MORPHOLOGICAL BOTTLENECKS AND PHYLOGENETIC RESTRUCTURING FOLLOWING THE END-CRETACEOUS MASS EXTINCTION: TESTING PRIORITY EFFECTS IN THE DEVELOPMENT OF THE MODERN MARINE BIVALVE FAUNA

Mass extinctions disrupt the balance of clade richness across the tree of life and ecologies. Certain modes of life are permanently lost, others persist through the survival of lineages or by independent re-evolution. Both scenarios may induce priority effects, where the first and/or taxonomically richest clade in an adaptive zone diversifies to the greatest extent. However, our analyses show that some bivalve families do not fit this model; a family's genus richness today is not tightly predicted by the size of its taxonomic bottleneck through the end-Cretaceous mass extinction (KPg) or its number of survivors into the Paleocene. Families with relatively low survivorship and high proportional extinction have reached similar levels of genus richness today as those with high survivorship and low proportional extinction. This pattern may arise because surviving genera in either scenario have high morphological disparity, which increases the probability of taxonomic and morphologic evolution in multiple directions.

However, families with highest genus richness today did not have exclusively high morphological disparity of shell shapes in their surviving genera across the KPg. Eight of the top-10 families in extant genus richness had low disparity of survivors (only 6-20% of the maximum measured disparity for families); thus, the accumulation of high taxonomic diversity today is not strictly tied to high initial disparity in the Cenozoic. Nor is high extant richness tied to low changes in morphospace range: 5 of those top-10 families experienced some of the greatest range reductions across the KPg. Further, 2 of 4 "dead-clade-walking" families survived the KPg with full morphological range. Thus, changes in a family's morphospace occupancy across the KPg appear to have a many-to-one mapping with its extant genus richness. However, families bottlenecked to one genus—total reduction in morphospace range and variance—and had lateral shifts in their morphospace position are restricted to low genus richness today. High taxonomic survivorship and/or persistence of morphospace may elevate a clade's diversification after a mass extinction, but neither scenario holds for all bivalve families across the KPg. Instead, an interaction of KPg bottleneck effects with Cenozoic processes generate the modern bivalve fauna.