Well, that's ostensibly what this paper is. I need to give it another
read-through or three, but on my first pass, I'm not convinced by
Lingham-Soliar et al.'s conclusions. Is their analysis a good one? Yes,
actually -- they do present some interesting arguments, certainly ones that
need to be tested further; unlike some previous BAND papers, this one is
actually pretty thorough in its analysis, though it is far too narrow in
scope -- that is, it's good for what it _is_ examining, but it doesn't
examine enough (though for all I know, more papers are forthcoming; I also
expect that the brevity and narrow focus was, in large part, a result of
_Proceedings B_'s short page limits!) They do open the paper with a good
point: these structures really haven't been subjected to a rigorous analysis
perviously -- lots of talk, and a few sketches, but not much else,
particularly in _Sinosauropteryx_, and I think this paper is actually a good
start, but far from conclusive. My main concerns about it are:

* they don't examine enough taxa -- as others here have noted (about this
paper and in the past), they don't look at the similar structures on, say,
_Beipiaosaurus_, _Sinornithosaurus_ (where they have well-documented tufting
and branching morphologies), _Dilong_, etc.

* they seem to continually interpret all the structures are body midline
structures, but Currie et al. long ago pointed out that in the holotype, the
structures are _not_ preserved along the body midline, particularly in the
head area. Yes, the skin doubtless sloughed off during the decay process,
and not having any real resistance (like the bones did), would have been
compacted more than the skeleton, and could easily make it look like midline
structures are not on the midline, but this argument cuts both ways:
non-midline structures could also be displaced to appear as if they're on
the midline, especially when the entire body is laterally compressed and
exposed in lateral view. Currie & Chen's paper document (though doesn't
adequately picture) patches of these structures on the caudolateral margin
of the cranium, and in another specimen on the ribs and side of the tail.
Unless these animals were covered by the kinds of ribbon-like frills of
collagen in places other than the body midline (a la
http://lhs.lidgerwood.k12.nd.us/LHS/BlakesWebPage/Graphics/Pictures/Comp.jpg
-- certainly possible, but seems very odd to me...certainly without a
modern analog), the argument that the fibers are collagenous and restricted
to the body midline is weakened.

* much of their argument seems to focus on the continual linearity --
interpreted as inflexibility -- of the structures around _Sinosauropteryx_;
in particular, they seem bent on disproving the statement by Currie & Chen
that the structures are "soft and pliable." Well, disproving that statement
depends on it being correct in the first place, and I'm not convinced it is.
If these structures are proto-feathers, and in particular rather like a
barb- and barbule-free, "naked" rachis, then it wouldn't be "soft and
pliable"; one would expect it to be rather stiff and, hey, linear! I can't
recall offhand what prompted Currie & Chen to think that they might be "soft
and pliable" (as in mammalian fur, I suppose, though even fur can be very
wiry), but I've never thought of proto-feathers as anything but very
superficially like fur, and more like feather rachises, myself. Beyond all
this, many of the structures in, at least, the _Sinosauropteryx_ holotype
look curvilinear to me; some of the ones in the new specimen shown in the
Lingham-Soliar paper appear slightly wavy, too.

* I fail to see why linearity, and in particular non-perpendicularity to the
long axes of various body part, like the tail, could not also be induced in
the structures by even very weak bottom currents (or even follicular
orientation), which may well have been present. Such currents could have
swept the structures along parallel to current direction; weak currents may
even have been redirected and channeled along the body simply as an effect
of interference, kind of like how a sizeable object deflects air currents,
though I'm not remotely enough of a physicist to even be able to know if
there's a basis for this kind of argument...

