app006332019

Discovery of a crocodyliform tooth from the upper Eocene Ergilin Dzo Formation, Mongolia

MASAYA IIJIMA, TAKEHISA TSUBAMOTO, KHISHIGJAV TSOGTBAATAR, TSOGTBAATAR CHINZORIG, and SOYOL BAASANKHUU

Although the distribution of Asian crocodyliforms was extended northwards during the Paleocene–Eocene greenhouse world, the fossil record in northern Asia becomes scarce towards the end of the Eocene. We here report the first crocodyliform (an isolated tooth) from the upper Eocene Ergilin Dzo Formation of southeastern Mongolia, the mammalian fauna of which defines the Ergilian Asian Land Mammal Age. The conical non-recurved crown, the near complete root with the central resorption facet, and its late Eocene age suggest the crocodyliform affinity of the tooth. The current finding represents one of the northernmost occurrences of crocodyliforms in the upper Eocene of Asia (paleolatitude ca. 49°30’ N), and demonstrates that SE Mongolia probably met thermal requirements of crocodyliforms during the late Eocene.

Introduction

The early Cenozoic, before the abrupt climate deterioration at ca. 34 Ma during the Eocene–Oligocene transition, was characterized by higher mean global temperature and reduced latitudinal temperature gradient (Zachos et al. 2001; Cramer et al. 2009), and crocodyliforms were extended into middle and even high latitudes over North America, Europe and Asia (Estes and Hutchison 1980; Markwick 1998, 2007; Mannion et al. 2015). In northern Asia, several crocodyliform taxa were reported from the late Paleocene through late Eocene (Mook 1940; Dong 1974; Efimov 1976, 1982, 1983, 1988, 1993; Snetkov 2011; Skuchas et al. 2015; Ni et al. 2016). However, the fossil record becomes poorer in younger formations, and only eastern Kazakhstan and northwestern China yielded late Eocene crocodyliforms (Dong 1974; Efimov 1988; Lucas et al. 2012; Stidham and Ni 2014). Here, we report an isolated crocodyliform tooth from the upper Eocene Ergilin Dzo Formation of southeastern Mongolia. Crocodyliformes is considered as a biological climate proxy (Markwick 1998, 2007; Mannion et al. 2015; Tennant et al. 2016), and its occurrence can help discuss the regional paleoclimate of Mongolia during the late Eocene.

Institutional abbreviations.—MPC, Institute of Paleontology and Geology (= formerly Mongolian Paleontological Center), Mongolian Academy of Sciences, Ulaanbaatar, Mongolia.

Fig. 1. Locality map of the Paleocene–Eocene crocodyliforms in northern Asia. 1, Ergilin Dzo, southeastern Mongolia (ca. 49°30’ N in the late Eocene); 2, Zaysan Basin across eastern Kazakhstan and northwestern China (ca. 48°18’ N, 49°54’ N, and 50°55’ N in the early, middle, and late Eocene, respectively; after Efimov 1982, 1988; Lucas et al. 2012; Stidham and Ni 2014; Skuchas et al. 2015); 3, upper reach of Manasi River, southern Junggar Basin, Xinjiang, China (ca. 48°18’ N in the late Eocene; Dong 1974); 4, near Erenhot, Inner Mongolia, China (ca. 50°06’ N in the middle Eocene; Mook 1940); 5, Mangyshlak Peninsula of western Kazakhstan (ca. 41°00’ N in the middle Eocene; Snetkov 2011); 6, Osh, Kyrgyzstan (paleolatitude incomputable; Efimov 1982); 7, northeastern Junggar Basin, Xinjiang, China (ca. 46°06’ N in the late Paleocene; Ni et al. 2016), 8, Tsagan Hushu, southern Mongolia (ca. 46°18’ N in the late Paleocene; Efimov 1976, 1983). Paleolatitudes of the crocodyliform localities in the late Paleocene (58 Ma) and the early, middle, and late Eocene (50, 45, and 35 Ma, respectively) were calculated using the paleolatitude calculator (van Hinsbergen et al. 2015).

Systematic palaeontology

Eusuchia Huxley, 1875

Crocodyliformes Hay, 1930

Crocodyliformes gen. et sp. indet.

Fig. 2.

