Every other animal that has walked on land, including humans, elephants, lizards and theropod dinosaurs - a family that includes the T Rex, had a moveable thigh bone.
The find may force paleontologists to reconsider their long-held belief that modern birds are direct descendants of ancient, meat-eating dinosaurs.
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'It's amazing we didn't understand a basic aspect of bird biology,' said US researcher Professor John Ruben.
'A velociraptor did not just spout feathers at some point and fly off into the sunset.
bird
Running and walking is generated mostly at knee and ankle joints in birds
'Birds probably evolved on a parallel path alongside dinosaurs, starting that process before most dinosaur species even existed.'
Characteristics such as feathers, wings and a bird's unique way of moving therefore developed separately.
Dinosaurs and birds both thrived in the Cretaceous period between 145 and 65 million years ago.
The Archaeopteryx, long considered the oldest bird, lived 150million years ago, while dinosaurs emerged earlier around 200million years ago.
However, Professor Ruben said he didn't discount birds and dinosaurs sharing an ancient ancestor such as the small, reptilian 'thecodonts' that roamed Earth 250million years ago.
'It just seems pretty clear now that birds were evolving all along on their own and did not descend directly from the theropod dinosaurs,' he concluded.
There study has been published in The Journal of Morphology.
http://www.dailymail.co.uk/sciencetech/article-1192019/Why-birds-NOT-descended-dinosaurs.html