The molecular evolution of feathers with direct evidence from fossils

Dinosaur fossils possessing integumentary appendages of various morphologies, interpreted as feathers, have greatly enhanced our understanding of the evolutionary link between birds and dinosaurs, as well as the origins of feathersand avian flight. In extant birds, the unique expression and amino acid composition of proteins in mature feathershave been shown to determine their biomechanical properties, such as hardness, resilience, and plasticity. Here, we provide molecular and ultrastructural evidence that the pennaceous feathersof the Jurassic nonavian dinosaur Anchiorniswere composed of both featherβ- keratinsand α- keratins. This is significant, because mature feathersin extant birds are dominated by β- keratins, particularly in the barbsand barbules forming the vane. We confirm here that featherswere modified at both molecular and morphological levelsto obtain the biomechanical properties for flight during the dinosaur–bird transition, and we show that the patterns and timing of adaptive change at the molecular level can be directly addressed in exceptionally preserved fossils in deep time.