THE BIOGEOGRAPHY OF ‘SLUGGISH’ EVOLUTION: THE IMPACT OF GEOGRAPHIC RANGE SIZE ON EXTINCTION SELECTIVITY IN PENNSYLVANIAN BRACHIOPODS OF THE NORTH AMERICAN MIDCONTINENT

Wide geographic range has long been shown to buffer species against extinction. This relationship is borne out during times of both background and mass extinctions, but remains unexamined for sluggish periods of evolution when extinction rates are extremely low. Consequently, we tested the association between geographic range size and species’ survivorship during the Pennsylvanian to early Permian of the North American Midcontinent, a time and region characterized by depressed turnover rates and cyclical, though broadly stable, climatic conditions. Analyses were conducted using high-resolution spatiotemporal data for 141 brachiopod species, encompassing over 2400 spatially unique occurrences. Data were gathered from the University of Kansas and Yale Peabody Museum collections, which were recently georeferenced and digitized as part of a multi-institution Paleoniches Thematic Collections Network project funded by the NSF Advancing the Digitization of Biological Collections initiative. Geographic ranges were reconstructed to the stage level using PaleoGIS v.4.2 and normalized by rock outcrop available during the corresponding stage. We evaluated the relationship between extinction patterns and geographic range size using binary logistic regressions for each of six stages. Geographic range size consistently failed to identify extinction risk in all stages but the Virgilian. To further evaluate extinction selectivity, we tested whether patterns of abundance impact survivorship both independently and in combination with geographic range size. Once again, we found no relationship for any stage except the Virgilian. The Virgilian had the highest per-million-year extinction rate of any examined stage, indicating there is a possible cut-off below which extinction selectivity is no longer influenced by geographic range size. Absence of significant environmental drivers during sluggish periods of extinction may cause this pattern to break down, mirroring a similar but opposite breakdown sometimes observed during mass extinctions when climatic changes are often global and catastrophic.