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.