Shells were broken experimentally by compressive and compactional loading to determine the best predictors of shell strength among a number of morphologic (length, width, height, weight, thickness) and taphonomic (presence of a drillhole, exposure to seawater, point contacts of shells) features. Compressive strength of fresh (Mercenaria mercenaria, Mytilus edulis) and beach-collected (Anadara ovalis) shells was measured by placing a single valve flat on a surface and compressing it. Compactional strength of beach-collected Mulinia lateralis shells was measured by placing isolated valves and shell layers in fine sand and compacting the sand and shell mixture.
The most effective predictor of compressive shell strength of fresh shells was thickness; it was significant for all three species and it easily can be interpreted as an obvious defense against a shell-crushing predator. Size was comparatively less important. The presence of a drillhole reduced shell strength significantly, and strong correlations of weight and shell strength among beach collected Anadara shells may reflect different states of degradation of the original amounts of organic matrix. Supporting this conclusion is the observation that immersion in seawater for seven weeks significantly decreased shell strength of organic-rich shells (Mytilus), but did not affect shell strength of organic-poor shells (Mercenaria). Fracture patterns varied considerably between and within taxa. The breakage pattern of Anadara and Mercenaria shells generally consisted of a set of fractures radiating from the point of loading to the shell margin, including even the thick dorsal margin. The breakage pattern in Mytilus usually consisted of one fracture only, which did not extend from the point of loading but whose path was highly unpredictable. In compaction experiments with beach-collected Mulinia lateralis, the most important factor determining whether or how a shell would break was the contact between shells. Isolated shells, both drilled and undrilled, did not break. Among drilled valves in simulated shell beds, only 26 percent of the fragments > 2 mm fractured through the drillhole.