Extinction and rebound: Evolutionary patterns in late Cretaceous and Cenozoic bivalves.

摘要

Despite the importance of extinction in the history of life, the evolutionary consequences of extinction are not well understood. In this dissertation, I have taken a comparative approach to extinction, examining multiple events and their effects on the diversity, morphology, and ecology of bivalves.; First, I explored the relationship between abundance and survivorship across the end-Cretaceous (K/T) extinction in bivalve subgenera from the North American Coastal Plain. Abundance did not promote survivorship, regardless of the abundance metric, spatial scale, taxonomic scale, data treatment, or statistical test used. Several explanations for this pattern were considered, including the possibility that, in contrast to modern extinctions, abundance exerts little influence on survivorship during mass extinctions.; Next, I examined macroevolutionary patterns of extinction and rebound across the K/T in veneroid bivalve subgenera. The extinction was not morphologically selective, although survivors did have significantly larger geographic ranges than victims. The rebound was biased towards veneroids with elliptical shells, deep pallial sinuses, and small sizes, indicating that deeper burrowing taxa were more likely to originate after the K/T. Several explanations for this pattern were suggested, including increased predation pressure and the refilling of ecological niches. The rebound in morphospace proceeded step-by-step, beginning below the area occupied by the survivors and diffusing outward. Finally, I examined the effects of multiple extinctions (K/T, mid-Eocene (mid-E), and end-Eocene (E/O)) on the evolution of veneroids. The K/T rebound was strongly biased according to shape, but the K/T extinction and mid-E and E/O events showed no such selectivity. The K/T rebound was biased towards smaller veneroids, while the mid-E and E/O rebounds showed the opposite pattern. These patterns may result from the relationships among veneroid size, temperature change, and latitude. Veneroids with larger geographic ranges were more likely to survive the K/T and mid-E, but not the E/O. It is clear that the K/T, though short-lived geologically, exerted a stronger influence on veneroid diversity, morphology, and ecology than the other two events. The evolutionary effects of the K/T were not tied to the extinction itself, but to the rebound.