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

Abstract

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.

Nearshore–offshore–basin species diversity and body size variation patterns in Late Permian (Changhsingian) brachiopods

“…This approach has the potential to reduce the influence of juvenile or incomplete specimens and has been employed in previous body size studies (e.g., Rego et al, 2012;Zhang and Payne, 2012;Payne et al, 2013). We tested whether or not the size frequency distribution in each of the three environments followed the normal distribution using the Shapiro-Wilk test and found that none satisfied the test (nearshore: W=0.9784, p<0.0001; offshore: W=0.993, p=0.0496; basin: W=0.9845, p=0.0208) (Fig.…”

Constraints on the Adult-Offspring Size Relationship in Protists

“…However, because proloculus size ranges over about eight orders of magnitude within each order, our measurement error is small relative to the overall distribution of proloculus sizes used in the analyses and will not affect any of our conclusions. Similarly, size variation within foraminiferan species is small relative to size variation among species (Rego et al ) and so the uncertainty introduced by using a single specimen to approximate a species will be small at the scale of the present analysis, which includes species spanning eight orders of magnitude in biovolume. Importantly, the proportional error in size measurements should not vary significantly with proloculus or test size because images of specimens are magnified to approximately the same size in the Ellis and Messina catalog.…”

A Shift in the Long-Term Mode of Foraminiferan Size Evolution Caused by the End-Permian Mass Extinction

“…Although the scope of this catalogue (>40,000 records) prohibits comprehensive taxonomic standardization, size trends in data designated as from the Superfamily Fusulinoidea in Ellis and Messina corresponded well with trends in a taxonomically and stratigraphically standardized North American dataset (Payne et al 2012). Analyses of size variation among specimens within species and species maxima within genera for Triassic foraminifera from south China show that the great majority of size variation is among species and genera rather than within them (Rego et al 2012). Comparison of monograph illustrations and bulk samples for bivalves and brachiopods shows that monographed specimens correlate with but are typically larger than the median or mean size of the bulk population (Krause et al 2007).…”