Scales, teeth, feathers, hair, rhino horn.

Lynnis' post on the keratin in rhino horn brings to mind a little known but recently established fact that a sequence of evolutionary steps evolved fish scales into teeth (calcified scales that moved into the mouth), scales into feathers (dinos/birds), and feathers into hair (mammals). Of course the keratin in rhino horn is just consolidated hair. The roughly same gene system is known to behind all those evolutionary variants.