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Geological Society, London, Memoirs; 1990; v. 12; p. 181-187;
DOI: 10.1144/GSL.MEM.1990.012.01.16
© 1990 Geological Society of London

Early Palaeozoic Biogeography

Brachiopod zoogeography across the Ordovician-Silurian extinction event

P. M. Sheeman & P. J. Coorough

Department of Geology, Milwaukee Public Museum, 800 West West Wells St., Milwaukee, WI 53233 USA

The Extinction event at the close of the Ordovician, one of the largest of the Phanerozoic, had a profound effect on the zoogeographic distribution of brachiopods. Strong endemism in the Ashgill was particularly apparent in the epicontinental seas covering the primary lithospheric plates. Areas in the open ocean and around the margins of the continents had more widely distributed faunas. Glacio-eustatic decline in sea level and climatic deterioration caused by the developing North African glaciation have been widely invoked as the cause of the extinction event. The Hirnantian Stage roughly corresponds to the glacial maximum, and Hirnantian faunas were somewhat more cosmopolitan than were early-middle Ashgill faunas. At the end of the Hirnatian, sea level rise and climatic amelioration accompanying the end of the glacial maximum apparently caused another wave of extinctions. Early Silurian brachiopods were far more cosmopolitan, especially in the epicontinental seas, than they had been in the Late Ordovician. There is a strong correlation between wide geographic distribution of genera and survival of the extinction event. Provincial patterns are consistent with the symposium maps.