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Summary
Trends in body size are a rich source of information for evolutionary studies. This is because body size not only has numerous implications for function and life history but also has necessary limits that differ between groups of organisms. Moreover, there are many evolutionary patterns that might underlie trends, and these patterns need not be constant over time. On page 812 of this issue, Sallan and Galimberti (1) show that trends in the body sizes of vertebrates during the Devonian and Mississippian (about 420 to 325 million years ago) not only are markedly different at different times but also likely reflect a variety of different evolutionary mechanisms.