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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

Published online by Cambridge University Press:  08 February 2016

Brianna L. Rego
Affiliation:
Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, United States of America. E-mail: [email protected]
Steve C. Wang
Affiliation:
Department of Mathematics and Statistics, Swarthmore College, Swarthmore, Pennsylvania 19081, United States of America, and Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, United States of America
Demir Altiner
Affiliation:
Department of Geological Engineering, Middle East Technical University, Ankara 06531, Turkey, and Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, United States of America
Jonathan L. Payne*
Affiliation:
Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, United States of America. E-mail: [email protected]
*
∗∗Corresponding author

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.

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Articles
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Copyright © The Paleontological Society 

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