Material.—MPC-MX 1/108, an isolated tooth. This specimen was collected by one of the co-authors, TC, during the paleontological expedition on 4th September, 2008 (Tsubamoto et al. 2010) from Ergilin Dzo (= Ardyn Obo) located in the southern part of Dornogobi Aimag (eastern Gobi Desert), southeastern Mongolia (43°19’40’’N, 109°07’42’’E, map datum: WGS84; Yanovskaya et al. 1977; Dashzeveg 1993; Tsubamoto and Tsogtbaatar 2008; Fig. 1: 1) in Ergilin Dzo Formation (= Ardyn Obo Formation by Berkey and Granger 1923; Ergilin Dzo svita/suite by Yanovskaya et al. 1977 and Dashzeveg 1993); late Eocene (Ducrocq 1993; Meng and McKenna 1998; Tsubamoto et al. 2004, 2008).

Measurements.—Crown height = 11.9 mm; mesiodistal crown width at the base = 9.3 mm; labiolingual crown width at the base = 8.2 mm.

Description.—The isolated tooth (Fig. 2) preserved with its root and representing a functional tooth. The blunt-tipped conical crown without apparent recurvature suggests it is a posterior maxillary or dentary tooth. The crown surface is badly worn and neither carinae nor striation is observed. The crown-root boundary is slightly waisted, and the root flares towards the base. The presence of the resorption facet in the root (Fig. 2A3) indicates the root resorption by the next generation. The root neck shows uniform thickness.

Remarks.—The presence of the resorption facet (Fig. 2A3) characterized by a uniformly thin neck, and its late Eocene age suggest the crocodyliform affinity of the tooth (Frey and Monninger 2010). The near complete root suggests the postmortem tooth loss, which is common in crocodyliforms and mammals, but rare in other tetrapods (Brand et al. 2003; LeBlanc and Reisz 2013). The non-recurved, conical crown excludes the possibility of a mammalian canine. The lack of distinctive features in the tooth precludes the diagnosis at a lower taxonomic level (i.e., Crocodylia). Because basal crocodyliforms (putative dyrosaurids) were reported from the Paleogene of China and Myanmar (Buffetaut 1978; Ni et al. 2016), the possibility of a non-crocodylian crocodyliform cannot be excluded.

Fig. 2. Crocodyliform gen. et sp. indet. (MPC-MX 1/108), a tooth from the upper Eocene Ergilin Dzo Formation of Mongolia; in mesial or distal view (A1, stereo pair), distal or mesial view (A2, stereo pair), diagonal root view showing the resorption facet of the root (A3), occlusal view (A4, stereo pair), labial or lingual view (A5, stereo pair), lingual or labial view (A6, stereo pair).

Discussion and conclusions

MPC-MX 1/108 is the first crocodyliform from the Ergilin Dzo Formation, and demonstrates that a crocodyliform inhabited in southeastern Mongolia during the late Eocene. The Ergilin Dzo Formation is interpreted as fluvial deposits (Saneyoshi et al. 2010) and is dated as the late Eocene on the basis of its fossil mammalian fauna (Ducrocq 1993; Meng and McKenna 1998; Tsubamoto et al. 2004, 2008). The formation yields many vertebrate fossils including amiiform and siluriform fishes, amphibians, chelonians, lacertilians, avians, and more than 80 species of mammals (e.g., Kurochkin 1976; Yanovskaya et al. 1977; Russell and Zhai 1987; Dashzeveg 1993; Tsubamoto et al. 2006, 2008, 2011a, b, 2012, 2013a, b; Tsubamoto and Tsogtbaatar 2008; Egi et al. 2009, 2016; Tsubamoto 2010; López-Torres and Fostowicz-Frelik 2018; and references therein). The mammals from this formation define the type fauna of the Ergilian Asian Land Mammal Age (Ergilian ALMA) (Russell and Zhai 1987; McKenna and Bell 1997; Meng and McKenna 1998). Despite such richness and importance of the vertebrate taxa and specimens in the Ergilin Dzo Formation, crocodyliforms have never been reported so far even in the comprehensive faunal lists of the formation (e.g., Yanovskaya et al. 1977; Devyatkin 1981). Efimov (1983, 1988) and Skuchas et al. (2015) reviewed and newly described many fossil crocodyliforms from Mongolia and the former Soviet Union; in these papers, no crocodyliform was reported from the Ergilin Dzo Formation (= Ergilin Dzo suite).

