pro- et al. 2014), and it has more advanced dental morphology than toconid is long and extends almost to the base of the metaconid. H. nomogenesis. H. chimidae also ranges from the Early Oligocene The mesoconid, mesostylid and hypoconulid are more distinct (biozone A) to the Early Miocene (biozone D), and it shows a than on m1. 4 Q. Li et al.

Table 1. Measurements of teeth of Heosminthus (mm).

H. nomogenesis H. primiveris H. chimidae H. borrae H. sp Range Mean n Range Mean n Range Mean n Range Mean n Range Mean n M1(L) 0.95–1.10 1.07 3 0.93–1.04 0.99 7 0.96–1.25 1.14 433 0.87–1.06 0.97 30 1.27–1.39 1.31 12 M1(W) 0.95–1.10 1.02 3 0.82–0.90 0.87 6 0.87–1.22 1.05 437 0.80–0.99 0.92 29 1.18–1.34 1.23 12 M2(L) 1.00 1.00 3 0.92–1.03 0.98 6 0.94–1.22 1.08 381 0.82–1.01 0.92 19 1.22–1.29 1.26 4 M2(W) 0.95–1.00 0.98 3 0.89–0.99 0.93 6 0.89–1.22 1.05 381 0.80–0.99 0.89 19 1.11–1.32 1.24 4 M3(L) 0.65 0.65 1 0.81 0.81 1 0.68–1.01 0.80 213 0.64–0.78 0.71 21 0.94 0.94 1 M3(W) 0.75 0.75 1 0.77–0.78 0.78 2 0.64–1.04 0.86 209 0.66–0.85 0.74 21 0.99 0.99 1 m1(L) 1.05–1.35 1.17 5 1.07 1.07 2 0.96–1.36 1.20 444 0.94–1.11 1.04 31 1.39–1.46 1.43 7 m1(W) 0.80–0.95 0.84 5 0.72–0.73 0.73 2 0.73–1.13 0.89 448 0.71–0.94 0.79 31 0.99–1.13 1.05 6 m2(L) 1.15–1.20 1.17 3 1.03–1.10 1.07 2 0.99–1.34 1.19 457 0.87–1.06 0.98 14 1.36–1.48 1.41 9 m2(W) 0.90 0.90 3 0.78–0.85 0.82 2 0.71–1.20 0.93 453 0.87–0.94 0.85 13 1.01–1.18 1.11 9

Note: Data for H. primiveris from Wang (1985); Data for H. chimidae, H. borrae and H. sp. from Daxner-Höck et al. (2014).

Figure 3. Cheek teeth of Sinosminthus sp. occlusal view. Notes: (a) V 17812.1, right M2; (b) V 17812.2, right M2; (c) V 17812.3, right mandible fragment with m1-2.

wide variability of dental morphology and size; however, the of the protoconid is complete and connected to the metaconid. dental morphology of H. nomogenesis is primitive and invariable. The anteroconid and the metaconid are isolated to each other.

