DYNAMICS OF THE BODY SIZE EVOLUTION OF CROCODYLIFORMES

Twenty-four species of crocodile populate the globe today, but this richness represents a minute fraction of the diversity and disparity of Crocodyliformes since their origin early in the Triassic. Across this clade, three major diversification events into the aquatic realm occurred. Differences in the physical and chemical conditions between aquatic and terrestrial habitats have the potential to cause differing selective pressures on diversification and morphology. To assess these effects, we compiled an extensive body size and habitat database of fossil and modern crocodyliformes. Phylogenetic ancestral state reconstruction reveals an overall trend of increasing minimum and average body size in crocodyliformes through time, coupled with dramatic fluctuations in skewness, possibly in response to other biotic factors. Furthermore, diversification rates do not show a significant relationship with body size or habitat, nor do they present any trends through time. Finally, there is evidence for a history of repeated body size increase and convergence coupled with increases in strength of selection and decreases in variance following shifts to a fully aquatic, diving lifestyle, suggesting common selective pressures on life in water spanning multiple aquatic clades. Lung volume, which has long been proposed as the main determining factor for how long these animals can dive, is only a constraint at sizes greater than 10 kg, whereas the rate of cooling constrains diving time at masses less than 10 kg. Therefore, we propose that limitations on diving time due to heat loss have imposed a minimum size in the evolution of fully aquatic, diving crocodyliformes, while the scaling of lung capacity with body mass has encouraged even larger sizes within these clades.