Bernissart Dinosaurs and Early Cretaceous Terrestrial Ecosystems

25. The Age of Lycoptera Beds (Jehol Biota) in Transbaikalia (Russia) and Correlation with Mongolia and China

453 25 The Age of Lycoptera Beds (Jehol Biota) in Transbaikalia (Russia) and Correlation with Mongolia and China Evgenia V. Bugdaeva* and Valentina S. Markevich Volcanogenic sedimentary deposits, which contain abundant fossils of lacustrine fauna and flora, are widespread in the territory of Transbaikalia (Russia), Mongolia, and northeastern China. These fossil ecosystems are known as the Jehol Biota. The Jehol Biota of the Yixian Formation in western Liaoning province (China) are particularly famous because of the discovery, from 1996, of feathered dinosaurs and of the earliest angiosperms. However, the age of the Yixian Formation is still the object of discussions. The possibilities of correlation of the fossil flora of Lycoptera beds of Transbaikalia with potential coeval floras of Yakutia, Mongolia, and northeastern China are considered. Comparison with the Transbaikalia flora allows us dating the beds with Jehol Biota, including the Yixian Formation, as Barremian to Aptian. The volcanogenic sedimentary deposits, which contain abundant remains of lacustrine ecosystems, are widespread in the territory of Transbaikalia (Russia), Mongolia, and northeastern China. In the nineteenth century, A. P. Gerassimov was the first to discover fossils from the Jehol Biota in socalled fish shales, outcropping in the basin of the Turga and Byrka rivers. Gerassimov transmitted his findings to A. Th. Middendorf, who accomplished a journey through Siberia between 1842 and 1845. It is possible to say that the history of paleontology of the region began from that time. The outcrop in the bank of the Turga River (Figs. 25.1 and 25.2) is often called Middendorf’s outcrop, although it was the less famous local, Gerassimov, who discovered and collected fossils. Also in Alexander Theodore Middendorf ’s honor are the name of the fish from these layers, Lycoptera middendorfii Müller (Fig. 25.3), and the representative of the conchostracans, Bairdestheria middendorfii Jones. The fossiliferous layers in this locality are dominated by diverse insects , the most abundant of which is Ephemeropsis trisetalis (Fig. 25.4). Subsequently, Reis (1909) published a description of new animal and plant taxa from the fish shales in the basins of the Turga and Vitim rivers from materials collected during Middendorf’s expedition. The author noted the scarcity and fragmentary nature of the plant remains. He came to the conclusion that these sediments were Late Jurassic to Early Cretaceous in age. This dating of fossil-bearing strata was the first in a chain of opinions of researchers. Stratigraphic correlation of the Upper Mesozoic deposits was hampered by complicated tectonics of Transbaikalia, facies diversity, and Introduction Bugdaeva and Markevich 454 high endemism of the biota. Curiously, the high diversity of animals and plants at numerous sites did not facilitate the age determination of the layers containing organic remains, but resulted in a debate among specialists about the dating and position of beds in the sequence. Paradoxically, some paleontologists who studied the flora or the same groups of faunas came to diametrically opposite conclusions. A more complete study of fossil plants of Transbaikalia was undertaken by Prynada (1950, 1962). He regarded the Turga Formation as the horizon of the same name and considered it as facies of coal-bearing formation completing the sequence of the continental Mesozoic in the isolated basins of Transbaikalia. The localities cropping out on the banks of the Turga and Vitim rivers, and near Shivia town (Fig. 25.1), were included by Prynada in the Turga facies. In his conclusion, “there are no plants in the Turga flora, which can be considered as guide fossils for a certain time,” so he considered the age of the coal-bearing formation to be the Late Jurassic or Early Cretaceous (Prynada, 1962, 15). In the assemblage of the Turga Formation, Prynada included fossil plants from Bukachacha, Holbon, Chernovskoe, Duroi, Kharanor, Tarbagatai, Tugnui, Gusino-Uda, and Bayangol coal mines, as well as from deposits outcropping in the basins of Alyangui, Bukukun, and Chikoi rivers. Martinson (1961) substantiated a different age for the above-mentioned localities. He thought that the Turga–Vitim Formation , set up by him, was Early Cretaceous in age. However, Kolesnikov 25.2. The Middendorf’s outcrop locality (Transbaikalia, Russia). Photo by R. Korostovsky. Lycoptera Beds in Transbaikalia and Correlation with Mongolia and China 455 (1964) recognized four correlative biostratigraphic horizons in Transbaikalia and considered that the Turga horizon was Late Jurassic in age. Perhaps his opinion had a major impact on the development during the 1950s and 1960s of the Mesozoic stratigraphy in northeastern China...