TEMPORAL AND SPATIAL ANALYSIS OF NATICID DRILLING PREDATION ON SCAPHARCA LESUEURI FROM THE EARLY OLIGOCENE BYRAM FORMATION IN MISSISSIPPI

Previous work examining patterns of faunal stability at an outcrop of the Oligocene Byram Formation near Vicksburg, MS, showed very similar fossiliferous horizons reappearing through time (Visaggi & Ivany, 2010). Molluscan paleocommunities were interpreted as persistent using data based on presence-absence and guilds, particularly when rare species were excluded. However, faunal change was suggested by fluctuating abundances. All assemblages were dominated by bivalves Scapharca lesueuri and Corbula laqueata, both of which were preyed upon by naticid gastropods. The purpose of this study is to examine patterns in drilling predation of these primary taxa in comparison to existing interpretations of stability or flux in these paleocommunities, as well as to data collected on drilling in the Byram at other localities.

Ten replicate samples were collected from each of three horizons (C, E, G) by Visaggi in 2003. Earlier work utilized all specimens with umbos, but the current study restricted data to nearly whole valves (>85%) for documentation of naticid drilling. Valves were size binned in 5 mm increments into four groups (20 mm maximum), and were examined for complete or incomplete drillholes. Data from the 10 replicates were combined into three aggregate samples consistent with prior work. Drilling frequencies (DF: number of drilled valves divided by half the total valves) were calculated.

Scapharca lesueuri from horizons C and E yielded ~1300 and ~2850 valves respectively, representing 57% and 70% of the samples from the original work. DFs varied between 7–10% for C and 11–16% for E. Most specimens were <10 mm (C=88%; E=92%) and >90% of all drillholes were recorded in these size classes for both horizons. Chi-square analysis indicated no differences in drilling between pairs of aggregate samples from the same horizons except for E2 (11%) vs. E3 (16%) at p = 0.019. Differences were detected for overall DFs between C (8%) and E (13%) at p=0.001; pairwise comparisons of aggregate samples across different horizons revealed mixed results. Scapharca collected in the Byram by Kelley & Hansen (1993) exhibited comparable DFs of 13% (MGS 106E) and 16% (MGS 106N), with drilling focused on the most populated size classes <10 mm. Future work will incorporate data from horizon G and DFs for the other dominant bivalve, Corbula laqueata.