ECOLOGICAL DETERMINISM AND CONTINGENCY IN THE END-CRETACEOUS MASS EXTINCTION: SHAPING THE MODERN MARINE BIVALVE FAUNA

The functional ecology of the biosphere is both dynamic and seemingly resilient through the Phanerozoic. The number of ecological functional groups (FGs) has increased through time, and mass extinctions of taxa generally failed to remove entire groups. Even FGs lost through the total extinction of their constituent taxa were sometimes re-evolved by other lineages. While FGs appear to be resilient, and even entrenched into the adaptive landscape, the number of taxa within them might be highly labile, particularly across mass extinctions. Any changes in the relative dominance of FGs through time could reflect changes in the structure of resources and thus influence the longer-term accommodation of taxa. Comparing the phylogenetic structure of changes in the relative dominance can also test the role of evolutionary determinism in the re-diversification of taxa within FGs following mass extinctions.

The end-Cretaceous mass extinction (KPg) bottlenecked Mesozoic diversity by ~50% at the genus level on a short evolutionary timescale, providing potential for ecological restructuring. In our analysis of marine bivalves, most FGs persisted across the KPg via the survival of at least one phylogenetically continuous lineage. However, the KPg equalized the distribution of genera among FGs. Diversification through the Cenozoic restored unevenness among the FGs but produced a different distribution of genera among FGs compared to the pre-extinction fauna, although the rank-order of genus richness remained similar. The FGs that originated in the Cenozoic (~20% of extant FG richness) tend to have low genus richness today, typically comprising <10% of global genus richness. This pattern is consistent with minimal changes to the structure and abundance of the resources underlying FGs, with the newest FGs appearing to more finely segment previously established modes of life. Within FGs, the phylogenetic structure of genus-richness among families also changes, with both distantly and closely related families replacing the lineages dominant before the KPg. Thus, repopulation of the ecological landscape appears deterministic at the level of FGs, but which lineages diversify within FGs appears to be more contingent in the generation of the modern bivalve fauna.