After squeezing and baking beetle wings, or soaking them in mud to let them decay, scientists think they’re closer to being able to reconstruct the original brilliant hues of some fossilized insects.
Some insects keep their colors after they become fossils, in some cases for millions of years. But others turn varying shades of brown and black. Scientists interested in the evolution of insect colors — and their role in things like camouflage, mating, and defense — want to better understand how colors change after fossilization.
What really turns a beetle brown, it turns out, is warm temperatures, a team of scientists reported Feb. 20 in Geology. “Temperature is the key to destroying the colors of fossils,” said paleontologist Maria McNamara, a study coauthor at the University of Bristol. McNamara and her colleagues based their conclusion on a battery of tests known as maturation experiments, during which scientists watched what happened when they subjected beetle bits to a variety of conditions that mimic those a dead insect might encounter after many millennia buried under dirt and debris.
“This opens potential pathways for recovering the color signature from specimens which have since lost their coloration,” said paleoentomologist Michael Engel of the University of Kansas. ”In time we may be able to look upon a drawer of fossils rendered black by preservation, but which we know were once colored, and reconstruct their lost hues and patterns.”
McNamara has been studying fossil insect colors for years. After identifying some trends in how fossil colors change, she decided to test some of the conditions that could produce color changes after a bug gets buried. To do this, McNamara and her colleagues took advantage of a Yale University lab equipped to do maturation experiments, a facility normally used by geochemists. Here, the high temperatures and pressures that can affect buried sediments are produced by autoclaves, instruments that heat- and pressure- sterilize lab equipment.
Except McNamara removed the forewings from jewel beetles and weevils and put them in the autoclave.
The shiny colors of the beetles’ outer cuticles come from microscopic structures. Some beetles, like the green jewel beetle (above left), get their shimmer from multiple layers of reflective compounds. Others, like the weevil (above right), derive their colors from tiny 3-D biophotonic crystals. These crystals, McNamara says, are among the most complex structures known – so complicated that scientists haven’t figured out how to replicate them artificially. Determining when the crystals showed up in the fossil record is a different question, since most fossils show no evidence for the structure.
The jewel beetle’s shiny covering fared well when subjected only to high pressure conditions, McNamara found. But turning up the heat as well as pressure produced a predictable color change, from green, to cyan, to blue, to indigo. And then, brown or black.
“Cook anything long enough and it’ll all end up black,” McNamara said.
The weevil’s outer layer responded similarly. Placing both types of insect cuticles in dirt and water for 18 months produced no color change, leading the team to conclude that post-burial temperatures are the most important factor in color change. High temperatures alter color-producing structures, shrinking layers and changing chemical compositions, which causes the tissue to bend light differently. “The color they produce is really dependent on how much the structure bends light,” McNamara said.
In support of her conclusion, McNamara points to fossils unearthed from various sites — buried at different depths and under different conditions — whose colors conform to the hypothesis.
Not everyone is convinced, though. Some scientists suggest McNamara is generalizing too much, and that color changes vary on a case-by-case basis, depending in part on species and precise post-burial conditions.
McNamara is working on resolving how these factors can influence a fossil’s color, and is planning on testing additional species and tissues. For now, she points to a tantalizing piece of evidence that emerged from her studies: Some of the black fossils she studied retain their original color-producing structures, which means that — with more information — scientists could eventually backtrack from those structures and determine what colors may have adorned paleo-insects.
Resurrecting the Rainbow Colors of Insect Fossils
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Resurrecting the Rainbow Colors of Insect Fossils
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Resurrecting the Rainbow Colors of Insect Fossils
