CETACEAN BODY MASS DISTRIBUTIONS: DISPARITY, EXTINCTION, AND SUPERWHALES

The evolutionary dynamics of mammal species body mass distributions are well approximated by a mathematical model of cladogenetic diffusion with hard lower and soft upper constraints. The accuracy of this model provides a novel approach for formalizing and testing macroevolutionary hypotheses related to species body masses and taxonomic diversity. Here, we combine a large database of fossil cetacean mass estimates with the time-dependent solution of the model to explore the macroevolutionary history of cetaceans, from the early Eocene onward. We find that the dramatic increase in size disparity among archaeocetes over the Eocene is fully explained by diffusion away from a lower boundary, not by changes in species counts. Furthermore, despite the apparent loss of disparity and diversity in the Oligocene, a model-based estimate of the likelihood of a cetacean mass extinction during this time is close to zero. Finally, during the Miocene, we show that the Cetacean body mass distribution has reached equilibrium, implying a coupling between body mass disparity and species count. As a result, the mid-Miocene peak in cetacean taxonomic diversity implies a concordant maximum in body mass disparity, with the model predicting a species 2.9 times heavier, or about 46% longer, than Balaenoptera musculus.