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Paleogeographic, paleoceanographic, and tectonic controls on early Late Ordovician graptolite diversity patterns

By
Daniel Goldman
Daniel Goldman
Department of Geology, University of Dayton, 300 College Park, Dayton, Ohio 45469, USA
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Wu Shuang-Ye
Wu Shuang-Ye
Department of Geology, University of Dayton, 300 College Park, Dayton, Ohio 45469, USA
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Published:
April 01, 2010

The Katian Age (early Late Ordovician) was a time of significant decline in marine biodiversity, but whether this decline was a real phenomenon or an artifact of the relatively few studies devoted to this interval requires further research. We examined the pattern of graptolite faunal changes across the boundary between the Climacograptus bicornis and Diplacanthograptus caudatus graptolite zones in North America and on several other continents. A sharp decline in species diversity occurs in the Appalachian Basin. Scores for normalized diversity dropped from 20 in the C. bicornis Zone to 7 in the D. caudatus Zone. Only 11% of the species present in the C. bicornis Zone carry over into the D. caudatus Zone. A similar pattern occurs in central Oklahoma. Regions at higher paleolatitude, such as Wales and Baltoscandia, exhibit low graptolite diversity in lower Katian strata, and then diversity declines further in higher strata. In other regions at low paleolatitude, such as Australasia and Scotland, however, diversity is fairly constant across this interval (although the percentage of carryover taxa remains low). We conclude that seawater temperature change or disruption of the oceanic density structure, which might accompany temperature change, provides explanations for the similarity between Laurentian and higher paleolatitude diversity patterns. Flooding of the Laurentian craton through the Sebree Trough by cool, subpolar Iapetus seawater may have adversely affected graptolite diversity there. Regions at high paleolatitudes likely underwent cooling associated with Katian climate deterioration. Thus seawater cooling, albeit driven by different mechanisms, may have produced similar diversity patterns at different paleolatitudes.

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Contents

GSA Special Papers

The Ordovician Earth System

Stanley C. Finney
Stanley C. Finney
Department of Geological Sciences, California State University at Long Beach, Long Beach, California, USA
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William B.N. Berry
William B.N. Berry
Department of Earth and Planetary Science University of California, Berkeley, Berkeley, California, USA
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Geological Society of America
Volume
466
ISBN print:
9780813724669
Publication date:
April 01, 2010

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