ISOTOPIC AND TAPHONOMIC STUDY OF FOSSIL GLYCYMERIS BIVALVE SHELLS FROM THE NORTH CAROLINA PLEISTOCENE: EXAMINING SEASONALITY OF PREDATION OF NATICIDS AND DUROPHAGES

Taphonomy, drilling predation, and repair scars were studied for 534 Glycymeris bivalves from the Pleistocene lower Waccamaw Formation near Old Dock, North Carolina. Drillholes on the bivalves were caused by naticid gastropods and repair scars likely formed in response to attacks by durophagous predators. We aimed to understand seasonality of predation by naticid and durophagous predators on Glycymeris prey.

 

Shells with significant taphonomic damage had a high drilling frequency indicating that drilled shells are no more susceptible to breakage than shells without drillholes. However, we found repair scars to be less recognizable on shells with heavy taphonomic alterations. Shells with taphonomic damage were excluded from isotopic study.

 

Isotopic profiles along the growth axis for six Glycymeris specimens indicated the seasonal variation in δ¹⁸O values. Sub-samples milled from the margin of 189 well-preserved shells (74 with complete drillholes and 115 undrilled) provided δ¹⁸O values indicating the last season the bivalves secreted shell material (a proxy for season of death). The warm season (δ¹⁸O < 0.61‰) yielded 44 drilled and 48 undrilled, the intermediate season (0.61‰ < δ¹⁸O < 2.02‰) yielded 22 drilled and 58 undrilled, and the cold season (δ¹⁸O > 2.02‰) yielded 8 drilled and 9 undrilled shell deaths. Significantly more drilling occurred and significantly more total deaths occurred in warm than cold seasons. The fewest deaths occurred in cold seasons. It is likely naticid gastropods more actively drilled Glycymeris in warm water temperatures due to higher metabolism. These results also suggest that, aside from drilling predation, heat stress contributed to mortality of Glycymeris.

 

Samples were milled from 30 repair scars on 25 shells with scars. The δ¹⁸O values from the site of the scar indicated the season the bivalve was damaged and started to repair its shell (i.e., season of durophagous attack on the bivalve). The δ¹⁸O values at the repair scars showed no significant difference in number of repairs by season (10 repairs from warm, 13 from intermediate, and 7 from cold seasons). These results suggest that durophagous predators were active at all times of the year. The naticid gastropods and durophages were active during the same season; therefore, the potential for emergent effects of predators exists.