DO PREDATORY BOREHOLES SIGNIFICANTLY WEAKEN SHELLS: AN ACTUOTAPHONOMIC STUDY USING A ROCK TUMBLER

A large body of paleontological research has been conducted using the boreholes of boring gastropods on durable, shelly prey. For instance, drilling frequency of boreholes has been used to explore evolution and escalation between predator and prey. Drilling frequency is a function of bored valves versus all valves, so any taphonomic or diagenetic process that preferentially destroys either bored valves or unbored valves will affect the calculated drilling frequency. Compression experiments have confirmed that bored valves are less durable than unbored valves undergoing compressive stress. In this study, we will use a rock tumbler to examine whether or not bored valves are also less durable in an impact loading and abrasive environment like the intertidal zone.

We used a total of 129 whole valves of Spisula raveneli (Conrad, 1832) collected from the high tide strandline on the west coast of Egmont Key, Florida. The specimens were sorted by size and then split into three samples with the same size distributions. Each sample was rolled in a rock tumbler with water and three small pebbles for 5 hours at a time for a total of 100 hours. After each 5 hour interval, the specimens were dried and those that were still either whole or fragments that had an intact hinge were weighed and examined for breakage. The specimens were then returned to the tumbler along with all of the broken detritus and rolled for another 5 hour period. At the end of the hundred hours of tumbling, the remains will be examined to determine if any recognizable bore holes are preserved in the fragments.

Mann-Whitney U analysis will be used to determine if the bored valves broke more quickly, and thus were less durable, than valves lacking bore holes. Drilling frequency values will be determined for the pristine and rock tumbled specimens (e.g., at 20 hours, 40 hours, etc.) and those values will then be compared.