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Early Palaeozoic Biogeography |
1 Department of Geological Sciences, California State University, Long Beach, CA 90840, USA
2 Institute of Geology and Palaeontology, Academia Sinica, Nanjing, People's Republic of China
Graptolite biogeography is examined for the late Arenig-Llanvirn when provincialism was at its greatest and the Llandeilo-Caradoc when graptolites were cosmopolitan. This examination incorporates new data from China and recently revised palaeogeographic base maps. Water-mass specificity is considered to be the primary factor affecting graptolite biogeography. On the recently revised base maps, the distribution of Pacific and Atlantic provinces in the late Arenig-Llanvirn is in general consistent with the model of water-mass specificity and the hypothesis that climate cooling associated with the onset of continental glaciation lead to the development of provincialism. However, a small clockwise rotation of South America and a slight northward shift in the position of North America is recommended. The climatic-cooling hypothesis includes the proposal that continued cooling confined graptolites to the tropics where the uniform environment precluded provincialism. The distribution of the cosmopolitan fauna of the Nemagraptus gracilis Zone in relationship to the Llandeilo-Caradoc palaeogeography is consistent with this hypothesis, except for the positions of the European platform, Britain, Armorica, and the Carpathians in the mid to high southern latitudes. Either the latitudinal positions of these areas must be shifted substantially to the north, or the climatic-cooling hypothesis mnst be in error.
