Testing extinction rates across various modes of life in Bivalvia during the end-Cretaceous mass extinction

Abstract

Mass extinctions disrupt the balance of clade richness across the tree of life, dramatically and permanently altering the ecological landscape (Foote, 1997; Jablonski, 2005; Sclafani et al., 2018). Certain groups of organisms are lost to extinction, while others persist through the survival of individual lineages (Jablonski, 2002; Jablonski, 2005; Sclafani et al., 2018; Wan et al., 2021). The difference between survival and extinction could be connected to three key traits–mobility, feeding, and tiering–which combined make up an organism’s mode of life (MOL). I analyzed extinction selectivity across all bivalve genera known to exist during the end-Cretaceous mass extinction (KPg) to determine which modes of life fared better and which of the three MOL traits most contributed towards genus survival.

Of the fifteen unique MOLs, and the thirteen MOLs that had enough samples to properly analyze, only one went extinct (immobile, intermediate epifaunal, suspension/photosymbiotic feeder). Despite the relative stability of the MOLs, there were clearly groups hit harder than others. Semi-infaunal suspension feeders suffered the second greatest extinction level regardless of whether the taxa were mobile or sessile (81% for mobile taxa and 72% for sessile taxa) while immobile suspension feeding boring bivalves had the lowest extinction rate (<25%). Separating the three MOL traits, I found tiering to be the most important trait in guarding against extinction with feeding and mobility being of secondary and tertiary importance respectively. More exposed bivalves (intermediate epifaunal or semi-faunal) suffered the greatest extinction rates (100% and 77% respectively) while less exposed bivalves (boring and deep infaunal) had the lowest extinction rates (<25% and 43% respectively). Bivalve groups that relied on a combination of suspension feeding and photosynthesis (i.e. rudists) suffered a complete extinction whereas groups that used a chemosymbiotic strategy to feed suffered a less than 25% extinction rate. There is also a slight trend towards higher levels of mobility increasing survival rates (58% of immobile bivalves went extinct and 50% of mobile bivalves went extinct). There appears to be intersections between tiering and feeding that affects extinction rates; greater amounts of natural coverage and more reliable sources of food likely protected certain groups against extinction.