Discovering the colors of fossilized animals used to be complete conjectures, even in the rare finds that contain pieces of feathers, scales or fur, the original tones in such soft tissues usually disappear. Now, for the first time, researchers have been able to identify the chemical signature of the pigment that gives color to red hair in the fossil of an ancient mouse, using a new technique that leaves intact the precious fossil samples.
"The mouse fossil, it looks good. It is a beautiful specimen. But then you scan it, and this is the eureka moment, "says Roy Wogelius, a geochemist at the University of Manchester in the United Kingdom, who together with his colleagues developed the technique.
Using a variety of techniques, scientists have been able to gather clues about the colors of fossils, including dinosaur feathers and dinosaur eggs. A decade ago, scientists used high-energy synchrotron X-rays to identify the key chemical signatures of a pigment called eumelanin, which colors the skin, hair and other tissues of black, brown and gray. But his brother pigment called pheomelanin, which gives the skin and hair a pink or red hue, has been harder to nail.
Part of the problem, says Wogelius, was that relatively little was known about the chemistry of pigment in modern tissues. In a paper published in 2016, he and his colleagues carefully analyzed the different trace metals in the pigments of modern pens and found that while eumelanin contains copper, pheomelanin contains sulfur and zinc. They wondered if tracking those metals could allow them to find signs of the reddish pigment in the fossils as well.
To test the idea, the scientists analyzed two exceptional fossils, with soft tissues and still visible hairs, of an extinct mouse called Apodemus atavus who lived 3 million years ago in what is now Germany. The close relatives of the species that live today, like the European wood mouse, have a reddish coat, so the researchers thought that the fossil mouse could have had a similar color. Indeed, when they scanned the fossils of mice, they found the characteristic superposition of sulfur and zinc in regions where the hair was visible in the fossil. They report their findings today in Communications of nature.
Now that scientists know what to look for, says Wogelius, he is confident that the signature of pheomelanin will be detectable in much older fossils. The new data support the team's previous claim of evidence of pheomelanin in a 30-million-year-old fossil tadpole. "I'm sure we can go back 30 million years, and probably even more than that."
The technique is "a very elegant method to analyze the entire fossil in a non-destructive way," says Jasmina Weimann, a molecular paleobiologist at Yale University. "It's very cool."
Other methods of chemical analysis require researchers to take small samples of fossils. That not only damages part of the fossil, it also means that the whole image is still conjecture. "If you take a sample of square millimeter of zebra skin, you could know if the sample was black or white, but you would not understand what a zebra is like," says Wogelius.
Weimann hopes similar techniques will allow researchers to identify not only pigments, but also other chemical signatures, for example, of proteins specific to certain tissues.