Evolutionary and ecophenotypic controls on bivalve body size distributions following the end-Permian mass extinction

Mass extinction events are characterized by major losses in faunal diversity and are associated with several other ecological effects, e.g., reductions in tiering, selective losses of ecological lifestyles, and body size reductions. The latter 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 (the Gulliver absence effect), a size decrease within surviving genera and species (the Lilliput effect), or a combination of these factors. Here, we investigated both evolutionary and ecophenotypic controls on body size distributions following the end-Permian mass extinction event by investigating size changes in bivalves (Claraia, “Unionites”, and Neoschizodus) from the Werfen Formation (Dolomites, Italy) and the Bódvaszilas Sandstone Formation (Aggtelek Karst, Hungary). Our results show that the Early Triassic shell size increase of bivalve genera was driven by both evolutionary and ecophenotypic responses. First, genera Claraia and “Unionites” show significant increases in body size with the appearance of the new species C. clarai and “U”. canalensis, respectively, suggesting that evolution played a role on Early Triassic bivalve body size dynamics. Further, the same genera record significant within-species increases in average and maximum body size into the late Griesbachian and Dienerian stages (the Brobdingnag effect), indicating that within-species ecophenotypic changes were also involved on observed long-term body size trends. These increases are associated with invigorated ocean circulation, improved oxygenation of the seafloor, and probably increased food supply. We, therefore, hypothesize that both growth-limiting environmental conditions (deoxygenation and reduced primary productivity) and the extinction-induced absence of large species led to small-sized species dominating early Griesbachian benthic invertebrate communities.