Fossils of a feathered dinosaur have given wings to the idea that complex quilled feathers might have evolved for courtship, not flight, in at least one dinosaur species.
The species concerned, the ostrich-like Ornithomimus, seems to have had vaned feathers but neither it, nor its ancestors, ever seem to have taken to the air.
The new insights come from re-examination of a series of fossils found in North America. After finding two fossils in stream-bed deposits in Alberta, Canada, one in 2008 and the other in 2009, a local businessman contacted University of Calgary palaeontologist Darla Zelenitsky.
Technicians at the Royal Tyrrell Museum in Midland Provincial Park, Alberta, then exposed filamentary feathers on the Ornithomimus remains as they removed the rock encasing the bones.
That discovery led them to check an adult Ornithomimus discovered in 1995 that had odd marks on its forearms. "We went back and found the traces on the forearms looked like marks left by the vaned feathers of modern birds," Zelenitsky says.
Feathers with central vanes or shafts are required for flight, but ornithomimids were bipedal dinosaurs and grew to 3.5 metres long – far too large to fly. Falling between two feathered groups, birds and tyrannosaurs, on the evolutionary tree, they are not thought to have had flying ancestors.
Ornithomimids split from the evolutionary branch of birds long before flight evolved, so palaeontologists expected ornithomimids to have downy or filamentary feathers like tyrannosaurs.
The vaned feathers were only found on the arms of a complete adult fossil; the arms of a year-old juvenile 1.5-metres long had only filamentary feathers. Why did the adults have stiff feathers like flying birds?
One possibility, Zelenitsky's team suggests in a paper published today in Science, is that the long arm feathers served as sexual displays. Another is that they shielded the young during brooding, an idea proposed more than a decade ago by biotechnology consultant and amateur palaeontologist Thomas Hopp.
Whatever their purpose, evolution preserved them for some 80 million years from the time ornithomimids evolved (about 155 million years ago) until the dinosaurs in the area died 70 million to 75 million years ago.
"Their interpretation looks good to me," says Richard Prum, a Yale University ornithologist. "If it is confirmed… then it implies an early origin of a non-aerodynamic 'wing' of long forelimb feathers [and] an exciting role for display and communication in the origin of feather diversity."
But the preservation of details in the coarse sandstone fossil is poor, says palaeontologist Thomas Holtz of the University of Maryland in College Park. He wants to see better-preserved specimens to be sure what type of feather structures were on the ornithomimids' arms.
Journal reference: Science, DOI: 10.1126/science.1225376
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