Experimental measurement of shell strength and its taphonomic interpretation

Shells were broken experimentally by compressive and compactional loading t o determine the best predictors of shell strength among a number of morphol ogic (length, width, height, weight, thickness) and taphonomic (presence of a drillhole, exposure to seawater, point contacts of shells) features. Com pressive strength of fresh (Mercenaria mercenaria, Mytilus edulis) and beac h-collected (Anadara ovalis) shells was measured by placing a single valve flat on a surface and compressing if. Compactional strength of beach-collec ted Mulinia lateralis shells was measured by placing isolated valves and sh ell 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 b e interpreted as an obvious defense against a shell-crushing predator. Size was comparatively less important. The presence of a drillhole reduced shel l strength significantly and strong correlations of weight and shell streng th among beach collected Anadara shells may reflect different states of deg radation of the original amounts of organic matrix. Supporting this conclus ion is the observation that immersion, in seawater for seven weeks signific antly decreased shell strength of organic-rich shells (Mytilus), but did no t affect shell strength of organic-poor shells (Mercenaria). Fracture patte rns varied considerably between and within taxa. The breakage pattern of An adara and Mercenaria shells generally consisted of a set of fractures radia ting from the point of loading to the shell margin, including even the thic k dorsal margin. The breakage pattern in Mytilus usually consisted of one f racture 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 s hell would break was the contact between shells. Isolated shells, both dril led and undrilled, did not break. Among drilled valves in simulated shell b eds, only 26 percent of the fragments > 2 mm fractured through the drillhol e.