On the geological age of mammalian fossils from Shanmacheng, Gansu Province

ZHANG Zhao-Qun WANG Jian

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

On the geological age of mammalian fossils from Shanmacheng, Gansu Province

ZHANG Zhao-Qun WANG Jian

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

The mammalian fossils (MimolagusandAnagalopsis) from Shanmacheng in the Jiuxi Basin described by Bohlin in 1951 have long been enigmatic in systematics and controversial in the geological age. New survey in this area suggests the fossils were discovered from the Shanmacheng Member of the Huoshaogou Formation. Comparisons of the Shanmacheng fossils with new specimens ofMimolagusfrom the Irdin Mahan Formation, andAnagalefrom the Late Eocene Ulan Gochu Formation, Nei Mongol, suggest an Eocene age for the Shanmacheng fossils. Reinterpretation of the fossil horizon and the paleomagnetic data of Dai et al. (2005) indicates the polarity zones from Huoshaogou section can be correlated to GPTS Chrons 13-18. Therefore, the Huoshaogou Formation may cover late Middle Eocene to Late Eocene and Bohlin’s fossil horizon can roughly be correlated to Chron18n (～39-40 Ma) of late Middle Eocene. The late Middle Eocene age ofMimolagus rodensnarrows the time gap with its close relativeGomphosthat have been recorded from Early to Middle Eocene. The archaic group Anagalidae may have become extinct before Oligocene.

Shanmacheng, Jiayuguan, Gansu; Eocene;Mimolagus,Anagalopsis; geological age

1 Introduction

Bohlin (1951) described three mammalian fossil taxa, e.g.Mimolagusrodens,Anagalopsis kansuensisand an ungulate, from Shih-her-ma-ch’eng (Shanmacheng), Hui-hui-p’u area, western Kansu (Gansu), China collected during the Sino-Swedish expedition. However, due to the uniqueness of the taxa, the geological age of this locality was undetermined, and a tentative correlation to Early Tertiary was made. Later references of its geological age are controversial, with correlation either to Early Tertiary (Hu, 1962), Early Oligocene (Zhou et al., 1977), earlier than Oligocene (Meng et al., 2009), or to Late Eocene or earlier (Wang, 1997). Stratigraphic position of the fossil horizon is also controversial. It has been associated either to Baiyanghe Formation (Zhai and Cai, 1984) or Huoshaogou Formation (Liang et al., 1992). The Jiuxi Basinwhere the fossils were found is an important area for studying the uplift processes of Qinghai-Tibet Plateau and also for oil exploration (Dai et al., 2005; Zhu et al., 2006). Therefore, the geochronology of this fossil locality is important for both geology and evolution of mammals. To relocate the fossil locality, we made a short survey in the early April, 2015. In this paper, we will provide our relocation of the fossil horizon, and discuss the biochronology based on updated understanding of related taxa, biostratigraphy and magnetostratigraphic data.

2 Geographic and geological relocation of the fossil locality

Bohlin (1951) stated that the fossils were collected at Shih-her-ma-ch’eng, on the right bank of the Po-yang-ho (Baiyanghe), some hundred meters north of the main road. We extracted part of Bohlin’s illustration from 1940 (Bohlin, 1940: pl. I-1) and compared it with the Google satellite image (Fig. 1). The gulley Bohlin figured is now locally called Shanmacheng gulley after the name of Shanmacheng castle while the Hui-hui-p’u village is named as Xinminpu. Terrestrial sediments are well exposed along the right bank of the gulley, dipping towards south as Bohlin observed. Cretaceous sediments are well exposed in the lower reaches of the gulley. The overlying Paleogene sediments of Huoshaogou, Baiyanghe, and Shulehe formations are well developed. After about 80 years, the main road Bohlin mentioned is not visible, but may be near the castle relics. Tracing from the castle relics northwards, we found “the brick red sandstone or fi ne conglomerate laid down in very thick bed” on the right bank of the gulley. Referring to the photo (Bohlin, 1951: pl. VII-7), we relocated the fossil horizon and the possible locality (N39°55′31″, E97°46′44″) (Fig. 1). Stratigraphically, the brick red sandstone bed is in the lower part of Huoshaogou Formation, about 40 m above its lower boundary with Cretaceous sediments.

