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, recent fossil discoveries reveal a much more interesting story that is beginning to be corroborated by biomechanical studies. According to this story, the development of flight was chaotic, with different dinosaurs experimenting with different airborne behaviors using different airfoil and feather arrangements (see the figure), until ultimately only modern birds survived.