Feathered "wings" may first have originally evolved in dinosaurs for courtship displays rather than flight, a study suggests.
The evidence comes from the first feathered dinosaurs to be discovered in north America.
Scientists recovered the 75-million-year-old ornithomimids from rocks in Alberta, Canada.
One juvenile and two adult fossils of the ostrich-like creatures were found. All were covered in downy feathers that may have been used for insulation, but only the adults had larger feathers on their arms, forming wing-like structures.
"This pattern differs from that seen in birds, where the wings generally develop very young, soon after hatching," said lead researcher Dr Darla Zelenitsky, from the University of Calgary in Canada.
Details of the find are published in the journal Science.
Experts know that ornithomimids were not capable of flight, so the discovery of early wings in the specimens is a mystery.
Co-author Dr Francois Therrien, from the Royal Tyrrell Museum of Palaeontology in Drumheller, Alberta, said: "The fact that wing-like forelimbs developed in more mature individuals suggests they were used only later in life, perhaps associated with reproductive behaviours like display or egg-brooding."
Until now the fossil skeletons of feathered dinosaurs have been found almost exclusively in fine-grained rocks in China and Germany.
"It was previously thought that feathered dinosaurs could only fossilise in muddy sediment deposited in quiet waters, such as at the bottom of lakes and lagoons," said Dr Therrien. "But the discovery of these ornithomimids in sandstone shows that feathered dinosaurs can also be preserved in rocks deposited by ancient flowing rivers."