Stromatoporoids of the upper Hirnantian (Upper Ordovician) Shiqian Formation of South China: implications for environmental interpretation and the Ordovician–Silurian stromatoporoid transition

The diversification of Palaeozoic stromatoporoid sponges was interrupted by the Late Ordovician glaciation, which caused the first of the five major extinctions during the Phanerozoic. Glaciation and the subsequent expansion of anoxia during deglaciation are interpreted to have resulted in a persistent scarcity of stromatoporoids during the Hirnantian. Previously, Hirnantian stromatoporoids were only known from two regions: Estonia in Baltica and Anticosti Island in eastern Laurentia. Here we report stromatoporoids from the upper Hirnantian Shiqian Formation of South China, adding to the rare fossil record of Hirnantian stromatoporoids. Three new stromatoporoid taxa are identified: Ecclimadictyon ancipitum Jeon sp. nov., Camptodictyon contortus Jeon & Kershaw sp. nov., and Cystostroma rallus Jeon sp. nov. The appearance of these stromatoporoids in South China is stratigraphically later than solitary rugose corals, but coincides with tabulate corals in the aftermath of the end-Ordovician mass extinction. Compared to the wider palaeogeographical and environmental distributions of rugose and tabulate corals during the Hirnantian, stromatoporoids presumably had a narrower environmental tolerance. The stromatoporoid assemblage in South China during the Hirnantian is dominated by clathrodictyids, which exhibit similar faunal patterns to those found in Baltica and Laurentia during the same period. The rise of clathrodictyid-dominated assemblages began in the middle to late Katian of the Late Ordovician, particularly in peri-Gondwanan regions, and rapidly expanded worldwide during the subsequent Hirnantian and Silurian onwards. The Late Ordovician glaciation significantly hastened the global Ordovician–Silurian stromatoporoid transition, as evidenced by the disappearance of numerous labechiidtype stromatoporoids and the expansion of clathrodictyid-type stromatoporoids in the Hirnantian and the subsequent Silurian.