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Address : 1Chinese Academy of Sciences, China;2Guilin Institute of Technology, China;3Xi’an Institute of Geology and Mineral Resources, China
Celebration date : 21 Sunday September 2008
Author name : Zhou, Zhi Yi1; Yuan, Wen Wei1; Han, Nai Ren2; Zhou, Zhi Qiang3

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  • Article title : Trilobite faunas across the late Ordovician mass extinction event in the Yangtze Block
    Article type : Paleontology and Stratigraphy
    Location : International Geological Congress,oslo 2008

    Fulltext :

    Altogether 56 trilobite genera belonging to 28 families and 19 superfamilies have been recorded from the middle Ashgill (Ordovician)-Aeronian (Silurian) of the Yangtze Block. On the basis of representative collections made from measured sections, 11 trilobite associations are recognized along onshore-offshore gradients. According to the faunal composition, diversity and lithofacies exhibited in the region, the palaeogeographic distribution and bathymetric range of each association are inferred. The trilobite diversity was high in the mid Ashgill (interval of pre-extinction), but was suddenly decreased after the first phase of the mass extinction in the Hirnantian (interval of survival-recovery) and further declined to the early-mid Rhuddanian (interval of survival) minimum following the second phase of the mass extinction; this was followed by a limited diversity rise in the late Rhuddanian-early Aeronian (interval of recovery), and, then, an initial Silurian peak occurred during the mid-late Aeronian (interval of radiation). There were 22 trilobite families occurred in the middle Ashgill of the block. Among them, 12 are assigned to the Ibex Fauna and almost all became extinct prior to or during the first phase of the mass extinction. Other 10 families belong to the Whiterock Fauna, of which 8 survived the mass extinction and formed part of the Silurian fauna. The Hirnantian trilobite fauna includes 5 families, all of which are in the Whiterock Fauna. Elements of the fauna are composed of cold-water taxa, or immigrants from high latitude Gondwana, mixed with fortunate relicts. Most of them survived the second phase of the mass extinction into the early-mid Rhuddanian. It is, therefore, suggested that the major extinction that trilobites suffered was at the Rawtheyan-Hirnantian boundary in the Yangtze region, with Ibex Fauna components totally replaced by those of the Whiterock Fauna. Trilobites of different environments were all disrupted during the end Ordovician mass extinction. Outer-shelf dwellers and deep-water mesopelagic cyclopygids were severely affected and became totally extinct, but some shallow-water forms survived. Trilobites were rather rare in the early-middle Rhuddanian, and all are referred to a single near-shore association founded on a trilobite faunule from the Meitan-Tongzi area of northern Guizhou. The faunule consists mainly of the Hirnantian relicts, such as Dalmanitina, Dicranopeltis, Eoleonaspis and Niuchangella. Trilobite recovery from the mass extinction took place in the late Rhuddanian-early Aeronian, when an initial Silurian fauna formed in an oxygenated inner shelf area in northern Guizhou and northern Zhejiang, including some newly evolved endemic genera and new immigrants. The Silurian trilobite fauna further developed and became flourished for the first time in the middle-late Aeronian. Genera of the fauna occupied a variety of benthic niches and colonized the inner and shallow outer shelf in the Yangtze area.