Confirmation of a Middle Jurassic age for the Eedemt Formation in Dundgobi Province, southeast Mongolia: constraints from the discovery of new spinicaudatans (clam shrimps)

Abstract

Two spinicaudatan species, Triglypta eedemtensis Li sp. nov. and Dundgobiestheria mandalgobiensis Li gen. et sp. nov., are described on the basis of scanning electron microscopy (SEM) imaging of newly collected specimens from paper-thin laminated black shale of the Eedemt Formation exposed at the Eedemt locality in the Khootiin Khotgor coal mine region of Dundgobi Province in southeast Mongolia. Triglypta eedemtensis is ornamented mainly with puncta and a punctate fine reticulum; radial lirae occur only on two or three growth bands near the venter. The small spinicaudatan Triglypta is a common component of the Middle Jurassic Euestheria ziliujingensis fauna and Sinokontikia fauna, and is a typical taxon in the Middle Jurassic lacustrine sequences of northern Hebei and the Junggar and Turpan basins of the Xinjiang Autonomous District of China; however, it does not occur in stratigraphically higher units elsewhere. Therefore, the age of the Eedemt Formation should be considered Middle Jurassic rather than Early Cretaceous. The Eedemt Formation is much older than the Early Cretaceous Shinekhudag Formation in the Shine Khudag area of southeast Mongolia.

本文描述了两个针尾类种：Triglypta eedemtensis Li sp. nov. 和 Dundgobiestheria mandalgobiensis Li gen. et sp. nov.。该工作基于用扫描电子显微镜对出露于蒙古东南部Dundgobi省Khootiin Khotgor煤矿区的Eedemt组的纸层状黑色页岩中采集的标本进行的（SEM）成像。Triglypta eedemtensis 的主要装饰是疹点和点状细网;径向条脊只出现在靠近腹侧两或三个增长带上。小型针尾类Triglypta是中侏罗世Euestheria ziliujingensis动物群和Sinokontikia动物群的共同分子，是河北省北部和新疆地区准噶尔盆地和吐鲁番盆地的中侏罗世湖相序列的典型类群; 然而，它不会出现在其它地区地层序列上更高的地层单元中。因此，Eedemt组的时代应被视为中侏罗世，而不是早白垩世。Eedemt组比蒙古东南部Shine Khudag区的早白垩世地层Shinekhudag组老得多。

Key words:

• Jurassic spinicaudatans
• Conchostracans
• clam shrimps
• taxonomy
• biostratigraphy
• lacustrine deposits
• Dundgobi Province
• Mongolia.

Acknowledgements

This study was supported by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (B21403008, leader: H. Ando), the Major Basic Research Projects of the Ministry of Science and Technology, China (National 973 Project 2012CB822004), National Natural Science Foundation of China (41172010). We thank Dr. B. Delgertsogt (Mineral Resources Authority of Mongolia), Dr. R. Barsbold and Dr. Y. Khand (Paleontological Center, Mongolian Academy of Science) for their fruitful discussions and help in the field survey. Great thanks go to Prof. Hiromichi Hirano (Department of Earth Sciences, School of Education, Waseda University) for providing study room and necessary equipment, and to Prof. Yanbin Shen and Prof. Peiji Chen (Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences) for constructive communications. Thanks also go to Prof. O.F. Gallego (Universidad Nacional del Nordeste, Argentina) and Dr. A.L. Stigall (Ohio University, USA) for their critical reviews and constructive comments. Special thanks go to Dr. S. McLoughlin (Swedish Museum of Natural History, Sweden) for carefully smoothing the English text. The SEM micrographs were taken with the help of Mr. H. Miura and Mr. H. Koyasu, through the courtesy of the Hitachi S-2380 and 3400 facilities (Department of Earth Sciences, School of Education, Waseda University).