In northern Asia, several crocodyliform taxa were reported from the upper Paleocene through upper Eocene, although most of which need taxonomic revisions. In Central Asia, Pristichampsus birjukovi, Pristichampsus kuznetzovi, Asiatosuchus zajsanicus, and Tomistoma borisovi from the lower to upper Eocene of the Zaysan Basin, eastern Kazakhstan (Efimov 1982, 1988), and Ferganosuchus planus from the lower Eocene Alay beds of Osh, Kyrgyzstan (Efimov 1982; Fig. 1: 2, 6) were known. The Zaysan Basin of Kazakhstan (Aksyir Formation) also produced numerus crocodylian coprolites (Lucas et al. 2012). Additionally, the upper part of the Irtysh River Formation (late Eocene) in the Zaysan Basin across the border of China, Xinjiang, yields a crocodylian (Stidham and Ni 2014). In East Asia, Dzungarisuchus manacensis from the upper Eocene of Xinjiang, northwestern China (Dong 1974; Fig. 1: 3), and Asiatosuchus grangeri from the earlier middle Eocene Erdin Manha Formation of Inner Mongolia, northern China (Mook 1940; age based on Meng and McKenna 1998; Tsubamoto et al. 2004; Fig. 1: 4) were known. Many other fragmentary materials of indeterminate crocodyliforms were also reported from the upper Paleocene to upper Eocene of northern Asia (Efimov 1976, 1983; Skuchas et al. 2015; Ni et al. 2016; Fig. 1: 5, 7, 8). However, crocodyliform records become scarce towards the end of the Eocene in northern Asia, and only eastern Kazakhstan and northwestern China have yielded the late Eocene crocodyliforms (Dong 1974; Efimov 1988). To our knowledge, Ergilin Dzo is the third crocodyliform bearing locality in the upper Eocene of northern Asia (Fig. 1).

During the late Eocene (at 35 Ma), the Ergilin Dzo fossil locality (approx. 43°20’N at present) was likely located at ca. 49°30’N according to the paleolatitude calculator (van Hinsbergen et al. 2015). Together with the Zaysan Basin across Kazakhstan and China (located at ca. 50°55’N; Efimov 1988; Lucas et al. 2012; Stidham and Ni 2014) and southern Junggar Basin, Xinjiang, China (located at ca. 48°18’N; Dong 1974), Ergilin Dzo represents the northernmost locality for the late Eocene crocodyliforms in Asia (paleolatitudes according to van Hinsbergen et al. 2015). Crocodyliforms are ectothermic animals, and the distribution of living crocodylians suggests that temperatures (coldest month mean temperature of at least 5.5°C, mean annual temperature of at least 14.2°C, relatively lower mean annual range of temperature) as well as the presence of standing water are important factors for their survival (Markwick 1998). Therefore, southeastern Mongolia (northern part of East Asia; estimated paleolatitude ca. 49°30’N) most likely met the thermal requirements of crocodyliforms during the late Eocene (Ergilian ALMA); thereafter, crocodyliforms were increasingly constrained to lower latitudes around the world (Markwick 1998, 2007).

Acknowledgements.—We thank Christopher Brochu (University of Iowa, Iowa City, USA) for his valuable comment on the taxonomic identification of the tooth. Comments from the two journal referees, Stéphane Jouve (Sorbonne Université, Paris, France) and Jonathan Tennant (University of Southern Denmark, Odense, Denmark), significantly improved the manuscript. This research was partly supported by JSPS KAKENHI Grant Numbers 15K05330 and 18H01327 to Naoko Egi (Kyoto University, Inuyama, Japan).

References

Berkey, C. and Granger, W. 1923. Later sediments of the desert basins of Central Mongolia. American Museum Novitates 77: 1–16.

Brand, L.R., Hussey, M. and Taylor, J. 2003. Decay and disarticulation of small vertebrates in controlled experiments. Journal of Taphonomy 1: 69–95.

Buffetaut, E. 1978. A dyrosaurid (Crocodylia, Mesosuchia) from the Upper Eocene of Burma. Neues Jahrbuch für Geologie und Paläontologie, Monatshefte 1978 (5): 273–281. Crossref

Cramer, B.S., Toggweiler, J.R., Wright, J.D., Katz, M.E., and Miller, K.G. 2009. Ocean overturning since the late cretaceous: Inferences from a new benthic foraminiferal isotope compilation. Paleoceanography 24: PA4216. Crossref

Dashzeveg, D. 1993. Asynchronism of the main mammalian faunal events near the Eocene–Oligocene boundary. Tertiary Research 14 (4): 141–149.

