Natural Science Forum / Biology / Paleontology / October 2004

Have you tried the authors' web sites? People commonly post their own
publications. Failing that, you should just write to them and ask for a
PDF directly. Here's the abstract in case you haven't found that. I
don't see any email addresses, but again you could look that up on the
university web site.

Avian skin development and the evolutionary origin of feathers
Roger H. Sawyer *, Loren W. Knapp *
Department of Biological Sciences, University of South Carolina,
Columbia, South Carolina 29208

*Correspondence to Roger H. Sawyer, Department of Biological Sciences,
University of South Carolina, Columbia, SC

*Correspondence to Loren W. Knapp, Department of Biological Sciences,
University of South Carolina, Columbia, SC

Abstract
The discovery of several dinosaurs with filamentous integumentary
appendages of different morphologies has stimulated models for the
evolutionary origin of feathers. In order to understand these models,
knowledge of the development of the avian integument must be put into an
evolutionary context. Thus, we present a review of avian scale and
feather development, which summarizes the morphogenetic events involved,
as well as the expression of the beta (
<http://www3.interscience.wiley.com/giflibrary/12/bgr.gif> ) keratin
multigene family that characterizes the epidermal appendages of reptiles
and birds. First we review information on the evolution of the
ectodermal epidermis and its beta (
<http://www3.interscience.wiley.com/giflibrary/12/bgr.gif> ) keratins.
Then we examine the morphogenesis of scutate scales and feathers
including studies in which the extraembryonic ectoderm of the chorion is
used to examine dermal induction. We also present studies on the
scaleless (sc) mutant, and, because of the recent discovery of
<http://www3.interscience.wiley.com/giflibrary/12/ldquo.gif> four-winged
<http://www3.interscience.wiley.com/giflibrary/12/rdquo.gif> dinosaurs,
we review earlier studies of a chicken strain, Silkie, that expresses
ptilopody (pti),
<http://www3.interscience.wiley.com/giflibrary/12/ldquo.gif> feathered
feet. <http://www3.interscience.wiley.com/giflibrary/12/rdquo.gif> We
conclude that the ability of the ectodermal epidermis to generate
discrete cell populations capable of forming functional structural
elements consisting of specific members of the
<http://www3.interscience.wiley.com/giflibrary/12/bgr.gif> keratin
multigene family was a plesiomorphic feature of the archosaurian
ancestor of crocodilians and birds. Evidence suggests that the discrete
epidermal lineages that make up the embryonic feather filament of extant
birds are homologous with similar embryonic lineages of the developing
scutate scales of birds and the scales of alligators. We believe that
the early expression of conserved signaling modules in the embryonic
skin of the avian ancestor led to the early morphogenesis of the
embryonic feather filament, with its periderm, sheath, and barb ridge
lineages forming the first protofeather. Invagination of the epidermis
of the protofeather led to formation of the follicle providing for
feather renewal and diversification. The observations that scale
formation in birds involves an inhibition of feather formation coupled
with observations on the feathered feet of the scaleless (High-line) and
Silkie strains support the view that the ancestor of modern birds may
have had feathered hind limbs similar to those recently discovered in
nonavian dromaeosaurids. And finally, our recent observation on the
bristles of the wild turkey beard raises the possibility that similar
integumentary appendages may have adorned nonavian dinosaurs, and thus
all filamentous integumentary appendages may not be homologous to modern
feathers. J. Exp. Zool. (Mol. Dev. Evol.) 298B:57-72, 2003. © 2003
Wiley-Liss, Inc.