THE INFLUENCE OF SUCCESSION ON EXTINCTION SURVIVORSHIP -- LOCAL PROCESS WITH A BIG PUNCH

A critical question for paleobiology is whether local-scale processes or emergent properties play major roles in extinction survivorship. We examine the influence of multiple attributes, operating at different scales, on survivorship.

Previous research by the authors established a method for evaluating successional order in the fossil record. The mean diversity of those communities in which a taxon is present provides a proxy for successional order. Early-stage taxa tend to have lower mean community diversity (MCD) than late-stage taxa because late-stage taxa are restricted to ecosystems with low disturbance and time-averaging preserves the diversity of all successional stages in such ecosystems. This method has been tested by independent assessment of successional order in fossil and Recent epibiont communities.

In the present study, we analyzed the effect of multiple taxon attributes on mass extinction survivorship, using 240 brachiopod assemblages from the Late Pennsylvanian and Early Permian of 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 both pulses of the Permo-Triassic mass extinction and the subsequent “transitional phase” (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.

Unlike several prior studies, those taxa with large geographic ranges, high abundance, and broad environmental tolerances did not experience enhanced survivorship. In fact, rare and stenotypic taxa were more likely to survive. However, the strongest factors influencing survivorship were small size (t-test, p = 0.01) and high MCD (t-test, p < 0.02). These results suggest that small, late-stage taxa, which have a high tolerance for limited resources, were more likely to survive the long-term environmental stresses of the Late Permian and Earliest Triassic. More importantly, this example indicates that attributes operating at the scale of individuals and populations, and local-scale processes, can scale up to influence broad, global patterns.