Mixed Warm-Water Cathaysian and Cool-Water Gondwanan Elements in an Asselian - Sakmarian Brachiopod Fauna from the Houziguan Limestone of South China

The systematic study of the brachiopod fauna from the Bianping section (Ziyun County, Guizhou, South China) in the Houziguan Limestone, of carbonate platform environment, reveals unusually high diversity, with 105 species belonging to 76 genera and seven orders: Productida, Orthotetida, Orthida, Rhynchonellida, Athyridida, Spiriferida and Terebratulida. In terms of species and specimen abundance, the fauna is dominated by productids and spiriferids. Stratigraphically, it corresponds to the Proanidanthus enaagardi-Buxtonia-Linoproductus Association recognised in the Maping Limestone of Guangxi, and its age is constrained as Asselian–Sakmarian by correlation with the fusulinid Pseudoschwagerina Zone. The base of the Sakmarian, which is commonly difficult to identify in South China, can be detected using fusulinids at the base of the Sphaeroschwagerina moelleri Zone. At this level, brachiopods record a marked diversity increase, with the first appearance of numerous productellids, orthids and spiriferids that are absent in the lower part of the section. From a palaeobiogeographical viewpoint, the fauna is dominated by typical warm-water Cathaysian elements (e.g., Enteletes, Liosotella, Linoproductus), as known from other South Chinese localities. However, minor but significant cold-water components are also present, including antitropical taxa such as Globiella, Spirelytha, Kochiproductus and Yakovlevia, as well as clear Gondwanan affinities evidenced by species of Callytharrellaand “Aequalicosta” bisnaini. The co-occurrence of warm-water Cathaysian and cool- to cold-water Gondwanan/antitropical brachiopods within the same assemblage is remarkable. These findings confirm previous hypotheses that cold upwelling currents may have intermittently influenced parts of South China during the Cisuralian, which was then located at palaeoequatorial latitudes. This would have allowed cold-water Gondwanan taxa to temporarily settle among the indigenous warm-water Cathaysian fauna. An alternative or complementary explanation is that the presence of these cold-water elements requires a re-evaluation of the palaeogeographical position of South China during the Early Permian, or at least a reconsideration of the thermal structure of the eastern Palaeotethys. The Bianping fauna thus represents a key dataset for understanding Permian biogeographical dynamics, trans-equatorial faunal migrations, and the possible far-field climatic influence of Gondwanan glaciation at low latitudes.