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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