Patterns, processes and likely causes of the Ordovician trilobite radiation in South China

Abstract

AbstractThe Ordovician genus‐diversity trajectories of trilobites and the radiation patterns previously proposed in South China are reviewed on the basis of an extensive and intensive study of 32 representative collections assigned respectively to the suecicus, hirundo and clavus graptolite‐zone intervals and made along shelf environment gradients from 15 re‐measured representative early‐Middle‐Ordovician sections. Analyses of new data indicate that components of the Whiterock Fauna (Paleozoic Evolutionary Fauna) spread across the whole of the environmental spectrum and in general increased steadily through the Ordovician. The major faunal innovations appeared first in the inner shelf during the Early Ordovician and the novelties then expanded seawards into the outer shelf. The Whiterock Fauna first exhibited a higher representation in shallow outer‐shelf faunas and made its early appearance in deep outer‐shelf habitats during the early Middle Ordovician. However, it was not until the clavus interval (late early Middle Ordovician) that the onset of the major Ordovician radiation or the radiation of the Whiterock Fauna as a whole commenced in deep outer‐shelf environments. From the Darriwilian onwards, members of the Whiterock Fauna had further developed and dominated all environments, but it was not until the Hirnantian that they comprised the whole fauna. The beginning of the radiation in South China was dominated by the expansion of reedocalymenine calymenids, trinucleids and cyclopygids. All of these three trilobite clades had their early diversification centred in high latitude zones of Gondwana. In terms of the macroevolution of trilobites, these specialized trilobites were only diagnostic of a regional Ordovician radiation. All of them represented a subset of the Paleozoic Evolutionary Fauna, which became extinct before or during the latest Ordovician mass extinction and bore no relationships to the Silurian Fauna. Therefore, the Whiterock Fauna of South China only played a minor role in the global macroevolution of the Paleozoic Evolutionary Fauna. As revealed by close investigations, the Ordovician radiation in South China may have been directly triggered by two major transgressive‐regressive cycles during the Tremadoc/Arenig and Lower/Middle Ordovician. The increase of oxygen levels in oceans during the early Middle Ordovician may have provided an essential condition for the radiating fauna to eventually inhabit the new ecological niches of the deep outer shelf. Copyright © 2007 John Wiley & Sons, Ltd.