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Scale-Dependence of Cope's Rule in Body Size Evolution of Paleozoic

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Scale-Dependence of Cope's Rule in Body Size Evolution of Paleozoic Scale-dependence of Cope’s rule in body size evolution of Paleozoic brachiopods Philip M. Novack-Gottshall* and Michael A. Lanier Department of Geosciences, University of West Georgia, Carrollton, GA 30118-3100 Edited by Steven M. Stanley, University of Hawaii at Manoa, Honolulu, HI, and approved January 22, 2008 (received for review October 10, 2007) The average body size of brachiopods from a single habitat type volume) for 369 adult genera [see supporting information (SI) increased gradually by more than two orders of magnitude during Appendix, Tables 1 and 2] from deep-subtidal, soft-substrate their initial Cambrian–Devonian radiation. This increase occurred habitats demonstrates that brachiopod body size increased sub- nearly in parallel across all major brachiopod clades (classes and stantially and gradually during the Early and Mid-Paleozoic (Fig. orders) and is consistent with Cope’s rule: the tendency for size to 1), from a Cambrian mean of 0.04 ml (Ϫ1.40 log10 ml Ϯ 0.27 SE, increase over geological time. The increase is not observed within n ϭ 18 genera) to a Devonian mean of 1.55 ml (0.19 log10 ml Ϯ small, constituent clades (represented here by families), which 0.06, n ϭ 150). The magnitude of size increase between periods underwent random, unbiased size changes. This scale-dependence is statistically significant. We evaluated within-phylum dynamics is caused by the preferential origination of new families possessing by using maximum-likelihood comparisons among three evolu- initially larger body sizes. However, this increased family body size tionary models: directional (driven, biased, general random does not confer advantages in terms of greater geological duration walk) change (DRW), unbiased (passive) random walk (URW), or genus richness over families possessing smaller body sizes. We and stasis (22), with DRW generally resulting in a pattern of suggest that the combination of size-biased origination of families Cope’s rule when there is a positive directionality parameter (a and parallel size increases among major, more inclusive brachiopod maximum-likelihood estimate of the magnitude of the rate of clades from a single habitat type is best explained by long-term, size change). The brachiopod phylum-level size trend is over- secular environmental changes during the Paleozoic that provided whelmingly supported by the directional model (SI Appendix, opportunities for body size increases associated with major mor- Table 3), with a constant and positive rate of size increase of phological evolution. 0.013 log10 ml/Myr Ϯ 0.005. This rate of change is small but is sufficient to gradually increase brachiopod size by an order of body volume ͉ origin of clades ͉ macroevolutionary trend ͉ magnitude every 77 Myr, on average. Sparse sampling during the species selection ͉ maximum likelihood Cambrian makes it impossible to resolve here whether size increase was continuous throughout the Cambrian–Devonian or ncreasing body size is a pervasive predictor of population-level was delayed until the Ordovician Radiation. Regardless, the Iselection (1), and although macroevolutionary trends of in- increasing minimum size of brachiopods overall is consistent creasing size, including Cope’s rule (2–7), are known for many with Cope’s rule, excluding a passive, diffusional trend of fossil groups, the mechanisms by which short-term size advan- increasing variance through time (7–10). These increases are not tages are manifested at longer time scales remains poorly the result of sampling heterogeneities because they are observed understood. Macroevolutionary size increases can occur via two when sampling is standardized by rarefaction (SI Appendix, distinct pathways: (i) maximum and mean size can increase Fig. 5). within a clade because of passive diffusion from an unchanging Such a phylum-wide increase can result from the accumulation lower size (sometimes termed ‘‘increasing variance’’) or (ii) size increase can be driven and accompanied by increases in mini- of within-clade processes where constituent clades are all tending mum size (7–10). Here, we confine Cope’s rule to this second, toward larger size or from among-clade processes where con- driven pathway because it implies that clades with larger body stituent clades remain at a constant size throughout each of their sizes have greater evolutionary fitness than smaller clades. Such histories but clades with small body-sized genera are replaced advantages can arise in several ways, including from the biolog- over time by clades with larger-sized genera (14, 15). Because ical benefits of larger, optimal sizes (1, 4, 7); from size-linkages recent brachiopod classifications use cladistically informed stan- with changing environmental conditions (11–13); or from pref- dards and recognize many monophyletic groups (17–20), we erential sorting of clades associated with larger body size (14, 15). treated classes, orders, and families as representative clades of Analyses of size trends have focused on post-Paleozoic groups, varying levels of hierarchical nestedness. Although many of these limiting our understanding of size evolution during the otherwise clades, especially families, are likely paraphyletic, our results well studied Cambrian and Ordovician radiations of animals and remain adequate summaries of the evolutionary relationship during the Paleozoic in general. Brachiopods offer a natural between morphological and body size evolution in Paleozoic exemplar for such studies because of their fundamental contri- brachiopods because these taxonomic groups are defined by bution to these Lower Paleozoic radiations, their ecological morphological similarity. dominance in most Paleozoic benthic marine communities, and We evaluated the underlying mechanism for this trend by their unrivaled fossil record (16–19). This study uses the largest using two distinct tests. The first test applies the maximum- and temporally most extensive database of fossil brachiopod likelihood approach used above to differentiate DRW, URW, sizes assembled to date to evaluate the existence and causes of size increases during the Cambrian through Devonian: the 170-million-year (Myr) interval covering the initial ascent of Author contributions: P.M.N.-G. and M.A.L. designed research; P.M.N.-G. and M.A.L. per- brachiopods to their zenith of Phanerozoic diversity. Recent formed research; P.M.N.-G. and M.A.L. analyzed data; and P.M.N.-G. wrote the paper. cladistically based brachiopod classifications (17–20) allow de- The authors declare no conflict of interest. tailed testing of the mechanisms responsible for these trends. This article is a PNAS Direct Submission. *To whom correspondence should be addressed. E-mail: [email protected]. Results and Discussion This article contains supporting information online at www.pnas.org/cgi/content/full/ Size Trends Within Large Clades (Phylum, Classes, and Orders). A 0709645105/DC1. database of brachiopod body sizes (measured here as shell © 2008 by The National Academy of Sciences of the USA 5430–5434 ͉ PNAS ͉ April 8, 2008 ͉ vol. 105 ͉ no. 14 www.pnas.org͞cgi͞doi͞10.1073͞pnas.0709645105 Downloaded by guest on October 1, 2021 210 _ _ _ _ _ ____ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ _ _ _ _ _ _ _ _ _ _ _ ___ — _ _ _ _ _ _ _ _ _ _ ___ __ ____ _ ____ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ — — _ _ _ _ ___ ___ _ —_ _ _ _ _ _ _ _ _ ______ _ _ _ ) _ _ _ _ ___ —_ —_ — _ _ _ _ _ ___ ___ __ _ lm gol( em gol( lm _ _ _ _ _ _ _ —_ _ _ _ _ _ —_ _ _ __ _ __ _ ___ ___ _ —___ _ —___ ___ __ _ _ _ ___ —___ —___ _ ___ _ __ ——___ —_ —_ ___ _ _ _ _ — _ __ _ _ _ _ _ _ —_ —___ _ _ _ _ _ _ _ _ _ _ _ _ _ _ —_ — _ _ _ _ _ _ _ _ _ 1 _ _ _ _ _ _ _ _ _ _ _ _ -2- — — — _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ ulov ydoB ulov _ _ _ _ _ —— — — _ _ _ _ _ _ _ _ _ — _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 3 -4- _ _ _ _ C4C3C2 O3O2O1 O4 O5 S2S1 D2D1 D3 D4 D5 Cambrian Ordovician Silurian Devonian 520 500 480 460 440 420 400 380 360 Geologic age (Ma) Fig. 2. Mean size trends among brachiopod clades (classes, orders, and Fig. 1. Increasing size trend in Cambrian–Devonian brachiopod genera from families). Different clades are distinguished by different shades of color, with deep-subtidal, soft-substrate habitats. Time scale from ref. 21 with 11-Myr line width corresponding to level of hierarchical nestedness. Large clades bins used in the Paleobiology Database. Each data point is the observed body display parallel trends of increasing size (i.e., classes parallel other classes, volume for a single genus plotted at its bin midpoint age; several points orders parallel other orders), whereas trends within small, constituent clades overlap. Simultaneously increasing minimum, mean, and maximum sizes (families) display more stochastic dynamics. Only clades with a minimum of 10 through most durations are consistent with Cope’s rule. SE bars around means occurrences over five intervals are colored, except for the addition of order are 1 SD from the distribution of 2,000 bootstrap replicates. Trend in median Paterinida, included to allow additional documentation of Cambrian trends, sizes (data not shown) is nearly identical to mean trends. Ma, megaannum and the omission of class Craniata, which suffers from inconsistent sampling.
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