Palaeobiogeography of Middle Palaeozoic organic-walled phytoplankton

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Abstract
Potential advantages to the use of Palaeozoic organic-walled phytoplankton over other fossil groups are their presence in large numbers in small samples (particularly cores) and their partial independence from lithofacies. Reasons why this potential has not been fully realized are discussed, and the history of study is briefly reviewed. The 'latitude parallel' distribution model proposed for Silurian phytoplankton does not hold up when plotted on a more recent plate reconstruction, although some important insights are forthcoming. Ordovician microfloras appear broadly consistent with proposed plate models, and the diachronous distribution of the genusFrankeacan be explained by combining the plate model with a restricted, high southern latitude distribution. Ten Frasnian microfloras are compared in detail using G. G. Simpson's Index and cluster analysis. Microfloras from Brazil, Ghana and Algeria cluster together, distinct from outlying areas, while the Carnarvon Basin microflora in Australia is considered relatively endemic. The Frasnian phytoplankton data support a plate reconstruction in which a large seaway separated Africa and South America from North America and Europe.