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Re: Reversals



 
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.
Well, when a feature appears and then disappears, that's called a reversal, too, isn't it?
(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.  [...])
Well, most flightless birds don't need grasping hands. And those that needed them might have had them to some extent... Titanis?
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.)
To be precise, phylogenetic bracketing: It's normal for vertebrates to have 4 types of cones in the retina, for red, green, blue and ultraviolet. Mammals never have the last as far as is known, and all except some primates (e. g. us) don't have the first either -- to be red-green-blind is a synapomorphy of mammals. In those that have a red one it is clearly a recent, slightly altered copy of the green one (the gene got duplicated).
Can anyone suggest an example of a true reversal?
That something dis- and then reappears? I can't, at least at the moment.