Diversification patterns of Early and Mid Ordovician graptolites in South China

During the Early and Mid Ordovician, graptoloids underwent a dramatic diversification. Many clades (e.g. anisograptids, dichograptids and biserials) originated and subsequently diversified during this interval, playing the most important roles in the evolution history of graptolites. In this study we conduct an analysis of the generic and species diversity history of Early to Mid Ordovician graptolites from the Yangtze Region (platform) and Jiangnan Region (slope) of South China, and provide evidence that the regionally distinct biodiversification patterns are related to the different representative environments of each region. Graptolite diversity measured across graptolite zones displays a remarkable increase in both regions during the Early to Mid Ordovician, but with distinct scales and patterns in each region. In the Upper Yangtze Region, the diversification is less rapid and can be divided into four phases. The second major diversification phase (early Arenig) is characterized by the rapid expansion of dichograptids, and the fourth diversification phase (mid-late Darriwilian), which is preceded by a decline (third phase), is typified by the flourishing of biserial graptolites. The diversification of graptolites in the Jiangnan Region is more dramatic and shows a distinct pattern of rapid, progressive and sustained diversity rise from the early Arenig to the early Darriwilian. This rapid diversification is initially driven by the expansion of dichograptids and subsequently enhanced by the significant addition of glossograptids and diplograptids. A comparison of diversity patterns between the Upper Yangtze Region, the Jiangnan Region, Australasia, Baltoscandia and Avalonia indicates: (1) the global Arenig expansion of dichograptids is also recorded in South China, (2) the diversity pattern of the Upper Yangtze Region is largely comparable to that of Baltoscandia and Avalonia, but is considerably different from that of Australasia, and (3) the sustained rapid increase of graptolite diversity in the Jiangnan Region is not shared by any of the other regions. Copyright (c) 2007 John Wiley & Sons, Ltd.