A new mimotonidan Mina hui (Mammalia, Glires) from the Middle Paleocene of Qianshan, Anhui, China

LI Chuan-KuiwANG Yuan-QingZHANG Zhao-QunMAO Fang-YuanMENG Jin2,

(1Key Laboratory of Vertebrate Evolution and Human Origins of Chinese Academy of Sciences,Institute of Vertebrate Paleontology and Paleoanthropology,Chinese Academy of SciencesBeijing 100044, China lichuankui@ivpp.ac.cn)

(2Division of Paleontology,American Museum of Natural HistoryNew York, NY 10024, USA)

A new mimotonidan Mina hui (Mammalia, Glires) from the Middle Paleocene of Qianshan, Anhui, China

LI Chuan-Kui1wANG Yuan-Qing1ZHANG Zhao-Qun1MAO Fang-Yuan1MENG Jin2,1

(1Key Laboratory of Vertebrate Evolution and Human Origins of Chinese Academy of Sciences,Institute of Vertebrate Paleontology and Paleoanthropology,Chinese Academy of SciencesBeijing 100044, China lichuankui@ivpp.ac.cn)

(2Division of Paleontology,American Museum of Natural HistoryNew York, NY 10024, USA)

Here we report a new genus and species,Mina huigen. et sp. nov., of basal Glires from the Middle Paleocene of Qianshan, Anhui, China. The new taxon is characterized by combination of the following characters: medium-sized mimotonidan; upper dental formula 2.0.3.3; dI2 transversely narrow and having smooth labial surface without longitudinal groove; M1 the largest cheek tooth and other cheek teeth decreasing in size considerably away from M1 so that the external margin of the upper cheek tooth row is distinctly arched labially; lingual side of upper molars unilaterally hypsodont and bearing no hypostria; hypocone being slightly distolingual to protocone; presence of a mesostyle; upper incisor with double-layered enamel structure; posterior border of anterior root of zygoma situated lateral to M1-2 and infraorbital foramen positioned low.M. huiis one of the earliest known Glires, co-existing withHeomysandMimotonain Qianshan geographically and Middle Paleocene (ca. 61 Ma) chronologically. we consider that the Mimotonida would include two families: the monotypic Mimotonidae that containsMimotonaand Mimolagidae that includesMimolagus,Gomphos,Anatolimys,Minaand possiblyAmar aleator.Among known mimotonidans,Mimotonaprobably represents a primitive “morphotype”as the ancestor of lagomorphs, whereas Mimolagidae includes a side branch diverged from the clade evolved toward lagomorphs. Future research may show that Mimolagidae is not a natural group, and may possibly submerge into Mimotonida, or involve more than one family-level clade. The occurrence ofHeomys,MimotonaandMinafrom Qianshan show that Glires had already diversi fi ed by the Middle Paleocene.

Qianshan, Anhui; Paleocene; basal Glires, Mimotonidae, Mimolagidae

1 Introduction

Glires Linnaeus, 1758 as a superordinal taxon includes two extant mammalian orders: Rodentia (rats, squirrels, guinea pigs, and relatives) and Lagomorpha (rabbits and pikas).Members of both orders are characterized by possessing a pair of enlarged, evergrowing upper and lower incisors, respectively, in which enamel is primarily restricted to the buccal surface. The relationship of Rodentia and Lagomorpha has been a long-standing problem in mammalian systematics (Tullberg, 1899; Gidley, 1912; wilson, 1949; wood, 1957; Dawson, 1967; Landry, 1999; Luckett and Hartenberger, 1985, 1993; Li, 1977; Li and Ting, 1985, 1993; Novacek, 1985, 1990; Jaeger, 1988; Meng and wyss, 2001, 2005; Meng et al., 2003; Asher et al., 2005). In addition to their sister-group relationship, each group has for a time been allied with other mammalian groups. Rodents were thought to be related to multituberculates, mixodectids, tillodonts and taeniodonts, primates, leptictids and eurymylids, whereas lagomorphs have been regarded relatives of triconodonts, artiodactyls, condylarths, zalambdodont insectivorans, pseudictopids, anagalids, macroscelidids, eurymylids, zalambdalestids and mimotonids, respectively (see Meng and wyss, 2005 for references).

