EFFECTS OF VALVE MICROSTRUCTURE ON STRENGTH AND PRESERVATION POTENTIAL OF TWO MODERN CHITON (MOLLUSCA, POLYPLACOPHORA) SPECIES

Most chiton (Mollusca, Polyplacophora) fossils consist of disarticulated, relatively poorly preserved, and often fragmented valves. Resistance to fragmentation and other taphonomic processes depends partly on shell strength, which is defined and measured as the maximum compressive force needed to break the shell. The strength of modern Katharina tunicata and Mopalia muscosa valves collected from San Juan Island, Washington, USA was experimentally measured by breaking disarticulated valves in the laboratory using a rock press. Katharina tunicata intermediate valves were significantly stronger (p < 0.001) than M. muscosa intermediate valves, requiring more force to fracture. Observed differences in preservation potential between the two species in previous studies thus is at least partly due to the relatively greater strength of K. tunicata valves relative to M. muscosa valves. The strength of head and tail valves was not significantly different between species, but M. muscosa head and tails tended to be stronger than K. tunicata head and tails, consistent with the preservation potential differences previously observed. Thickness was the most consistent predictor of valve strength, although mass, length, width, height and thickness all significantly (p < 0.05) correlated to strength for intermediate valves of the two species. Only thickness of the head valves and height of the tail valves significantly correlated to strength for both species. Qualitative differences in the fracture patterns between species and among valve types suggested strong microstructural control of fracture propagation. Analysis of microstructure using etched thin sections and scanning electron microscopy showed that the type of structure used to construct the intermediate valves differs. Katharina tunicata valves consist mostly of complex crossed lamellar (CCL) whereas mostly crossed lamellar (CL) structure comprise Mopalia muscosa valves. Experimental results in the literature on bivalves suggest that shells with CCL are stronger and more resistant to taphonomic processes of abrasion and dissolution than shells with CL structure. Microstructure thus is an important factor in the higher preservation potential of K. tunicata intermediate valves relative to M. muscosa valves observed in previous studies.