ALL HAPPY FAMILIES ARE ALIKE: FUNCTIONAL VS PHYLOGENETIC COMMONALITIES IN MARINE BIVALVE DIVERSIFICATION FOLLOWING THE END-CRETACEOUS MASS EXTINCTION

Mass extinction events cause extreme crashes in taxonomic and morphologic diversity. Unrelated and morphologically dissimilar clades can suffer similar bottlenecks, but recovery patterns can differ strongly. The Late Cretaceous-Recent fossil record of the marine Bivalvia provides an ideal case-study of a major extinction and its subsequent long-term diversity dynamics in a phylogenetic framework. Using a taxonomically standardized database of marine bivalve genera from the latest Cretaceous (Maastrichtian) to the Recent, we estimated genus-level extinction across the end-Cretaceous and standing diversity through the Cenozoic using the stage-boundary crossing method (Foote 2000). Trajectories of genus diversity within families through the Cenozoic were characterized by third-order polynomial fits and then classified by hierarchical clustering. Phylogenetic signal in extinction and recovery dynamics was estimated from the family-level molecular phylogeny of Combosch et al. (2017). The end-Cretaceous (K/Pg) mass extinction removed ~65% of marine bivalve genera, and extinction severity within families was proportional to the pre-extinction standing richness in the clade but had limited family-level phylogenetic signal. The taxonomic recoveries of families, however, are not predicted by the size of their respective bottlenecks, and recovery trajectories are also not phylogenetically partitioned—a mixture of families in each major branch of the bivalve tree crash, remain steady, or increase in genus diversity to today. For example, the Veneroidea suffered 76% genus-level extinction but rebounded to become the most diverse bivalve clade today (~130 genera, ~750 species), whereas their sister group, Arcticoidea, lost 67% of its genera but only 6 genera and 13 species exist today. Bottlenecks and recovery dynamics appear to be inversely related for feeding modes that are unrelated to phylogeny. Suspension-feeders experienced much tighter bottlenecks than chemosymbiotics, carnivores, and deposit-feeders, but rebounded with more vigor to become the dominant feeding group today. Comparison of victims and survivors of the K/Pg event along both phylogenetic and functional axes furthers our insight into the evolutionary and ecological consequences of post-extinction recovery.