Three Evolutionary Stages of the Jehol Biota in Transbaikalia

The Lower Cretaceous non-marine deposits in Transbaikalia are known to be a stratigraphic analogue of the North-East Chinese strata containing the famous Jehol Biota. The age of the Upper Mesozoic non-marine deposits of Transbaikalia is still a subject of debate due to the contradictory stratigraphic conclusions obtained from different groups of fauna and flora, as well as due to the lack of radiometric dating. The new U-Pb dates (LA-ICP-MS) obtained by the authors, together with a re-examination of the fossil biota, made it possible to reliably establish all three phases (JBS I – III) of the Jehol biota evolution in Transbaikalia, as well as to correlate the Lower Cretaceous deposits of Transbaikalia and northeastern China. The Ust–Karsk Formation is developed in the Ust-Karsk depression in Eastern Transbaikalia. Its fossil assemblage includes spinicaudatans Nestoria and Defretinia, and mayfly larvae Proameletus. Nestoria is one of the key fossils of the JBS I. U-Pb dates obtained from two tuff layers indicate the Valanginian/Hauterivian boundary age ─ 132.58±0.8 and 132.47±0.75 Ma. Therefore, the Ust-Karsk Formation correlates with the Valanginian-Hauterivian Dabeigou and Huajiying formations characterized by a fossil assemblage of JBS I and distributed in Northern Hebei.The Turga Formation is developed at the Southeastern Transbaikalia. It contains an abundant fossil assemblage consisting of fishes Lycoptera and rare peipiaosteids, spinicaudatans Eosestheria, and mayfly larvae Ephemeropsis. This assemblage represents JBS II – JBS III, which can be distinguished only based on vertebrates and occurring of spinicaudatan Yanjiestheriain JBS III. New U-Pb dating from a bentonite interlayer in the stratotype of the Turga Formation showed the early Barremian age of 124.23±0.53 Ma. This suggests that the Turga Formation correlates with the Yixian Formation of Western Liaoning (JBS II). The Semen Locality, represented by the outcrop of the upper part of the Doronino Formation, is located in Central Transbaikalia. This site contains the most diverse fossil assemblages among Transbaikalian sites. It includes Lycoptera, peipiaosteid, and polyodontiid fossil fishes, with teleost Irenichthys being the most common. Among invertebrates, spinicaudatans Eosestheria and diverse insects, including mayfly Ephemeropsis, are found. Based on the abundance of fishes Irenichthys with rare Lycoptera, we can correlate the Doronino Formation with the Lower part of the Jiufotang Formation, where Lycoptera are replaced by most advanced teleosts. The new δ13Corg curve obtained from the Semen section demonstrates a trend that we interpret as part of the C2 and the beginning of the C3 segments of the δ13C upper Barremian-Aptian composite curve. Thus, the upper part of the Doronino Formation outcropped in the Semen section correlates with the lower part of the Jiufotang Formation characterized by the fossil assemblage of JBS III. The presence of the JBS I representatives in Eastern Transbaikalia allows us to consider this region as a center of Jehol Biota origin along with Northern Hebei. The fact that Transbaikalian Cretaceous deposits contain all three stages of Jehol Biota suggests that they have great potential for discovering more exceptionally preserved ecosystems. This is a contribution to the Russian Science Foundation project № 25-77-10059, https://rscf.ru/project/25-77-10059/.