Sinosminthus Wang, 1985 The entoconid is situated further forward than the hypoconid. The Sinosminthus sp. hypolophid is complete and extends to the anterior arm of the Specimens. IVPP V 17812.1, right M2; V 17812.2, right M2; V hypoconid. The posterolophid is well-developed. The ectolophid 17812.3, right mandible with m1-2. is straight and low, connected to the protoconid at its base. The Locality and Horizon. Erden Obo, Nomogen, Siziwangqi, Nei mesolophid is long and low, and slightly oblique anterolingually. Mongol; upper part of the ‘Upper Red’ beds. The m2 is rectangular in occlusal view and has four main cusps. The anteroconid is absent. Both anterolophids are well Description and comparison developed, but the labial one is short. The metalophid is complete and connects with the anterior arm of the protoconid. The poste- The occlusal surface of M2 is rectangular. Its anterior border is rior arm of the protoconid is long and extends posterolingually straight, whereas the posterior one more rounded (Figure 3(a) toward the base of the metaconid (Figure 3(c)). The mesolophid and (b)). The four main cusps are robust. The metacone is slight is short and the mesoconid is weaker than that in m1. The talonid lower than the paracone, and the hypocone is smaller than the of m2 is similar to that of m1. protocone. The anteroloph has long labial and short lingual arms. The new specimens from the Erden Obo section shares with A short anterolophule connects the anteroloph to the anterior Sinosminthus inapertus (Wang 1985) from Qujing the following side of the protocone. Two protoloph connections exist between features: protoloph I and II on M2 are subequal; sinus on M2 is the paracone and the protocone, and the protoloph I joins the top open and less oblique; metaloph on M2 is obviously extends to of the paracone. The two protolophs are subequal in length but anterior arm of the hypocone, and mesolophid is long on m1, and protoloph II is slight lower than the protoloph I. Furthermore, it is shorter than posterior arm of protoconid on m2. Compared the single metaloph joins the anterior arm of the hypocone. The to the holotype of S. inapertus, the specimens reported here are entoloph is distinct and complete. The mesoloph ranges from larger (see Table 2), and the anterior spur of the hypoconulid medium-length to long. The thin posteroloph is long, delimiting on m1 is absent. These specimens are tentatively referred to a large posterosinus. The broad sinus is less oblique anteriorly. Sinosminthus, but cannot be identified to the specific level. The ventral edge of the mandible is convex below the cheek Allosminthus Wang, 1985 teeth, with the ventral-most point situated below m1 (2.7 mm). Allosminthus cf. A. majusculus Wang, 1985 There is a mental foramen on the lateral surface of the right man- Holotype: IVPP V 17813.1-3, left M1; V 17813.4-8, right M1; V dible with m1 and m2, which is anteroventral to m1. The m1 17813.9-14, left M2; V 17813.15, right M2; V 17813.16-20, left is trapezoidal in occlusal view, and is longer than wide (Figure m1; V 17813.21, right m1; V 17813.22-27, left m2; V 17813.28- 3(c)). The anteroconid is small and low, and a crest extends to 29, right m2. meet the protoconid. The protoconid and metaconid are sube- Locality and Horizon: Erden Obo, Nomogen, Siziwangqi, Nei qual, with the latter positioned more anteriorly. The posterior arm Mongol; lower and middle part of the ‘Upper Red’ beds. Historical Biology 5

Table 2. Measurements of teeth of Sinosminthus (mm). The M2 is subquadrate in occlusal outline, but with a slightly Sinosminthus inapertus narrower posterior side (Figure 4(d) and (e)). The anteroloph has Sinosminthus sp. (Wang 1985 ) a short lingual arm and a longer labial arm. Both the protoloph V17812.1 V17812.2 V17812.3 Range Mean I and the metaloph are directed slightly forward. They connect M2(L) 2.00 1.95 1.34–1.65 1.53 with the paracone and the anterior arm of the hypocone, respec- M2(W) 1.65 2.00 1.32–1.50 1.42 tively. The transverse mesoloph is long (4/7) (Figure 4(d)) or of m1(L) 1.85 1.41–1.73 1.63 m1(W) 1.50 1.06–1.33 1.22 medium length (3/7) (Figure 4(e)). Other characters of M2 are m2(L) 1.80 1.48–1.75 1.66 similar to M1. m2(W) 1.65 1.16–1.38 1.29 The m1 is narrow in its anterior part. The anteroconid is smaller and lower than any of the main cuspids, and it is isolated in most cases. The protoconid and metaconid are in opposite Differential diagnosis: Differs from other known species of positions, and are connected by the posterior arm of the proto- Allosminthus in being large, having a medium-length to long conid (Figure 4(f) and (g)). The hypoconid and the entoconid are mesoloph in M1-2 and a long mesolophid in m1, a developed in alternating position, and more widely spaced. The hypolophid mesostyle in M1 and mesostylid in m1 of some cases. Differs is straight and extended to the anterior arm of the hypoconid. from A. diconjugatus, A. minutus and A. khandae in lacking a The ectolophid is complete and low, connected to the protoconid protoloph II on M1. at its base. The mesoconid on the ectolophid is distinct and the mesostylid is small. The mesolophid is long in most cases (5/6) Description (Figure 4(f) and (g)). In a few cases there is a short ectomesolophid (Figure 4(g)). The posterolophid is continuous and extends The M1 is identified by being wider posteriorly than anteriorly, to the base of the entoconid, and the hypoconulid is weak. but the maximum width and length of M1 are nearly equal. The The m2 is rectangular in occlusal outline. The anterior and contact facet on the anterior surface is consistent with presence of posterior pair of main cusps are located almost opposite to each a small P4. Four main cusps are distinct and crests are low (Figure other. The anterolophid is developed. The short anterior arm 4(a)–(c)). The anterior cingulum is present in some cases (5/8) of the protoconid extends to the anterolophid, and the metal- (Figure 4(a) and (c)) and short. The anterior arm of the protocone ophid is short (3/8) (Figure 4(h)) or absent (5/8) (Figure 4(i)). is complete and extends labially to connect the anterior part to The posterior arm of the protoconid is prominent and meets the the paracone. In some specimens, a small protoconule is present metaconid. The talonid is similar to that of m1. The mesolophid

