UNEXPECTED ESCALATION OF ANTIPREDATORY SHELL DEFENSES IN STROMBID GASTROPODS IN THE PLEISTOCENE OF FLORIDA

A general pattern is emerging that selection pressures conducive to escalation (adaptive improvement) existed in the Pliocene western Atlantic but not in the Pleistocene. The Pliocene supported a diverse fauna of abundant and highly specialized predators, and witnessed widespread diversification of minor adaptive innovations, such as features and behaviors that enable predatory gastropods to subdue shell-bearing prey more quickly. The decline of these predators and innovation rates during an end-Pliocene regional mass extinction event has been taken as evidence that such selective conditions were less severe in the Pleistocene.

Here, we evaluate the generality of this consensus view with strombid gastropods (S. alatus complex) from Florida. We examined more than 7500 adult shells from four formations spanning the Plio-Pleistocene. We calculated the frequency of shell repair (an index of selection for shell armor) and measured morphological traits known to confer an advantage against durophagous predators.

Our preliminary data reveal unexpected escalation in anti-predatory traits of strombid gastropods (S. alatus complex) during the post-Pliocene of Florida. Adult body size and lip thickness, the percent of individuals with knobs on the last whorl, and the maximum number of knobs on the last whorl all increased in the Pleistocene relative to the Pliocene. Repair frequency shows a similar pattern, but is complicated by exposure time (as inferred from oxygen isotope analysis of shell growth rates) to enemies.

These data suggest that the consensus view of “de-escalation” following the late Pliocene extinctions is an oversimplification. Our results point towards a complex biotic history, biased by habitat, with escalation progressing for one subset of the overall fauna (sand habitats) and remaining static (or even regressing) for another (seagrass habitats), even though they are adjacent to one and often inter-finger one another. These complex patterns would be obfuscated in an assemblage-level approach to studying predator-prey interactions in fossil record, which mixes different patterns from different communities.