In the last few decades we’ve realized that birds descended from theropod dinosaurs, and that the evolution of feathers preceded the evolution of flight. Indeed, many biologists still consider birds to be dinosaurs, since the group “dinosaurs” leaves out some of the descendents of dinosaurs—birds. Regardless of whether one adheres to this convention, we know from genetic and fossil evidence that birds are united with reptiles in one group: the Archosauria, which includes the common ancestor of crocodilians and birds, and all the descendants of that ancestor up to and including modern birds and crocodilians. That means that crocodilians are the living reptiles most closely related to birds (if, that is, you don’t consider birds to be reptiles).
Comparing birds with archosaurs, then, can tell us something about the evolutionary changes that produced our feathered comrades. And this is what was done in a new paper in Nature by Bhart-Anjan Bhullar
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