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Unusual feathers preserved in 100-million-year-old Cretaceous amber could have been used as defensive decoys.

Pierre Cockx/RSM

Ancient bird fossils have ‘the weirdest feathers I have ever seen’

One hundred million years ago, the sky was filled with birds unlike those seen today, many with long, streamerlike tail feathers. Now, paleontologists have found examples of these paired feathers preserved in exquisite detail in 31 pieces of Cretaceous amber from Myanmar. The rare 3D preservation reveals the feathers’ structure is completely different from that of modern feathers—and hints that they may have been defensive decoys to foil predators.

Such tail streamers—in some cases longer than the bodies—have been observed in early bird fossils from China for several decades, in particular, the 125-million-year-old Confuciusornis sanctus. They may also be present in some feathered dinosaurs. Scientists have long thought the feathers were ornamental, similar to the tail feathers in some modern hummingbirds and birds of paradise—and that they may have been unique to either males or females, as only a subset of fossils of some species possess them.

But most of those fossils are squished almost flat, making the structure of the feathers near impossible to study. “These new discoveries change the game—the fossils are astoundingly beautiful,” says Steve Brusatte, a palaeontologist at the University of Edinburgh who was not involved in the work.

Now, an international team of researchers, led Lida Xing at the China University of Geosciences in Beijing, have analyzed these feathers, many of them found paired, in 31 pieces of 100-million-year-old amber from Myanmar. “They are the weirdest feathers I have ever seen,” says co-author Jingmai O’Connor, who studies fossil birds at the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing. 

Many Cretaceous birds, such as Confuciusornis (center) and these enantiornithines, had pairs of tail streamer feathers that may have easily been detached in moments of peril.

Cheung Chung Tat

In most of the conventional fossils with tail streamers from China, the birds and their feathers are squashed almost totally flat. “The way we interpreted these feathers from compression fossils was basically completely, entirely wrong,” O’Connor says. “Looking at them in three dimensions preserved in amber, I was astonished.”

In all modern feathers, the central shaft or rachis is a hollow tube. But the ancient ribbonlike tail feathers are fundamentally different, with a shaft that’s more like a half-cylinder, flattened and open on one side. They also have significantly reduced feather barbs on either side of the shaft, compared with modern flight feathers. These tail streamers would have stuck out straight and rigid, like an extended tape measure, explains co-author Ryan McKellar of the Royal Saskatchewan Museum in Regina, Canada.

But the streamers were also insanely thin. “The thickness of the rachis in some specimens is 3 microns thick. That’s less than the size of the average cell,” O’Connor says. (Human red blood cells are about 7.5 microns thick.) “How could something be so thin and maintain structural integrity?” She believes the thinness and the half-formed shaft were a way to economize, making the feathers much less energetically demanding to produce.

As the authors argue today in the Journal of Palaeogeography, the thin shaft and other clues suggest these feathers were highly disposable, similar to the detachable tail of a lizard, and may have helped ancient birds escape the clutches of predators. The fact that so many paired streamers have been found in amber—fossilized tree resin—without the body of the bird, suggests they were plucked out easily when stuck in the resin. To McKellar, that suggests a defensive role. “You’re giving the predator a nonlethal target that’s half the size of your body.”

That’s a speculative idea, says Gerald Mayr, an ornithologist at the Senckenberg Research Institute and Natural History Museum in Frankfurt, Germany. But he finds the half-open feather shafts highly intriguing. “This [structure] suggests significant developmental and functional differences to the feathers of living birds.”