WITHIN- AND AMONG-GENUS COMPONENTS OF FORAMINIFERAN SIZE DYNAMICS DURING MASS EXTINCTION, RECOVERY, AND BACKGROUND INTERVALS

The evolution of body size has long been of interest to paleobiologists. One aspect that has received particular attention is change in size distributions during and after mass extinction events. Because body size correlates with important ecological and physiological characteristics such as geographic range, life span, reproductive rate, and metabolic rate, the pattern of size change may provide insight into the causes of mass extinction events and controls on subsequent recovery, beyond what can be gleaned from diversity data alone.

Previous work has examined extinction selectivity by comparing size distributions of taxa before and after extinction events and by studying size trends within surviving lineages. However, the relative importance of within- and among-lineage processes for driving shifts in overall size distributions has yet to be determined. Here we investigate the dynamics of size evolution in foraminifera from the Late Permian through the Early Jurassic, a time interval that includes two mass extinctions and their recoveries as well as intervening background times. We explicitly partition changes in body size distributions into three components: biased extinction, within-lineage trends, and biased origination.

We find that within-genus size change was the most important factor driving foraminiferan size change during and after major extinction events. Following a dramatic decrease in size during the end-Permian mass extinction, the mean and maximum sizes of foraminiferan species remained smaller than Late Permian values through the Early Triassic, but returned to near–pre-extinction levels early in the Middle Triassic. This size recovery, like the decrease during the extinction, resulted primarily from within-genus size change. A similar pattern is found for the end-Triassic mass extinction and subsequent recovery. In between the two mass extinctions, foraminiferan size shows a pattern of stasis, with all of the three components contributing minimally to size change.