Biodiversity patterns in the fossil record are often interpreted as functions of only origination and extinction whereas the migration of taxa among regions or paleocontinents is rarely considered. A null biogeographic model is presented that evaluates the role of migration in shaping global biodiversity patterns across evolutionary time scales. As taxa are allowed to originate, go extinct, and migrate among continents, the model keeps track of global richness and differentiation diversity (the diversity gained by pooling continents). The model's results highlight the difference between global-scale and continental-scale origination and extinction rates. Intuitively, origination and extinction have opposite effects on global richness at the global scale, but they interact with migration at the continental scale to influence differentiation diversity and global richness in surprising ways. The model shows that the migration of taxa among paleocontinents can facilitate an increase in global richness, even when continental extinction is greater than continental origination. Additionally, the model shows that differentiation diversity reaches a dynamic equilibrium that is dictated by the combination of migration, origination, and extinction rates. A test of the model with Ordovician macroinvertebrate data indicates that migration rates were low during the Ordovician and that differentiation diversity was high and varied little. Overall, the Ordovician was an interval of high provinciality. It also shown that widespread genera were less prone to global extinction, even though extinction of genera on individual paleocontinents was common.