DRILLING PREDATION AND DIVERSITY IN CRETACEOUS AND CENOZOIC TURRITELLINE GASTROPODS OF THE U.S. GULF AND ATLANTIC COASTAL PLAIN

Predation has been considered a factor affecting diversity of clades, either by increasing extinction rates or by affecting speciation rates (suppressing speciation by elimination of incipient species or increasing speciation through the "fission effect"). We explored the relationship of predation, diversity, and abundance for turritelline gastropods of the US Gulf and Atlantic Coastal Plain (CP).

Turritellines were favored prey of drilling naticid gastropods, suffering significantly greater drilling frequencies (DF) than other gastropods. We compared frequency of naticid drilling on turritellines with diversity and relative abundance of turritellines for 16 formations of Cretaceous through Pliocene age from the Gulf and Atlantic Coastal Plain. We used three measures of diversity: "alpha diversity" (number of turritelline species in our bulk samples of each formation); "gamma diversity" (number of species existing across the CP during the time represented by each formation; and global turritelline species diversity for each interval. Relative abundance was measured as percent of the total gastropod fauna from bulk samples represented by turritellines.

CP and global diversity curves parallel one another with several exceptions (early Paleocene, when CP diversity rose but globally fell; late Paleocene to early Eocene, when global diversity increased and CP diversity decreased; and late Miocene, when a decline in CP diversity was not found globally). DF showed no relationship to alpha diversity and a weak negative correlation with gamma diversity. We found a significant (p<0.05) negative correlation, however, between DF and global turritelline diversity. Relative abundance of turritellines was not correlated with DF, but a positive correlation between abundance and gamma diversity was nearly significant (p<0.10).

These results suggest that diversity inversely tracks drilling predation through time, especially in the global diversity data set. This conclusion is contingent upon CP drilling predation reflecting global temporal patterns in predation, and available data support that inference. The apparent inverse correlation between predation and diversity suggests that drilling predation decreases diversity in turritellines, either by increasing extinction or suppressing speciation.