A continuous Ordovician-Silurian boundary section from the upper Wufeng Formation through the Kuanyinchiao Formation to the lower Lungmachi Formation has been carefully measured and collected at Shuanghe of Changning, southern Sichuan Province. For the first time, the temporal changes of α- and β-diversities of the Hirnantia fauna have been discussed in great detail. The general trend of brachiopod diversity change, increasing upward, is consistent with the regional trend of the Yangtze Platform, which had been controlled by both intrinsic and extrinsic factors. However, the sudden drop of diversity for a short period in the upper Kuanyinchiao Formation might have been controlled by environmental factors rather than normal faunal turnover. Synecological analysis using numerical methods recognizes two brachiopod-dominated associations of the Hirnantia fauna, the Dalmanella-Kinnella Association and the Mirorthis Association, both living in an offshore, deeper water environment corresponding to BA3-upper BA4, particularly lower BA3.
The Hirnantia Fauna is a globally-represented,cool water brachiopod fauna that originated and flourished in shallow marine environments at intermediate to low latitudes after the first episode of the end-Ordovician mass extinction.It was well-developed,widely distributed,long in stratigraphical range,and complex in palaeoecological differentiation on the Upper Yangtze Platform of the South China paleoplate,where it has been extensively studied.Qualitative and quantitative analyses show that the FAD of the Hirnantia Fauna in South China becomes stratigraphically higher from nearshore to offshore localities on the Yangtze Platform,while the range of the fauna becomes shorter whereas the taxonomic diversity increases.Over its stratigraphical range the diversity of the Hirnantia Fauna at nearshore localities consistently decreases,but trends in the opposite direction at offshore,deeper water localities.The fauna was represented by different communities,subcommunities or associations with changing environmental factors(such as water depth and substrate) at different localities or horizons.
