The Experts below are selected from a list of 69 Experts worldwide ranked by ideXlab platform
Xu Chen - One of the best experts on this subject based on the ideXlab platform.
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preliminary report on the late ordovician graptolite extinction in the yangtze region
Palaeogeography Palaeoclimatology Palaeoecology, 2007Co-Authors: Junxuan Fan, Xu ChenAbstract:Abstract The present work is mainly based on five continuous Ashgillian to earliest Llandovery sections together with data from more than 30 other published sections from South China. Rarefaction analysis indicates a sudden drop in diversity from the upper part of the Paraorthograptus pacificus zone to the Akidograptus ascensus zone. We combined the species range data from some of these sections into a single graptolite composite standard sequence (CSS) using graphic correlation technique. A temporal scale for graptolite zones in the Ordovician and Silurian was adopted and the CSS ranges were plotted against this temporal scale. The new CSS reveals that the mass extinction was gradual or stepwise and began with a major extinction spanned an interval from near the top of the Tangyagraptus typicus subzone to the middle of the Normalograptus extraordinarius zone. A secondary, more minor pulse of extinction (minor extinction) took place late in the interval of the upper Normalograptus persculptus zone. Using temporally scaled range data, species diversities can be calculated more precisely. According to the species-area relationship, we can calculate that reduction in the area of the Yangtze sea only led to less than 21.5% decrease in graptolite diversity. At least 41% decline of the species diversity was due to the destruction of the favorite habitat of graptolites, such as a sudden change from dysoxic to fully oxygenated waters, decreasing of water temperature, reduction of primary producers and interruption of food web.
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Ashgillian graptolite fauna of the yangtze region and the biogeographical distribution of diversity in the latest ordovician
Bulletin De La Societe Geologique De France, 2003Co-Authors: Xu Chen, Junxuan Fan, Michael J Melchin, Charles E MitchellAbstract: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.
Francesco Leone - One of the best experts on this subject based on the ideXlab platform.
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Replacements of benthic associations in a sequence stratigraphic framework, examples from Upper Ordovician of Sardinia and Lower Devonian of the Massif Armoricain
Palaeogeography Palaeoclimatology Palaeoecology, 2006Co-Authors: Arnaud Botquelen, Rémy Gourvennec, Alfredo Loi, Gian Luigi Pillola, Francesco LeoneAbstract:Sedimentological, palaeoecological and taphonomic analyses were carried out to establish the impact of relative sea-level variations on benthic faunas in a platform depositional context and to propose a reliable palaeoecological model for different palaeogeographic areas during different periods of the Palaeozoic. One Caradocian–Ashgillian section in Sardinia and one Lower Emsian section in the Massif Armoricain have been selected. The approach combines sequence stratigraphy, taphonomy and delineation of recurrent fossil associations. Several sedimentary facies have been recognized in relation to the distance from the shoreline. The succession consists of the stacking of genetic sequences related to sea-level changes linked to cyclic variations of orbital parameters. Several fossil associations, identified by means of multivariate analyses, are recurrent in similar facies. Eleven associations are recognized. These are the Longvillia, Nicolella, Paucicrura and Iberomena associations in the Upper Ordovician of Sardinia; the Adolfia, Stenorhynchia, Chonetid, Crinoid–Brachiopod, Ostracod–Tentaculitid, Bryozoan and Dacryoconarid associations in the Lower Devonian of the Massif Armoricain. The results of multivariate analyses based on the faunal contents are in good accordance with the clustering based on the sequence stratigraphic analysis. Benthic associations are closely related to relative sea-level variations. When the sea-level fluctuations were gradual and moderate, the association replacement is characterized by reorganization of the taxonomic composition, changes in diversity and trophic webs (association replacement by reorganization). When the sea-level changes rapidly, the association replacement is characterized by renewal of the ecological structure (association replacement by renewal). Palaeoecological changes are recognized to be strongly related to sea-level variations.
Junxuan Fan - One of the best experts on this subject based on the ideXlab platform.
