Turnover and ecological release drove biogeographic shifts during the Ordovician/Silurian transition

Abstract Whether mass extinctions consistently drive global homogenization or instead produce phase-dependent restructuring remains debated. Here, we evaluate the impact of the Late Ordovician mass extinction on global biogeographic structure and its relationship to biodiversity dynamics across the Ordovician/Silurian transition. Using 26,532 taxonomically vetted occurrences of 2474 genera, we quantify diversity partitioning and bipartite network topology using formation-based sampling standardization and equal-area geographic grid cells. We find that biogeographic homogenization peaks across the Hirnantian/Rhuddanian transition, when reduced spatial turnover and increased genus occupancy indicate the expansion of surviving taxa. This interval is characterized by declines in beta diversity driven by turnover loss, accompanied by a collapse in network modularity and path length, indicating the breakdown of provinciality. Geographic occupancy increased markedly after the extinction, driven by the persistence and range expansion of tolerant survivors. We interpret this pattern as crisis cosmopolitanism, whereby environmental disruption and the reduction of dispersal barriers facilitated transient global range expansion. Although taxonomic diversity rebounded in the Aeronian, biogeographic structure recovered more slowly, indicating a lag between taxonomic and spatial reorganization. The reestablishment of provinciality likely required the rebuilding of ecological barriers, such as increased competition and predation, during the recovery phase. Similarities with other mass extinction intervals suggest that biotic homogenization is a recurrent consequence of global crises.