This 3-million-year-old mouse just gave scientists the key to decoding ancient red pigments

Figuring out the colors of fossilized animals used to be complete guesswork—even in the rare finds containing bits of feathers, scales, or fur, the original hues in such soft tissues are usually long gone. Now, for the first time, researchers have been able to identify the chemical signature of the pigment that gives red hair its color in the fossil of an ancient mouse—using a new technique that leaves precious fossil specimens intact.

“The mouse fossil, it looks nice. It’s a beautiful specimen. But then you scan it, and it’s this eureka moment,” says Roy Wogelius, a geochemist at the University of Manchester in the United Kingdom, who with his colleagues developed the technique.

Using a variety of techniques, scientists have been able to gather hints 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 skin, hair, and other tissues black, brown, and gray. But its sister pigment called pheomelanin, which gives skin and hair a pink or red hue, has been tougher to nail down.

Part of the problem, Wogelius says, was that relatively little was known about the chemistry of the pigment in modern-day tissues. In work published in 2016, he and his colleagues looked carefully at the different trace metals in pigments from modern feathers and found that whereas eumelanin contains copper, pheomelanin contains sulfur and zinc. They wondered whether tracing those metals might allow them to find signs of the reddish pigment in fossils as well.

To test the idea, the scientists analyzed two exceptional fossils—with soft tissues and hair still visible—of an extinct mouse called Apodemus atavus that lived 3 million years ago in what is now Germany. Close relatives of the species alive today, such as the European wood mouse, have reddish fur, so the researchers thought the fossil mouse might have had similar coloring. Sure enough, when they scanned the mouse fossils, they found the characteristic overlap of sulfur and zinc in regions where hair was visible on the fossil. They report their find today in Nature Communications .

Now that scientists know what to look for, Wogelius says, he’s confident the pheomelanin signature will be detectable in much older fossils. The new data support the team’s previous claim of evidence for pheomelanin in a 30-million-year-old fossil tadpole. “I’m certain we can go back 30 million years, and probably even longer than that.”

The technique is “a very elegant method for analyzing the whole fossil in a nondestructive way,” says Jasmina Weimann, a molecular paleobiologist at Yale University. “It’s very cool.”

Other chemical analysis methods require researchers to take tiny samples from fossils. Not only does that damage part of the fossil, it also means the full picture is still guesswork. “If you took a square-millimeter sample of zebra skin, you might be able to tell if the sample was black or white, but you wouldn’t understand what a zebra looks like,” Wogelius says.

Weimann hopes similar techniques might allow researchers to identify not only pigments, but also other chemical signatures, for example of proteins specific to certain tissues.