Coloring the past will do more than just improve the accuracy of museum displays and dinosaur movies. Across the living world, creatures use color for camouflage, to signal their species and sex, to advertise their toxicity to predators, or to show off to potential mates.
They undoubtedly did so in the past as well.
"So far we've been living in a black-and-white world," said Luis Chiappe, a paleontologist from the Natural History Museum of Los Angeles County, who was not part of the team. "Now we can get a sense of the colors of dinosaurs and other extinct animals."
The project brought together paleontologists, geochemists, and physicists through a bit of serendipity that started when a fossil hunter in North Dakota heard a report on NPR. The segment was about some physicists who had used an X-ray generator called a synchrotron to help decipher a 2,200-year-old text - the oldest surviving writing of Archimedes.
The original writing had mostly been painted over. But physicists found a way to read it with the synchrotron by the way that the X-rays it generated interacted with atoms of iron in the original ink used by the Greek engineer and mathematician.
Natural pigments from living things also incorporate heavy metals. If any had somehow stuck with the fossils, perhaps the high-intensity X-rays could find them as well.
There was already some circumstantial evidence for the existence of pigments in extinct birds, said team member Phillip Manning, a paleontologist at the University of Manchester, England, who is now on sabbatical at Penn.
The group that did that work, based at Yale University, used a scanning electron microscope to map out remnants of cells, called melanocytes, in 100-million-year-old fossil imprints of feathers. These structures serve as minuscule paint pots that hold pigments, Manning said.
In 2008, the Yale team reported that the melanocytes were arranged in striking bands.
Manning and his colleagues thought they could go further, deciphering the actual chemistry of the pigments. "They could find the containers but they couldn't analyze their contents," he said of the previous finding.
The idea to use a synchrotron to analyze fossils began when the paleontologist who heard the NPR report, Pete Larson, got in touch with physicist Uwe Bergmann, who had worked on the Archimedes text at the SLAC National Accelerator Laboratory in Menlo Park, Calif.
"I thought immediately it would be possible to learn something because of the sensitivity of this technique," said Bergmann.
The synchrotron is a type of particle accelerator, similar to those used to create exotic subatomic particles. The one in this study accelerates electrons around an oval tube nearly 800 feet in circumference. The acceleration of the electrons generates X-rays, which can be focused on a small target.
The fossils are analyzed in tiny segments, said Bergmann, a process that took between eight and 12 hours and had to be done with exquisite care to avoid damaging the specimen.
The atoms of different metals absorb and reemit X-rays at very specific frequencies, he said, allowing scientists to trace the patterns of iron, copper, calcium, and other elements.
"You can get images and maps that you overlay . . . and by doing that you start to re-create the composition of what is left from the original animals," said Bergmann. In a sense, he said, "you bring the fossil back to life."
The first sample the paleontologists tried was of a primitive bird called Archaeopteryx, which lived during the Jurassic era and had teeth rather than a beak. Though the fossil looked like a mere impression, the synchrotron revealed some traces of sulfur and phosphorus that the scientists think made up part of the original feathers.
But there were limits to what they could see in Archaeopteryx, said Manning, because when the fossil was discovered in the 1800s, it had been brushed, hard, likely removing most of the remaining organic matter.
So they brought in two fossils that had not been brushed and included well-preserved imprints of feathers. One was a bird called Confuciusornis sanctus, which lived 120 million years ago and represents one of the first birds to have a beak rather than teeth. Manning describes it as similar to a modern magpie.
The other bird, called Gansus yumensis, is 110 million years old and looked like a modern grebe.
The first element that the scientists looked for with the synchrotron was copper, which is typically present in the dark pigment eumelanin. It adds darkness to blacks, browns, and similar colors. Copper was indeed concentrated in the feather imprints of the fossil birds.
Going a step further, the scientists were able to study the patterns of X-rays that were reemitted from the fossils to get information about the other atoms that were bound to the copper. That revealed that the copper was incorporated into organic compounds similar to those associated with eumelanin in living animals.
That gives them strong evidence that the copper didn't come from the rock surrounding the bird but most likely was part of a pigment, said the paper's lead author, Roy Wogelius, a geochemist at the University of Manchester. "The copper chemistry in these preserved pigments is extremely similar to melanin we've sampled from living organisms."
The copper-containing compounds may have survived so long because they are so toxic, he said, and thus were not consumed by microorganisms.
The next step will be to try to detect other pigments beyond eumelanin, Wogelius said: "There just may be some hints of some other chemicals that give us the full color palette."
Contact staff writer Faye Flam at 215-854-4977 or email@example.com.