Trace metals in fossils have allowed researchers to recontruct the shading of Confuciusornis sanctus. Credit: Science/AAAS

In the quest to probe ever deeper into fossil clues left behind by extinct animals, an international group of scientists has developed a new technique to detect hints of pigment left behind in fossilized feathers, skin and scales.

The non-destructive X-ray technique used powerful synchrotron-generated X-ray technology at the US Department of Energy's SLAC National Accelerator Laboratory in Menlo Park, California, to detect trace metals left behind by soft tissue. The technology could one day provide a window on mechanisms of camouflage and colour-based sexual selection that could have shaped the evolution of dinosaurs and other ancient animals.

Ghosts of colour

"It's one of those techniques which allows us to map chemical ghosts," says palaeobiologist Phil Manning of the University of Manchester, UK, who is one of the authors on the paper published today in Science1.

In the past few years, scientists have used scanning electron microscopes to find hints of pigment by detecting ancient melanosomes – the sub-micrometre-sized packages within cells that contain pigment — in fossils. Spherical melanosomes are assumed to have contained reddish-brown pigment called phaeomelanin, whereas oval ones are interpreted as having once contained the dark-black/brown eumelanin. But that method requires chipping away at precious fossils and indicates pigment only in tiny areas.

By contrast, the powerful X-rays generated by a synchrotron can scan a whole fossil without destroying it. They pick up on minute amounts of pigment-derived elements that stick around even after the melanosomes themselves have been degraded.

In this experiment, the finding of copper indicated the presence of the dark eumelanin pigment. But because copper is also present in non-pigmented keratin, the protein that gives feathers their structure, the team confirmed that two other pigment-associated elements, zinc and calcium, occurred in the same places. Other scans made sure that the copper was bound to other atoms in a way that indicated it was from an organic source, meaning that it was part of the feathers rather than being added as an inorganic mineral during fossilization.

Finding a pattern

By scanning a feathered skeleton of Confuciusornis sanctus, a bird that lived about 120 million years ago, the team found that the darkest areas were in the downy feathers of the body and in the tips of wing feathers. The rest of the wing feathers were probably white or coloured by some other pigment, they conclude.

To check that they were interpreting the scans correctly, the researchers scanned animals that had died recently and found the same chemical signatures for eumelanin. For example, scans of a fossilized squid ink sac full of eumelanin looked just like ones of squid fresh from the fish market.

"I'm quite convinced that what they've been able to detect are in fact traces of the fossilized melanin," says palaeobiologist Jakob Vinther from Yale University in New Haven, Connecticut. He was one of the first scientists to use melanosomes to infer the type of pigment in fossilized feathers2. "I think that's a really good verification of the methodology that we have developed looking at fossilized melanosomes."

Even though the team only reported on eumelanin, it is also looking for signatures characteristic of other pigments for future research.

"It's a first step," says Roy Wogelius, a geochemist from the University of Manchester and an author on the Science paper. "Now the research will head into trying to pick out finer shades and maybe more subtle coloration."

Once the technology is more sophisticated, it might shed light on taxonomically important traits, says palaeontologist Luis Chiappe, director of the Dinosaur Institute at the Natural History Museum of Los Angeles County. With more information, he says, researchers could differentiate between species on the basis of colour patterns, even if their bone morphology is very similar.