The discoveries ofHeomysandMimotonafrom the Middle Paleocene of Qianshan, Anhui Province, China provided perhaps the most critical evidence for the origin and relationship of Glires. In reportingHeomysandMimotona, the primary conclusion reached by Li (1977) was thatHeomyswas closely related to rodents primarily because, in addition to several other diagnostic craniodental features, it has one pair of enlarged upper incisors (presumably dI2 Luckett, 1985). In contrast,Mimotonawas closely related to lagomorphs because, in addition to other craniodental features, it has two pairs of upper incisors, with the enlarged pair (dI2) followed by a pair of minor ones (I3). This view was further supported by later work (Li and Ting, 1993) in which additionalMimotonaspecimens showed that the enlarged dI2 has a longitudinal groove on the labial surface, a typical lagomorph feature. These studies onHeomysandMimotonaand on other Asian Paleocene-Eocene Glires have refocused the discussion about Glires origin on the Asian record during the last a few decades.

Until now, five Glires species have been reported from the Paleocene of Qianshan, includingHeomyssp.,H. orientalis,Mimotona wana,M. robusta, andM. lii. Reported specimens of these species were collected from three horizons.Heomyssp. (IVPP V 4323; Li, 1977),Mimotona wana(V 4326; Li, 1977) andM.lii(V 4327; Dashzeveg and Russell, 1988) were discovered from the lower part of the Upper Member of the wanghudun Formation, representing the earliest known record of unambiguous Glires.M.robusta(V 4329; Li, 1977) was from the Lower Member of the Doumu Formation, which overlies the wanghudun Formation.Heomys orientalis(V 4321, V 4322 and V 4331; Li, 1977) andMimotona wana(V 4324, V 4325.1, V 4325.2 and V 4328, Li, 1977; V 7416 and V 7416.1, Li and Ting, 1993) were from the Upper Member of the Doumu Formation. Their geological age ranges from the late Early through Middle Paleocene (wang et al., 2016). Outside of the Qianshan Basin but also in Anhui Province,Mimotonasp. (V 11359) was discovered from the Late Paleocene Tujinshan Formation of Mingguang City (previously known as Jiashan County) (Huang, 2003).

New specimens, including those ofHeomysandMimotona, have been collected from both wanghudun and Doumu formations since the work of Li and Ting (1993). Here we report a new genus and species of basal Glires, represented by two fragmentary specimens collected from the upper part of the Upper Member of the wanghudun Formation. The new form enriches the diversity of the earliest Glires from the Qianshan Basin and provides additional evidence for understanding the origin of Glires.

For higher-level taxonomy we follow McKenna and Bell (1997). “Mimotonida” may well be a paraphyletic group, as shown in some phylogenetic analyses (Meng et al., 2003; Asher et al., 2005; O’Leary et al., 2013), but before a better phylogeny and taxonomy including the most basal Glires become available, it is still convenient to use “Mimotonida” to refer the basal Glires that have two pairs of incisors in both upper and lower dentitions.

2 Systematic paleontology

Type genusMimolagusBohlin, 1951.

Diagnosis Mimotonidans with two pairs of upper incisors; larger thanMimotona; dI2 without groove on the anterior surface (primitive), cheek teeth square-shaped, less hypsodont than eurymylids, conical cusps with relatively weak crests, where known, the posterior border of anterior root of the zygomatic arch located more anteriorly than inMimotona.

Included generaMimolagus(Bohlin, 1951),Gomphos(Shevyreva et al., 1975; Zhegallo and Shevyreva, 1976; Dashzeveg and Russell, 1988),Anatolimys(Shevyreva, 1994; Averianov, 1994, 1998),Minagen. et sp. nov., and possiblyAmar(Dashzeveg et al., 1987).

Distribution The Paleogene, Asia.

Comments The use of Mimolagidae is tentative, as we elaborate in some details in the discussion. The main feature to set mimolagids apart fromMimotonain basal duplicidentates is absence of the longitudinal groove on the labial surface of dI2. This feature is likely plesiomorphic for Glires; thus it is probable that mimolagids may not form a natural group.

Minagen. nov.

Type speciesMina huigen. et sp. nov., the only species of the genus.

Etymology The genus name is dedicated to Prof. Minchen Chow (Zhou Ming-Zhen) in recognition of his contributions to the study on Chinese Paleocene biostratigraphy and mammalian fauna. Min, derived from what his family called him affectionately.

