IMPACT OF ENVIRONMENTAL VARIABILITY ON DRILLING PREDATION: A COMPARISON AMONG TURRITELLA-DOMINATED ASSEMBLAGES

Studies on drilling predation on turritelline gastropods from the Cretaceous onward are relatively common. However, few studies exist that examine predation of turritellines from the unique occurrences known as Turritella-dominated assemblages (TDAs). A TDA is an assemblage in which turritelline gastropods are 20% of the macroscopic individuals and also are at least twice as abundant as any other macrofauna in the assemblage. In this study, a TDA from the Pliocene Jackson Bluff Formation of Florida was examined to quantify the percentage of individuals drilled in this assemblage (i.e., drilling frequency; DF) by members of the family Naticidae and Muricidae on the gastropod genus Turritella. The Jackson Bluff Formation assemblage has a DF value of 14.6% that is comparable to the mean DF of Turritella during the Pliocene derived from five published studies representing 27 different species within the clade where the number of individuals was at least 15. However, a 1997 study on a TDA assemblage from the Pliocene Pinecrest Beds of Florida documented a DF of 30.9%. Another TDA assemblage from the latest Maastrichtian Infratrappean bed of Rajahmundry, India, found a DF of 24.1% among turritellids. Variability in DF within an assemblage can vary due to prey or predator availability. Owing to the fact that these assemblages are Turritella dominated, the effect of prey abundance is expected to be inconsequential. Therefore, any variability in DF seen among these three TDAs may be attributed to environmental controls of predator abundance. Whereas the other two studies show higher DFs than the Jackson Bluff Formation regardless of copious available prey, the environments vary remarkably. The low-salinity lagoonal setting of the Jackson Bluff Formation likely had lower abundances of shell-drilling predators, whereas the Pinecrest and Infratrappean beds represent normal marine conditions and likely a greater prevalence of Turritella predators. It is possible that environmental conditions play a larger role in predation frequencies than prey abundance and that these environmental variations can account for the spatial and temporal variability in DFs recorded in the fossil record.