3 Systematics of Mimolagus and Anagalopsis

Mimolagushas long been an enigmatic taxon. Bohlin (1951) described in detail the morphology of the partial skull, named it asMimolagusrodens, suggesting both lagomorph and rodent characters, and listed it as a duplicidentate. However, he referred the postcranial bones from the same locality as rodent inc. sed. Bleefeld and McKenna (1985) revised these materials and consideredMimolagusas a genus of Lagomorpha. When studying the fossils from Qianshan, Anhui Province, Li (1977) named a new family Mimotonidae, includingMimotonaandMimolagus, showing their similarity and close relationship.Mimolaguswas also considered as a monotypic genus in the family Mimolagidae (Szalay, 1985; Erbajeva, 1986). Asher et al. (2005) placedMimolagusat the basal position of Lagomorpha in Glires, which was followed and reinforced by Wible (2007) and Fostowicz-Frelik et al. (2015).

Recently Fostowicz-Frelik et al. (2015) reported a new species,Mimolagusaurorae, from the lower beds of Irdin Mahan Formation, Erlian Basin, Nei Mongol. With only a few teeth and some postcranials, the new species show strong similarity with the type speciesM.rodensin many characters. The dI2 is narrow and lacks the anterior groove which is developedin Lagomorpha, upper cheek teeth are unilateral hypsodonty. The ankle bones are close in both morphology and size. The calcaneus size ofM.rodensfalls in the variation ofM.aurorae, though the astragalus is slightly smaller. Therefore, the two species are very close if not conspeci fi c.

Fig. 1 Geographic map of the fossil locality near Shanmacheng, west of Jiayuguan City, Gansu Province The Google satellite image shows the location of the fossil locality with the partial fi gure from Bohlin (1940) for comparison. The fi eld photo at the right bank of Shanmacheng gulley indicates exposure of“the brick red sandstone or fi ne conglomerate laid down in very thick bed”. The red star marks the fossil level of Bohlin (1951)

Anagalopsiswas also an enigmatic taxon for Bohlin. He cautiously listed it under Mammalia insertae sedis. In the description and comparison, Bohlin stated the similarities withAnagaleof Anagalidae, but refrained from referring it to Anagalidae. With more materials found, Zhou et al. (1977) comparedAnagalopsiswithLinnanialofoensisfrom the Paleocene of Nanxiong Basin and grouped them in Anagalidae, and further statedAnagalopsishas more derived characters such as larger size, more molarized P4/p4, and reduced M3 thanLinnaniaandAnagale. Hu (1993) confirmed the systematic position ofAnagalopsisusing cladistic analysis, which is nested in the clade of Anagalidae.

4 Geological age of Anagale

The type specimen (well preserved skull, jaws and postcranials) was discovered from the lower part of Ulan Gochu Formation, at Twin Oboes, Nei Mongol (Inner Mongolia) by Chinese assistant “Buckshot” during Central Asian expedition in 1928 (Simpson, 1931). Together withAnagale, some remains ofEmbolotheriumof Brontotheriidae were also found. Ulangochuian age was named by Romer (1966) after the fauna from Ulan Gochu Formation, as representing the Early Oligocene Asian Land Mammal Age. The Ulangochuian was correlated with Chadronian age of North America and Sannoisian age of Europe. Hence, Ulangochuian has been widely cited as Early Oligocene till 1997 Wang Banyue introduced the updated dating of the E/O boundary and the realigned age of Chadronian of North American Land Mammal Age, and revised the Ulangochuian as representing Late Eocene.

5 Magnetostratigraphic study in Jiuxi Basin

Huang et al. (1993) did magnetostratigraphical work in Jiudong and Jiuxi basins including a section in Huoshaogou (Fig. 1), where the Huoshaogou Formation was originally named. Referring the mammal fossils and pollen data from Huoshaogou Formation, they correlated the magnetozones 10-12 to Early Oligocene. The most recent work in the Huoshaogou section was carried out by Dai et al. (2005). Totally 11 normal and 11 reversal polarity zones were recognized in the Huoshaogou Formation. They cited the Shanmacheng fossils in the basal Baiyanghe Formation, as indication of no later than Early Oligocene. They further cited typical Oligocene mammalian data from a locality of Baiyanghe Formation a few hundred kilometers away. Hence, they correlated the polarity zones in the Huoshaogou Formation to Chrons 13-18n respectively.

