THE RELATIONSHIP BETWEEN POPULATION ABUNDANCE AND SURVIVORSHIP: AN UNEXPECTED RESULT FROM THE P-TR EXTINCTION

Numerous studies document that extinction survivorship is increased by three organismal attributes: (1) greater population abundance maintains viable gene pools and reduces the risk of “gamblers' ruin”; (2) broad environmental tolerance (eurytopy) increases the likelihood that some members of a taxon will live in a habitat less affected by the extinction; and (3) broad environmental range increases the likelihood that some individuals will live outside the most impacted regions. These attributes may be interrelated, e.g. area and population often correlate, and dispersal across a broad geographic range may owe to eurytopy. However, local-scale ecological studies are not examining mass extinctions, and the use in paleontological studies of geographic range as a proxy for abundance or eurytopy is often assumed, not tested. This study examines the influence of local abundance and rarity on mass extinction survivorship, using 220 Late Pennsylvanian through Early Permian brachiopod assemblages from Kansas and Texas. Specimens were surface-counted on sq.-meter plots on limestones or sampled and counted in shales of approximately equivalent volume. The units represent a wide range of environmental conditions. Genera were divided into two categories reflecting whether the genus belonged to a lineage that survived the Permo-Triassic mass extinction (survivor, n=5) or not (victim, n=14). Analyses were limited to taxa present in both Kansas and Texas throughout the Late Pennsylvanian-Early Permian. Victims were *more* abundant than survivors (t-test, p = 0.047). Similarly, victims appeared in more beds than survivors (t-test, p = 0.025). No genus present in > 56 beds (~26% of beds) survived. These results are not due to differences in shell mineralogy (all taxa have low-Mg calcite shells). Rarity is partly a function of rare taxa having narrower niches – rare taxa had a stronger substrate preference. The unexpected result that rare and stenotopic brachiopods exhibit greater survivorship (5/10 taxa appearing in < 57 beds survived) suggests that the relationship between abundance, eurytopy, geographic range, and extinction needs to be explored in more detail. The relationship between greater abundance and survivorship may apply to background extinction but not to mass extinctions, or at least not to the P-Tr event.