Flying dinosaur fossil with intact feathers reveals how first birds took flight - Researchers say set of never-before-seen feathers on upper arm proved key to Archaeopteryx’s flight

Most modern birds solved this issue by evolving shorter appendages to accompany their tertial feathers. But even though Archaeopteryx possessed those long arm bones, it also had proportionately sized tertials. Many of its dinosaur relatives evolved feathers, but they stopped at the elbow and lacked the tertials necessary for flight.