Which skeletal part was a key element in showing that birds are descended from dinosaurs?

As scientists look at dinosaur fossils, they find similarities with the skeletons of birds. A theory was discussed that of all of the bones that were similar in so many vertebrates for a dinosaur to be able to fly would include the flexible and correctly placed wishbone {furcula} to allow for the transition from "arms" to that of "wings."

Indeed, after decades of trying to find this particular missing link, it was found in particularly well-preserved fossils in volcanic ash in northeast China. The results of analyzing those fossils was that some dinosaurs used their front appendages {arms} to grasp and pull prey back towards their mouths. This exact series of motions matches the patterns needed for flight with wings.

Along with the feathers that some dinosaurs already had to help keep them warm, the essential structure of a skeleton needed for flight had evolved.