Maximum body size has increased enormously over the Phanerozoic, but the pattern and timing of this size increase is poorly known. Consequently, the limits to body size and the extent to which size evolution is passive or driven have been difficult to evaluate. We compiled a database of the largest known fossil Arthropods, Chordates and Molluscs in each Epoch in order to empirically determine the pattern of body size increase since the Cambrian.
Results/Conclusions
We found that size increased rapidly following the appearance of multicellular animal life, reaching 9.8 log10 biovolume units (BV; approx. 6300kg) during the Middle Ordovician. Since that time, however, maximum size has been maintained at approximately 10-11 BV, while global animal diversity has continued to increase. The apparent upper bound to global maximum body size is conserved across both time and animal phyla. Many clades have attained this maximum size independently, including cephalopods, reptiles, fishes, and mammals. In each class, maximum size was reached quickly from a substantially smaller ancestor. In the oceans the largest animals were habitually motile predators or filter feeders, whereas on land herbivores have out-sized predators since the Triassic. Correlation of maximum size patterns to motility, habitat, and trophic level indicate that primary productivity and trophic efficiency may provide clues to the limit of maximum size on earth.
