Traces of Predation/Parasitism Recorded in Eocene Brachiopods from the Castle Hayne Limestone, North Carolina, U.S.A.

Abstract

The Castle Hayne Limestone (Middle Eocene, North Carolina), noted for its diverse macro-invertebrate fossils, was sampled to assess if early Cenozoic brachiopods from eastern North America record any traces of biotic interactions. Systematic surveys of two North Carolina quarries yielded 494 brachiopods, dominated by one species: Plicatoria wilmingtonensis (Lyell and Sowerby, 1845). Despite subtle variations in taphonomy, taxonomy, and drilling patterns, the two sampled quarries are remarkably similar in terms of quantitative and qualitative paleoecological and taphonomic patterns. Ninety-two brachiopod shells (18.6% specimens) contained a single circular hole. Majority of drillholes were singular, perpendicular to shell surface, and drilled from the outside. In addition, ventral valves were drilled slightly more frequently than dorsal ones and larger brachiopods contained more drillholes than smaller ones. However, the size of drillholes did not correlate with the size of brachiopods. The drillholes record "live-live" biotic interactions, which may represent either predatory attacks or parasitic infestations or combination of those two types of interactions. A notable fraction of specimens bears multiple drillholes, which is consistent with either parasitic nature of interactions or frequent failed predatory events. Drilling frequency was high in both quarries (24.5%); this high frequency reinforces other recent reports (from other continents and Cenozoic epochs) that drilling organisms may be a frequent predator or parasite of brachiopod prey or hosts. The number of case studies reporting high frequencies of drilling in brachiopods is still limited and thus insufficient to draw reliable generalizations regarding the causes and consequences of these occasionally intense ecological interactions.