UNIVERSAL PROCESSES GOVERN BODY MASS EVOLUTION IN MARINE AND TERRESTRIAL ENVIRONMENTS

Across terrestrial mammals, mammalian sub-clades, birds, and insects, body size distribution exhibits a common right-skewed pattern. The maximum size is orders of magnitude larger than the most common body size, which is only slightly larger than the smallest size. While this pattern is common in terrestrial environments, it is unclear if mammalian macroevolutionary processes and body mass distributions are the same in marine systems. In terrestrial environments, this distribution can be explained mechanistically as the outcome of macroevolutionary “diffusion” of body sizes constrained by: (1) a minimum viable size, (2) size-dependent extinction risk, and (3) Cope’s rule. Using a novel dataset of morphological measurements from 214 extinct cetacean species and 77 extant species, we compare mammalian marine and terrestrial extant body mass distributions, the evolution of their distributions over time, and quantify the commonality of macroevolutionary processes across terrestrial and marine environments. The extant cetacean body mass distribution is shown to be canonical and right-skewed, mirroring that of terrestrial mammals but shifted right by 3 orders of magnitude. This rightward shift is a result of a larger minimum size (6.8 kg compared to 2 g) caused by greater heat loss in marine environments. We find that the size-dependent extinction risk is present in marine environments, and at the same strength as in terrestrial environments, creating the universal canonical shape of body mass distributions. While Cope’s Rule causes lineages to increase in size through time in terrestrial environments, we find no evidence of Cope’s Rule in marine environments. This pattern could be the result of marine resources aggregating in shallow near-shore waters, causing the traditional selective forces for larger sizes to equalize against selective forces for smaller sizes. Finally, we compare the evolution of marine and terrestrial body mass distributions over the past 55 million years using a macroevolutionary diffusion model and show that diffusion, constrained only by (1) a minimum size and (2) size-dependent extinction risk, predict cetacean body mass distributions and maximum observed sizes through time.