PUTTING THE ANTHROPOCENE INTO CONTEXT: A 3.5 MYR BASELINE OF BIOTIC INTERACTIONS BETWEEN DRILLING GASTROPODS AND THEIR BIVALVE PREY (Invited Presentation)

Studies of the Neogene fossil record of gastropod drilling predation have benefited greatly from field observations and experimental manipulations of living animals. Experimental studies of extant muricid gastropod drillers in Florida, for example, have revealed that edge drilling traces on venerid bivalve prey increase predictably in frequency relative to wall drills when threats from enemies (food stealing competitors, other predators, etc.) are elevated. Conversely, naticid drillers produce greater frequencies of incomplete drill holes when faced with similar threats. Because muricids and naticids in this region produce readily distinguished drill holes, biotic interactions for each predator type can be tracked separately through time.

Here, we examine the Pliocene-to-Anthropocene record of drilling predation on 40,000 bulk-collected shells of the prey venerid bivalve Chione along the west coast of Florida, a time span that encompasses dramatic climate and oceanographic changes as well as anthropogenic stressors such as trawling, eutrophication, and overfishing. Data were collected on drilling frequency (d.f.), incomplete d.f., edge d.f., and predator-prey size selectivity. Data were also collected for Recent shells separated by taphonomic grade for finer temporal resolution within the Anthropocene.

Our results show that both seagrass-dwelling muricids and open sand-dwelling naticids decreased their d.f. after the Pliocene, although naticid d.f. rose in the Recent while muricid d.f. decreased. Different patterns for muricid and naticid d.f. in the Recent is consistent with complex habitat changes causing expansion of open-sand drilling gastropod populations at the expense of taxa associated with seagrass. Incomplete d.f. and edge d.f., our proxies for the intensity of interactions between drilling snails and their own enemies (escalation), show similar declines after the Pliocene. However, unlike the complex pattern for d.f., both escalation metrics increased in the Anthropocene to a level unprecedented in the last 2.5 myr and with a magnitude of change comparable to that observed at the Pliocene-Pleistocene boundary, a period of ecological adjustment preceding a regional mass extinction event roughly 2 Ma. The Anthropocene may be a similar period of adjustment preceding extinction.