The upper Pennsylvanian Naqing and Narao carbonate successions were deposited in intra‐platform slope‐to‐basinal settings across the Kasimovian–Gzhelian boundary in Guizhou, South China. Conodont faunas consist of a mixture of the endemic taxa of the Idiognathodus luodianensis group and cosmopolitan species of the I. simulator group. The I. luodianensis group includes the new species I. fengtingensis, I. luodianensis, I. naqingensis and I. naraoensis. On platform landmark analysis the species of the I. luodianensis group differ in morphological features from co‐occurring species of the I. simulator group. Both groups display similar increasing asymmetry in P1 element pairs across the Kasimovian–Gzhelian boundary, as recognized on the basis of the first occurrence of I. simulator. Many Kasimovian Idiognathodus species disappear and several Gzhelian species first appear with I. simulator, including two new species of Streptognathodus: S. nemyrovskae and S. zhihaoi. Just below the base of the Gzhelian, carbonate δ13C falls from 4‰ to 2‰ in both sections. The combination of an abrupt faunal turnover immediately above the prominent negative δ13C excursion might represent an oceanic event in South China, perhaps recognizable on a global scale. One of these two South China sections may be the best location to place the GSSP for the base of the Gzhelian Stage.