The ancient lake beds of the lower part of the Yixian Formation, Liaoning Province, northeastern China, have yielded a wide rangeof well-preserved fossils: the ‘feathered’ dinosaurs Sinosauropteryx1, Protarchaeopteryx and Caudipteryx2, the primitive birds Confuciusornis3 and Liaoningornis4, the mammal Zhangheotherium5 and the reportedly oldest flowering plant, Archaefructus6. Equally well preserved in the lake beds are a wide range of fossil plants, insects, bivalves, conchostracans, ostracods, gastropods, fish, salamanders, turtles, lizards, the frog Callobatrachus7 and the pterosaur Eosipterus1,8. This uniquely preserved assemblage of fossils is providing newinsight into long-lived controversies over bird–dinosaur relationships1,2, the early diversification of birds3,9,10 and the origin and evolution of flowering plants6. Despite the importance of this fossil assemblage, estimates of its geological age have varied widely from the Late Jurassic to the Early Cretaceous. Here we present the first 40Ar/39Ar dates unambiguously associated with the main fossil horizons of the lower part of the Yixian Formation, and thus, for the first time, provide accurate age calibration of this important fauna. The results of this dating study indicate that the lower Yixian fossil horizons are not Jurassic but rather are at least 20 Myr younger, placing them within middle Early Cretaceous time.
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We thank all the members of the IVPP field crew, and M. M. Chang, Z. Luo and S. Kelley for discussion and comment on this study. This research was funded, in part, by the Chinese Academy of Sciences and the National Science Foundation, Earth Science Program.
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