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 Sinosauropteryx, Protarchaeopteryx and Caudipteryx, the primitive birds Confuciusornis and Liaoningornis, the mammal Zhangheotherium and the reportedly oldest flowering plant, Archaefructus. 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 Callobatrachus and the pterosaur Eosipterus. This uniquely preserved assemblage of fossils is providing newinsight into long-lived controversies over bird–dinosaur relationships, the early diversification of birds and the origin and evolution of flowering plants. 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.
Swisher, Carl C., Wang, Yuan-qing, Wang, Xiao-lin, Xu, Xing, Wang, Yuan. 1999. Cretaceous age for the feathered dinosaurs of Liaoning, China. Nature. 400:58-61.