EXPERIMENTAL ABIOTIC SHELL FRAGMENTATION IN MERCENARIA MERCENARIA

The study of crushing predation on shelly fossils is confounded by the effects of post-mortem, abiotic fragmentation. To address this, we tested three fragmentation processes and characterized the resulting damage on clam shells.

We purchased live Mercenaria mercenaria (“littleneck” clams), then shucked, cleaned, and separated the valves. Three modes of breakage were tested: crushing, tumbling, and dropping. Shell crushing was accomplished by placing the valve, convex-up, between the plates of a metal vise. The vise was tightened until the shell failed. For dropping damage, the valve was dropped 2 m onto a cement floor. For tumbling, the valve was placed in a rock tumbler with large quartz grains (~2-6 cm diameter); the tumbler was run for 30-60 minutes, with at least one stop part-way through to monitor shell damage. For each breakage mode, the damaged valve was retrieved and photographed. Breakage edges were described as straight, divoted (v-shaped), or scalloped (curved). The broken surfaces were categorized as sharp or rounded. Breakage location on the valve was also noted.

Among the crushed and dropped shells, breakage edges were mostly straight, and breakage tended to originate from the central area of the valve (the point in contact with the vise plates, or the point that hit the ground first). The tumbled shells exhibited a combination of straight damage originating from the center of the shell and scalloped or divoted damage concentrated along the shell margin. This is likely because tumbled valves had many points of repeated contact with the surrounding rocks.

Tumbling was most likely to result in predation-like damage, with scalloped chips along the shell margin that could be confused with those made by “nibbling” predators. However, the broken surfaces were rounded, indicating that abrasion happens very quickly in a high-energy environment where such “tumbling” may take place. Thus, scalloped-but-sharp breakage may rule out natural tumbling as a cause of damage (and strengthen the argument for predation). Previous studies have found that tumbling in finer-grain media (such as granules or sand) does not result in such breakage, but rather just abrades the shell. Thus, scalloped-but-sharp breakage in a lower-energy (finer-sediment) environment may be a good indicator of predation.