Devyatkin, E. [Devâtkin, E.] 1981. The Cenozoic of Inner Asia (stratigraphy, geochronology and correlation) [in Russian]. Trudy Sovmestnoj Sovetsko-Mongol’skoj Naučno-Issledovatel’skoj Geologičeskoj Èkspedicji 27: 1–196.

Dong, Z. 1974. Fossil crocodylian from Xinjiang [in Chinese]. Vertebrata PalAsiatica 12: 187–188.

Ducrocq, S. 1993. Mammals and stratigraphy in Asia: is the Eocene–Oligocene boundary at the right place? Comptes Rendus de l’Académie des Sciences, Paris, Série II 316: 419–426.

Efimov, M. 1976. On first occurrence of Alligator (Crocodilia) in the Lower Paleogene of Mongolia [in Russian]. Trudy Sovmestnoj Sovetsko-Mongol’skoj Paleontologičeskoj Èkspedicji 3: 105–106.

Efimov, M. 1982. New fossil crocodiles from the USSR [in Russian]. Paleontologičeskij žurnal 1982 (2): 146–150.

Efimov, M. 1983. Review of fossil crocodiles from the USSR [in Russian]. Trudy Sovmestnoj Sovetsko-Mongol’skoj Paleontologičeskoj Èkspedicji 24: 76–96.

Efimov, M. 1988. The fossil crocodiles and champsosaurides of Mongolia and the USSR [in Russian]. Trudy Sovmestnoj Sovetsko-Mongol’skoj Paleontologičeskoj Èkspedicji 36: 1–108.

Efimov, M. 1993. The Eocene crocodiles of the GUS—a history of development. Kaupia 3: 23–25.

Egi, N., Tsubamoto, T., and Tsogtbaatar, K. 2009. New amphicyonid (Mammalia: Carnivora) from the Upper Eocene Ergilin Dzo Formation, Mongolia. Paleontological Research 13: 245–249. Crossref

Egi, N., Tsubamoto, T., Saneyoshi, M., Tsogtbaatar, K., Watabe, M., Mainbayar, B., Chinzorig, T., and Khatanbaatar, P. 2016. Taxonomic revisions on nimravids and small feliforms (Mammalia, Carnivora) from the Upper Eocene of Mongolia. Historical Biology 28: 105–119. Crossref

Estes, R. and Hutchison, J.H. 1980. Eocene lower vertebrates from Ellesmere Island, Canadian Arctic Archipelago. Palaeogeography, Palaeoclimatology, Palaeoecology 30: 325–347. Crossref

Frey, E. and Monninger, S. 2010. Lost in action—the isolated crocodilian teeth from Enspel and their interpretive value. Palaeobiodiversity and Palaeoenvironments 90: 65–81. Crossref

Hay, O.P. 1930. Second Bibliography and Catalogue of the Fossil Vertebra of North America. 916 pp. Carnegie Institute of Washington, Washington D.C.

van Hinsbergen, D.J., de Groot, L.V., van Schaik, S.J., Spakman, W., Bijl, P.K., Sluijs, A., Langereis, C.G., and Brinkhuis, H. 2015. A paleolatitude calculator for paleoclimate studies. PLoS ONE 10 (6): e0126946. Crossref

Huxley, T.H. 1875. On Stagonolepis robertsoni, and on the evolution of the Crocodilia. Quarterly Journal of the Geological Society 31: 423–438. Crossref

Kurochkin, E. 1976. A survey of the Paleogene birds of Asia. Smithsonian Contributions to Paleobiology 27: 75–86.

LeBlanc, A.R. and Reisz, R.R. 2013. Periodontal ligament, cementum, and alveolar bone in the oldest herbivorous tetrapods, and their evolutionary significance. PLoS ONE 8 (9): e74697. Crossref

López-Torres, S. and Fostowicz-Frelik, Ł. 2018. A new Eocene anagalid (Mammalia: Euarchontoglires) from Mongolia and its implications for the group’s phylogeny and dispersal. Scientific Reports 8: 13955. Crossref

Lucas, S.G., Spielmann, J.A., Hunt, A.P., and Emry, R.J. 2012. Crocodylian coprolites from the Eocene of the Zaysan basin, Kazakstan. Bulletin of New Mexico Museum of Natural History 57: 319–324.

