Graptolite diversification during the Floian and Dapingian (Early-Middle Ordovician): A case study from the Ningkuo Formation of Hunan Province, China

A detailed study of the graptolite fauna from the Lower to Middle Ordovician Ningkuo Formation in the Nanba section, Yiyang, Hunan Province, reveals variable but generally increasing diversity during the Floian-Dapingian, peaking in the Corymbograptus deflexus Biozone. The diversification trend of graptolites in the Nanba section is broadly consistent with that in the Jiangnan region of South China and Avalonia, but is delayed compared with that in the Yangtze region of South China, Australasia, and Baltica. Furthermore, the first peak in diversity of graptolites in the Nanba section slightly post-dates that of brachiopods and conodonts, but is earlier than that for trilobites and acritarchs. Comparison of diversity trends with the sea-level curve for the Yangtze Platform reveals that the Early and Middle Ordovician graptolite radiation coincided with a transgression. Graptolite diversity trends through this interval initially reflect replacement of Anisograptid fauna by Dichograptid fauna and subsequent commencement of evolution within the Dichograptid fauna.