The preditor Deinonychus had feathered arms that worked just like a bird!
“...Dinosaurs such as Velociraptor might have partly folded their feathered arms to protect such plumage from harm’s way, researchers explained. The wrists and the feathers in the lineage that led to birds then became more extreme, laying the groundwork for flight, they added.
Although birds are most known for their feathers, wings and toothless beaks, another distinctive feature is a wrist joint that is extremely flexible, although only in one direction. A bird can bend its wrist to the point where the side of the hand where the little finger would be can lie closely alongside the forearm, so any fingertips would point back almost towards the elbow, but the wrist cannot bend in the opposite direction, nor even fully straighten….It remains unclear if the foldable wrist allowed dinosaurs to evolve longer feathers or if the evolution of longer feathers drove the need for more flexible wrists.
Still, it is clear “that wing-folding, or at least feathered arm folding, significantly preceded flight,” said lead researcher Corwin Sullivan, a vertebrate paleontologist at the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing. “This pattern of flexibility originally evolved in a terrestrial context and just happened to be present and available for use when birds took to the air.
This discovery highlights the fact “that some characteristics that biologists used to think of as distinctively avian — feathers and air sacs are other good examples — are actually quite deeply rooted in theropod evolution,” Sullivan added.
To cement their findings, he suggested measuring wrist bones for more non-avian theropod species and measuring feather lengths whenever possible to get a better idea of the exact interplay between the evolution of wrists and feathers….”