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preliminary report on the late ordovician graptolite extinction in the yangtze region
Palaeogeography Palaeoclimatology Palaeoecology, 2007Co-Authors: Junxuan Fan, Xu ChenAbstract:Abstract The present work is mainly based on five continuous Ashgillian to earliest Llandovery sections together with data from more than 30 other published sections from South China. Rarefaction analysis indicates a sudden drop in diversity from the upper part of the Paraorthograptus pacificus zone to the Akidograptus ascensus zone. We combined the species range data from some of these sections into a single graptolite composite standard sequence (CSS) using graphic correlation technique. A temporal scale for graptolite zones in the Ordovician and Silurian was adopted and the CSS ranges were plotted against this temporal scale. The new CSS reveals that the mass extinction was gradual or stepwise and began with a major extinction spanned an interval from near the top of the Tangyagraptus typicus subzone to the middle of the Normalograptus extraordinarius zone. A secondary, more minor pulse of extinction (minor extinction) took place late in the interval of the upper Normalograptus persculptus zone. Using temporally scaled range data, species diversities can be calculated more precisely. According to the species-area relationship, we can calculate that reduction in the area of the Yangtze sea only led to less than 21.5% decrease in graptolite diversity. At least 41% decline of the species diversity was due to the destruction of the favorite habitat of graptolites, such as a sudden change from dysoxic to fully oxygenated waters, decreasing of water temperature, reduction of primary producers and interruption of food web.
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Ashgillian graptolite fauna of the yangtze region and the biogeographical distribution of diversity in the latest ordovician
Bulletin De La Societe Geologique De France, 2003Co-Authors: Xu Chen, Junxuan Fan, Michael J Melchin, Charles E MitchellAbstract: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.
Milada Vavrdova - One of the best experts on this subject based on the ideXlab platform.
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early ordovician provincialism in acritarch distribution
Review of Palaeobotany and Palynology, 1997Co-Authors: Milada VavrdovaAbstract:Abstract Palynological evidence on the distribution of fossil marine microplankton corroborates presumed provincialism during the late Early Ordovician. Closely similar acritarch assemblages and other palynomorphs, namely cryptospores, were recovered from coeval sedimentary sequences within a sedimentary belt rimming the northern margin of Gondwana. A barrier dividing the cool, high-latitude Perigondwanan region from the warm water Baltoscandinavia and Laurentia may be inferred from the spatial distribution pattern of acritarch paleocommunities. Provincialism of the organic-walled microfossils of earliest and latest Ordovician (Tremadocian, Ashgillian) age is less evident. The Coryphildium bohemicum acritarch bioprovince of Arenigian-Llanvirnian age, which extends from northwestern Argentina to the Yangtze Paraplatform, is here proposed.
Arnaud Botquelen - One of the best experts on this subject based on the ideXlab platform.
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Replacements of benthic associations in a sequence stratigraphic framework, examples from Upper Ordovician of Sardinia and Lower Devonian of the Massif Armoricain
Palaeogeography Palaeoclimatology Palaeoecology, 2006Co-Authors: Arnaud Botquelen, Rémy Gourvennec, Alfredo Loi, Gian Luigi Pillola, Francesco LeoneAbstract:Sedimentological, palaeoecological and taphonomic analyses were carried out to establish the impact of relative sea-level variations on benthic faunas in a platform depositional context and to propose a reliable palaeoecological model for different palaeogeographic areas during different periods of the Palaeozoic. One Caradocian–Ashgillian section in Sardinia and one Lower Emsian section in the Massif Armoricain have been selected. The approach combines sequence stratigraphy, taphonomy and delineation of recurrent fossil associations. Several sedimentary facies have been recognized in relation to the distance from the shoreline. The succession consists of the stacking of genetic sequences related to sea-level changes linked to cyclic variations of orbital parameters. Several fossil associations, identified by means of multivariate analyses, are recurrent in similar facies. Eleven associations are recognized. These are the Longvillia, Nicolella, Paucicrura and Iberomena associations in the Upper Ordovician of Sardinia; the Adolfia, Stenorhynchia, Chonetid, Crinoid–Brachiopod, Ostracod–Tentaculitid, Bryozoan and Dacryoconarid associations in the Lower Devonian of the Massif Armoricain. The results of multivariate analyses based on the faunal contents are in good accordance with the clustering based on the sequence stratigraphic analysis. Benthic associations are closely related to relative sea-level variations. When the sea-level fluctuations were gradual and moderate, the association replacement is characterized by reorganization of the taxonomic composition, changes in diversity and trophic webs (association replacement by reorganization). When the sea-level changes rapidly, the association replacement is characterized by renewal of the ecological structure (association replacement by renewal). Palaeoecological changes are recognized to be strongly related to sea-level variations.
