The Sihetun Vertebrate Assemblage consists of fossil fishes, amphibian s, reptiles, birds and mammals, featuring birds Confuciusornis and Liaonin gornis , and feathered theropod dinosaurs Sinosauropteryx, Protarchaeopteryx, Caudipt er yx and Beipiaosaurus. This assemblage was found from the lake shales interbedd ed with tuffs of the lower Yixian Formation, western Liaoning, China. Result of ra diometric dating of tuff samples, 124.6±0.3 Ma and 124.6±0.2 Ma, suggests the m iddle Barremian age (J—K boundary = 144 Ma) or the late Valanginian age (J—K b ou n dary = 136 Ma) of the Early Cretaceous for the Sihetun Vertebrate Assemblage. A lmost all vertebrate fossils are exceptionally wellpreserved. This type of pre se r vation reflects mass mortality and rapid burial of the animal bodies in the lacu strine environment, and indicates that the intermediate—acid volcanic eru ptions are m ain cause of the mass mortality events.