The feathers from the young Similicaudipteryx specimen and the adult are strikingly different. Credit: Xing Lida and Song Qijin.

Fossils that capture snapshots of a dinosaur species at different growth stages show that its feathers changed dramatically during its development, according to Chinese researchers in a paper published in Nature today1.

Two Similicaudipteryx specimens were recovered, one from early adulthood and one younger juvenile. Both belong to a group of egg-stealing dinosaurs known as oviraptorosaurs, and they seem to follow a maturation pattern not seen in modern birds.

"This baby dinosaur has bizarre flight feathers, which are strikingly different from those of adults," says first author Xing Xu from the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing.

The wing and tail feathers of the more mature dinosaur resemble quill pens. These 'pennaceous' or contour feathers have a central shaft that runs through its entire length. Conversely, the feathers of the younger dinosaur have a flat, ribbon-like stem at one end, but the more familiar pennaceous feather at the tip. The early juvenile also has smaller wing feathers than tail feathers, but this size difference is less significant in the elder specimen.

These 125-million-year-old fossils "expand our knowledge of feather evolution", says Xu. The specimens were collected by local farmers from the Lower Cretaceous Yixian Formation of western Liaoning in China.

Flight of fancy?

Palaeobiologists say that if the team's interpretation of the fossils is correct, it would be the first time that juvenile dinosaurs have been shown to have a different type of feather from adults. "Modern birds don't make such a transition," says Mike Benton from the University of Bristol, UK. Apart from the downy feathers of newborns, all later stages of modern birds are characterized by the same flight feathers. "This paper marks the first step in attempts to disentangle the evolution of developmental sequences among birds and their ancestors."

The fossils of the juvenile (left) and adult dinosaurs. Credit: Zheng Xiaoting.

But some ornithologists and developmental biologists who study feathers question whether the younger fossil shows a ribbon-like feather or is instead from the bird's moulting phase. "Feathers are complicated," says ornithologist Richard Prum from Yale University in New Haven, Connecticut. When birds regenerate their feathers, the new ones grow rolled-up in a tube sheath. Prum says that the fossilized feathers of the younger dinosaur could be interpreted as a preserved image of feathers emerging from their sheath — like modern feathers in active moult.

But Xu maintains that this finding is "not an artefact of preservation or temporary morphology" based on the proportions of the feathers. If the juvenile feathers were simply in active moult, he would expect the ribbon-like part of the feather to be shorter.

"If we give authors the benefit of doubt," says developmental biologist Cheng-Ming Chuong from the University of Southern California in Los Angeles, "it will be the first demonstration that these feathered dinosaurs can undergo changes of plumages in life."

Using work done by Prum, Chuong and others, Xu and his colleagues further suggest that the unusual partially-pennaceous feather might be a result of the delayed expression of genes that are activated earlier in modern birds.

"Dinosaurs had been experimenting with feathers and feather-like structures for a period of at least 25 million years before the dinosaurs described here," says Philip Currie, a dinosaur palaeobiologist from the University of Alberta in Edmonton, Canada. "The wonderful thing about this paper is that it provides developmental clues that may force palaeontologists, ornithologists and developmental biologists to recognize a broader spectrum of possibilities rather than looking for one simple answer."