New chronostratigraphic constraints on the Yixian Formation with implications for the Jehol Biota

The fossils of the Cretaceous Jehol Biota exhibit excellent preservation and extraordinary diversity. Terrestrial fossils, including plants, insects, dinosaurs, birds, mammals and freshwater invertebrates have been discovered from the Dabeigou, Yixian and Jiufotang Formations (and correlative strata) in Central and East Asia. Significant specimens have been recovered from the Yixian Formation of Inner Mongolia, Hebei Province, western Liaoning Province of NE China. The Yixian Formation is generally divided (from bottom to top) into the Lujiatun Unit, Lower Lava Unit, Jianshangou Unit, Upper Lava Unit, Dawangzhangzi Unit and Jingangshan Unit. However, previously reported age data obtained through a range of methods and applied to different units of the Yixian Formation do not consistently integrate with the Yixian's commonly accepted stratigraphic interpretation. Here we summarize previous age data for the Yixian Formation and recalibrate previously reported 40Ar/39Ar ages by using new age interpretations for the same standard (i.e. FCs = 28.294). We also present new high-precision 40Ar/39Ar ages of 125.8 ± 1.0 Ma and 126.0 ± 0.8 Ma for two basaltic samples from the Lujiatun Unit, a unit of previously uncertain age. The new age data and re-calibrated published data indicate that the Lujiatun Unit was deposited contemporaneously with the Jianshangou Unit. The widely accepted stratigraphic column for the Yixian Formation therefore requires reinterpretation. Our study provides significant clues for reconstructing sedimentary environment for the fossil-rich Yixian deposits and allows to constrain evolutionary rates for early mammals, primitive birds and feathered dinosaurs.