The molecular evolution of feathers with direct evidence from fossils

Abstract

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 feathers and avian flight. In extant birds, the unique expression and amino acid composition of proteins in mature feathers have been shown to determine their biomechanical properties, such as hardness, resilience, and plasticity. Here, we provide molecular and ultrastructural evidence that the pennaceous feathers of the Jurassic nonavian dinosaur Anchiornis were composed of both feather β-keratins and α-keratins. This is significant, because mature feathers in extant birds are dominated by β-keratins, particularly in the barbs and barbules forming the vane. We confirm here that feathers were modified at both molecular and morphological levels to 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.

Taphonomic experiments resolve controls on the preservation of melanosomes and keratinous tissues in feathers

“…; Pan et al . , ) and organic matrices surrounding fossil feather melanosomes have been interpreted as degraded residues of originally keratinous feather tissue (Li et al . ; Zhang et al .…”

“…; Pan et al . , ) in both modern and palaeontological contexts in order to understand better the underlying drivers in the evolution of feathers and flight. Keratin is a fibrous protein rich in cysteine, which promotes extensive cross‐linking (and thus confers chemical stability) via disulfide bonds (Wang et al .…”

Feather evolution: looking up close and through deep time

“…The conclusions from the second study, which was published by Pan et al [10] in the February 19, 2019 issue of Proceedings of the National Academy of Sciences of the United States of America, are even more unexpected. Pan et al [10] examined the ultrastructure and molecular composition of fossil feathers and claw sheaths of several non-avialan dinosaurs and early birds by using scanning electronic microscope (SEM), transmission electronic microscope (TEM), and immunohistochemistry methods. Surprisingly, they find that: (1) the simple filamentous structures of Shuvuuia, an early-diverging maniraptoran theropod, lack feather b-keratins;…”

“…The second study by Pan et al [10] will face even more challenges. The dominance of a-keratins in the feathers of Anchiornis is unusual because the same study indicates that other fossil feathers from both non-avialan theropod dinosaurs and early birds, and even the fossil claw sheath of earlier-branching theropods are dominated instead by b-keratins.…”

“…Even with these significant new fossils and associated data, important issues such as when the first feathers evolved [7] and the microscopic composition of early feathers [8] remain unclear ( Fig. 1), although two recent studies provide new information on these issues [9,10].…”