Cretaceous age for the feathered dinosaurs of Liaoning, China

The ancient lake beds of the lower part of the Yixian Formation, Liaoning P rovince, northeastern China, have yielded a wide range of well-preserved fo ssils: the 'feathered' dinosaurs Sinosauropteryx(1), Protarchaeopteryx and Caudipteryx(2), the primitive birds Confuciusornis(3) and Liaoningornis(4), the mammal Zhangheotherium(5) and the reportedly oldest flowering plant, A rchaefructus(6). Equally well preserved in the lake beds are a wide range o f fossil plants, insects, bivalves, conchostracans, ostracods, gastropods, fish, salamanders, turtles, lizards, the frog Callobatrachus(7) and the pte rosaur Eosipterus(1,8). This uniquely preserved assemblage of fossils is pr oviding new insight into long-lived controversies over bird-dinosaur relati onships(1,2), the early diversification of birds(3,9,10) and the origin and evolution of flowering plants(6). Despite the importance of this fossil as semblage, estimates of its geological age have varied widely from the Late Jurassic to the Early Cretaceous. Here we present the first Ar-40/Ar-39 dat es unambiguously associated with the main fossil horizons of the lower part of the Yixian Formation, and thus, for the first time, provide accurate ag e calibration of this important fauna. The results of this dating study ind icate that the lower Yixian fossil horizons are not Jurassic but rather are at least 20 Myr younger, placing them within middle Early Cretaceous time.