Mannion, P.D., Benson, R.B.J., Carrano, M.T., Tennant, J.P., Judd, J., and Butler, R.J. 2015. Climate constrains the evolutionary history and biodiversity of crocodylians. Nature Communications 6: 8438. Crossref

Markwick, P.J. 1998. Fossil crocodilians as indicators of Late Cretaceous and Cenozoic climates: implications for using palaeontological data in reconstructing palaeoclimate. Palaeogeography, Palaeoclimatology, Palaeoecology 137: 205–271. Crossref

Markwick, P.J. 2007. The palaeogeographic and palaeoclimatic significance of climate proxies for data-model comparisons. In: M. Williams, A.M. Haywood, F.J. Gregory, and D.N. Schmidt (eds.), Deep-Time Perspectives on Climate Change: Marrying the Signal from Computer Models and Biological Proxies. The Micropalaeontological Society Special Publications, 251–312. The Geological Society, London. Crossref

McKenna, M. and Bell, S. 1997. Classification of Mammals Above the Species Level. xii + 631 pp. Columbia University Press, New York.

Meng, J. and McKenna, M. 1998. Faunal turnovers of Palaeogene mammals from the Mongolian Plateau. Nature 394: 364–367. Crossref

Mook, C.C. 1940. A new fossil crocodilian from Mongolia. American Museum Novitates 1097: 1–3.

Ni, X., Li, Q., Stidham, T.A., Li, L., Lu, X., and Meng, J. 2016. A late Paleocene probable metatherian (?deltatheroidan) survivor of the Cretaceous mass extinction. Scientific Reports 6: 38547.

Russell, D. and Zhai, R. 1987. The Paleogene of Asia: mammals and stratigraphy. Mémoires du Muséum National d’Histoire Naturelle, Série C, Sciences de la Terre 52: 1–488.

Saneyoshi, M., Tsubamoto, T., Watabe, M., and Tsogtbaatar, K. 2010. Lithostratigraphic and sedimentological analysis of the upper Eocene Ergilin Dzo Formation of Ergilin Dzo locality, Mongolia. Hayashibara Museum of Natural Sciences Research Bulletin 3: 149–153.

Skuchas, P. [Skučas, P.], Efimov, M., and Frisky, A. 2015. Suborder Crocodyliformes [in Russian]. In: E. Kuročkin, A. Lopatin, and N. Zelenkov (eds.), Iskopaemye pozvonočnye Rossii i sopredel’nyh stran (Čast’ 3, Iskopaemye, Reptilii i Pticy), 17–45. GEOS, Moskva.

Snetkov, P. 2011. Vertebrae of the sea snake Palaeophis nessovi Averianov (Acrochordoidea, Palaeophiidae) from the Eocene of Western Kazakhstan and phylogenetic analysis of the superfamily Acrochordoidea. Paleontological Journal 45: 305–313. Crossref

Stidham, T.A. and Ni, X. 2014. Large anseriform (Aves: Anatidae: Romainvilliinae?) fossils from the Late Eocene of Xinjiang, China. Vertebrata PalAsiatica 52: 98–111.

Tennant, J.P., Mannion, P.D., and Upchurch, P. 2016. Environmental drivers of crocodyliform extinction across the Jurassic/Cretaceous transition. Proceedings of the Royal Society B 283: 20152840. Crossref

Tsubamoto, T. 2010. Additional specimens of the Anthracotheriidae (Mammalia; Artiodactyla) from the upper Eocene Ergilin Dzo Formation of Mongolia. Hayashibara Museum of Natural Sciences Research Bulletin 3: 143–147.

Tsubamoto, T. and Tsogtbaatar, K. 2008. New specimens of anthracotheriid artiodactyls from the upper Eocene Ergilin Dzo Formation of Mongolia. Paleontological Research 12: 371–386. Crossref

Tsubamoto, T., Saneyoshi, M., Tsogtbaatar, K., Chinzorig, T., Khatanbaatar, P., Mainbayar, B., and Suzuki, S. 2010. Report of the HMNS-MPC Joint Paleontological Expedition in 2008. Hayashibara Museum of Natural Sciences Research Bulletin 3: 29–39.

