Nei Mongol, China) and the Premolar Morphology of Anagalidan Mammals at a Crossroads
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diversity Article A Gliriform Tooth from the Eocene of the Erlian Basin (Nei Mongol, China) and the Premolar Morphology of Anagalidan Mammals at a Crossroads Łucja Fostowicz-Frelik 1,2,3,* , Qian Li 1,2 and Anwesha Saha 3 1 Key Laboratory of Vertebrate Evolution and Human Origins, Institute of Vertebrate Paleontology and Anthropology, Chinese Academy of Sciences, 142 Xizhimenwai Ave., Beijing 100044, China; [email protected] 2 CAS Center for Excellence in Life and Paleoenvironment, Beijing 100044, China 3 Institute of Paleobiology, Polish Academy of Sciences, Twarda 51/55, 00-818 Warsaw, Poland; [email protected] * Correspondence: [email protected]; Tel.: +48-22-6978-892 Received: 25 October 2020; Accepted: 3 November 2020; Published: 5 November 2020 Abstract: The middle Eocene in Nei Mongol (China) was an interval of profound faunal changes as regards the basal Glires and gliriform mammals in general. A major diversification of rodent lineages (ctenodactyloids) and more modern small-sized lagomorphs was accompanied by a decline of mimotonids (Gomphos and Mimolagus) and anagalids. The latter was an enigmatic group of basal Euarchontoglires endemic to China and Mongolia. Here, we describe the first anagalid tooth (a P4) from the Huheboerhe classic site in the Erlian Basin. The tooth, characterized by its unique morphology intermediate between mimotonids and anagalids is semihypsodont, has a single buccal root typical of mimotonids, a large paracone located anteriorly, and a nascent hypocone, characteristic of advanced anagalids. The new finding of neither an abundant nor speciose group suggests a greater diversity of anagalids in the Eocene of China. This discovery is important because it demonstrates the convergent adaptations in anagalids, possibly of ecological significance. Keywords: Euarchontoglires; Anagalidae; dental morphology; Eocene; Irdinmanhan; Nei Mongol 1. Introduction Glires (rodents and lagomorphs) is the most numerous clade of Euarchontoglires which, in turn, is one of the four main clades of placental mammals, recognized on the basis of molecular and morphological data—e.g., [1–3]. The “basal Glires” is an operational term for a paraphyletic group of primitive members of the clade, which do not display clear rodent or lagomorph synapomorphies. However, this group can be partitioned into simplicidentates (Eurymylidae) and duplicidentates (Mimotonidae), based on the number of incisors [4–6]. The basal Glires usually cluster in the phylogenetic analyses with Anagalidae [4,7], an enigmatic grouping of basal Euarchontoglires. The concept of “Anagalida” was proposed originally by Szalay and McKenna [8] as including Anagalidae, Pseudictopidae, Zalambdalestidae, and possibly, Didymoconidae; later emended by the addition of Macroscelida and Lagomorpha [9]. Macroscelida in the light of molecular data are considered Afrotheria [10]; thus, the Anagalida sensu McKenna [9] are now disproved. Meng and Wyss [11] defined the term “Gliriformes” as all taxa are phylogenetically closer to Glires than to any other Recent placental group—a notion endorsed, e.g., by Missiaen and Smith [12]. Whether Anagalidae themselves form a monophyletic group or are also a paraphyletic group is still a matter of debate [13]. Nevertheless, with respect to the cranial and especially dental morphology, Diversity 2020, 12, 420; doi:10.3390/d12110420 www.mdpi.com/journal/diversity Diversity 2020, 12, x FOR PEER REVIEW 2 of 8 Diversity 2020, 12, 420 2 of 8 morphology, they can be viewed as a morphological transitional zone in the morphospace between the basal primates and scandentians on one side and basal Glires on the other, but they are closer to theythe last can group be viewed [13]. as a morphological transitional zone in the morphospace between the basal primates and scandentiansAll representatives on one sideof Anagalidae and basal Gliresand almost on the all other, early but Glires they are closerknown to exclusively the last group from [13 the]. PaleogeneAll representatives of Asia (mainly of Anagalidaefrom China) and with almost the only all earlyexception Glires of are the known rodentiaform exclusively Alagomyidae from the Paleogenelineage which of Asia is (mainlyalso known from China)from North with the America only exception [14]. Most of the anagalid rodentiaform taxa are Alagomyidae restricted lineagespatio- whichtemporally is also to known the Paleocene from North of southern America and [14 central]. Most China anagalid [13,15] taxa. The are restrictedEocene representatives spatio-temporally were toconfined the Paleocene to northern of southern China andand centralMongolia China [13,16 [13–,1518]]. and The their Eocene findings representatives are scarce. were