EARLY MESOZOIC EXTINCTION SELECTIVITY AND THE RISE OF THE MODERN FAUNA

The interval from the Middle Permian to Early Jurassic contained four extinction events associated with flood basalt eruptions and carbon cycle disruption, including two of the largest mass extinctions of the Phanerozoic, and a pivotal shift to marine ecosystems containing abundant mollusks of the modern evolutionary fauna. Ecosystems shifts were closely linked to extinction, especially during the end-Permian, but the mechanism by which global change (warming, anoxia, or ocean acidification) led to those faunal changes is less understood. Activity level in marine organisms may be an important predictor of vulnerability to global change, with more active taxa often faring better under experimental hypercapnia and having higher thermal limits during experimental warming. To test whether activity level played an important role in selectivity during early Mesozoic extinctions, I first assigned benthic marine genera an activity score on the basis of life position, motility speed and frequency, and feeding style. I used logistic regression to test the relationship between activity score and extinction probability using Paleobiology Database occurrence data at stage-level resolution in the Permian and Triassic and at zone-level resolution in the Early Jurassic. Although activity level did not influence extinction risk during background intervals, genera with higher activity scores were significantly more likely to survive during extinctions in the Permian, Triassic, and Early Jurassic. Differences in activity level between the Paleozoic fauna and the modern fauna, which includes bivalves, gastropods, and echinoids that tend to be more motile and to have more energetic feeding strategies, provides a physiological mechanism to link extinction selectivity to abundance shifts in marine ecosystems. The taxonomic composition of today’s marine ecosystems has been shaped in part by repeated extinction events when global change favored survival of more active members of the modern fauna.