Tsubamoto, T., Saneyoshi, M., Watabe, M., Tsogtbaatar, K., and Mainbayar, B. 2011a. The entelodontid artiodactyl fauna from the Eocene Ergilin Dzo Formation of Mongolia with comments on Brachyhyops and the Khoer Dzan locality. Paleontological Research 15 (4): 258–268. Crossref

Tsubamoto, T., Takai, M., and Egi, N. 2004. Quantitative analyses of biogeography and faunal evolution of middle to late Eocene mammals in East Asia. Journal of Vertebrate Paleontology 24: 657–667. Crossref

Tsubamoto, T., Tsogtbaatar, K., Chinzorig, T., Mainbayar, B., Egi, N., Saneyoshi, M., and Nishido, H. 2013a. Dental morphology of “Pterodon sp.” (Mammalia; Hyaenodontidae) described from the Eocene Ergilin Dzo Formation, Mongolia. The Bulletin of Research Institute of Natural Sciences, Okayama University of Science 39: 43–44.

Tsubamoto, T., Tsogtbaatar, K., Saneyoshi, M., Mainbayar, B., Watabe, M., Chinzorig, T., Khatanbaatar, P., and Nishido, H. 2013b. New specimens of Entelodon gobiensis (Mammalia; Artiodactyla; Entelodontidae) from the Eocene Ergilin Dzo Formation, Mongolia. The Bulletin of Research Institute of Natural Sciences, Okayama University of Science 39: 37–41.

Tsubamoto, T., Tsogtbaatar, K., Saneyoshi, M., Mainbayar, B., Watabe, M., Chinzorig, T., Suzuki, S., Khatanbaatar, P., Ishigaki, S., and Barsbold, R. 2011b. A new specimen of Gobiopithecus khan (Mammalia; Pantolesta) from the Eocene Ergilin Dzo Formation, Mongolia. Journal of Fossil Research (Japan) 44: 20–23.

Tsubamoto, T., Tsogtbaatar, K., Saneyoshi, M., Mainbayar, B., Watabe, M., Chinzorig, T., Suzuki, S., Khatanbaatar, P., Ishigaki, S., and Barsbold, R. 2012. Fossil evidence of a mesonychid mammal from the upper Eocene Ergilin Dzo Formation, Mongolia. Paleontological Research 16: 171–174. Crossref

Tsubamoto, T., Watabe, M., and Tsogtbaatar, K. 2008. Hyaenodon chunkhtensis and the hyaenodontid fauna from the upper Eocene Ergilin Dzo Formation of Mongolia. Journal of Vertebrate Paleontology 28: 559–564. Crossref

Tsubamoto, T., Watabe, M., Suzuki, S., and Tsogtbaatar, K. 2006. Fossil vertebrate assemblage from the upper Eocene Ergilin Dzo Formation of Mongolia [in Japanese]. Journal of the Geological Society of Japan 112 (3): v–vi. Crossref

Yanovskaya, N. [Ânovskaâ, N.], Kurochkin, E. [Kuročkin, E.], and Devjatkin, E. [Devâtkin, E.] 1977. Ergeleen-Dzo locality—the stratotype of lower Oligocene in South-East Mongolia [in Russian with English summary]. Trudy Sovmestnoj Sovetsko-Mongol’skoj Paleontologičeskoj Èkspedicji 4: 14–33.

Zachos, J., Pagani, M., Sloan, L., Thomas, E., and Billups, K. 2001. Trends, rhythms, and aberrations in global climate 65 Ma to Present. Science 292: 686–693. Crossref

Masaya Iijima [miijima8@gmail.com], School of Resources and Environmental Engineering, Hefei University of Technology, 193 Tunxi Road, Baohe, Hefei, Anhui 230009, China.

Takehisa Tsubamoto [tsubamoto@sci.ehime-u.ac.jp] (corresponding author), Graduate School of Science and Engineering, Ehime University, 2-5 Bunkyo-cho, Matsuyama, Ehime 790-8577, Japan.

Khishigjav Tsogtbaatar [tsogtmondin@gmail.com], Tsogtbaatar Chinzorig [chinzorig.tsogtbaatar@gmail.com], and Soyol Baasankhuu [baaska_970@gmail.com], Institute of Paleontology and Geology, Mongolian Academy of Sciences, P.O. Box 46/650, Chingeltei District-15160, Horoo-4, S. Danzan Street 3/1, Ulaanbaatar, Mongolia.

Received 6 May 2019, accepted 11 June 2019, available online 14 October 2019.

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