Thus, confined any new to northerndiscovery China of this and enigmatic Mongolia group [13,16 –is18 of] andconsiderable their findings interest are scarce.to our Thus,understanding any new discovery of evolution of this of enigmaticgliriform mammals. group is of considerable interest to our understanding of evolution of gliriform mammals. HereHere wewe presentpresent anan upperupper premolarpremolar ofof anan unknownunknown genusgenus andand speciesspecies ofof thethe anagalidanagalid branchbranch ofof thethe EuarchontogliresEuarchontoglires fromfrom thethe middlemiddle EoceneEocene (Irdinmanhan(Irdinmanhan AsianAsian LandLand MammalMammal Age)Age) HuheboerheHuheboerhe localitylocality in in the the Erlian Erlian Basin, Basin, Nei Nei Mongol, Mongol, China. China. The specimenThe specimen shows shows strong strong similarities simila bothrities to Eoceneboth to representativesEocene representatives of basal Gliresof basal and Glires some and more some evolved more Anagalidae.evolved Anagalidae. 2.2. MaterialsMaterials andand MethodsMethods TheThe isolatedisolated gliriformgliriform premolarpremolar (IVPP(IVPP V26960)V26960) isis housedhoused inin thethe collectioncollection ofof thethe InstituteInstitute ofof VertebrateVertebrate Paleontology Paleontology andand PaleoanthropologyPaleoanthropology ofof thetheChinese Chinese AcademyAcademy ofof Sciences,Sciences, Beijing,Beijing, ChinaChina (IVPP).(IVPP). TheThe toothtooth waswas examinedexamined withwith aa scanningscanning microscopemicroscope JSMJSM 61006100 (at(at thethe IVPP).IVPP). TerminologyTerminology ofof dentaldental structuresstructures followsfollows MengMeng andand WyssWyss [ 11[11]]. GeologicalGeological setting:setting: The The tooth tooth was was found found at at the the Huheboerhe Huheboerhe localit localityy (Figure (Figure 1)1, ),which which lies lies ca. ca. 37 37km km SW SW of ofErenhot Erenhot (Nei (Nei Mongol, Mongol, China), China), close close to to the the China/Mongolia China/Mongolia boundary [[19]19].. The sitesite isis locatedlocated atat thethe “Huhebolhe“Huhebolhe CliCliff”ff”( (sensusensuRussell Russell andand ZhaiZhai [[20]20])) andand isis alsoalso knownknown asas “Huhe“Huhe Bulak”Bulak” oror “locality“locality 7 mi7 mi west west and and southwest southwest of Camp of Camp Margetts”, Margetts” as marked, as duringmarked the during Central the Asiatic Cent Expeditionral Asiatic (CAE)Expedition led by (CAE) the American led by Museumthe American of Natural Museum History of (AMNH)Natural inHistory 1930 [19 (AMNH)]. Paleogeographically, in 1930 [19]. HuheboerhePaleogeographically, is located Huheboerhe in the Erlian is Basin located [21, 22in], the one Erlian of non-marine Basin [21,22] basins, one of of late non Mesozoic-marine originbasins inof northeasternlate Mesozoic China. origin Thein northeastern Huheboerhe China. section The includes Huheboerhe three sediment section includes series divided three sediment by upper series and lowerdivided hiatuses: by upper the and “upper lower beds”, hiatuses: consisting the “upper of a lower beds” part, consisting of the Irdin of a Manhalower part Formation of the Irdin (IM-1) Manha and underlyingFormation upper(IM-1) bedsand underlying of the Arshanto upper Formation beds of the (AS), Arshanto the “middle Formation beds” including(AS), the deposits“middle ofbeds” the Arshantoincluding Formation, deposits of and the the Arshanto “lower beds” Formation, consisting and of thethe upper“lower Nomogen beds” consisting Formation of sediments the upper of theNomogen Bumbanian Formation Asian Landsediments Mammal of the Age Bumbanian [23]. Asian Land Mammal Age [23]. FigureFigure 1.1. Anagalid-bearingAnagalid-bearing localitieslocalities inin NeiNei MongolMongol (gray(gray area),area), China.China. ((AA).). SchematicSchematic mapmap ofof ChinaChina withwith thethe approximate approximate location location of of the the Erlian Erlian Basin Basin marked marked by by red red circle. circle. (B ).(B Localities). Localities in thein the vicinity vicinity of Erenhot,of Erenhot, which which yielded yielded anagalid anagalid remains remains (yellow (yellow dots). (( dots).B), after ((B Fostowicz-Frelik), after Fostowicz et al.-Frelik [24], modified).et al. [24], modified). Diversity 2020, 12, 420 3 of 8 Diversity 2020, 12, x FOR PEER REVIEW 3 of 8 The fossil mammal fauna of Huheboerhe is rich; the lower beds yielded many Gomphos The fossil mammal fauna of Huheboerhe is rich; the lower beds yielded many Gomphos specimens [25], the Arshantan sediments include numerous remains of perissodactyls [19,23], and the specimens [25], the Arshantan sediments include numerous remains of perissodactyls [19,23], and Irdinmanhan deposits yielded, among others, artiodactyls, primates, cricetids, ischyromyids, many the Irdinmanhan deposits yielded, among others, artiodactyls, primates,