6 Reinterpretation of the geological age of Mimolagus and Anagalopsis

By comparison with the EoceneAnagaleandMimolagusfrom Nei Mongol,MimolagusrodensandAnagalopsiskansuensisfrom Shanmacheng may also be Eocene in age. Our survey shows the locality should be in the lower part of Huoshaogou Formation, not in the lower part of Baiyanghe Formation. The brick red sandstone and conglomerate level in the Shanmacheng section can be correlated to the Shanmacheng Member of Huoshaogou Formation in the Huoshaogou gulley. With the updated understanding of the biostratigraphy, the polarity zones of Dai et al. (2005) showing dominantly normal polarity, can be correlated to GPTS Chrons 13-18. Therefore, Bohlin’s fossil horizon can roughly be correlated to Chron 18n (～39-40 Ma), late Middle Eocene in age (Fig. 2).

Fig. 2 Reinterpretation of the paleomagnetic data of Dai et al. (2005) with the fossil horizon marked

7 Conclusive remarks

With the updated data and new interpretation of the paleomagnetic data, the Huoshaogou Formation in the Jiuxi Basin may be referred to late Middle Eocene-Late Eocene in age. The late Middle Eocene age ofMimolagusrodensnarrows the time gap with its close relativeGomphosthat have been recorded from Early to Middle Eocene (Meng et al., 2004, 2009). The archaic group Anagalidae may have become extinct before Oligocene.

Acknowledgements Sincere thanks are due to Dr. Wang Weitao, Zhang Zhuqi, Zhang Ning from Institute of Geology, China Earthquake Administration for their collaboration in the field work. Discussions with Prof. Li Chuankui from IVPP and Dr. Fostowicz-Frelik from Institute of Paleobiology, Polish Academy of Sciences greatly improve the understanding of fossils concerned in the text. We also thank reviewers and Dr. Anu Kaakinen for constructive comments.

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甘肅騸馬城哺乳動物化石地質(zhì)年代的新認(rèn)識

張兆群 王 健

(中國科學(xué)院脊椎動物演化與人類起源重點(diǎn)實(shí)驗(yàn)室，中國科學(xué)院古脊椎動物與古人類研究所 北京 100044)

步林(Bohlin, 1951)描述的酒西盆地騸馬城(又譯為十二馬廠)化石(Mimolagus和Anagalopsis)在分類位置與地質(zhì)年代上長期存在爭議。步林認(rèn)為僅依據(jù)這些化石無法確定其年代，只是推測可能為第三紀(jì)的早期。后人根據(jù)Anagalopsis與內(nèi)蒙古發(fā)現(xiàn)的Anagale對比，認(rèn)為該化石點(diǎn)大致時代為早漸新世。化石出自哪個層位也有不同的認(rèn)識。有人認(rèn)為化石發(fā)現(xiàn)在白楊河組下部，也有人認(rèn)為化石層位屬于火燒溝組。鑒于其重要意義，我們于2015年4月赴酒西盆地進(jìn)行了調(diào)查。根據(jù)步林的地理位置描述與照片(Bohlin, 1940, 1951), 我們初步確定化石地點(diǎn)應(yīng)位于騸馬城溝內(nèi)，火燒溝組下部騸馬城段。對比內(nèi)蒙古二連盆地伊爾丁曼哈組新發(fā)現(xiàn)的Mimolagus以及晚始新世烏蘭戈楚組的Anagale, 認(rèn)為步林描述的化石時代也應(yīng)為始新世。根據(jù)對化石產(chǎn)出層位的重新認(rèn)識，并對比戴霜等(2005)在火燒溝剖面的磁性地層學(xué)資料，推測火燒溝組的時代為中始新世晚期至晚始新世，騸馬城化石層位的地質(zhì)年代大致為中始新世晚期。Mimolagus的生存年代與其近親Gomphos的差距大大減小，古老的Anagalidae也應(yīng)該在漸新世之前就已全部滅絕。

甘肅嘉峪關(guān)騸馬城，始新世,Mimolagus,Anagalopsis, 地質(zhì)年代

Q915.873, P534.613

A

1000-3118(2016)04-0351-07

2015-06-12

Zhang Z Q, Wang J, 2016. On the geological age of mammalian fossils from Shanmacheng, Gansu Province. Vertebrata PalAsiatica, 54(4): 351-357

國家自然科學(xué)基金(批準(zhǔn)號：41472003)和國家重點(diǎn)基礎(chǔ)研究發(fā)展計(jì)劃項(xiàng)目(編號：2012CB821904)資助。