The Early–Middle Ordovician graptolite genus Azygograptus in South China: New material and paleogeographic implications

Abstract

This study presents a new analysis of three species of the Ordovician graptolite genus Azygograptus Nicholson & Lapworth from the Wenxiakou section in the Zhongxiang area of Hubei Province. The paleogeographic occurrences of the genus within South China were plotted on an up-to-date paleogeographic map based on a revised data set of graptolite occurrences from 68 localities compiled by the online Geobiodiversity Database (GBDB). The paleogeographic distribution of Azygograptus follows a nearshore-offshore trend, and the taxonomic diversity of Azygograptus is higher in nearshore high-energy environments than in offshore low-energy environments. However, graptolites also gathered in deeper-water environments surrounding oldlands partly due to the high-nutrient inputs and rapid deposition. Azygograptus is found not only in high-latitude cold-water regions as endemic taxa but also in low-latitude regions. Oceanic circulation may account for the rapid dispersal of Azygograptus from high-latitude regions, such as Britain and Scandinavia, to low-latitude regions, such as South China, eventually resulting in their observed paleogeographic distribution.