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Ashgillian graptolite fauna of the Yangtze region and the biogeographical distribution of diversity in the latest Ordovician

¹ Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing, 210008, China.
² Geology Department, St. Francis Xavier University, PO Box 5000, Antigonish, Nova Scotia, B2G 2W5, Canada.
³ Department of Geology, Box 603050, SUNY at Buffalo, Buffalo, NY14260-3050, USA.

Abstract

Ashgillian graptolites have been described and recorded globally from 15 different paleoplates. The most diverse graptolite faunas are from the Yangtze region, South China, including 28 genera and 96 species. Among them, 25 genera and 73 species belong to the DDO fauna (Dicranograptidae – Diplograptidae – Orthograptidae fauna), and only three genera but 23 species belong to the N fauna (Normalograptidae fauna). Among the Yangtze graptolite fauna there are six endemic genera and 51 endemic species, which represent 21% and 52%, respectively, of the Ashgillian fauna in the region. This is an unusually high level of endemism. Endemic species are present in both the endemic and the cosmopolitan genera.

A late Ashgillian stepwise extinction event has recently been recognized, based on graptolite diversity changes and graphic correlation. This begins with a major extinction from the Diceratograptus mirus Subzone to the middle Normalograptus extraordinarius-N. ojsuensis Zone, and ends with a minor extinction prior to the end of the Normalograptus persculptus Zone. Graptolite faunal replacement of the DDO fauna by the N fauna occurred throughout this interval.

A comparison of Ashgillian graptolite diversity between the Yangtze region and other regions indicates that two different biogeographical realms existed in mid-Ashgillian time. A moderate-diversity graptolite fauna is present in the low-middle latitude realm, which includes South Scotland, Kazakhstan, Kolyma, Nevada, Yukon, Canadian Arctic, SE Australia. The Yangtze region was located in this realm, but was characterized by a very high-diversity fauna. Some other localities, including eastern Avalonia (Wales and England), the Argentina Precordillera, and Bohemia, which mainly represent the mid- to high-latitude realm, contain the lower-diversity mid-Ashgillian assemblages. This biogeographical distribution suggests a latitudinal diversity gradient, which may be controlled mainly by water temperature. This climate gradient becomes much less evident by late Hirnantian time in which most parts of the world have a relatively low diversity fauna totally dominated by normalograptid species, many of which appear to have been eurytopic. Throughout the Ashgillian, however, the Yangtze platform shows a high diversity and long persistence of DDO taxa the mass extinction interval. This may be a consequence of the semi-restricted nature of the basin in which conditions relatively favorable to graptolite survival and speciation existed throughout all or most of the Hirnantian.

Key Words: Ashgillian • Ordovician • Graptolites • Biogeography • Yangtze region • Gondwana • Extinction

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