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Reversals



I visited Yale's Peabody Museum today.  (This is a holiday called Presidents' Day, honoring Washington and Lincoln.)  As part of the festivities called Dino Days, teachers had the opportunity to meet the public.
In a discussion with one teacher, anonymous in tribute to his sacrifice, he indicated that it is difficult to think of a true reversal, when a lost anatomical attribute disappears and then reappears.  (This was in context of noting that a flightless bird's arms do not grow into grasping weapons, but frequently disappear or retain limited functions from before flight was lost.  Not a BCF'er)  
He mentioned as an example the fact that mammals lost the ability to see color.  (How this was determined I forbore asking; we talked for only about 3 hours, with interruptions, and cladistics was included.)  He said that mammals did not regain this capability as it was priorly, but instead developed a new mechanism, a 'patch'.  (I'm wondering how mammals 'knew' there were colors to be seen, but that's a different discussion also.)
Can anyone suggest an example of a true reversal?
The above quotations are paraphrases, and any mistakes are mine, of course.