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Upper Darriwilian (Middle Ordovician) radiolarians and ostracods from the Hulo Formation, Zhejiang Province, South China

Yuhao Yi1, 2, Aihua Yuan*1, Jonathan C. Aitchison3 & Qinglai Feng1, 2
1. School of Earth Sciences, China University of Geosciences, Wuhan 430074, China;2. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China;3. School of Earth and Environmental Sciences, University of Queensland, Brisbane, Queensland 4072

Considerable research has been done on the Ordovician marine fossils from South China, including macrofossils such as brachiopods, graptolites, bivalves and trilobites and some microfossils like conodonts and acritarches. However, radiolarians and ostracods that are also important constituents of the Ordovician marine ecosystem have been paid little attention in this region. In this study, ten species radiolarians belonging to four genera and sixteen species ostracods grouped into nine genera were found from the Hulo Formation at the Hengdu Section of the Jiangshan District, western Zhejiang Province, South China. The fossil-bearing strata belong to the graptolite Pterograptus elegans Zone which indicates the late Darriwilian age. This radiolarian fauna is the first record of the Middle Ordovician radiolarian body fossils and also the earliest Ordovician radiolarian fauna reported from South China. The occurrence of Beothuka in this fauna extends the stratigraphic range of the genus to the upper Darriwilian. Review of the previous literatures suggests that the diversity of Beothuka was greater during the Early Ordovician, and then declined gradually from the Early Ordovician to the Middle Ordovician before its extinction. The co-occurring ostracod fauna belongs to a shallow-water ecotype. This is contrary to the host lithofacies of the ostracod fauna which represent a deep-water environment. Therefore, these ostacods may have been transported from the shallow-water environments, most likely from the Yangtze Carbonate Platform. If this conjecture is the truth, then the Jiangshan District was near the shallow-water carbonate platform and received its sediments during the Darriwilian. It is still essential to do more work in the future to help us understand better the ecology of the Ordovician ostracod fanuas and their role in the sedimentary system of South China.

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Keywords:radiolarians, ostracods, ecotype, Ordovician, South China

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