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Re: New in Nature: Birds Galore!



Do we not live in wonderful times!! The shades of
Huxley, Colbert, Heilmann and others would finally
taste blood, and the living like Paul would have a
strong reason to be vindicated. I deeply envy the like
of Dr. Holtz and others who are privy to much more
than the humble lay.

To me the feathers on the Hind limbs are not
surprising : it is merely a case of serial homology.
When a new structure arises it does not come with all
the developmental pathways to fine tune its
expression. The recent work has shown that feather
development appears to be regulated by a pattern
forming field of Sonic hedgehog and BMP2. Now, these
pathways are even more  ancient as they even function
in the mammalian hair follicle's formation. Slight
adjusments to the expression pattern of these ancient
regulators has resulted in the diversity of amniote
epidermal structures. But the basic pathway is the
same and is regulated at a higher level by the general
patterning pathways of the limb. So when a large
flight feather evolves its pathway is likely to be
still under the control of the higher level generic
limb development pathways. Accordingly it is expressed
 in both the fore and hind limbs. This fossil
Microraptor pauli appears to have precisely captured
this stage in evolution. Subsequently natural
selection allowed the origin of specific pathways that
restricted this kind of feathers to the fore limb
only. To me this is very remniscent of the evolution
of the haltere and forewing of the dipterans from a
generic forelimb like structure (reverse in
strepsiterans).

how little we know even of the well studied theropods!
-E

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