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Scale, feather, hair -- what's the diff?



Jeff Hecht wrote:
<< Longisquama shows scales have considerably more potential for interesting 
structure than is generally assumed. Whether or not those are true 
"feathers", they clearly evolved from scales. They may very well have evolved 
from scales independently of the feathers on true birds. Maybe we really need 
to take a closer look at scales to better understand feathers. >>

Nicely understated. I have been keeping an eye on the skin appendage 
literature, and watch out! a revolution is coming. For instance, there is a 
family of genes called "Wnt" that have been implicated in dictating the 
shapes of feathers, hair follicles, and teeth. There is a mutant of the 
chicken that has flight feathers on the toes of the feet (booted mutant). A 
single gene transferred into dermal cells causes bony ossicle formation where 
there was none . . .
    Fossil evidence and attendant controversies aside, there will come a time 
when we will be able to reconstruct, from DNA, the evolutionary homologies 
between different skin structures. As I have said on this list in the past, I 
am hedging for a common ancestor for all of the above-mentioned projecting 
structures, and dermal scutes as well.
    Gene mutations seem to change these structures with remarkable ease. Duh, 
let's see, would there be any advantage to vertebrates evolving a highly 
CHANGEABLE integument? Mix'n'match parts. Variations on a theme. Feathers 
jumping from wings to toes to tails to backs. Feathers becoming hairs. Scales 
becoming feathers. Ossified dermal nodules evolving from -- any of the above? 
What a great way to adapt to changes in the environment!
    That Longisquama developed an interesting variation on the continuum of 
projecting skin structures is, while not surprising, still way cool. And the 
WHEN it did so, -- well, that gets one to wondering just how far back fancy 
dermal structures go. I'm still holding out for the Devonian. Molecular 
biology ought to get at this issue in the near future as the human genome 
wraps up and the chicken, salmon and whatever-the-heck-else genome projects 
get rolling. Can't wait.
- Tom Hopp