COS 13-5
500 million years of increasing body size, ecological complexity, and metabolic activity of marine animals
Marine animals have dramatically increased in taxonomic diversity since the “Cambrian Explosion” (543 mya – present). However, taxonomic diversity is an incomplete proxy for how marine ecosystems have changed over evolutionary time. Here, we ask whether there have been concomitant increases in mean body size, ecological complexity, and mean per taxon metabolic activity in the marine animal fossil record over the Phanerozoic that may be related to the known increase in taxonomic diversity. To this end, we generated a genus-level dataset of body sizes (including over 16,500 genera, or ~55% of known diversity) and functional ecological traits (including over 22,500 genera, or ~85% of known diversity) of fossil marine invertebrates, fishes, reptiles, and mammals. We recorded the lengths of the three major body axes from published images of specimens (typically holotypes of the type species) to determine body size. We assigned ecological traits using the first principles of functional morphology, coding each genus to an “ecospace” defined by three fundamental niche axes (habitat tiering in relation to the seafloor, feeding mode, and motility level). To address changes in metabolic activity across time, we used living analogues to calculate trends in mean per-genus metabolic rate across genera.
Results/Conclusions
Our data clearly demonstrate that the ecological diversification history closely parallels the taxonomic diversity history across all major clades of marine bilaterian animals. However, increase in taxonomic diversity is not simply the result of even filling of new ecospaces as they arise. Some ecospaces have always had far greater diversity than others, but recent oceans have higher evenness in diversity across ecospaces than did ancient oceans. In parallel, our data show that animals are on average about 27 times larger in modern seas than in Cambrian times, with a fairly continuous increase since the “Cambrian Explosion.” This increase in size is strongly coupled to ecology; for example, larger sizes are often associated with ecological shifts towards more motile, predatory organisms that have left the seafloor and moved into the water column. These data also suggest that the mean metabolic rate across marine animal genera increased by several orders of magnitude during the Phanerozoic. Taken together, these new data suggest the increase in taxonomic diversity was enabled by an increase in ecological diversity focused on strategies associated with larger body size and higher metabolic demand. Ecological interaction thus appears to be a key driver of taxonomic diversity over geological time.