on the anterior arm of the protocone (3/8) (Figure 4(b)), which is is middle-length and shorter than that in m1. The mesostylid more distinct on the worn M1. The protoloph II is never present. is absent. The metaloph is transverse and connected to the anterior arm of the hypocone in most of the teeth (6/8) (Figure 4(a) and (c)). The Comparisons mesocone is distinct in most cases and the mesoloph is long or of medium length. Sometimes a small mesostyle occurs at the labial The described specimens differ from dipodoids such as margin between the paracone and metacone (Figure 4(b)). The Parasminthus, Shamosminthus, Sinosminthus and Gobiosminthus entoloph is complete and joins the protocone, except in V17813.3 (Huang 1992) in having a metaloph extending to the anterior arm where it is short and does not extend to the protocone (Figure of the hypocone in M1-2, and a developed posterior arm of the 4(b)). The sinus is broad and shallow. The posteroloph is long protoconid in m1-2. The Erden Obo specimens appear morpho- and extends to the posterolabial corner. logically advanced over Primisminthus (Tong 1997) in having a

Figure 4. Cheek teeth of Allosminthus cf. A. majusculus in occlusal view. Notes: (a) V 17813.2, left M1; (b) V 17813.3, left M1; (c) V 17813.5, right M1; (d) V 17813.11, left M2; (e) V 17813.15, right M2; (f) V 17813.16, left m1; (g) V 17813.17, left m1; (h) V 17813.26, left m2; (i) V 17813.29, right m2. 6 Q. Li et al.

Figure 5. Measurements of the molars of Allosminthus cf. A. majusculus from the Erden Obo. Notes: The measurements are compared with the type material of Allosminthus ernos from Wang, 1985; the type material of Allosminthus diconjugatus and Allosminthus uniconjugatus from Tong 1997; Allosminthus majusculus from Wang, 2008; and Allosminthus khandae and Allosminthus minutus from Daxner-Höck, Badamgarav and Maridet, 2014.

complete entoloph, a more developed metaloph and mesoloph A. ernos and A. majusculus are named and described by Wang in M1-2, a complete posterior arm of the protoconid in m1-2, (1985) based on specimens from Qujing in Yunnan. Later, Wang and a long mesolophid in m1. The anterior arm of the protocone (2008) found some materials of A. majusculus from the Houldjin of Heosminthus is short and separated from the paracone by a Formation of Erenhot, Nei Mongol. The Erden Obo cheek teeth shallow groove, whereas that of the Erden Obo specimens is differ fromA. ernos in several aspects, including larger size connected with the paracone. In Heosminthus, the mesoloph and (Figure 5), longer mesoloph and more developed mesostyle in mesolophid is longer than in the Erden Obo specimens. M1-2, longer mesolophid in m1-2 and more distinct mesostylid The Erden Obo specimens share the following features with in m1. The only materials ofA. majusculus found are m1 and Allosminthus: crown brachyodont, with obtuse main cusps and m2. The individual size differences of A. majusculus are within low crests, a strong anterior arm of the protocone of M1 con- the range of variation of the species from Erden Obo; however, nected with the paracone, a metaloph of M1-2 extended to the the Erden Obo specimens differ from A. majusculus in having anterior arm of the hypocone, and a complete hypolophid in shorter or weaker metalophid on m2 and longer mesolophid m1-2. on m1. Allosminthus includes six previously described species: A. Based on the specimens from Shanxi and Henan, China, ernos, A. majusculus, A. diconjugatus, A. uniconjugatus, A. minu- Tong (1997) erected a new genus Banyuesminthus, including tus and A. khandae. B. uniconjugatus and B. diconjugatus. Wang (2008) pointed out Historical Biology 7

