Stratigraphic refinement unveils high-resolution shallow marine biotic dynamics immediately preceding the Late Ordovician Mass Extinction in South China

Exceptionally preserved Late Ordovician successions in South China offer a globally significant archive for investigating both the Great Ordovician Biodiversification Event (GOBE) and the Late Ordovician Mass Extinction (LOME). In contrast to the graptolitic facies that dominate much of the region, the late Katian carbonate deposits in the Jiangshan–Changshan–Yushan (JCY) area of East China (South China paleoplate), formerly referred to as the Sanqushan Formation (or its equivalents), are virtually the only strata in South China preserving diverse shallow marine biotas, providing rare ecological snapshots of the final biodiversity peak of the GOBE immediately preceding the LOME. Despite their importance, the lithostratigraphic framework and age constraints of these fossiliferous rocks remain debated. The prevailing view interprets the Xiazhen Formation as a nearshore equivalent of the Sanqushan Formation, with both units broadly assigned to a generalized late Katian age. Based on a critical review of integrated sedimentologic and paleontologic data, supplemented by new field observations, we support the interpretation that the Xiazhen Formation represents the upper portion of the ‘Sanqushan Formation’, and propose to elevate the latter to group rank to represent platform facies of this entire interval. The revised Sanqushan Group comprises, in ascending order, the Yaojiakeng, the Jitoushan, and the Xiazhen formations. The Yaojiakeng and the Jitoushan formations correlate more precisely with the Dicellograptus complexus Biozone, while the Xiazhen Formation aligns with the Paraorthograptus pacificus Biozone. This refined stratigraphic framework enables high-resolution reconstruction of biotic evolution prior to the LOME, offering new insights into regional ecological dynamics and their broader global significance.