Diagnosis Medium-sized mimotonidan (with M1 length and width being 2.2 and 2.99 mm, respectively); upper dental formula 2.0.3.3. Differs from eurymylids and rodents in having two pairs of upper incisors. Differs fromMimotonaand lagomorphs in having transversely narrow dI2 with smooth labial surface (no longitudinal groove). Differs from other mimotonidans in having transversely narrow dI2 and I3; M1 (probably P4 as well) the largest cheek tooth and the other cheek teeth decreasing size remarkably away from P4-M1 so that the external margin of the upper cheek tooth row is distinctly arched labially; lingual side of upper molars sub-hypsodont but bearing no hypostria; hypocone being distolingual to protocone; presence of a mesostyle; upper incisor with double-layered enamel; posterior border of anterior root of the zygoma situated lateral to M1/2 and the infraorbital foramen positioned low.

(Figs. 1-3)

Holotype A partial right rostrum with dI2 and I3 (IVPP V 7509.1) and a fragmentary left maxilla with M1, M2 and alveoli of P2-4 (V 7509.2). They were found in a small (mugsized) block of rock when collected. Because they have comparable sizes, gliroid dental morphologies, and similar distinctive sutures in each specimen showing a similar ontogenetic stage, we consider them probably from the same young adult individual.

Locality and horizon Fujiashanzui, Gujing, Qianshan County, Anhui Province; the upper part of the Upper Member of wanghudun Formation; early Middle Paleocene (wang et al., 2016).

Etymology The species name is in memory of the late Dr. Yaoming Hu who made a great contribution to the collection of Qianshan Paleocene mammals.

Diagnosis Same as for the genus.

Measurements of teeth (length/width in mm; *=estimated from the alveolus) dI2, 4.05/1.76; I3, 2.36/1.09; P2, 1.33*; P3, 1.50/3.06*; P4, 1.91*/4.30*; M1, 2.52/2.99; M2, 2.39/2.60.

Description The partial rostrum (V 7509.1) has the anterior external parts of the maxilla and premaxilla preserved (Fig. 1A-D). The dI2 and I3 were preserved in their anatomical position and exposed medially because of the breakage (Fig. 1B). In lateral view, a zigzag suture between premaxilla and maxilla is distinctive and runs dorsoventrally, differing fromMimotonaand lagomorphs in which the suture is curved anteriorly and then far backward to form a slender wedge-like posterodorsal process of the premaxilla.

The general shape of dI2 is similar to that of rodents rather than lagomorphs, in being a gentle arc (about one third of a circle) and extending posteriorly into the maxilla instead of being contained within the premaxilla. The cross section of dI2 is roughly an elongated oval. Compared to the mesiodistal length of the tooth, dI2 is transversely narrow. The width-tolength ratio of dI2 is 0.44, considerably less than that ofMimolagus(0.60-0.63) andGomphoselkema(0.67) (Fostowicz-Frelik et al., 2015). The pulp cavity in the tooth tip is a slit that nearly bisects the dentin (Fig. 1D, E).

The labial surface of dI2 is smooth and lacks a longitudinal groove (Fig. 1C); thus it differs from that ofMimotonaand lagomorphs. The enamel band primarily covers the mesiolabial surface, with only a narrow strip visible on the medial surface of dI2 (Fig. 1A, B). The wear facet on the tip of dI2 has two parts. The mesial one is small, uneven and orientated more steeply, whereas the distal one is large (long) and concave (Fig. 1E). I3 is separated from dI2 by a diastema of 1.3 mm and rooted completely within the premaxilla. It is transversely compressed (see Measurements). A very thin, if any, enamel covers the “crown” but not the “root” which is lighter in color, although there is no distinct boundary between the two portions. In occlusal view, I3 bears wear at least on its mesial part, suggesting that the tooth was somewhat functional.

V 7509.2 is a fragmentary left maxilla with a small part of the palatine attached. The anterior root of the zygomatic process of the maxilla was also preserved, which is lateral to M1-2. In lateral view, the infraorbital foramen, although broken, is large and measures 1.4 mm in diameter; its position is near the level of the tooth alveolus, lower than that ofMimotonaandGomphos(Li and Ting, 1993; Asher et al., 2005). The anterior surface of the zygomatic process gradually converges to the snout, showing no indentation at the transition. The posterior border of the anterior root of the zygomatic process appears to have an acute angle to the longitudinal line of the skull, contrasting to a more open angle inMimotona(Fig. 1G). The posterior margin of the anterior root aligns with the distal half of M1, similar to that ofGomphos. In ventral view, the palatine-maxillary suture is distinct and the palatine extends anteriorly at least to the level of P3. The palate foramen is sizable and levels with the M1-2 junction. A narrow groove leads anteriorly from the foramen on the palatine.

