INVESTIGATION OF DAMAGE PATTERNS INDUCED BY CRUSHING OF BIVALVE SHELLS

Ecologists and paleoecologists use predatory traces to determine a wide variety of biotic interactions such as mortality patterns, prey preference, and attack technique. Due to the important and widespread study of predatory traces, apparent damage on fossils should be classified in order to distinguish between biotic and abiotic damage. Due to the difficulty in determining if predatory damage is present in taphonomically crushed fossils, different observers can interpret the cause of damage on hard shell fossils disparately. Here, we examined the differences in abiotic breakage patterns among temperature conditions to determine if similarities existed with biotic crushing. We chose to look at temperature to determine if it affected the shells’ structural integrity and thereby the way in which they break.

In order to investigate this question, we placed freshly-killed littleneck clam (Mercenaria mercenaria) shells into one of three ten-gallon tanks, set up with cool, room temperature, and warm water. After three weeks, four shells from each tank were removed and crushed using a vise. Damage was classified based on the number of shell fragments, orientation of damage to growth lines, location on the shell, and the shape of the damage. Damage shapes included scalloped, straight, and divoted. Care was taken to use the minimum pressure needed to crush the specimens for three shells of each temperature. The remaining shell from each temperature condition was then subjected to an additional two full turns of the vise after its initial break.

Across all conditions breaks were predominantly straight and perpendicular to growth lines. In scenarios where excessive pressure was applied, breaks were straight and perpendicular near the commissure but became increasingly parallel near the umbo. We did not observe the scalloped or V-shaped breakage patterns that are considered indicative of predatory attack. Based upon our investigation, temperature alone would not cause misidentification of the source of shell damage. Future investigation will increase the sample size of shells examined, and introduce additional environmental factors (e.g., salinity) and abiotic damage sources (e.g., tumbling).