Despite being tens of millions of years old, some beetle fossils appear almost as they did in life. Not only are their shape and structure preserved, but so are the actual colors of their shells, which have changed only slightly in the intervening eons.
Though relatively little-known, these fossils represent the purest of biological colors retrieved from deep time, far richer than much-celebrated pigment traces of dinosaur plumage and more varied than the hues of a few ancient plants.
In a study published Sept. 27 in Proceedings of the Royal Society B, researchers led by Yale University paleogeologist Maria McNamara analyzed 10 of these spectacular beetle fossils, ranging from 15 million to 47 million years old, which owe their enduring shades to the phenomenon of structural coloration. Unlike pigments, which generate color from light bouncing off a chemical, structural colors are produced by the interaction of light with nanometer-scale surface geometries.
If especially fine-grained sediments replace a dead beetle’s decomposing body, the resulting fossil should replicate its hues, too. “Structural colors don’t need chemicals at all,” said McNamara. “What we wanted to find out was, what kind of structures in the fossils make the color? And are the colors we’re seeing today in the fossils the same as when beetles were alive millions of year ago?”
McNamara’s team took .00008 millimeter-wide samples of the fossils’ surfaces, too small to see unaided but enough to determine surface shape when viewed under an electron microscope. From that shape, they used models derived from modern beetle shells to calculate how the fossils ought to look. Prediction and reality didn’t quite match: According to their calculations, the fossils now appear just slightly more reddish than they ought to. The fossils don’t perfectly replicate the beetle’s original carapaces, and subtly change how light refracts as it passes through shell layers.
“You need to mentally redshift the color. If it’s green, it’s actually a little more on the yellow side. If it’s blue, it’s a little greener,” said McNamara, who next plans to analyze the colors of moth fossils.
“Holding these fossils is amazing,” she said. “But what I really get a kick out of is investigating the details that are preserved on such a tiny scale. It’s one thing to look at a colored fossil beetle, but quite another to realize the level of preservation extends right down to the level of structures that are smaller than a cell.”
On the following pages are examples of the fossils and their structures.
Images: McNamara et al./Proceedings of the Royal Society B
Citation: “The original colours of fossil beetles.” By Maria E. McNamara, Derek E. G. Briggs, Patrick J. Orr, Heeso Noh and Hui Cao. Proceedings of the Royal Society B, September 27, 2011.