The maxilla preserves M1-2 and the alveoli of P2-4. The maxilla posterior to M2 was broken (no alveolus preserved). However, on the distal surface of M2 there is a distinct contact facet for M3, which justi fi es our identi fi cation of M1-2 and the upper cheek tooth formula. The general pattern of the upper dentition is that P4 and M1 are transversely wide and other cheek teeth reduce size mesially or distally away from P4-M1. Because of this, the cheek teeth series appears proportionally short in length. The total alveolus length of P2-M2 is 9.64 mm.

From the alveoli it can be estimated that P2 is a circular and bud-like tooth with one root. P3 and P4 are mesiodistally short and transversely wide. Judging from the alveoli, P4 appears as wide as or slightly narrower than M1. The width ratio of M1 to M2 is 115% (2.99/2.60), whereas inMimotona wana(V 7500) it is 102% (2.80/2.75). The same ratio of P4 to P3 is 141% (4.30/3.06, measurements of alveoli) forMina huiand 107% (1.88/1.76) forM. wana(V 7500). These ratios show that the size reduction of cheek teeth (mesially away from P4 and distally from M1) is proportionally more signi fi cant inMinathan inMimotonaandGomphos.Thus, the external outline of the dentition ofMinais strongly arched labially (Fig. 1F), incontrast to a gentle curve inGomphosandMimotona(Fig. 1G).

Fig. 1 The holotype ofMina huiand comparison withMimotona wanaA-E.Mina hui(IVPP V 7509.1): A. lateral view of the right rostrum showing the maxilla-premaxillary suture, B. medial view of the broken rostrum showing the exposed dI2 and I3, C. anterior view showing no longitudinal groove, D. occlusal (ventral) view of the rostrum showing the cross outline of the upper incisors, E. the wear facet of the upper incisor; F.Mina hui(V 7509.2): occlusal view of the left palate and M1-2; G. occlusal view ofMimotona wana(IVPP V 7500) for comparison

In the description of upper molars below, we follow Zhou et al.’s (1975) suggestion in using the terminology for tooth crown structures that is illustrated in Fig. 2.

The occlusal outline of M1 is in a rounded square shape. The tooth shows some degree of hypsodonty on the lingual side, and the tip of the protocone is more labially positioned than that ofMimotonaandGomphos(Figs. 1F, 2). There is no hypostria along the lingual surface of M1, nor are labial and lingual cingula. In occlusal view, the protocone is subequal to the hypocone and more labial than the latter, but is higher than the hypocone. The preprotocrista extends from the protocone to the lingual side of the paracone. A narrow mesial cingulum, lower than the preprotocrista, extends from the protocone to the anterolabial corner of thetooth. There is no indication of paraconule. The paracone and metacone are labially positioned and separated by a rounded mesostyle that blocks the labial side of the trigon basin. The basin is transversely long and bears no structure and is broader than that ofMimotonaorGomphos. The paracone is transversely wider than mesiodistally long, with the labial side being wider and rounded. The labial border of the metacone is slightly lingual to the paracone and mesostyle. The metacone is smaller than the paracone; it is oval in occlusal view and bears a wear facet on its tip. Differing fromMimotonaandGomphos, the metaconule ofMinais as large as the metacone, but not as tall. Lingually, the metaconule is separated from the protocone by a narrow gap, whereas its labial base is confluent with the lingual base of the metacone. The hypocone is slightly lower than the protocone and extends labially as the postcingulum that gradually narrows labially and terminates at the distal side of the metacone.

M2 is narrower than M1, with the distal occlusal outline of the tooth being a broad curve convex distally (Figs. 1F, 2). Although the general pattern of M1 observed, all the cusps and ridges of M2 are transversely shorter and the lingual side of M2 is proportionally longer in relation to M1. The metacone is equal to the metaconule in size and the mesostyle is miniscule on M2. The metaconule has a weak connection with the protocone.

