AN EXPERIMENTAL APPROACH FOR INTERPRETING SHARK TOOTH TAPHONOMY

Late Cretaceous shark teeth recently collected from the Judith River Formation (JRF) in Montana exhibit different breakage and wear patterns, some of which [presumably] occurred postmortem. To simulate postmortem wear on shed shark teeth, a sample of 24 Carcharias taurus (shark) teeth and 8 Rhinoptera bonasus (ray) teeth were left in lapidary tumblers in a mixture of sand (from the JRF) and salt water for a period of 500 hours (approximately equivalent to 465 km calculated from the size and rotation rate of the barrels). Digital images and measurements (height, width, and mass) of teeth before and after tumbling show a marked difference. The approximate rates of loss are, height (mm/hr): 0.0045 (shark), 0.0005 (ray); width (mm/hr): 0.0045 (shark), 0.0003 (ray); and mass (g/hr): 6e ^-5 (shark), and 9e ^-6 (ray) where one hour of travel is approximated to be 0.931 km.

Mean values of measurements for all teeth were taken at 50 hour intervals throughout the entire 500 hour length of the experiment. For the shark teeth (n = 24), the means measured at each 50 hour interval varied significantly (minimum p = 0.0005) from each succeeding or preceding interval. The ray teeth (n = 8) in general showed less significant changes at 50 hour intervals, although some individual teeth did exhibit significant changes from one interval to the next. Additionally, significance correlation for the rate of loss (height, width, and mass) for each individual tooth was found to be 99% with the exception of 4 teeth (3 of which are significant at 95% and one tooth which is not found to be significant) out of a total of 32 teeth.

This type of experimental approach provides data that (1) help distinguish autochthonous from allochthonous fossil vertebrate hard parts, (2) quantify the amount of transport and wear, and (3) illustrate the potential of postmortem effects to cause the loss or distortion of diagnostic skeletal features.