CHEMICAL FINGERPRINTING OF PIGMENT-BASED COLOR IN FOSSIL INSECTS

The color of ancient organisms is typically elusive. This obscures the evolutionary history of the diverse colors and color patterns of modern animals. Recent synchrotron-X-ray-fluorescence (XRF) studies of fossil feathers have revealed chemical evidence of melanin, with significant implications for reconstructions of the original coloration and visual ecology of feathered dinosaurs and fossil birds. The extent to which trace element chemistry can identify evidence of pigments in other taxa, however, is uncertain. Fossil insects commonly exhibit monochromatic patterning that resembles melanin-based patterning in extant insects, but the origins of the fossil patterning has not been investigated. Here, we resolve these issues using macro and micro-XRF and electron microprobe analyses of fossil and modern insects that exhibit striking color patterning. All analyses of fossil insects confirm that the spatial distributions of melanin-chelated metals, particularly Cu (and, to a lesser extent, Ca), correspond to darker tones. Analyses of modern insects, however, reveal the opposite trend (i.e. Cu is concentrated in regions with light tones) or are inconclusive. These data suggest that inferences of the original coloration of fossil insects based upon trace element distributions alone should be treated with caution. Combination of elemental mapping techniques with other chemical analyses and data on depositional context and diagenetic history is critical to resolving the origins of color patterning in fossil insects.