Here is something you do not often hear: The dead field mouse looks incredibly close to its age. It lived and died three million years ago in what is now Germany, but layers of stone preserved almost the entire skeleton along with most of the fur and skin on its body, feet and tail. Even its small, delicate ears were preserved.
Thanks to new imaging methods and a better understanding of chemistry behind the pigment in animal hair and feathers, we now know that it had reddish brown fur with a white underbelly. Paleontologists have been given the tools to detect light and dark color patterns in fossil feathers for a few years, but this is their first true glimpse of a colored pigment.
It comes in color
The color length of animal fur comes from different amounts of two types of a pigment called melanin. Eumelanin produces black or dark brown color, while pheomelanin creates reddish or yellow shades. Pheomelanin does not tend to quit well over the millions of years most fossils are buried; Eumelanin is more robust and therefore we have a decent idea of light and dark patterns in Archeopteryx feathers and some of the other ancestors of today's birds.
For several years, the pigment chemistry studied in modern animals, the University of Manchester geochemist Roy Wogelius and his colleagues discovered that chemical compounds with poor sulfur are a dead giveaway for the presence of reddish pheomelanin in newly-killed animal skins or feathers. But when the animal falls, these sulfur ring molecules break down.
"We discovered an important relationship between zinc and sulfur, which we believe could be used as a" tag "to solve pheomelanin in fossils," co-author Nick Edwards, a researcher at the SLAC National Accelerator Laboratory, told Ars Technica. When pheomelanin breaks down, small spores of the metal remain embedded in the fossil hair fibers, and Wogelius and his colleagues found a way to look for these traces to reconstruct the staining of the long-dead fossil field mouse.
It seemed like a perfect way to test the method. "We thought the conservation in this case was good enough to allow comparison with modern tissues, but old enough that it would be a good test of our new protocols for fossil analysis," Wogelius said. Using a technique called synchrotron x-ray fluorescence imaging, the team bombed the fossil with x-rays and looked at how they interacted with the metals in the fossilized mouse skin threads. The patterns of metal tracks matched those they had previously seen in animals with very high concentrations of the reddish pigment – everywhere except on the white underbelly.
Fossils Preserve Chemistry, Not Just Forms
"This study, in addition to what we can see with the visible part of the electromagnetic spectrum, preserves the original biological chemistry also within these structures," said Edwards Ars. It also shows that paleontologists can miss important information if they only check a few spots for the presence of pigment. Instead, Wogelius and his colleagues, mapping how pigments vary (and often blend) across the body of an organism, are the key to really understanding the coloration.
Of course, it depends on having fossilized fur or feathers to analyze in the first place. When it died 3 million years ago, the field mouse came in a small deep basin, where the sediment darted quickly, burying the mouse and sealing it away from oxygen. Happiness does not happen every day.
But it happens. "Ordinary? No. But there will be hundreds of units, either stored in museums or yet to be tested, which will have appropriate material for study," says Wogelius. X-ray fluorescence imaging does not require scientists to destroy fossil samples to study them, so the team is optimistic about the will of the museums to look more closely at their best-preserved entities. "We are embroiled in a number of new projects, some related to pigments and some related to other aspects of softening tissue extending beyond 3 million years"
"It has taken us over a decade to really understand the information that our primary technique (synchrotron X-ray fluorescence imaging) can provide," added Edwards.
Nature Communications 2019. : 10.1038 / s41467-019-10087-2; dois).