Figure 6. Cheek teeth of Allosminthus ernos in occlusal view. Notes: (a) V 17814.1, right maxillary fragment with P4-M1; (b) V 17814.2, right maxillary fragment with M2-3; (c) V 17814.5, left M1; (d) V 17814.10, left M1; (e) V 17814.14, right M1; (f) V 17814.15, left M2; (g) V 17814.18, right M2; (h) V 17814.3, left m1-2; (i) V 17814.25, left m1; (j) V 17814.38, right m2; (k) V 17814.40, left m3; (l) V 17814.41, left m3. that the differences between A. ernos and Banyuesminthus fall are similar to A. khandae in that the mesostyle and relatively within the individual variation of A. ernos, and she pointed that developed mesoloph are present on M1-2. However, there are Banyuesminthus is a junior synonym of Allosminthus. Daxner- also important differences between the two taxa. In the Erden Höck et al. (2014) also confirmed this synonymy. The Erden Obo Obo specimens, the protoloph II of M1 is absent, the metalophid specimens are larger than A. uniconjugatus and A. diconjugatus of m2 is short or absent, and the posterior arm of the protoconid (Figure 5). They can be clearly distinguished from A. diconjugatus of m2 is complete. The Erden Obo specimens are larger thanA. by the following characters: no protoloph II in M1, longer mes- khandae (Figure 5). oloph in M1 and M2 and mesolophid in m1, more distinct Daxner-Höck et al. (2014) suggested that Heosminthus minu- mesostylid in m1. In M1-2 of A. uniconjugatus the entoloph is tus (Daxner-Höck 2001) is Allosminthus rather than Heosminthus, incomplete and the mesoloph is short. In upper molars of the based on the primitive dental structures of specimens. A. minu- Erden Obo specimens, by contrast, the entoloph is complete and tus is the smallest species in Allosminthus (Figure 5). A. minu- the mesoloph is middle length or long. In addition, the lower tus differs from the Erden Obo specimens in their smaller size, teeth of the Erden Obo specimens differ from the A. uniconjuga- and in that M1-2 have a pronounced protoloph II and short tus in having a short metalophid in m2 of some cases and longer to medium length mesoloph. In A. minutus the mesolophid on mesolophid of m1. m1-2 is absent and the mesoconid is weak or absent. Tatalsminthus khandae and Heosminthus minutus were estab- Based on the morphological similarities, the Erden Obo spec- lished by Daxner-Höck (2001) based on the specimens collected imens belong to A. majusculus. The differences between them from the Early Oligocene Hsanda Gol Formation of Valley of are mainly focused on the development of the mesolophid in Lakes in Mongolia. Wang (2009) considered Tatalsminthus m1-2 and metalophid in m2, but the mesolophid and metalo- as a junior synonym of Allosminthus, and Daxner-Höck et al. phid are variable in Allosminthus. The only known material of (2014) also confirmed the synonym. The Erden Obo specimens A. majusculus is composed four isolated teeth, so information 8 Q. Li et al.

Table 3. Measurements of teeth of Allosminthus ernos (mm).

