Research on Conodont Biostratigraphy near the Bottom Boundary of the Middle Triassic Qingyan Stage in Southern Guizhou ProvinceYao Jianxin, Ji Zhansheng, Wang Liting , Wang Yanbin andWu Guichun(1. Institute of Geology, Chinese Academy of GeologicalSciences, Beijing, 100037; 2. Bureau of Geological and MineralResources Survey of Guizhou Province, Guiyang, Guizhou550004)