CAUSES OF EPIZOAN LOSS AND SELECTIVE EXCLUSION OF EPIBIONTS IN THE ARTHROPOD FOSSIL RECORD

Examination of fossil and modern crabs demonstrates that epibionts are significantly underrepresented in the fossil record. Attention has focused on the dorsal carapace of the crabs because this is the region that is typically exposed in fossil preparations. The claw and finger fragments that are frequently preserved in the fossil record are difficult to identify and are not heavily invested with epibionts in living forms. Preliminary statistical analysis indicates that fewer than 1 percent of fossil remains exhibit epibionts whereas living crabs are fouled at a higher rate. This pattern of selective exclusion of epibionts from fossil crabs can be assigned different causes. Epibionts adhere largely to the epicuticle, a thin, external layer that readily spalls off during post-mortem degradation. When the epicuticle is lost, the attached epibionts are lost as well. Further, many of the epibionts observed on living crabs, such as bryozoans and many worms, are not strongly calcified. Thus, their potential for preservation is low even if the epicuticle is not lost. Finally, it is possible that a higher proportion of fossil crabs possess epibionts, but they are not readily observed on specimens. Many, if not most, crabs are preserved in concretions. When concretions are opened, the separation of part and counterpart typically occurs between the exocuticle and the endocuticle; with the outermost layers of the cuticle adhering to the counterpart. If epibionts were present, they would not be readily visible for this reason. The analysis of biotic interactions involving attaching and encrusting organisms on fossil decapods can only be advanced by much more careful preparation of material and by more thoroughly examining counterparts of the fossils.