Experimental fragmentation of gastropod shells by sediment compaction: Implications for interpreting drilling predation intensities in the fossil record
Abstract Compaction is one of the taphonomic processes responsible for fragmenting invertebrate shells, thereby removing them from the fossil record. Studies of drilling predation intensities in the fossil record depend on equal preservation of drilled and undrilled shells. However, predatory drill holes may weaken shells, and these shells may be preferentially destroyed during compaction, thus overprinting predation signals in the fossil record. Previous studies have experimentally compacted drilled and undrilled bivalve valves. Such experiments have not previously been performed with gastropod shells, which may respond differently to compaction. To test if drilled gastropod shells are preferentially removed by compaction, 227 undrilled and 28 drilled Olivella biplicata shells were subjected to compaction (70,602 N) within a sediment column. Multiple shells were tested together but were given ample space so as not to rest on one another, thereby avoiding forming point contacts. Many drilled and undrilled shells possessed aperture damage, removed apices, pinhole bores, and spionid worm bores, which in conjunction with shell size, were hypothesized to lower compactional shell strength. Shell strength was assessed by using the frequency of shell failure. Binomial logistic regression models were utilized to identify the variables controlling shell failure. Three models were created each using the whole O. biplicata sample. Each model used a unique and different definition of shell strength: any damage, breakage, and obliteration (