Fig. 2 Occlusal view of M1-2 ofMina hui(IVPP V 7509.2) A. The photograph of M1-2 ofM. huiin occlusal view; B. Stippling and line drawing showing terminology for the upper molar crown structures ofM. huiAbbreviations: hy. hypocone; me. metacone; mel. metaconule; mss. mesostyle; pa. paracone; pog. postcingulum; pr. protocone; preprc. preprotocrista; trb. trigon basin

Despite the many cracks on the cross-section of the investigated dI2, the general morphology of the enamel microstructures is still recognizable in cross section (Fig. 3A, C) and longitudinal section (Fig. 3B, D). The enamel is about 200 µm thick from the enamel-dentine junction (EDJ) to the outer enamel surface (OES) and has double-layered schmelzmuster with Hunter-Schreger bands (HSB) in the inner layer of enamel (the portio interna, PI). In the cross-section, it is clear that the inner and outer layers are subequal in thickness. In longitudinal section, the HSBs are distinctive; they are generally 3-6 prisms wide and as long as about half thickness of the enamel, but the width and length are irregular. TheHSB shave a small inclination angle to the EDJ and transition zones between HSBs are present (Fig. 3D). The outer layer of enamel (the portio externa, PE) consists of radial enamel. There is no clear boundary between HSB of PI and the radial enamel of PE. The detailed description of the enamel microstructure and an in-depth discussion about its signi fi cance are presented in Mao et al. (2016).

Fig. 3 The enamel structures of the right dI2 ofMina hui(IVPP V 7509.1, holotype) A. The cross section showing the general shape with many cracks; B. The longitudinal section; C. The close-up view of the cross section showing the prism and interprismatic matrix (IPM); D. The close-up view of the longitudinal section showing the HSBs in the inner layer and the radial enamel in the outer layer of incisor enamel, roughly divided along the dashed line. See also Mao et al. (2016)

3 Discussion

Comparison withHeomysAlthough two pairs of upper incisors can distinguishMina huifrom all the simplicidentate glires (only one pair of incisors in both upper and lower dentitions), it is still worth comparingMinawithHeomysfrom roughly the same stratigraphic level in the Qianshan Basin. The general morphology of upper molars of these two genera issimilar. Their buccal cusps, paracone and metacone, are blunt and rounded, and both have a large metaconule instead of a metaloph. On the other hand, the cranial and dental morphologies show distinct differences that can distinguishMinafromHeomys. Based on the molar size,Heomys orientalisis distinctly smaller thanMina hui. In the lateral view of the maxilla or anterior part of skull, the infraorbital foramen is positioned relatively much higher inHeomysthan that inMina.H.orientalishas only two premolars, whereasM.huihas three premolars. Compared withMina, P3-M1 inH.orientalisare less transversely widened so that the external outline of the upper dentition is less arched labially. M1 ofHeomysis relatively wider and forms a rhomboidal shape with the paracone area extending anterobuccally. The distal half of M2 is proportionally less reduced.The hypocone shelf ofHeomysmolars extends slightly more lingually and the mesostyle is smaller.

Taxonomy of traditional mimotonids Among the basal non-rodent and nonlagomorph Glires,Mina huiis taxonomically assignable to the traditional Mimotonidae within Duplicidentata instead of Eurymylidae within Simplicidentata, regardless whether or not each of the two clades is monophyletic, simply becauseM. huipossesses two pairs of upper incisors, one of the critical diagnostic features that distinguish Mimotonidae from Eurymylidae. Thus, our discussion will focus onM. huias a member of basal duplicidentatans, including those commonly placed in the traditional Mimotonidae or Mimotonida.

After the proposal of Mimotonidae (Li, 1977), different points of view on the content of the family emerged, some of which have been discussed by Averianov (1998). The family was considered variously to consist of four genera (Mimotona,Mimolagus,Gomphos, andHypsimylus) (Li and Ting, 1985), two genera (MimotonaandGomphos) (Dashzeveg and Russell, 1988), 5 genera (Mimotona,Mimolagus,Gomphos,Hypsimylusand ?Tsaganolagus wangi) (Li and Ting, 1993) or six genera (“Anatolmylus”,Aktashmys,Mimotona,Gomphos,Zagmys, andMimolagus) (Averianov, 1994). McKenna and Bell (1997) includedMimotona,MimolagusandAnatolimys(=Anatolmylus, see below) in the family Mimotonidae. Averianov (1998) realized that “Anatolmylus”rozhdestvenskiiAverianov, 1994 and “Romanolagus”hekkeriShevyreva, 1995 (placed in Palaeolagidae), both from the Early Eocene Andarak 2 locality in Kyrgyzstan, are junior subjective synonyms ofAnatolimys rozhdestvenskiiShevyreva, 1994.Anatolimyswas originally placed in Eurymylidae by Shevyreva, 1994, but Averianov (1998) regarded it as a member of Mimotonidae. In a more recent study, Averianov and Lopatin (2005) consideredAktashmysas a lagomorph, withValerilagus reshetoviShevyreva, 1995 from the same locality (Andarak 2 locality in Kyrgyzstan) being a junior synonym, and placedAktashmysin a new family Strenulagidae Averianov & Lopatin, 2005, that is based on the genusStrenulagusTong & Lei, 1987.