Allosminthus ernos (from Erenhot) Allosminthus ernos (from Erden Obo) Allosminthus ernos (from Qujing) (Wang 1985) (Wang 2008) n Range Mean n Range Mean n Range P4(L) 1 0.45 0.45 P4(W) 1 0.40 0.40 M1(L) 12 0.90–1.15 1.01 9 0.92–1.14 1.05 M1(W) 12 0.80–1.10 0.94 10 0.81–0.96 0.90 M2(L) 7 1.00–1.15 1.03 6 0.95–1.09 1.02 M2(W) 7 0.90–1.10 0.97 7 0.80–0.96 0.88 M3(L) 1 0.75 0.75 M3(W) 1 0.80 0.80 m1(L) 11 0.95–1.25 1.06 4 1.00–1.06 1.03 m1(W) 11 0.65–0.85 0.76 5 0.69–0.75 0.72 m2(L) 10 0.95–1.25 1.06 7 0.92–1.13 1.05 1 1.06 m2(W) 10 0.80–0.95 0.89 9 0.67–0.88 0.81 1 0.76 m3(L) 3 0.85 0.85 1 0.82 0.82 m3(W) 3 0.70–0.80 0.77 1 0.80 0.80

about variation of A. majusculus is lacking. We regard the Erden The m2 trigonid is subequal to the talonid in width. The ante- Obo material as Allosminthus cf. A. majusculus. rior arm of the protoconid is short and joins the anterolophid. Allosminthus ernos Wang, 1985 The metalophid is absent (4/10) (Figure 6(h)) or distinct and Specimens: IVPP V 17814.1, right maxilla with P4-M1; V extends to the anterior arm of the protoconid (6/10) (Figure 6(j)). 17814.2, right maxilla with M2-3; V 17814.3, left mandible with The posterior arm of the protoconid is long and connected to m1-2; V 17814. 4-11, left M1; V 17814. 12-14, right M1; V 17814. the protoconid and the metaconid. The mesolophid is short or 15-16, left M2; V 17814.17-20, right M2; V 17814. 21-27, left m1; absent. Other characters are similar to those in M1. V 17813.28-30, right m1; V 17814.31-35, left m2; V 17813.36-39, The m3 is the smallest lower molars. The anterior arm of the right m2; V 17813.40-42, left m3. protoconid and the metalophid are connected by the anterol- Locality and Horizon: Erden Obo, Nomogen, Siziwangqi, Nei ophid. The posterior arm of the protoconid varies from com- Mongol; ‘Upper Red’ beds. plete (2/3) (Figure 6(k)) to absent (1/3) (Figure 6(l)). There is no

mesolophid, but a hypolophid extends from the small entoconid Description and comparison to the anterior arm of the hypoconid (Figure 6(k)) or is weak (Figure 6(l)). The ectolophid is curved. The P4 is small, peg-like and single-rooted, with an oval contour The new specimens from the Erden Obo have a complete in occlusal view (Figure 6(a)). anterior arm of the protocone on M1 that forms the only con- Molars are low-crowned and have three roots. The four main nection between the paracone and the protocone, no protoloph II cusps are subequal in size. The most remarkable character is that on M2, a short mesoloph on upper molars, a complete posterior a complete anterior arm of the protocone exists between the par- arm of the protoconid on m1-2, a varied metalophid on m2, and acone and the protocone (Figure 6(a) and (c)–(e)). The metaloph a short or absent mesolophid on lower molars. All the features is complete and extends to the anterior arm of the hypocone in are identical with those of Allosminthus ernos. The size of the new most of the teeth. The mesocone is small, and the mesostyle is specimens falls within the range of variation of A. ernos (Wang weak. The mesoloph is usually short, rarely is long and reaches the 1985) (see Table 3). We report and describe P4 and M3 from the labial margin (Figure 6(d) and (e)). The sinus is broad and shallow. Erden Obo section, for the first time. The M2 is similar to M1, only the protoloph I exists, and Allosminthus cf. A. diconjugatus Tong 1997 the protoloph II is never present (Figure 6(b), (f), and (g)). The Specimens: IVPP V 17815.1-3, left M1; V 17815.4-5, right M1; V metaloph joins the anterior arm of the hypocone. The mesoloph 17815.6, left m1; V 17815.7, left m2; V 17815.8, right m2. is short and single or forks in some cases (4/8) (Figure 6(g)). Locality and Horizon: Erden Obo, Nomogen, Siziwangqi, Nei The M3 is small and narrowed posteriorly (Figure6 (b)). The Mongol; the lower part of the ‘Upper Red’ beds. protocone is anteroposteriorly extended, and the metacone and the hypocone are strongly reduced. The protoloph I is transverse Description and comparison and ends in the anterior arm of the protocone. The metaloph is short and turns backwards to the posteroloph. The mesoloph The M1 is almost square in occlusal outline. The anterior arm is short. of the protocone is prominent and extends to the anterior slope The anteroconid of m1 is small, and is isolated in most teeth of the paracone. The protoloph II is directed backwards and (Figure 6(h)), and rarely is joined to the protoconid by a weak connects with the posterior arm of the protocone (Figure 7(a) crest (Figure 6(i)). The protoconid and metaconid are directly and (b)). The metaloph is connected to the anterior arm of the connected by the posterior arm of the protoconid. The hypolo- hypocone in most teeth (4/5) (Figure 7(a)) or attached to the phid is transverse, and its junction with the ectolophid is very hypocone (Figure 7(b)). The mesoloph is medium length. The near to the hypoconid (Figure 6(h) and (i)). A distinct mesoconid entoloph is complete. is present on a low ectolophid, and the mesostylid and the mes- The m1 (Figure 7(c)) is rectangular in outline with a nar- olophid are absent. The posterolophid is well developed. row anterior border. The anteroconid is small and close to the Historical Biology 9

