TEMPORAL AND SPATIAL PATTERNS IN NATICID DRILLING PREDATION OF DOMINANT BIVALVE SPECIES IN THE BYRAM FORMATION OF MISSISSIPPI

Very similar fossiliferous horizons reappear up section at an exposure of the early Oligocene Byram Formation near Vicksburg, MS. Previously, Visaggi & Ivany (2010) analyzed >21,000 shells from this outcrop in studying the influence of data choice and analysis on interpretations of faunal stability. Use of presence-absence data and guilds supported the persistence of paleocommunities, especially if rare species were removed. Incorporating various forms of abundance data implied faunal change instead. Scapharca lesueuri and Corbula laqueata repeatedly dominated these molluscan assemblages, making up ~75% of individual samples collected from each horizon (C, E, G). These species showed evidence of predation by naticid gastropods, but drillholes were not explicitly studied in the original work. Data on predatory drilling by naticids in both arcid and corbulid bivalves are examined here in space and time.

Originally, ~10,200 Scapharca lesueuri and ~4725 Corbula laqueata (right valves only) were documented. However, in restricting the dataset to nearly whole valves (>85%) for analysis of drilling predation, only ~6475 S. lesueuri and ~4100 C. laqueata were retained. Specimens containing complete drillholes, incomplete drillholes, and multiple drillholes were recorded. Of the 10 replicate samples collected per horizon, data were aggregated into larger samples for comparative analysis consistent with prior work. Drilling frequency, prey effectiveness, and MULT were determined following other predation studies. Naticids were also examined as probable predators and for signs of cannibalism (noted in each horizon).

Drilling frequency (DF) for Corbula laqueata ranged from 9%-11% in C, 6%-13% in E, and 9%-12% in G. Scapharca lesueuri DFs were 8%-11% in C, 12%-16% in E, and 11%-15% in G. Kelley & Hansen (1993) reported similar DFs for S. lesueuri of the Byram Formation elsewhere in Mississippi at 13% (MGS 106E) and 16% (MGS 106N). Their work revealed comparable levels of drilling at 12% for C. laqueata as well. Prey effectiveness calculated by horizon in this study yielded 7% (C), 11% (E), 10% (G) for C. laqueata and 6% (C), 2% (E), 4% (G) for S. lesueuri. Multiple drillholes were not common among S. lesueuri, but combinations of both complete and incomplete drillholes were observed often in corbulid specimens.