Dashzeveg and Russell (1988) consideredHypsimylus beijingensisZhai, 1977 not to be a mimotonid, differing from the view of Li and Ting (1985), and placed it in a new subfamily Hypsimylinae under Eurymylidae. McKenna and Bell (1997) placedHypsimyluswithin Leporidae, whereas Averianov (1998) recognizedHypsimylusas Lagomorpha incertae familiae. The lagomorph af fi nity ofHypsimyluswas supported by Meng and Hu (2004) in a study based on additional material from Nei Mongol. Averianov (1998) provisionally referredH.beijingensistoDituberolagus venustusTong, 1997, a species originally assigned to ?Leporidae as to Mimotonidae.

Bleefeld and McKenna (l985) and Dashzeveg and Russell (1988) favored a true lagomorph membership forMimolagus, whereas Averianov (1998) endorsed Li and Ting (1985) to assignMimolagusto Mimotonidae, which was followed by Fostowicz-Frelik et al. (2015).

In our viewZagmys,DituberolagusandTsaganolagustentatively assigned to Mimotonidae in earlier studies, should be removed from the family.Zagmys insolitusDashzaveg et al., 1987 was based on a fragmentary mandible with the talonid of p4 and a complete m1. Although the mandible contains an elongated lower incisor, which indicates its af fi nity with Glires, m1 ofZagmysis quite tribosphenic, possessing a distinctive paraconid on the trigonid and three cusps (hypoconid, entoconid and hypoconulid) on the talonid. This lower molar morphology differs from those of any known mimotonids. In addition, the preserved mandible, lateral to the alveolus of the lower incisor, does not show any sign of i3 and the enamel microstructure of the incisor is a single layer. Those differ signi fi cantly from mandible of mimotonids (Li, 1977; Li and Ting, 1993; Meng et al., 2004; Asher et al., 2005). Thus, we thinkZagmys insolitusis unlikely a mimotonid and should be considered as Gliresincertae sedis.

Dituberolagus venustusTong, 1997 was based on several isolated lower teeth from the Middle Eocene (Irdinmanhan) Hetaoyuan Formation, Henan. A peculiar feature is the twin cusps of the protoconid and metaconid on the lower molars. Averianov (1998) remarked that the teeth ofD. venustusare noticeably similar to upper molars ofAktashmys montealbus, which constituted parts of the argument to referD. venustusto Mimotonidae at the time. with the reassignment ofAktashmysto the lagomorph family Strenulagidae, as discussed above, there is no suf fi cient evidence to keepD. venustusin Mimotonidae.

Tsaganolagus wangiwas tentatively listed under Mimotonidae (Li and Ting, 1993) because of its oval-shaped outline of the upper molar crown after considerable wear. Reexamination of the type specimens convinced us that the taxon has hypsodont teeth and the lower cheek teeth are of the typical lagomorph tooth morphology.T. wangishould be assigned to Lagomorpha.

Divergence of mimotonidans and Mimolagidae we consider the following genera to be reliably assignable to the “Mimotonida”: the PaleoceneMimotona(Li, 1977) andMina(this study), the EoceneGomphos(Shevyreva et al., 1975; Zhegallo and Shevyreva, 1976; Dashzeveg and Russell, 1988),Anatolimys(Shevyreva, 1994; Averianov, 1994, 1998), and the Eocene-?OligoceneMimolagus(Bohlin, 1951; Fostowicz-Frelik et al., 2015). A recent work shows that the temporal distribution of knownMimolagusis only in the Eocene (Zhangand wang, 2016). Nine species were known previously (see Fostowicz-Frelik et al., 2015 for a review).Minafrom the Middle Paleocene, coeval withMimotonaandHeomys, represents another earliest known basal Glires and suggests an early diversi fi cation of Glires. with the two pairs of upper incisors, it is readily referable to “Mimotonida”.

