The stratigraphy and paleoenvironment of a ‘Lycoptera Bed’ site in eastern Inner Mongolia, China: Correlation with the fossiliferous Lower Cretaceous strata in western Liaoning

Abstract

The stratigraphy and the fossil assemblage of a Lower Cretaceous ‘Lycoptera Bed’ of the Xinganling Group in Naketa, Arongqi area, Inner Mongolia, China are described, and the paleoenvironment is discussed as it relates to the Jehol Biota. The Naketa fossil bed belongs to the lower part of volcanic-sedimentary Jiufengshan Formation and preserves a typical EEL (Eosestheria-Ephemeropsis trisetalis-Lycoptera davidi) faunal assemblage, which indicates a strong affiliation to the Jehol Biota of western Liaoning, China. The fossil bed was deposited in a shallow to semi-deep lacustrine setting with occurrence of oil-shale facies, and this is different from the Lycoptera Bed of the Yixian Formation and more similar to the overlying Jiufotang Formation in western Liaoning. A layer of volcanic tuffite within the fossil bed is dated as 121.6 + 0.47 Ma using the Zircon U-Pb isotopic dating method. This age is contemporary with the age of the base of Jehol-Biota producing Jiufotang Formation in western Liaoning, thereby supporting the biostratigraphic correlation. However, the genus Lycoptera is almost entirely absent in western Liaoning, with only one species in the Jiufotang Formation, where the dominant fish genus is Jinanichthys. Therefore, Lycoptera in the Great Xing’an Mountain areas appears to have a longer record than in western Liaoning, which likely indicates that the fish fauna changed earlier in western Liaoning, and this observation supports the hypothesis that the Yanliao area was the diversification center of some Cretaceous new groups in NE Asia. Preliminary analysis of the taphonomy and the geology indicates that the mass mortality of fish at the Naketa site may not be related to volcanism. Additional taphonomic investigations would allow a better understanding of the Early Cretaceous aquatic paleoecology and paleoenvironment in the volcanic-rifting regions. The Great Xing’an Mountain area is a new opportunity for expanding the research on the Jehol Biota beyond the core localities in western Liaoning.