Feathered dinosaurs with something a lot like wings have for the first time been discovered in North America - but the feathers don't appear to have been used to aid flight.
The ornithomimids, which looked a lot like ostriches, were recovered from 75 million-year-old rocks in river deposits in Alberta, Canada.
"This is a really exciting discovery as it represents the first feathered dinosaur specimens found in the Western Hemisphere," says Darla Zelenitsky, assistant professor at the University of Calgary.
"Furthermore, despite the many ornithomimid skeletons known, these specimens are also the first to reveal that ornithomimids were covered in feathers, like several other groups of theropod dinosaurs."
Evidence of feathers was found on both a juvenile and two adult skeletons of Ornithomimus. However, while down-like feathers were discovered on the juvenile, only the adults had larger feathers on the arms, forming wing-like structures.
"This pattern differs from that seen in birds, where the wings generally develop very young, soon after hatching," says Zelenitsky.
This discovery of early wings in dinosaurs too big to fly indicates the initial use of these structures was not for flight.
"The fact that wing-like forelimbs developed in more mature individuals suggests they were used only later in life, perhaps associated with reproductive behaviors like display or egg brooding," says François Therrien from the Royal Tyrrell Museum of Palaeontology.
Until now feathered dinosaur skeletons have rarely been found outside China and Germany, contained in fine-grained rocks.
"It was previously thought that feathered dinosaurs could only fossilize in muddy sediment deposited in quiet waters, such as the bottom of lakes and lagoons," says Therrien.
"But the discovery of these ornithomimids in sandstone shows that feathered dinosaurs can also be preserved in rocks deposited by ancient flowing rivers."
Because sandstone is the type of rock that most commonly preserves dinosaur skeletons, the Canadian discoveries raise hopes of finding feathered dinosaurs worldwide.