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recapitulation & feathers



> The appearance of digits in the avian foot is 4-3-2-1,
> and it ought to be the same in the hand - and the fact that the order of
> appearance is 4-3-2-1-5 in most (but not all) nonavian tetrapods.

     What are the exceptions, and what digit do they start with?  I
thought the serial appearance of digits was the most interesting piece of
evidence in the paper.  I don't recall they even mentioned that there WERE
exceptions.
     In feathers, the dermis provides the pulp and blood vessels that feed
the growing feather, but the epidermis forms the actual feather. Feather
and scale development start out almsot identically.  The
dermis pushes up on the epidermis, forming a papilla, or "pimple".
After this, they differ; the scale is flattened, and the proto-feather
becomes elonmgate and stick up  The epidermial "cap", keeps growing, and
also forms a pit around the feather germ which will later become the
feather follicle.      
                                                      EEEE
                                                     EddddE
                                                     EddddE
        EEEEEEEEEE                                   EddddE
    EEEEEddddddddEEEEE                              EdddddE 
EEEEEddddddddddddddddEEEEEEE      =====>    EEEEEE EdddddddE EEEEEEE
dddddddddddddddddddddddddddd                ddddEE EdddddddE Edddddd
                                            ddddEE EdddddddE Edddddd 
                                            ddddddEEEddddddEEEdddddd
                                            dddddddddddddddddddddddd
     
      
    I was going to go into further detail, but I realized reading my
source that the differences in development between the neossoptile (natal
down feather) and the later teleoptile, or adult feather, has become hazy
to me since I took ornithology.  The explanation in the text for the
further development of a neossoptile isn't terribly clear to me.  I need
to do a little more digging.    
     One interesting note on feather coloration; birds can manufacture
melanin themselves to produce black, greys, browns and reddish browns.
Blues and irridescent colors are usually produced by structural
modifications of the barbs and barbules  combined with melanin
pigmentation. However, reds, oranges, and yellows are usually gotten
from plant carotenoids, and green is likewise usually produced through a
combination of caratenoids and barbule size and shape.  Therefore, since
theropods were mainly carnivorous, any forms with feathers may have been
more likely to have been brown, grey, black, reddish brown, or blue then
bright red, orange, or yellow.  However, it is important to note that
predatory birds can get some carotenoids from eating herbivorous birds
that have them, and that there is one modern bird, the touraco, that
manufactures a green pigment instead of relying on carotenoids.  Theropods
may have produced green or warm color pigments as well.  Just a thought.

Pettingil, O.S. 1985. Ornithology in laboratory and field, fifth edition.
     Academic Press, San Diego.  403 pages.


LN Jeff