Feathers Before Flight

@article{Clarke2013FeathersBF,
  title={Feathers Before Flight},
  author={Julia A. Clarke},
  journal={Science},
  year={2013},
  volume={340},
  pages={690 - 692}
}
Fossil data indicate that feathers and their precursors may have evolved over a much longer span than previously thought. Feathers are branched structures consisting of β-keratin—a rigid protein material formed by pleated β sheets—with a hollow central shaft. They are strikingly different from other forms of vertebrate integument such as scales, skin, and hair. Until recently, evolutionary hypotheses envisioned their origin through elongation of broad, flat scales driven by selection for aerial… 
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