A New Study on the Stratotype and Biostratigraphy of the Yixian Stage in the Yixian-Beipiao Region, Liaoning --Establishment and Study of the Yixian StratotypeWANG Wuli, ZHANG Lijun, ZHENG Shaolin, ZHENG Yuejuan, ZHANG Hong, LI Zhitong and YANG Fanglin (Shenyang Institute of Geology and Mineral Resources, Shenyang, Liaoning 110032)