Darla Zelenitsky, a paleontologist at the University of Calgary, says three years of study on dinosaur fossils found near Drumheller, Alta., suggest that the features most closely linked with flight evolved for a completely different reason.
"They may have initially evolved as a secondary sexual characteristic," says a paper published Thursday in the prestigious journal Science.
"These wing-like structures would have been used for reproductive activities (courtship, display, brooding) and were only later ... co-opted for other roles including flight."
The fossils in question — the oldest feathered dinosaurs ever found — are from three members of the species Ornithomimus edmontonicus, an agile, two-legged dinosaur that looked like a large ostrich and most likely lived off a mixed diet of bugs, fruit, leaves, eggs and the occasional small animal.
Ornithomimus aren't the direct ancestors of modern birds, but still offer clues to their development.
"Our specimens show the most primitive occurrence of wing-like structures," says Zelenitsky. "These specimens thus push back the occurrence of wing-like structures and give insight into their early uses."
On two of the individuals, one adult and one juvenile, Zelenitsky found traces of filamentous feathers — a primitive form of feather without the rigid central shaft.
That would have been exciting enough.
The fossils, found in 2008 and 2009, have pushed back the date of the earliest fossil feather by about 10 million years. As well, they were the first feathered dinosaur fossils found in North America and the first found in relatively coarse sandstone.
But things got even more interesting when Zelenitsky compared the two fossils with a third adult Ornithomimus unearthed in the same area in 1995. Despite being from exactly the same species, this dinosaur had much more advanced feathers.
"Their distribution and orientation are similar to the insertion pattern of covert feathers, which form the bulk of the feather covering in modern bird wings."
At about 150 kilograms, Ornithomimus was too big to fly, says Zelenitsky. And the fossils also suggested that real feathers weren't something Ornithomimus grew until it was fully adult.
That allowed her to make a couple of inferences:
"Because they're in these large dinosaurs, (wings) haven't evolved for flight," Zelenitsky says. "(And) because these wing-like structures develop later in life, that suggests they were used for purposes like display or courtship or egg-brooding."
Scientists had long wondered about why dinosaurs originally developed feathers. Previous theories suggested they helped dinosaurs glide through the air or hunt, but the feathers in Zelenitsky's ground-dwelling, plant-eating dinosaurs argue against that.
"There's been a number of different suggestions over the years as to why they've evolved. I think this just provides fossil evidence for wing feathers not initially evolving for flight."
Feathered dinosaurs were first discovered in the late 1990s. All came from fossil beds in China composed of fine-grained sedimentary rock.
Zelenitsky says finding the fossil feathers in an older specimen taken from relatively coarse-grained rock in North America could lead her fellow paleontologists to reconsider specimens tucked away in storage.
"We're hoping these discoveries will open up and make people realize there's new possibilities for finding feathered dinosaurs elsewhere.
"People will start looking at their collections."
Zelenitsky's next step will be to compare feathers on her specimens to those from the Chinese dinosaurs. And, of course, she'll be looking for more fossils.
"One of the big things will be looking for more feathered dinosaurs from Alberta."
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