with the new data accumulated since Li and Ting (1993), it seems that members of“Mimotonida” may contain two groups (or clades) that have different evolutionary trends. One group includesMina,Gomphos,AnatolimysandMimolagus, which is characterized by relatively larger size, dI2 with smooth enamel surface, and the posterior border of the anterior root of the zygoma aligned anterior to M2.Mimotona, on the other hand, stands alone as a taxon that is characterized by smaller size, has a distinctively grooved dI2 (a feature shared with lagomorphs), and the posterior border of anterior root of the zygoma is positioned lateral to M2/3. This division raises a taxonomic issue: whether it is necessary to recognize the family Mimolagidae within Mimotonida (Li et al., 1987). when comparing the foot structure ofMimolaguswith that ofPalaeolagus, Szalay (1985:120) noted: “Pal[a]eolagusis primitive among the leporoid lagomorphs (including leporids and ochotonids, in juxtaposition toMimolaguswhich should be placed in a family of its own, the Mimolagidae)”. The family was not formally diagnosed, nor was it used in his classi fi cation of Glires (Szalay, 1985). The same family name was used by Erbajeva (1986) and “Mimolagidae Erbajeva, 1986” was further cited as one of fi ve families of Lagomorpha (Erbajeva et al., 2011). The other four families are Leporidae Gray, 1821, Palaeolagidae Dice, 1929, Prolagidae Gureev, 1960, and Ochotonidae Thomas, 1897. McKenna and Bell (1997:108), however, did not recognize Mimolagidae, but placed the name under Mimotonidae.

If we follow the de fi nition of Lagomorpha as “the clade stemming from the most recent common ancestor ofOchotonaand Leporidae” (wyss and Meng, 1996; Meng and wyss, 2005), then there is no phylogenetic support for placing Mimolagidae in Lagomorpha. In fact, there has been no phylogenetic analysis that involved all the taxa that were taxonomically included in Mimotonidae. On the other hand, previous phylogenetic analyses that included selected mimotonid species often recognized mimotonids as a paraphyletic group (Meng et al., 2003; Asher et al., 2005). At present, it is premature to recognize Mimolagidae as a natural group, but this possibility cannot be ruled out. A systematic review of the existing Paleocene and Eocene basal Glires would be necessary to resolve this issue. However, in re fl ecting the new evidence accumulated recently (Fostowicz-Frelik et al., 2015; this study), we tentatively use Mimolagidae and provide a taxonomic diagnosis for the group in this study. In our taxonomic system, Mimotonida (Li et al., 1987) contains two families: the monotypic Mimotonidae that includesMimotonaand Mimolagidae (Szalay, 1985; Erbajeva, 1986) that consists ofMina,Gomphos,AnatolimysandMimolagus.

Amar aleatorfrom the Late Paleocene Zhigden Member of the Naran-Bulak Beds, Tsagan-Khushu, Mongolia was originally placed in Eurymylidae (Dashzeveg and Russell,1988), but this species, only two upper molars collected (M1-M2), bears some dental similarities withGomphosandMinain being relatively larger, unilaterally high crowned with less crested cusps. Therefore, there is the possibility thatAmarmay be placed in Mimolagidae, but this needs to be tested with better material with incisors preserved.

whenMimotonawas named, Li (1977) already recognized the similarities between it and lagomorphs, although at the time presence of I3 was not certain (see also Li and Ting, 1985). Additional material confirmed presence of two pairs of upper incisors inMimotona(Li and Ting, 1993), which further supported the view of “mimotonids” as a basal group of Lagomorpha. In our view,Mimotonahas relatively lophodonty molar morphology compared to other mimotonidans and has a grooved dI2, a derived feature shared with lagomorphs; it is more probable thatMimotonarepresents a primitive “morphotype” as the ancestor of lagomorphs, whereas genera of Mimolagidae were specialized and may represent a side branch that diverged in the Paleogene from the clade leading to lagomorphs.