Figure 7. Cheek teeth of Allosminthus cf. A. diconjugatus in occlusal view. Notes: (a) V 17815.1, left M1; (b) V 19815.3, left M1; (c) V 17815.6, left m1; (d) V 17815.7, left m2.

Table 4. Measurements of teeth of Allosminthus cf. A. diconjugatus (mm).

A. cf. A. diconjugatus (from Erenhot) A. cf. A. diconjugatus (from Erden Obo) (Wang 2008) A. diconjugatus (from Yuanqu) (Tong 1997) n Range Mean n Range n Range M1(L) 5 1.00–1.10 1.05 2 1.06–1.10 1 1.20 M1(W) 5 0.95–1.00 1.00 3 1.00–0.90 1 1.07 m1(L) 1 1.30 1.30 1 1.20 1 1.20 m1(W) 1 1.00 1.00 3 0.90–0.95 1 0.93 m2(L) 2 1.15–1.25 1.20 4 1.10–1.35 4 1.13–1.20 m2(W) 2 0.95–1.00 0.98 4 0.90–1.10 4 0.83–0.97

protoconid and the metaconid. The posterior arm of the protoco- Caijiachong Formation of the Qujing Basin, Yunnan. Later, Wang nid is short and complete. The distinct mesoconid is present on (2008) described the dipodids from the Houldjin Formation the ectolophid, and the mesolophid is medium length. The hypol- of Erenhot, Nei Mongol including Allosminthus majusculus, ophid is complete and joins the anterior arm of the hypoconid. Allosminthus cf. A. diconjugatus and Allosminthus ernos. The anterior arm of the protoconid and the metalophid of Cenozoic sediments and fossils of the Taatsiin Gol and m2 are connected to the anterolophid (Figure 7(d)). The pos- Taatsiin Tsagaan Nuur area of the Valley of Lakes in Mongolia

