Spatial Variation of Naticid Gastropod Predation in the Eocene of North-America

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 \u3c 0.01). Drilling frequency was significantly correlated with the percentage of naticids in each assemblage (r = 0.77, P \u3c 0.001) and the percentage of certain prey species of the bivalve families Lucinidae and Corbulidae, and the gastropod families Turritellidae and Hipponicidae (r = 0.80, P \u3c 0.001). These preferred species represented 40% of the naticid victims, but only 16% of the assemblages. Cook Mountain assemblages represented marine inner to middle shelf environments from Virginia, Alabama, Louisiana, and Texas. Naticid drilling frequencies of the entire molluscan fauna ranged from 6.8-38.7% with a mean of 22%. In the Cook Mountain, three preferred species (of the families Turritellidae, Lucinidae, and Noetiidae), representing 24% of the total individuals, accounted for 44% of the drilled individuals. As in the Moodys Branch Formation, Cook Mountain assemblages showed a significant correlation between relative abundance of preferred species and drilling frequency of samples (r = 0.585, P \u3c 0.05). The dependence of naticid drilling on the availability of preferred prey species indicates a possible behavioral inflexibility in naticid predation patterns. The Virginia Cook Mountain fauna exhibited significantly greater drilling frequencies (mean = 30.3%, n = 4 samples) than did the Gulf Coast assemblages (mean = 18.0%, n = 11 samples; Mann-Whitney U-test, P \u3c 0.01). This result is consistent with a trend found by some previous workers of an equatorward decrease in naticid drilling