Conodonts from the Cambrian–Ordovician boundary interval in the southeast margin of the Sichuan Basin, China

Conodonts from the Cambrian–Ordovician transition at the Liangcun section in Xishui County, Guizhou and at the Huangcao section in Wulong County, Chongqing are examined for the first time. Both sections are located at the southeast margin of the Sichuan Basin. A total of 1367 specimens were recovered, representing 30 species and 15 genera. Based on the ranges of conodonts generalized from these two sections and another six sections previously studied in the same region, three conodont zones, Cordylodus proavus, Monocostodus sevierensis and Cordylodus angulatus zones are recognized. The index species of the Cambrian–Ordovician boundary at the global stratotype section and point (GSSP), Iapetognathus fluctivagus and its substitute in China Iapetognathus jilinensis are not observed in the study sections, therefore it is impossible to determine the Cambrian–Ordovician boundary exactly. However, it probably lies within the lower part of M. sevierensis zone (the upper part of the Loushanguan Group), correlating with the GSSP in Canada and the Dayangcha section in China. Chronological sequences of the FAD (First Appearance Datum) of C. angulatus, Chosonodina herfurthi and Rossodus manitouensis are not obvious in the study, so the C. angulatus zone here is correlated with zones defined by C. angulatus, Ch. herfurthi and R. manitouensis in the lower Yangtze Platform.

► Conodont from the C–O boundary in the Sichuan Basin is examined for the first time.
► Cordylodus proavus, Monocostodus sevierensis and Cordylodus angulatus zones are recognized.
► The C–O boundary in study area probably lies in the lower M. sevierensis zone.