The 40Ar/39Ar step heating analyses of K-feldspar derived from the ignimbrite in Inner Mongolia, China indicates that they were formed at 159.8 ¿ 0.8 Ma, which provides a maximum age for the overlying fossil-bearing lacustrine deposits (Daohugou Bed). This result favors a post-Middle Jurassic (Late Jurassic or younger), rather than the Middle Jurassic age for the Daohugou Bed. Such a result is generally consistent with vertebrate biostratigraphic evidence, providing a maximum known age for the first appearance of several major animal groups such as Cryptobranchidae of Urodela (salamanders) and Maniraptora (birds and their closest dinosaurian relatives).