Area cladograms produced by parsimony analysis of endemicity illustrate historically developed biogeographical associations among Caradocian, Ashgillian, Llandoverian, and Wenlockian bryozoans. Areas in North America, Siberia, and Baltica were organized into three provinces and 12 biomes over a time interval of 35 million years. Six of these biomes belonged to the North American-Siberian Province and became extinct during the Ashgillian. Three biomes represent a successional series of biogeographical associations in the Late Ordovician of Baltica, and the middle biome of this succession is most closely related to that of the Wenlockian platform in North America. All four Silurian biomes are represented in Late Ordovician local areas, indicating that the associations important in the recovery radiation were already in existence prior to the extinction events. Three of these four biomes expanded their geographic extent in the wake of the Late Ordovician extinctions. Several biome extinction and replacement events took place during lowstands of sea level, suggesting that biogeographic reorganizations took place as a consequence of habitat loss in epeiric seas. Biome development largely depended on the extent of major lithotopes and their intersections with deep ocean and climatic barriers. The loss of regional habitats, associated with marine regression, was a key factor in biome extinction and reorganization, and indicates that biogeography played a significant role in the Late Ordovician mass extinctions and Silurian recovery radiations. Vicariance hypotheses are needed to account for the development of barriers subdividing ancestral areas, whereas hypotheses of congruent dispersal are required to explain the appearance of biomes in geographically disjunct areas.
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