PASSIVE DETERIORATION OF SHELLY MATERIAL - A STUDY OF THE RECENT EASTERN PACIFIC ARTICULATE BRACHIOPOD TEREBRATALIA-TRANSVERSA SOWERBY

The preservation potential of a brachiopod shell is in part determined by the post-mortem loss in its mechanical resilience to physical dest ruction. The loss of mechanical resistance, whether due to the decay o f the shell's organic matrix or other processes, can be observed using engineering testing techniques. Tests at various post-mortem interval s on evacuated shells of a population of the extant eastern Pacific te rebratulid brachiopod from the San Juan Islands, Terebratalia transver sa, revealed that a measurable decrease in preservation potential had occurred. This decrease is demonstrated by the loss of structural inte grity of both the hinging mechanism in articulated specimens, and of t he shell walls of the individual valves. For the hinging mechanism, th is loss in preservation was evidenced by an increase in the gape angle of the hinge at failure at greater post-mortem intervals when the hin ge was disarticulated by forcing the valves open. The loss of structur al integrity of the disarticulated valves was demonstrated by a tenden cy to fracture under compressive forces into a greater number of fragm ents for the same input of work at longer, but still relatively brief, postmortem intervals. The less massive, less convex brachial valves f ractured into a greater number of fragments at a lower input of energy than the more robust and convex pedicle valves, implying a natural pr eservational bias of pedicle over brachial valves for T. transversa in compression.