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Body size evolution in Mesozoic birds: little evidence for Cope’s rule
Cope’s rule, the tendency towards evolutionary increases in body size, is a long-standing macroevolutionary generalization that has the potential to provide insights into directionality in evolution; however, both the definition and identification of Cope’s rule are controversial and problematic. A recent study [J. Evol. Biol. 21 (2008) 618] examined body size evolution in Mesozoic birds, and claimed to have identified evidence of Cope’s rule occurring as a result of among-lineage species sorting. We here reassess the results of this study, and additionally carry out novel analyses testing for within-lineage patterns in body size evolution in Mesozoic birds. We demonstrate that the nonphylogenetic methods used by this previous study cannot distinguish between among- and within-lineage processes, and that statistical support for their results and conclusions is extremely weak. Our ancestor–descendant within-lineage analyses explicitly incorporate recent phylogenetic hypotheses and find little compelling evidence for Cope’s rule. Cope’s rule is not supported in Mesozoic birds by the available data, and body size evolution currently provides no insights into avian survivorship through the Cretaceous–Paleogene mass extinction.
‘Supertree B’ of Mesozoic birds demonstrating inter-relationships, the five major clades (Aves, Pygostylia, Ornithothoraces, Enantiornithes, and Ornithuromorpha) used in the analyses, and inferred branch lengths. Note that the majority (60%) of taxa included in the supertree are from just two formations: the Yixian Formation (late Barremian–early Aptian) and the Jiufotang Formation (Aptian) of China. The same bias is present in the nonphylogenetic data set of Hone et al. (2008), see Fig. 2. ‘Supertree A’ (19 taxa) differs in that there is no resolution of relationships within the clade labelled ‘X’ (which also includes the taxon Eoalulavis from the Yixian Format