Primary productivity, deoxygenation, and the Gulliver-absence effect determine bivalve body size following the end-Permian mass extinction

The impact of mass extinctions on the body sizes of animals has received considerable attention and debate, as to whether the reduced size of post-extinction organisms is due to the selective extinction of large species, absence of large species as a stochastic effect of low-diversity faunas, or a size decrease within surviving genera and species. Here, we investigated the body sizes of bivalves following the end-Permian mass extinction event and show that the shell size increase of bivalve genera was driven by both evolutionary and ecophenotypic responses. First, some genera show significant increases in body size with the evolution of new species. Further, the same genera record significant within-species increases in average and maximum body size into the late Induan, indicating that ecophenotypic changes were also involved on long-term body size trends. These increases are associated with invigorated ocean circulation, improved oxygenation of the seafloor, and probably increased food supply.