The Lujiatun bed of the Yixian Formation is famous for its extraordinary preservation of three-dimensional fossils and its implication for the most dramatic catastrophic mass mortality event in the Jehol Biota. The precise age of the fossil bearing deposits, however, remains to be established. 40Ar/39Ar step heating analyses on bulk K-feldspars from the fossil bearing tuff gave a weighted mean age of 123.2 ¿ 1.0 Ma (2σ). This date suggests the Lujiatun bed was most likely deposited at about the same time as the Jianshangou bed of the lower Yixian Formation, representing a stage when the dinosaurs displayed the most significant radiation in the Early Cretaceous.