Protoconodont fossils for refining the Cambrian bottom and the contribution to shale gas formation along the southwest margin of Yangtze Block oleh Jun-ping Liu, Si-cun Song, Wei Wang, Feng Tang, Jing Li, Xiang-dong Duan, Xiao-hu Wang, Bai-dong Sun, Sai-ying Yu, Shao-bin Hu, Wen-ting Duan