For nearly two centuries, people have struggled to imagine what the great extinct dinosaurs looked like. Thanks to modern paleontology and physiology, their shapes, masses and even how they might have moved and interacted have been deduced. But one of the most basic questions about their appearance, their coloring, seemed unanswerable.

A new study, however, proposes some of the first cellular hints. Extrapolating from primitive pigment-giving organelles known as melanosomes (which contain the coloring compound melanin and are still prevalent in modern animals) that have been found in fossilized dinosaur feathers from the Cretaceous period, a research team paints a picture of dark wings and brightly striped reddish tails. The findings will be detailed in the January 28 issue of Nature. (Scientific American is part of Nature Publishing Group.)

Using high-powered scanning electron microscopy, the researchers examined 125-million-year-old feathers found in the Jehol group, a geologic formation in northeastern China. One of the animals analyzed, the Sinosauropteryx, a small, meat-eating dinosaur, appears to have had alternating bands of dark and light along its tail. "In this case at least, the dark band was [a] russet, gingery color," Michael Benton, a professor of vertebrate paleontology at the University of Bristol in England and co-author of the study, said in a press conference Tuesday in London. "It's the first time anybody has had evidence of original color."

A prevalent pigment
The discovery of this pigment in ancient dinosaur and bird feathers did not begin at a dinosaur dig, but rather in a lab studying fossilized squid. Jakob Vinther, now a PhD student in paleontology at Yale University who was not involved with the new study, was examining preserved ink sacs from Jurassic period squid when he found that the fossilized melanosomes appeared identical to those in modern-day squid ink.

The similarity led him to propose that "there must be melanosomes preserved in other kinds of structures" in other animals. "So I said, 'It would be interesting to look at bird feathers,'" he said in a phone conversation. In these fossilized feathers he saw the very same structures.

After bringing his findings to the attention of his graduate supervisor, Vinther was told that he had only found a common bacteria that had long been known to exist in these feather fossils. But he was not convinced. After studying feathers with a known color pattern, he found that the dark-pigment particles appeared only where the black bands did—and in the fossils they aligned perfectly with the individual feather filaments, an unlikely arrangement for bacteria. In 2008 Vinther and colleagues published a paper in Biology Letters describing the find, in which they proposed, "The discovery of preserved melanosomes opens up the possibility of interpreting the color of extinct birds and other dinosaurs."

It was no great stroke of luck that these pigment particles were still left in the fossilized feathers. In fact, they are what had been fossilized from the feather, says Vinther: "The reason that you have a fossil feather is because there are pigments. So if you have a white feather, it would not leave a fossil."

What does seem like good fortune, at least to the researchers studying these ancient particles, are their shapes. "We're extraordinarily lucky that each of the pigments of the melanosomes are contained in a different-shaped organelle," Benton said at the press briefing.

The melanosomes that have been found no longer exhibit their original colors because their chemical properties have changed in the intervening millions of years. But researchers can still ascertain the extinguished hues by looking at the particles' sizes and shapes, which correspond to those in living animals. Those that are sausage-shaped are black, and rounder; smaller ones are responsible for red, rusty tints.

Cautious coloring
Although Benton and his colleagues have proposed some specific coloration patterns, Vinther is not convinced that paleontologists are quite ready to pick up a paintbrush. It is not yet known how different coloring particles work in combination—and in their original structure in feathers—to create a perceivable hue. "We are getting much closer to putting colors on dinosaurs," he says, but we are not quite there yet. And although more fossil sampling will be necessary, he says, the real missing data has been right in front of us the whole time: "We need to know more about modern birds" because birds are living dinosaurs.

Benton and his team admitted that despite the artistic representations published alongside their paper, "We can't say for sure [these are] all the colors, because there are other coloring agents other than melanosomes that may not be preserved with the fossils," he said. "Melanin is quite a tough protein," Benton pointed out at the press conference. But other coloring particles might not have survived these millions of years. Artistic representations of extinct animals often borrow from extant animals, taking into account their habitat camouflaging as well as possible display characteristics, so, as Benton noted, artistic representations are "not completely fatuous."

Vinther and his colleagues continue to search for more clues about coloration. By studying the structures of melanosomes' arrangement in more recent feathers, they were able to deduce when feathers would have iridescence, lending another level of sheen, and even purples, blues and greens to some surfaces. In a 2009 paper in Biology Letters, the team described evidence of iridescence they found on fossilized middle Eocene epoch feathers based on these structures, but older bird and dinosaur feathers have yet to yield any shiny clues.

Given these organelles' prevalence, their discovery in fossilized feathers will likely open the door to a rush of new coloration studies. "We can definitely take this to look at skin, as well," Vinther says. As Benton noted, however, although the pigmentation is widespread in reptile and fish scales today, it is unusual to find remnants of organic material in dinosaur fossils. What we know about their skin is generally deduced from imprints left in surrounding material, which would not contain the organelles needed to predict coloration.

Colorful clues
In the meantime, even if only the wings and tails of certain feathered dinosaurs and early birds can be colored in, it will help researchers deduce much more about extinct animals' ecology and behavior—and also about other physiological details such as vision. As Vinther notes, "If you do find spectacular colors in these animals, then you know they had an ability to see more colors than just black and white."

Beyond coloration, the presence of these melanosomes in early Cretaceous dinosaurs helps to confirm the presence of protofeathers in some dinosaurs, which some have argued were simply connective tissue. The Sinosauropteryx that they studied, one of the earliest Sinosauropteryxs to have featherlike structures had, what Benton described as "a very clear rim of feathers running down the head, down the back and along the tail."

These adornments were not feathers in the modern sense, such as those found on modern turkeys and peacocks. "These are very simple structures," Benton said. "They're sort of bristles" which were four to 10 millimeters long. But these unassuming bristles "really are feathers," he said.

"In terms of the sequence of evolution of feathers, we can now say they start as simple bristles," Benton noted. If this is the case, it would support the idea that the structures that were to become feathers originally developed not for flight but rather for display purposes. "That's a display function," Benton said. "It's clearly not for flight."