terior arm of the protoconid is connected with the metaconid yield important information about stratigraphy and mammal and closes the trigonid. The mesolophid is shorter than that in evolution in Asia. Based on the data on large and small mam- m1. The ectolophid is oriented longitudinally. The shape of the mals form the Valley of Lakes, the Mongolian informal biozones talonid is similar to that of m1. A, B, C, C1 were updated (Daxner-Höck et al. 2010). Recently, The teeth from the Erden Obo section and the specimens of Daxner-Höck et al. (2014) studied the taxonomy, evolution and A. diconjugatus (Tong 1997) from the Yuanqu Basin share general stratigraphic distribution of the Dipodidae from the Valley of features, and their sizes are similar (see Table 4). These molars Lakes, and pointed that the Dipodidae are the most abundant from the Erden Obo differ from those ofA. diconjugatus in hav- and species-rich rodent family of the Oligocene in Mongolia. The ing a more complete entoloph on M1 and more developed metal- dipodid fossils representing biozone A are Heosminthus chim- ophid on m2 that extended to the anterolophid. Allosminthus cf. idae, Heosminthus sp., Allosminthus khandae, Shamosminthus A. diconjugatus was described by Wang (2008) based on spec- sodovis, Onjosminthus baindi, and Bohlinosminthus parvulus. imens from the Houldjin Formation of Erenhot, and it differs Heosminthus nomogenesis is similar to Heosminthus primiv- from A. diconjugatus in having sharper main cusps and slim- eris from the Caijiachong Formation, and it is more primitive mer transverse lophs, and having a complete metalophid on m2. than Heosminthus chimidae of Mongolian biozone A (Daxner- Compared with the specimens from the Houldjin Formation, Höck et al. 2010, 2014). Allosminthus cf. A. majusculus from the the mesolophid on m1 from the Erden Obo teeth are longer, and Upper red differs from Allosminthus majusculus of the Houldjin other characters are similar. We include the specimens from the Formation in having a more variable mesolophid and metalo- Erden Obo section in Allosminthus cf. A. diconjugatus. phid in m1-2. Allosminthus ernos was found in the Upper red of the Erden Obo section, the Houldjin and the Caijiachong Discussion Formations. In addition, Allosminthus cf. A. diconjugatus was Geological age of the ‘Upper red’ of the Erden Obo Section present in the Upper red and the Houldjin Formation. Obviously, the dipodid assemblage of the upper red of the Erden Obo can The dipodid rodents described here include five species of be closely compared to the assemblage from the Caijiachong and three genera: Heosminthus nomogenesis sp. nov., Sinosminthus Houldjin Formations, but it is quite different and more primitive sp., Allosminthus cf. A. majusculus, Allosminthus ernos and than that of the Mongolian biozone A. Allosminthus cf. A. diconjugatus (see Table 5). Based on mammal fossils, the age of the Houldjin Formation Many dipodid fossils have been discovered in the Late Eocene was assigned to the Late Eocene (Wang 1997, 2001, 2007a, and Early Oligocene in Asia. Wang (1985) reported the dipodid 2007b, 2008; Qiu & Wang 2007). The age of the Caijiachong Heosminthus primiveris, Sinosminthus inapertus, Allosminthus Fauna is Ergilian (Naduan + Ulangochuan) of the Asian Land ernos, Allosminthus majusculus and Allosminthus sp. from the Mammalian Ages, which has been correlated with the North 10 Q. Li et al.

Table 5. Comparison of Myodonta fossil assemblages of the ‘Upper red’ from the Erden Obo with selected fauna in Asia.

A-biozone of Hsanda Gol and Caijiachong Formation, Qujing Houldjin Formation of Erenhot, Nei Loh Formations, Valley of Lakes, Upper red of Erden Obo, Nei Mongol Basin, Yunnan Mongol Mongolia Dipodoidae Dipodoidae Dipodoidae Dipodoidae Heosminthus nomogenesis Heosminthus primiveris Allosminthus ernos Heosminthus chimidae Sinosminthus sp. Sinosminthus inapertus Allosminthus majusculus Heosminthus sp. Allosminthus ernos Allosminthus ernos Allosminthus cf. A. diconjugatus Allosminthus khandae Allosminthus cf. A. majusculus Allosminthus majusculus Shamosminthus sodovis Allosminthus cf. A. diconjugatus Allosminthus sp. Onjosminthus baindi Bohlinosminthus parvulus Cricetidae Cricetidae Cricetidae Cricetidae Eucricetodon wangae Eocricetodon meridionalis Eocricetodon borealis Selenomys mimicus Eucricetodon sp. Oxynocricetodon leptaleos Oxynocricetodon erenensis Cricetops dormitor Pappocricetodon siziwangqiensis Eucricetodon caducus Eucricetodon asiaticus

