DRILLING PREDATION PATTERNS FROM THE MIOCENE CHIPOLA FORMATION OF FLORIDA: TAPHONOMIC VERSUS ENVIRONMENTAL OVERPRINTING

An underlying problem inherent in studying drilling predation is differentiating between environmental and taphonomic variables that may have influenced the patterns recorded in the fossil record. This study examines the frequency of drilling predation on bivalves at three different localities, Farley Creek (FC), Cooter Bluff (CB), and Alum Bluff (AB), within the lower Miocene Chipola Formation (~18.9 Ma), a highly fossiliferous, nearshore, marine unit from Florida’s Panhandle. The FC, CB, and AB have been reconstructed as a protected embayment, a back-reef deposit, and a shallow subtidal environment, respectively. The assemblage-level drilling frequency (ADF) of these environments displays the following trends: 1) FC shows little variability in drilling frequency; 2) CB is more variable, but statistically comparable to FC; and 3) AB had lower ADFs than the other two. Since this variation in ADFs likely results from a combination of environmental and taphonomic factors, to better constrain these processes the degree of etching and bioerosion, L:R valve ratios, as well as grain size were examined. CB and FC samples display low levels of etching and bioerosion and little variation in grain-size distribution. Moreover, the L:R ratio for both samples show values close to 1 (0.93 and 0.95, respectively), and the shell material is virtually pristine. In comparison, the well-preserved shell material from Alum Bluff displays slightly higher degrees of etching and bioerosion, and L:R ratio is lower (0.81) than the other two localities; these suggest that the AB sample has suffered more taphonomic overprinting than the other two which may, in turn, impact the ADF of that sample. In addition, AB is a more time-averaged unit with limited siliciclastic sediment (i.e., a shell bed). This study suggests that, within the Chipola, ADF is potentially impacted by a range of different processes including environmental differences and taphonomy. Thus, simply interpreting these as true biologic signals without considering other aspects that might influence the values could lead to erroneous conclusions.