A DRIVEN TREND TOWARD LARGER SIZE IN THE EVOLUTION OF MARINE ANIMALS

Body size is among the most important biological traits, but the history of animal size over evolutionary time remains poorly documented and the underlying controls are not well understood. We compiled a dataset of body sizes for 17,172 genera of fossil marine animals spanning the past 540 million years to constrain the pattern of size evolution in the ocean and test explanatory models. Mean biovolume across genera has increased by a factor of 100 since the Cambrian. Over the same interval, maximum biovolume has increased by five orders of magnitude and minimum biovolume has decreased by less than two orders of magnitude. We compared the observed trends in maximum, mean, and minimum body size to three stochastic branching models of body-size evolution. The models simulated (1) an unbiased branching model in which descendants are equally likely to be larger or smaller than their ancestors and there are no limits to minimum or maximum size, (2) an unbiased branching model containing a lower bound on minimum size, and (3) a size-biased branching model where descendants are more likely to be larger than their ancestors. The size-biased model best explains observed trends in maximum, mean, and minimum size. Neutral drift from a small initial value cannot explain observed patterns; rather, there has been a bias toward size increase in the evolution of marine animals ever since the Cambrian. Several factors may help to explain the driven trend toward larger body size. Progressive oxygenation of shallow-marine environments over geological time may have lifted physiological constraints on large body size; increases in the amount or nutritional quality of marine primary producers may have enabled the evolution of longer food chains with larger organisms at higher trophic levels; or competitive and predatory interactions may consistently select for larger organisms. Regardless of the precise causes, however, it is now clear that the size of the typical marine animal in the fossil record has increased substantially over the past 540 million years and that this size increase cannot be explained simply through neutral drift away from a lower bound.