American Chadronian or European MP19 (Wang & Zhang (1) P4 present or absent 1983; Wang 1985, 1992, 1997; Tong et al. 1995; Emry, Lucas The major difference traditionally recognized between et al. 1998). Recently, Maridet and Ni (2013) found the Early Dipodoidae and Muroidea is the presence of P4 in dipodoids and Oligocene cricetid from the Caijiachong Formation, but the new the absence of the tooth in muroids. Recent study shows that the fossiliferous layer is younger than the layer containing dipo- early cricetids Pappocricetodon antiquus (Wang & Dawson 1994), dids. Based on the dipodid assemblage, the age of the Upper Palasiomys (Tong 1997) have a small P4 or DP4, and the anterior red of the Erden Obo section is correlative to the Ergilian, and surface of M1 in some cases from Pappocricetdon neimongolensis in agreement with the case of cricetids from the same locality (Li 2012), Pappocricetdon kazakstanicus (Emry, Tyutkova et al. and layer (Li et al. 2016). 1998) and Pappocricetdon rencunensis (Tong 1997) has a small interdental wear facet. By the late Eocene, P4 is lost in the cri- The difference between the early dipodoids and muroids cetids and is present in all the genera of dipodoids. (2) anterocone and anteroconid The Dipodoidea has an Eocene origin, and constitutes the sis- The ratio of length to width of M1 in middle Eocene dipo-

ter group to the Muroidea (McKenna & Bell 1997). Based on the doids and most muroids is close to 1.1, but the ratio is from phylogenetic analysis to the Eocene Cricetidae, the split between 1.37 to 1.53 in the late Eocene muroids. In the early muroids, dipodoids and muroids occurred during the early middle Eocene (Gomes Rodrigues et al. 2010). Over the past 20 years, the molecu- the enlargement of the anterocone of M1 is coincident with an lar clock has commonly been used in estimating divergence times increase of the ratio of length to width of M1. Obviously, the in different taxa. Zhang et al. (2013) aimed to trace the fossil record size of the anterocone and the ratio of length to width of M1 and to provide molecular time estimates of Dipodoidea, and pro- are very different between the dipodoids and muroids in the posed that the diversification of modern dipodoids took place dur- late Eocene. ing the middle Eocene. A similar result was reported by Pisano et Huang (1992) pointed out that the anteroconid of m1 in al. (2015), and they inferred molecular phylogenetic relationships early dipodoids is small and rounded, while in the muroids it is for Dipodoidea based on five coding genes, made a time-cali- relatively large with a rounded anterior wall and flat posterior brated phylogeny, and suggested that modern Dipodoidea diver- one. The evolutionary trend of the anteroconid in cricetids is to sified during the middle Eocene. Moreover, the reconstruction of gradually enlarge (Wang 2007b; Gomes Rodrigues et al. 2011). ancestral areas and biogeographical events indicated that mod- However, the ratio of length to width of m1 in early dipodoids ern Dipodoidea originated in the Himalaya-Tibetan and Central and muroids are similar and close to 1.3, and the ratio did not Asian region. The aforementioned progress has certainly brought change significantly from middle to late Eocene. The anteroconid us much closer to understanding the early history of Dipodoidae is not a good referent to distinguish dipodoids and muroids in and Muroidea, and the timing of their common origin. However, the middle and late Eocene. stem Dipodoidea and Muroidea are almost indistinguishable on (3) anterior arm of the protocone in M1 the basis of the dental and cranial characters. Research on new, In the early dipodoids, the anterior arm of the protocone unequivocal characters was necessary to better discriminate them. usually joins the paracone (in Priminsminthus, Allosminthus, We carefully observed and compared the specimens of the and Sinosminthus), and sometimes it is short and free (in middle and late Eocene dipodoids (Allosminthus, Heosminthus, Heosminthus). In the early muroids, by contrast, the anterior arm Sinosminthus and Priminsminthus) and muroids (Eocricetodon, of the protocone is never connected to the paracone, but usually Oxynocricetodon, Palasiomys, Pappocricetodon, and Eucricetodon reaches to the anterolabial edge in M1 and is connected to the wangae). Some taxa (such as Elymys, Simimys and Aksyiromys) anterocone (in Pappocricetodon, Palasiomys, and Eocricetodon). of uncertain taxonomic position were not included in this study. In the late Eocene Oxynocricetodon and Eucricetodon wangae, The differences between the dipodoids and the muroids in the the anterior arm of the protocone is short and free, but it extends Eocene are as follows: anterolabially. Historical Biology 11

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