[Perspective] A Mesozoic aviary

The evolution of birds from a group of small dinosaurs between 170 million and 150 million years ago has emerged as a textbook example of a major evolutionary transformation in the fossil record (1). The attainment of powered flight—that is, active flapping that generates thrust—has been widely regarded, sometimes explicitly but often implicitly, as a long evolutionary march in which natural selection progressively refined one subgroup of dinosaurs into ever-better aerialists. However, recen