VERMEIJ CRUSHING ANALYSIS: ESTIMATING PREDATION MORTALITY IN FOSSIL GASTROPOD ASSEMBLAGES

Predation is a major force in the evolution and ecology of shelled animals, but the study of crushing predation is hampered by the difficulty of accurately identifying and quantifying predatory shell damage. The authors previously presented Vermeij Crushing Analysis (Vermeij, 1982; Stafford and Leighton, 2011) as a method of estimating crushing mortality in modern gastropod assemblages. Estimating crushing mortality from shell damage alone is impeded by taphonomy: some damage may be environmental rather than predatory. VCA establishes a baseline of taphonomic damage using drilled shells. Shells with complete predatory drillholes were killed by gastropod predators; any additional damage is taphonomic. The taphonomic baseline is subtracted from the damage frequency of undrilled shells, yielding the frequency of damage due solely to crushing predation.

In the present study, we apply VCA to fossil gastropods from the Miocene St. Mary’s Formation of Maryland, including Bulliopsis, a bullet-shaped burrowing predator and scavenger. Preliminary data on Bulliopsis suggest that certain types of damage occur more frequently in undrilled shells than drilled shells, indicating that this damage was predatory. “Extensive aperture chips” (chipping 10-90º around the body whorl, from the aperture) and “Major body whorl damage” (chipping 90-180º around the whorl) are more frequent among undrilled shells (23%, 17%) than drilled shells (8%, 8%). Using the drilled damage frequencies to correct for taphonomy, the resulting frequencies due to crushing predation are 15% and 9% (estimated crushing mortality = 15%). These two types of damage were also identified as predatory in our previous work with modern Olivella (estimated crushing mortality = 16%), a genus similar in shape, size, and life-mode to Bulliopsis, bolstering our conclusion that these particular types of damage were caused by crushing predators rather than taphonomy. Despite the fact that Bulliopsis is 11 million years older than the modern Olivella, the nature of the damage to the shells and the types of damage identified as predatory are remarkably similar. Further study of the St. Mary’s Formation gastropods, and comparison with similar modern forms, will illuminate the role of crushing predators in this ancient environment.