When animals turn into fossils, they become faded, dingy semblances of their former selves. Even if original tissue remains, pigments can be altered over the eons. Now a pair of researchers has shown a way to deduce the true color of iridescent arthropods by measuring reflective layers on their exoskeletons.
Many animals are tinted with pigments, but these compounds aren't the only source of color. Minuscule ridges called diffraction gratings reflect and split light into various wavelengths. Iridescent color can also arise from surfaces called multilayer reflectors, which are found on some butterfly wings, bird feathers, and beetle carapaces.
Sparkling colors have also been seen on 50-million-year-old beetles from Messel, Germany, a treasure-trove of exquisitely well-preserved fossils. Such color is rare, and indeed it fades away as soon as these water-rich fossils dry out. Andrew Parker, a zoologist at the University of Oxford, U.K., teamed up with David McKenzie, a physicist at the University of Sydney, Australia, to study a specimen with blue coverings on its wings. Looking with an electron microscope, they found flat, smooth layers. As in living beetles, the layers alternated between 80 and 95 nanometers thick and bend light in different ways.
A computer model predicted that as light rays hit the layers, they would interfere with each other and reflect light primarily at a wavelength of 490 nanometers. That was smack dab in the middle of the spectrum of light reflected by the specimen, Parker and McKenzie report this week in Biology Letters. They suggest that measuring fossil multilayers could also be used to identify true colors in drab specimens collected elsewhere. "This beetle might be a Rosetta stone for fossil colors," Parker says, adding that similar features have been seen in trilobites, crustaceans, and other fossil arthropods.
The approach is "a very elegant way of showing that--with exceptionally preserved material--you may be able to infer what the color was," says paleontologist Derek Briggs of Yale University. The method might be able to paint in details of fossil life, revealing for example whether extinct animals once had warning coloration. Or, a host of iridescent animals might indicate that an ancient environment was particularly sunny, says Parker. Briggs's hunch, however, is that chemical changes eventually wipe out the minuscule layers.