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 colour only because they could compare it with its modern relatives, the rollers. Tiny structures preserved in the fossilised feathers resemble those that give modern birds either blue or grey 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 colouration.
But since modern rollers are far likelier to have blue than grey feathers, the researchers conclude that the ancient bird was a deep blue. It’s the first time that such a feather colour 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 Babarovic, a PhD student at the University of Sheffield.
One man's dedication—and a very simple solution—have helped bring bluebirds back from the edge of extinction.
Because of the new findings, previous predictive models of fossil colours have now gone from 82 per cent accurate to 61.9 per cent accurate, since until now, these predictions were assuming that the fossil structures responsible for blue and grey gave rise to only grey. 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 palaeontologist who studies the feathered dinosaur Archaeopteryx, including its colouration. “It does increase the uncertainty ... but it went from an unknown unknown to a known unknown.”
Eocoracias brachyptera is most closely related to the rollers, a family of Old World birds that includes the lilac-breasted roller bird (Coracias caudatus), seen here soaring above Namibia's Etosha National Park.
PHOTOGRAPH BY MICHAEL MELFORD, NAT GEO IMAGE COLLECTION
The study marks the latest effort to unveil the colours of ancient animals, a field that has exploded in the last decade. Key to this colour revolution was the discovery that microscopic pigment sacs called melanosomes could fossilise. 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 colour 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 colours 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 colour, at work.
Prior studies have found evidence of structural colour 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 colour when they’re viewed at different angles, a property called iridescence. Not all structural colours 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 colours that make blue jays blue and help make some parrots green.
Blue and grey
In living birds, melanosomes associated with different colours 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. Babarovic wondered: Was there a distinctive melanosome shape associated with non-iridescent blues?
Researchers sampled this fossil of Eocoracias brachyptera in hopes of finding fossilised pigment sacs called melanosomes.
PHOTOGRAPH BY SVEN TRAENKNER, SENCKENBERG NATURE MUSEUM FRANKFURT
To find out, Babarovic 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 grey. To distinguish between the two, Babarovic realised he needed to plot a family tree of living birds and their feather colours, to figure out where blue or grey feathers predominated.
When he crunched the numbers, he found a 99-per cent chance that E. brachyptera had non-iridescent structural colour, and no better than a 19-per cent chance that the bird had grey feathers. That means the odds favour the fossil bird’s melanosomes granting it a blue hue.
Now that he’s started his PhD, Babarovic plans on taking a more sweeping look at the evolutionary history of the colour 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.”
Lead Image: Researchers predict that the plumage of Eocoracias brachyptera, a bird species that lived some 48 million years ago, was blue—the first time that blue feathers have been reconstructed from the fossil record.
ILLUSTRATION BY MARTA ZAHER, UNIVERSITY OF BRISTOL