LIFE IN THE FAST LANE: SELECTIVE SURVIVORSHIP OF MOLLUSKS WITH RAPID LIFE HISTORIES ACROSS A PLIOCENE EXTINCTION BOUNDARY

Stable isotope sclerochronology was used to trace the evolution of life history strategies and selectivity of survivorship in six lineages of the small predatory gastropod genus Urosalpinx across a late Pliocene (2.5 Ma) extinction boundary in Florida. The boundary coincides with an early, unusually intense glacial interval, but other changes, including a possible reduction in the intensity or predictability of upwelling, are also suspected.

Serial isotopic sampling of entire fossil shells revealed consistent differences in rates of both sexual development and growth between extinction survivors and losers but not in season of hatching. Prior to the extinctions, the Urosalpinx group was composed primarily (83%) of species that matured late (2+ yrs) and grew slowly (30-85 mm in 1st yr spiral distance). In contrast, post-extinction survivors all matured rapidly (1 yr) and grew rapidly (60-120 mm in 1st yr spiral distance). This pattern suggests that catastrophic mortality was the primary selective agent, and that delayed breeding - a trait associated with increased fitness of offspring and higher lifetime fecundity when conditions are favorable - became an impediment to species survival. Marked increases in first year growth rates, which may have compensated for reduced lifetime fecundity due to early maturation, hint that declines in the abundance or quality of food available were not likely involved.

Of several causal hypotheses examined, only a decrease in the predictability of upwelling at 2.5 Ma is supported. Many species of small-bodied Muricidae, such as Urosalpinx, are selective predators of small barnacles, bryozoans, and bivalve spat, which are available only seasonally when nutrient pulses arrive via coastal runoff or upwelling. Although many mollusks can survive for considerable periods without feeding, unpredictable prey availability would have been particularly harsh on Urosalpinx, which due to its small size likely had low tolerance for food shortages. The likelihood that such prey might not again become available to an individual before succumbing to starvation explains the selective advantage of having a rapid life history. Continued Pliocene upwelling after 2.5 mya, however, even if unpredictable, may be needed to explain the rapid first year growth rates of survivors.