SHUFFLING DIVERSITY, FORM, AND FUNCTION: PERSISTENCE BUT RESTRUCTURING OF BIVALVE FUNCTIONAL GROUPS AND THEIR MORPHOLOGICAL DISPARITY BETWEEN THE END-CRETACEOUS EXTINCTION AND PRESENT-DAY

Mass extinctions remove substantial numbers of taxa but appear to leave the variety of ecological functions largely intact. The structure of the ecological landscape can, however, shift in the wake of mass extinctions, with some formerly dominant groups failing to re-diversify taxonomically, and certain formerly marginalized groups rising in rank via spurred diversification of their constituent lineages. Comparing shifts in the richness of functional groups to the changes in morphological disparity of their taxa can test whether gains in functional dominance occur by radiations of disparate taxa or by piling up of similar ones.

In our analyses of marine bivalves, the end-Cretaceous mass extinction removed ~60% of genus richness (251 of 408) but only 14% of functional groups (5 of 36). The range of shell shapes (measured as the commissure outline) also persisted through the extinction, perhaps expected given that bivalve form loosely maps to its function. The persistence of functional groups through such high taxonomic loss necessarily evenned their richnesses. Subsequent taxonomic diversification through the Cenozoic restored that unevenness, but altered the rank order of functional-group richnesses. For functional groups that rose in rank from the Maastrichtian to today, most also gained morphological disparity, which is consistent with their polyphyletic diversification and apparent accessibility by multiple morphologies. Few functional groups gained rank while maintaining or losing morphological disparity, as would indicate a proliferation of many, morphologically similar taxa. Disparity increases with genus richness of functional groups in the latest Cretaceous, but neither is correlated in the Recent, suggesting that despite some increase in disparity of the richest functional groups today, most diversification has been concentrated towards the core of that group's morphospace. Thus, diversification of the richest functional groups in today's ocean is associated with some increases in disparity—perhaps reflecting relaxed or shifting accommodation of particular functions to morphologies or the polyphyletic invasion of groups by multiple, morphologically disparate clades—but has mostly proceeded as the accumulation of morphologies that were present before the K-Pg extinction.