ECO-PHYSIOLOGICAL DRIVERS OF BODY SIZE EVOLUTION IN MARINE ANIMALS

The size of the typical marine animal in the fossil record has increased in biovolume by more than two orders of magnitude since the Cambrian. Over this interval, maximum biovolume has increased by five orders of magnitude and minimum biovolume has decreased by less than two orders of magnitude. Statistically, these trends are best explained by active evolutionary forces driving animals toward larger sizes rather than neutral drift away from a lower bound. Here we explain this dramatic increase in mean and maximum size in terms of the ecological and physiological features of more than 18,000 Phanerozoic marine animal genera.

Our dataset consists of the five canonical paleontological phyla of solitary bilaterian marine animals: Arthropoda, Brachiopoda, Chordata, Echinodermata, and Mollusca. Each genus was coded for its tiering level, motility level, and feeding mode following the scheme of Bush & Bambach (2007). Additionally, we coded the respiratory anatomy of each genus along two binary axes: respiratory medium (i.e., air versus water) and open versus closed circulatory system. We used a linear mixed model to determine which factors are most influential in determining the body sizes of marine animal genera.

We find that the strongest correlates of body size in marine animals are breathing air and having a closed circulatory system. Being predatory has a small positive relation to body size while motility and being pelagic are associated with smaller body size. Considering temporal trends in mean and maximum size, we find that air breathers are the largest animals in the ocean when they first evolve and their average size does not change over the Mesozoic and Cenozoic. Animals with closed circulatory systems are, on average, smaller than animals with open circulatory systems during the Cambrian. However, by the middle Ordovician, these animals were the largest (measured by mean and maximum size) marine animals from Ordovician onward.

Our findings indicate that respiratory and circulatory anatomy constitute strong constraints on body size and played a central role in size evolution. We propose of model of size evolution where the evolution of increased complexity and metabolic efficiency permitted the diversification of metabolically demanding ecological modes of life.