Data from: Within- and among-genus components of size evolution during mass extinction, recovery, and background intervals: a case study of Late Permian through Late Triassic foraminifera

One of the best-recognized patterns in the evolution of organismal size is
the tendency for mean and maximum size within a clade to decrease
following a major extinction event and to increase during the subsequent
recovery interval. Because larger organisms are typically thought to be at
higher extinction risk than their smaller relatives, it has commonly been
assumed that size reduction mostly reflects the selective extinction of
larger species. However, to our knowledge the relative importance of
within- and among-lineage processes in driving overall trends in body size
has never been compared quantitatively. In this study, we use a global,
specimen-level database of foraminifera to study size evolution from the
Late Permian through Late Triassic. We explicitly decompose size evolution
into within- and among-genus components. We find that size reduction
following the end- Permian mass extinction was driven more by size
reduction within surviving species and genera than by the selective
extinction of larger taxa. Similarly, we find that increase in mean size
across taxa during Early Triassic biotic recovery was a product primarily
of size increase within survivors and the extinction of unusually small
taxa, rather than the origination of new, larger taxa. During background
intervals we find no strong or consistent tendency for extinction,
origination, or within- lineage change to move the overall size
distribution toward larger or smaller sizes. Thus, size stasis during
background intervals appears to result from small and inconsistent effects
of within- and among-lineage processes rather than from large but
offsetting effects of within- and among-taxon components. These
observations are compatible with existing data for other taxa and
extinction events, implying that mass extinctions do not influence size
evolution by simply selecting against larger organisms. Instead, they
appear to create conditions favorable to smaller organisms.