General remarks Based on morphologies ofHeomysandMimotonaand other early Glires, the most common view now is that Glires consists of two sister clades: Simplicidentata and Duplicidentata. Simplicidentata is defined as all Glires sharing a more recent common ancestor with Rodentia than with Lagomorpha, whereas Duplicidentata is defined as all members of Glires sharing a more recent common ancestor with Lagomorpha than with Rodentia (Meng and wyss, 2005). Taxa with one pair of upper incisors, such asHeomys, have been considered as stem members of Rodentia and often placed in the group eurymylids. Those with two upper incisors, such asMimotona, are stem members of Lagomorpha and are commonly grouped as mimotonids. Both eurymylids and mimotonids are probably paraphyletic although some eurymylids may form a monophyletic group (Meng et al., 2003; Asher et al., 2005; Meng and wyss, 2005). Phylogenetic relationships based on different morphological and/or molecular data sets do not always re fl ect the traditional classi fi cations of Simplicidentata and Duplicidentata, because some eurymylids were placed as basal members at the clade containing lagomorphs (Meng et al., 2003; Asher et al., 2005). There is no case to our knowledge, however, that a mimotonid species was clustered with the clade containing rodents. Although inconsistencies exist in the phylogeny and taxonomy of Glires, the monophyly of Glires generally holds. Given the monophyly of Glires,HeomysandMimotonafrom Qianshan show that the diversi fi cation of the group was as early as the late Early Paleocene. Because Glires were commonly nested within Placentalia (e.g., O’Leary et al., 2013), their occurrence suggests that other related placental groups had also diversified by at least the Middle Paleocene. In addition,Heomys,MimotonaandMinafrom Qianshan show that the earliest members of the larger group had a sympatric distribution and displayed a similar stage of morphological specialization in general dental morphology. Their enamel microstructures display more diverse morphologies, as shown here and the related study (Mao et al., 2016). we expect that a thorough analysis involving new specimens ofHeomysandMimotonaand otherknown eurymylids and mimotonids will be conducted to further our knowledge on the early evolution and phylogeny of Glires.

Acknowledgments we are grateful to Yu Ben’ai and Li Dingsheng of the Qianshan County Museum, Anhui, Xie Shuhua and Zhou Xiaoyuan of IVPP for their assistance in the fi eld work. we appreciate Dr. Mary R. Dawson of the Carnegie Museum of Natural History, Pittsburgh, USA and Dr. Lawrence J. Flynn of the Harvard University, Cambridge, USA for their helpful review comments. Thanks also go to Gao wei for taking specimen photos. This work was supported by the Major Basic Research Projects of MST of China (No. 2012CB821900) and the National Natural Science Foundation of China (Nos. 41404022, 41572013, 41572021).

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安徽潜山中古新世一新的模鼠兔类

李传夔1王元青1张兆群1毛方园1孟 津2,1

(1 中国科学院古脊椎动物与古人类研究所，中国科学院脊椎动物演化与人类起源重点实验室 北京 100044)
(2 美国自然历史博物馆古生物部 纽约 10024)

命名了中古新世的一个啮型类新属种——胡氏敏兽(Mina huigen. et sp. nov.)。新属种的正型标本为可能属于同一个体的前颌骨和上颌骨，采自安徽省潜山县古井乡傅家山嘴中古新统望虎墩组上段上部。属名来源于周明镇先生的曾用名周敏，以纪念他对我国古新世生物地层和哺乳动物研究的杰出贡献；种名源自胡耀明博士，他为潜山哺乳动物的采集和研究做出过重要贡献。胡氏敏兽的特征如下：中等大小的模鼠兔类，上齿列齿式2·0·3·3;第一对门齿(dI2)增大，无齿根、终生生长，釉质层双层，仅限于唇侧，表面无纵向浅沟；上颊齿列外缘显著凸出；颧弓前根后缘位于M1-M2之间，眶下孔位置低。以传统的模鼠兔目(Mimotonida)而论，它包括两个科：单型科模鼠兔科(Mimotonidae)只有模鼠兔(Mimotona)一属；模兔科(Mimolagidae)则包括模兔(Mimolagus),Gomphos,Anatolimys, 敏兽和可能的Amar。在已知模鼠兔类中，模鼠兔很可能代表原始的兔形类祖先的“形态型”，而模兔科则代表向兔形类进化的分支上分化出来的旁枝。晓鼠(Heomys)、模鼠兔和敏兽的共存说明啮形类在中古新世时已经分化。

安徽潜山，古新世，原始啮型类，模鼠兔科，模兔科

Q915.873

A

1000-3118(2016)02-0121-16

2015-12-15

Li C K, wang Y Q, Zhang Z Q et al., 2016. A new mimotonidanMina hui(Mammalia, Glires) from the Middle Paleocene of Qianshan, Anhui, China. Vertebrata PalAsiatica, 54(2): 121-136

国家重点基础研究发展计划项目(编号：2012CB821900)和国家自然科学基金(批准号：41404022, 41572013, 41572021)资助。