PHANEROZOIC EVOLUTION OF BODY SIZE IN BILATERIAN MARINE ANIMALS

An organism’s size is one of the most useful phenotypic traits for studying macroevolution, as it is often tightly correlated with metabolic rate, population size, extinction risk, and other traits. Furthermore, body size is easily gleaned from fossils and comparable across all taxa. We present here the first comprehensive dataset on the evolution of body size of Phanerozoic marine bilaterian animals, including the Arthropoda, Brachiopoda, Chordata, Echinodermata and Mollusca. The dataset contains more than 17,900 genera, each with a stage-resolved stratigraphic range and a measured maximum linear dimension (log-transformed for all analyses).

The body size history of marine animals exhibits several notable similarities and differences with diversity history. Like diversity, we find a Paleozoic plateau in mean body size after an initial Cambrian increase. However, mean body size begins a monotonic decline in the Early Devonian to a post-Cambrian low in early Carboniferous by a slow increase to a Triassic-Jurassic plateau (at higher mean than the Paleozoic plateau). Mean body size then declines through the Cretaceous, but has been steadily increasing since the Paleocene. Additional work is needed to determine if specific clades are driving trends in mean size during different intervals.

Fitting explanatory models to the data using the PaleoTS package for R reveals that the overall trend in mean body size as well as most clade-level trends are best fit by an unbiased random walk model. The two exceptions to this general pattern are ostracods and brachiopods, which show driven trends towards smaller and larger body size, respectively. These data suggest the net increase in mean body size over the Phanerozoic is better described by evolutionary drift (random walk) rather than by a consistent, driven trend toward larger size. Regardless of the underlying mechanisms, the dynamic history of mean body size has profound implications for energy fluxes through Earth’s marine biosphere because of body size’s influence on metabolic rate.