BIOGEOGRAPHIC ASSOCIATIONS AMONG BRYOZOANS THROUGH THE ORDOVICIAN-SILURIAN TRANSITION

Area cladograms produced by parsimony analysis illustrate the historically developed biogeographical associations among Caradocian, Ashgillian, Llandoverian, and Wenlockian bryozoan faunas. Faunas in North America, Siberia, and Baltica were organized into three provinces and 12 biomes. 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 the Wenlockian platform faunas of North America. All four Silurian biomes are represented by Late Ordovician faunas, 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 replacement and mass extinction events took place during lowstands of sea level, suggesting that biogeographic reorganizations took place as a consequence of habitat shrinkage in epeiric seas. Biome development largely depended on the extent of major lithotopes and their intersections with climatic gradients and oceanic currents. The loss of major habitat types, 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 biomes from sister areas, whereas hypotheses of congruent dispersal are required to explain the appearance of biomes in geographically disjunct areas.