The color of a fossil species is often its greatest secret, its pigmented tissues having decayed and returned to the earth long before its discovery. An anomaly in this pattern was the recent reconstruction of wing coloration from color-producing structures discovered in the wing scales of 47-million-year-old fossil lepidopterans (moths and butterflies) recovered from the Grube Messel oil shales in Germany.
The study, which was published in the journal PLoS Biology and involved researchers from the United States, Ireland, and Germany, is one of the first to describe the preservation of structural color in fossil lepidopterans. Structural color is responsible for the iridescent qualities of many beetles, birds, and butterflies.
The biological architecture that supports structural color is different from the mechanisms underlying pigmentation and from macrostructures capable of producing light and dark tones, which have been described in the wings of some fossil lepidopteran specimens. In moths and butterflies, structural color is imparted by modifications in nano-sized wing scale components (one nanometer = 10-9 meter). For example, there may be alterations in the spacing between tiny ridges or between stiff structures called microribs on the wings.
Nano-level changes in lepidopteran wing architecture influence the physical interaction between light waves and the wing surface. Such modifications create what is known as a biophotonic nanostructure. Other nano-sized biophotonic structures known in nature include nanochannels and nanospheres. The blue color of the little blue penguin is the result of feather barbs made up of dense bundles of keratin nanofibers that reflect and scatter light of blue wavelengths. In lepidopterans, biophotonic structures are made from a polymer called chitin, which is similar to keratin in function.
Biophotonic nanostructures fulfill an important ecological role. For example, in morpho butterflies, which have bright blue iridescent wings, modified wing scales affect the brightness of the wings and increase the angle over which light is reflected; this presumably allows the butterflies to communicate over long distances. Bright coloration in nature is also often considered to be a warning signal, and thus structural color may play a role in signaling moths' or butterflies' unpalatability or toxicity to predators such as birds.
The reconstruction of the structural color in the fossil lepidopterans revealed that the insects possessed wings with an iridescent yellow-green upper surface trimmed with blue and brown margins. The findings suggest that their coloration may have served either as a mechanism of communication or as warning signal during activities such as feeding. When at rest, with the wings folded upward together, the color would have been concealed.
Evidence of structural color in 47-million-year-old lepidopterans suggests that biophotonic nanostructures are not a recently evolved phenomenon. Such knowledge of ancient structural color provides scientists with an opportunity to investigate the habitat preferences and communication and signaling mechanisms used by moths and butterflies of the Eocene Epoch (55.8 million to 33.9 million years ago). Further investigation could also shed light on the evolution of biological structures and their functional roles in ecology and behavior.
Kara Rogers is a freelance science writer and senior editor of biomedical sciences at Encyclopaedia Britannica, Inc. She is a member of the National Association of Science Writers and author of Science Up Front on the Britannica Blog. She holds a Ph.D. in Pharmacology/Toxicology, but enjoys reading and writing about all things science. You can follow her on Twitter at @karaerogers.
This post also appears on the Britannica Blog.