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Early Palaeozoic Biogeography |
Marine Sciences Group, 3 Earth Sciences Building, University of California Berkeley. CA 94720, USA
Two faunal regions, a cool-water, Atlantic and a tropical-water, Pacific, may be distinguished among Tremadoc into Ashgill planktic graptolite faunas. The zenith of graptolite provincialism was during the Arenig-Llanvirn when Laurentia, Australia-New Zealand, North China, South China, Siberia, Argentine Precordillera, and parts of Kazachkstan were provinces within the Pacific Region. Coeval, Southern Hemisphere, Atlantic Region provinces were: England-Wales, Baltoscania, Bolivia-Peru-Northern Argentina, western Europe, North Africa, and possibly part of modern Tien Shan. South China Arenig-Llanvirn faunas include many incursions of Atlantic Region taxa, probably reflective of current circulation changes linked to development of seasonal monsoons. Mid-Ordovician plate motion included northward movement of Baltoscania into the tropics. That motion resulted in Baltoscania becoming a province in the Pacific Region in the Late Ordovician. Late Ordovician glaciation led to restriction of graptolites to the tropics (Pacific Region) during the latest Ordovician. Deglaciation was followed by re-establishment of a tropical, Pacific Region fauna and a cool-water, Atlantic Region fauna during the Llandovery and Wenlock. Graptolite faunas appear to have been only tropical from the Ludlow to their extinction in the Pragian (Early Devonian). The graptolite regions and provinces are essentially consistent with plate positions suggested by palaeomagnetic and lithofacies data.
