THE SEARCH FOR EPIBIONTS ON FOSSIL DECAPOD CRUSTACEA AND TRILOBITES: DOES THE CLUE LIE IN PRESERVATION OF THE EPICUTICULAR LAYER?

Epibionts on benthic organisms provide useful information regarding ecologic conditions that may not be readily inferred simply by studying the host organism. Within the decapod Crustacea, macroscopic epibionts, including corals, bryozoans, barnacles, and vermes, are typically limited to those taxa that dwell on the substrate most of the time; active burrowers tend to be far less frequently encrusted. Further, the attached organisms exhibit a non-random distribution on the host, seeking areas that are less likely to be fouled by sediment and more likely to benefit from water current flow generated by the host. Although common on many living decapods, epibionts are relatively rare on fossil decapods and are even more rare on trilobites. When present, they occupy positions predicted by examination of living decapods. It is hypothesized that the scarcity of epibionts on fossil decapods and on trilobites results from the attachment of the organisms to the epicuticle, the outermost layer of the cuticle, of the host, a layer that is extremely thin, non-calcified, and typically not preserved in the fossil record. The presence of epicuticle is universal in living decapods, has been observed to be the site of attachments of epibionts in living decapods, and has been observed occasionally in preparations of fossil cuticle. The layer has been described on a few extraordinarily preserved trilobites and is suggested to be a universal feature of trilobite cuticle, but is typically absent due to alteration and obliteration of shell structure during preservation. Current preliminary observations on polished sections suggest that epicuticle may be present beneath epibionts on both decapod and trilobite fossils, suggesting that this may be a good potential site to study the nature of the epicuticular layer in these groups. The preliminary observations will be tested using thin section and SEM techniques. The results may provide valuable insight into the architecture of trilobite cuticular structure. Further, the work will permit a more realistic assessment of the nature of epibiont preservation in the fossil record. Absence of epibionts may be largely a result of the poor preservation potential of epicuticle rather than original absence of the encrusters.