Spatial variation of naticid gastropod predation in the Eocene of North America

Abstract

Although, the fossil record of naticid gastropod drilling has played an important role in the controversy over predator-prey evolution, little is known about variation of drilling frequencies within single horizons or how predation patterns are influenced by environmental variables. Without an understanding of spatial variation in drilling, temporal patterns in drilling are difficult to interpret. We surveyed 27,554 specimens of molluscs from the Cook Mountain interval (upper middle Eocene) and Jackson Group (late Eocene) of the U.S. Gulf and Atlantic Coastal Plain to document spatial variation in naticid drilling frequencies. The Jackson Group assemblages from the Moodys Branch and Yazoo formations were sampled along a depth gradient in order to compare drilling frequencies to environmental variables within a single climatic zone. There was a statistically significant difference between drilling frequency of the inner to middle shelf Moodys Branch Formation (mean = 8.3%) and the outer shelf Yazoo Formation (mean = 21.2%; chi square = 49.58, P ≪ 0.001), but there was no significant bathymetric trend in drilling frequencies among the five assemblages of the Moodys Branch. Drilling frequency was not related to substrate type within the Moodys Branch. Drilling, however, was correlated with species diversity of Moodys Branch assemblages (r = 0.50, P < 0.01). Drilling frequency was significantly correlated with the percentage of naticids in each assemblage (r = 0.77, P < 0.001) an…