ISOTOPE SCLEROCHRONOLOGY OF LATE PLIOCENE TURRITELLIDAE (GASTROPODA) FROM SOUTHEASTERN NORTH CAROLINA

The hypothesis that a decline in nutrients led to evolutionary turnover among mollusks during the Plio-Pleistocene in the western Atlantic has been widely proposed (e.g., Allmon et al. 1993, Todd et al. 2001). Attempts to test this hypothesis have been made in other regions of the western Atlantic, such as Florida (Lavarreda et. al. 2007), but little has been done to test this hypothesis farther north, where temperature changes may have been more important. Turritelline gastropods (Family Turritellidae) are mainly suspension feeders, and so would be expected to be particularly sensitive to changes in primary productivity. We are thus examining growth rates in Plio-Pleistocene turritellines from North Carolina to compare to results of similar studies in Florida and elsewhere. Specimens were collected from two units: the Duplin Formation (pre-turnover) and lower Waccamaw Formation (following the first turnover pulse). The Duplin consists of shelly sand, sandy marl, limestone and is of mid-late Pliocene age. The Waccamaw consists of fossiliferous clay and fine-grained sand and is latest Pliocene. We performed δ¹⁸O isotope analysis of samples of shell material from each whorl of each specimen and used whorl number vs. age plots to estimate growth rate (a conservative estimate that is somewhat independent of body size). The species analyzed were the largest complete turritelline specimens from their respective formations. Analyses were conducted on one Turritella etiwanensis? specimen (length ~ 70 mm) collected from the Duplin near Lumberton, NC. Results revealed that this specimen had a lifespan of ~ 2.5 years and a growth rate slightly lower than two turritelline species from the mid-late Pliocene Pinecrest Sand of Florida, but slightly higher than that of a recent turritelline in high-nutrient areas of the Gulf of California. Preliminary analysis of several specimens of Turritella subannulata? (average length 28 mm), collected from the lower Waccamaw near Old Dock, NC, was inconclusive, but suggestive of average lifespans of 3-3.5 years and lower growth rates. If these results are confirmed, they would indicate that turritellines from the Duplin grew faster than those from the Waccamaw, consistent with the hypothesis of a drop in productivity as a leading cause of Plio-Pleistocene molluscan turnover.