Early Darriwilian graptolites from central and western China

Taxonomic restudy of Early Darriwilian graptolites from the Yangtze region, Chiadam and Alxa, has enabled a precise correlation of the Undulograptus austrodentatus Zone (including two subzones) across the three regions, and with the Darriwilian GSSP at Huangnitang in the Jiangshan-Changshan-Yushan (JYC) area. Based on specimens from 17 localities in the Yangtze region, the Exigraptus clavus and Undulograptus austrodentatus zones are recognised as replacements respectively of the Undulograptus sinodentatus (Lower) and U. austrodentatus (Upper) zones of Mu et al. (1979). The U. sinicus Subzone, the upper subzone of the U. austrodentatus Zone, is also recognised from the Sichuan-Guizhou area of the Yangtze region. Two subzones of the U. austrodentatus Zone have also been identified from the Chiadam region with more diverse graptolite fauna. The U. austrodentatus Zone fauna also occurs in the Alxa region. A general pattern of changing graptolite diversity from platform through basinal slope to deep basin is proposed, based on early Darriwilian bio- and lithofacies changes from the Yangtze Platform through the Jiangnan belt to the Zhujiang basin. The graptolite diversity increased with water depth and more off-shore setting. Key graptolite species are illustrated.