PATTERNS OF MOLLUSCAN PREDATION IN THE OLIGOCENE BYRAM FORMATION OF VICKSBURG, MISSISSIPPI

Multiple lines of evidence are necessary in testing faunal persistence in the fossil record. Visaggi & Ivany (2010) did an analysis of recurring fossiliferous horizons from the Oligocene Byram Formation in Vicksburg, MS, in which stability was examined using a variety of data and methodological approaches. Use of presence-absence data excluding rare species revealed stasis in paleocommunities, though incorporation of various abundance data more often indicated faunal change. Predation was not analyzed at the time, but the presence of complete and incomplete drillholes, as well as specimens with multiple drillholes was observed. Study of drilling was later revisited by several co-authors here in examining the two most abundant taxa, Scapharca leseuri and Corbula laqueata, which make up ~75% of individuals collected from all three horizons (C, E, and G). Drilling frequencies and prey effectiveness for these dominant taxa showed statistical similarities supporting ecological stability across horizons. This now expanded study investigates predation in the remaining 81 genera to provide a comprehensive comparison of drilling patterns across time.

Out of 83 bivalve and gastropod genera in the samples, 43 showed signs of drilling (34 in C, 30 in E, and 32 in G). Fourteen genera were drilled in all horizons, 16 were drilled in two horizons, and 17 were drilled in a single horizon. Absence of drilling more often reflected the lack of a rarer species in a horizon. All shells have not yet been recounted as whole vs. fragmented specimens, which is necessary to calculate drilling frequencies (DF). However, all 21,274 shells have been examined for drillholes (~1150 overall) and preliminary results indicate that the number of drillholes in whole shells compared to the number of total shells in each horizon indicate similar levels of drilling predation. If shell fragmentation is similar across horizons, DF should indicate ecological persistence. Taxa that are both common and more frequently drilled across horizons include Corbula, Crassinella, Euspira, Natica, Scapharca, and Turritella. Ongoing work will include separation of whole vs fragmented specimens in order to calculate DF and prey effectiveness as well as to do additional research in exploring relationships to abundance, size, and other data on drilling in this formation.