Somewhere over the rainbow 48 million years ago, a happy little blue bird flew—until it soared over a lake belching toxic gases and died. The lake’s sediments then entombed the bird’s body, exquisitely preserving the oldest fossil evidence of blue feathers ever found.
Described in a study published today in Journal of the Royal Society Interface, the feathers belong to an extinct bird, Eocoracias brachyptera, that was recovered from Germany’s Messel Pit. This wonderland of well-preserved fossils dates back to the Eocene period, which lasted from 56 to 33.9 million years ago.
Researchers could infer E. brachyptera’s blue color only because they could compare it with its modern relatives, the rollers. Tiny structures preserved in the fossilized feathers resemble those that give modern birds either blue or gray hues, depending on their arrangement. And as far as we know, blue feathers have been fairly uncommon through time: Of the 61 lineages of living birds, only 10 have species with E. brachyptera’s most probable coloration.
But since modern rollers are far likelier to have blue than gray feathers, the researchers conclude that the ancient bird was a deep blue. It’s the first time that such a feather color has been reconstructed from the fossil record.
“I would say that, for me, that was the most exciting and important part of this research,” says lead study author Frane Babarović, a Ph.D. student at the University of Sheffield.
Because of the new findings, previous predictive models of fossil colors have now gone from 82 percent accurate to 61.9 percent accurate, since until now, these predictions were assuming that the fossil structures responsible for blue and gray gave rise to only gray. This may sound like a step backward, but it’s actually providing valuable new context for understanding what ancient animals really looked like.
“The way that I see this whole paper is that it has a narrow direct application, but a broad indirect implication,” says National Geographic Emerging Explorer Ryan Carney, a University of South Florida paleontologist who studies the feathered dinosaur Archaeopteryx, including its coloration. “It does increase the uncertainty ... but it went from an unknown unknown to a known unknown.”
The study marks the latest effort to unveil the colors of ancient animals, a field that has exploded in the last decade. Key to this color revolution was the discovery that microscopic pigment sacs called melanosomes could fossilize. Melanosomes contain two varieties of the pigment melanin, which can create hues from red-brown to black. Melanosomes have been recovered from many prehistoric creatures, from birds to non-avian dinosaurs and even marine reptiles.
In addition, bird feathers can get their color from the fine structure of their feathers, rather than directly from pigments. Inside the feather’s barbules, layers of melanosomes and a type of the structural protein keratin can scatter incoming light in such a way that only certain colors reflect back out. If you’ve ever seen a peacock’s glittering tail or a starling’s rainbow sheen, you’ve seen this phenomenon, called structural color, at work.
Prior studies have found evidence of structural color in dinosaur feathers. The small theropod Caihong juji seems to have had a majestic mane of iridescent feathers, and the “four winged” dinosaur Microraptor likely had black feathers with a bluish sheen.
But if you’ve ever handled peacock feathers, you’ve probably noticed that they change color when they’re viewed at different angles, a property called iridescence. Not all structural colors behave this way. Some bird feathers have barbs that consist of three layers: an outer keratin sheath, a spongy middle layer, and an inner layer of melanosomes. This layering lets the feather reflect bluish light from many viewing angles, and it’s these non-iridescent structural colors that make blue jays blue and help make some parrots green.
Blue and gray
In living birds, melanosomes associated with different colors tend to have different shapes, and so the same should be true in the fossil record. Black ones look like sausages, while the red-brown ones look like meatballs. Babarović wondered: Was there a distinctive melanosome shape associated with non-iridescent blues?
To find out, Babarović and his colleagues examined the melanosomes in E. brachyptera and in 72 feathers from modern bird groups found around the world. In the fossil bird, the preserved melanosomes are about three times longer than they are wide, which is consistent with melanosomes involved with both non-iridescent blue and gray. To distinguish between the two, Babarović realized he needed to plot a family tree of living birds and their feather colors, to figure out where blue or gray feathers predominated.
When he crunched the numbers, he found a 99-percent chance that E. brachyptera had non-iridescent structural color, and no better than a 19-percent chance that the bird had gray feathers. That means the odds favor the fossil bird’s melanosomes granting it a blue hue.
Now that he’s started his Ph.D., Babarović plans on taking a more sweeping look at the evolutionary history of the color blue in birds. It’s a scientific quest that leaves him visibly beaming.
“There are nights when I couldn’t sleep because of it,” he says. “I just love it.”