X-rays reveal the colors of a fossil mouse 3 million years old

Here is something you do not hear often: the dead field mouse is amazing for his age. He lived and died three million years ago in what is now Germany, but rock layers have retained almost all his skeleton, as well as most of the fur and the skin of his body, feet and tail. Even his tiny, delicate ears have been preserved.

Thanks to new imaging methods and a better understanding of the chemistry of pigments found in the fur and feathers of animals, we now know that their fur is reddish brown with a white belly. Paleontologists have the tools to detect light and dark colors in fossil feathers for a few years, but this is their first glimpse of a colored pigment.

It comes in colors

The color range of animal fur comes from varying amounts of two types of pigments called melanins. Eumelanin produces a dark or dark brown color, while pheomelanin produces reddish or yellow tints. Pheomelin does not tend to resist for millions of years, most fossils are buried; Eumelanin is stronger, so we have a good idea of ​​the patterns of light and darkness in the feathers of Archeopteryx and some of the other ancestors of today's birds.

During several years of study of pigment chemistry in modern animals, University of Manchester geochemist Roy Wogelius discovered that chemical compounds containing rings containing sulfur were an indication of the presence of pheomelanin reddish fur or feathers recently dead. But when the animal breaks down, these sulfur cycle molecules break down.

"We have discovered an important relationship between zinc and sulfur that we believe could be used as a 'marker' for pheomelanin resolution in fossils," Nick Edwards, co-author of scientist at SLAC National Accelerator Laboratory. When pheomelanin breaks down, tiny traces of metal remain encrusted in the fossilized hair fibers, and Wogelius and his colleagues found a way to search for these traces in order to restore the color of the long dead fossil mouse.

It seemed like an ideal way to test the method. "We thought that the conservation in this case was good enough to allow a comparison with modern fabrics, but old enough to be a good test for our new fossil analysis protocols," Wogelius said. Using a technique called synchrotron X-ray fluorescence imaging, the team bombed the fossil with X-rays and observed how they interacted with the metals in the fossilized threads of mouse fur. The patterns of metallic traces corresponded to those they had previously observed in animals with very high concentrations of reddish pigment – everywhere except on its white belly.

Fossils preserve chemistry, not just forms

"This study presents evidence that, beyond what we can see with the visible part of the electromagnetic spectrum, the original biological chemistry is also preserved in these structures," Edwards told Ars. It also shows that paleontologists may miss important information if they only check a few points for the presence of pigment. According to Wogelius and his colleagues, it is essential to understand the coloring of pigments (and often their mixture) in the body of an organism.

Of course, it depends on the presence of fossilized fur or feathers to analyze. At his death, 3 million years ago, the mouse had fallen into a small, deep basin where sediment was accumulating rapidly, burying it and isolating it. oxygen. This kind of luck does not happen every day.

But it happens. "No, but there will be hundreds of specimens, either stored in museums or sampled, that will have the appropriate study materials," said Wogelius. The team is optimistic about the willingness of museums to look more closely at the best-preserved specimens. "We are busy with a number of new projects, some related to pigments and others related to soft tissue preservation, which extends over more than 3 million years, "said Wogelius.

"It took us over ten years to truly understand the information that our main technique (synchrotron X-ray fluorescence imaging) can provide," Edwards added.

Nature Communications, 2019. DOI: 10.1038 / s41467-019-10087-2; (About DOIs).