Can Predatory-Prey Arms Races Intensify during a Mass Extinction Event? Strombid Gastropods from Late Neogene of Florida

Mass extinctions often are triggered by environmental stress, including changes in physical conditions or food supply. During such times, a taxon's energy is directed towards survival, with opportunities for morphological change and adaptation supposedly suppressed. Any ongoing ‘arms race' should be halted in favor of survival. The present study examines the evolution of anti-predatory traits in strombid gastropods of the genus Macrostrombus from the Late Neogene of Florida in the context of a regional mass extinction event roughly 1.8 Ma. We focus on the fossil record of shell repair and the expression and occurrence of the dorsal knob, a large spine on the dorsal surface of the final whorl that functions in helping the snail right itself after accidental overturning and exposure to predators, such as crabs, turtles, fish, and other gastropods.

Nineteen morphological features were measured and assessed in 300 Macrostrombus specimens from museum and new collection efforts. Changes documented include a 50% increase in the average dorsal knob height from the Pliocene to the Pleistocene, in addition to a significant number of specimens acquiring this feature (4% vs. 94%). Evidence of shell repair due to durophagous predators also increased during the same time interval from 22% to 37%. The development of a large dorsal knob over the examined time periods is interpreted as an adaptive response to predation and is consistent with repair frequencies suggesting intensifying predation immediately after the extinction event.