ISOTOPIC AND TAPHONOMIC STUDY OF FOSSIL GLYCYMERIS BIVALVE SHELLS: UNDERSTANDING THE PREDATION HABITS OF NATICID GASTROPODS AND DUROPHAGOUS PREDATORS AND THE PALEOECOLOGY OF THE NORTH CAROLINA PLEISTOCENE

Taphonomy, drilling predation, and repair scars can be used to gain insight into predator-prey relationships. These tools were applied to 534 Glycymeris bivalves collected from the Lower Waccamaw Formation near Old Dock, NC. Drillholes on the bivalves were caused by naticid gastropods and repair scars were likely caused by durophagous (shell-crushing) predators. The initial stage of this study investigated the possible effects of taphonomic bias on predation studies. The taphonomic grade and size of the shells were analyzed and drilling frequency (DF) and repair scar frequency (RF) were assessed within each category and compared. A high DF of shells that accumulated significant taphonomic damage has allowed us to determine that shells with drillholes are no less susceptible to breakage than shells without drillholes. However, trends in RF indicate a bias exists and repair scars are less recognizable on shells with heavy taphonomic alterations.

The second stage of this study aims to understand the seasonality of predation by naticid and durophagous predators on the Glycymeris prey. The taphonomic study determined which shells were suitable for isotopic study. Milled and analyzed samples from the margin of the suitable shells will provide δO18 values, which will indicate the last season the bivalves secreted shell material; these values can be used as a proxy for the season of death of the bivalve. Season of death of bivalves with and without predation traces will be used to gain an understanding of Pleistocene ecology and to test hypotheses about the seasonality of predation. Undrilled shells dying in the summer could indicate bivalve death from heat stress or (if a scar is present) from durophages. A scarcity of drilled shells preyed upon in the summer would show that naticids did not feed on bivalves in the summer possibly due to the naticids being heat stressed, especially if drilled bivalves died at other times of the year. Alternatively, increased durophagous predation could have made bivalve prey unavailable to naticids in the summer. Isotopic work is ongoing.