UNDERSTANDING THE PRESERVATION OF STRUCTURAL COLOR IN FOSSIL INSECTS

Structural colors are generated by coherent scattering of light by complex nanometer-scale variations in tissue structure and are the most intense, reflective and pure colors in nature. In particular, iridescent and metallic colors are widespread among modern insects, but are preserved rarely in insect fossils. This suggests that a specific, unknown, set of taphonomic circumstances are required for preservation of structural colors in the fossil record. Here we present the results of the first systematic investigation of the ultrastructural and spectral fidelity of structurally colored tissues in fossil insects. Approximately 700 specimens of beetle taxa known to exhibit structural color were studied from six fossil Lagerstätten (from Oligocene to Pliocene in age). Differences in the quality of structural color preservation among these biotas are statistically significant. These differences do not correlate with the age of the biota; they are also independent of stratigraphic and sedimentological context and, by extension, short- to medium-term changes in environmental conditions. Instead, the variation between biotas is attributed to differences in the late diagenetic history (e.g., the nature of diagenetic fluid flow and recent weathering) of the host sediments. SEM and TEM analyses of cuticle from beetle specimens with well-preserved metallic colors demonstrate that the color is invariably generated by an epicuticular multilayer reflector. In modern insects, multilayer reflectors occur commonly in other cuticular layers, in addition to within the epicuticle. An epicuticular location may therefore enhance the preservation potential of fossil multilayer reflectors and of structural color in fossil insects. Further, preservation of an epicuticular multilayer reflector correlates positively with preservation of other cuticular ultrastructures. The latter is therefore the basis of a predictive model for the presence of multilayer reflectors in fossil beetles. This research has also identified a number of factors that cause, or contribute to, loss of structural color in fossil beetles after their collection. This has important implications for curatorial practice.