* they seem to make a big deal about the "light" areas between the dark
areas, particularly along the tail, to interpret the structures as the
remnants of a midline frill. However, they don't seem to address whether or
not the "bare" patches might not just be structures that lacked whatever
pigment and thus didn't have whatever chemical processes that enabled the
dark patches to show up better. After all, there are actual feathers
preserved in various places with color banding, and certainly the darker
areas show up much better; the non-dark areas can look almost absent (a la
http://www.scielo.br/img/fbpe/aabc/v72n4/0049img18.gif and, of course,
_Caudipteryx_, a la
http://www.yale.edu/peabody/images/explore/cfd/Feather1.jpg), and it's
largely the human propensity to connect subjective contours into entire
structures that makes the feather shape "appear." Of course, I have not
seen the specimens of _Sinosauropteryx_ in person and couldn't say at all
whether or not the bare patches really are bare or if there's actually
something there, but this doesn't seem to have been really tested in the
Lingham-Soliar et al. paper, either -- testing for pigment really is outside
the scope of their paper, so this isn't really a reasonable criticism, I
suppose... And, after all, it's certainly possible that pterylae along the
tail of _Sinosauropteryx_ really was interrupted by bands of apteria (though
again, this would be weird...)! Miniscule pigment differences could,
hypothetically, also be at least part of the reason some of the fibers seem
"beaded," as they call them, though again I emphasize that just 'cuz I say
this doesn't make it true!

* perhaps most importantly, and even beyond examining other theropods, it
would be wonderfully informative to do this exact same kind of analysis on
mammal fossils that preserve fur. Jehol Group mammals would be an ideal
test -- same environment, after all -- but Messel mammals, for example,
would also be good. Do individual fur fibers in these specimens also appear
linear and largely unidirectional, at least on common portions of the body?
Does it also appear preserved only on the midline?

* in the end, what does it mean if the structures in _Sinosauropteryx_
aren't proto-feathers? It would mean _Sinosauropteryx_ doesn't have
proto-feathers. It says little about whether or not other ostensibly
proto-feather-bearing animals, like _Beipiaosaurus_ and _Dilong_, also don't
have proto-feathers, and it especially doesn't mean that taxa like
_Sinornithosaurus_ didn't have homologous structures -- much more analysis
would be needed to make these conclusion (beware generalizations!). But even
if we assume -- and generalize -- that Lingham-Soliar's evidence correctly
demonstrates that _Sinosauropteryx_, _Dilong_, etc. are not covered in
proto-feathers, we still have theropods with true feathers on them. Those,
coupled with the massive quantity of osteological, behavioral, eggshell,
etc. evidence, still convincingly demonstrate that birds derive from within
the theropod lineage. Of course, Lingham-Soliar et al. (particularly the
Feduccia part of "et al.") would argue that all such taxa are birds, not
theropods, and all the ostensibly proto-feathered taxa are theropods,
unrelated to the bird lineage, but those phylogenetic positions aren't
supported by all the other evidence, either.

Some on the list have said things to the effect of "show me similar
structures in a non-theropod from the same unit and I'll believe it." I
don't find this to be a particularly effective argument, and here's why:
while other vertebrates do indeed have collagen in varying quantities, what
I think Lingham-Soliar et al. are arguing here, although they never come
right out and say this, is that theropods autapomorphically have frills of
collagen (or, at least, massive quantities of subdermal collagen) along
their bodies that are not present in other, non-theropod taxa. (Why
theropods would have excessive quantities of collagen, far beyond what any
other vertebrate has, I can't fathom, but it does seem to be what they are
arguing by interpreting the structures as collagenous frills exclusive to
theropods -- or maybe just _Sinosauropteryx_.) If true, then one would
_not_ expect to see similar structures in frogs, fish, or any other
vertebrate from the same deposits. Of course, this same argument applies to
the structures as proto-feathers, too, so really, it's a non-argument -
either proto-feathers or massive quantities, and possibly frills, of
collagen are restricted to theropods either way; their absence in other taxa
can't be used as an argument to show that something restricted to theropods
must therefore be feathery. Should someone come up with a collagen-frilled
frog, or an ostensibly proto-feathered fish, I'll be all eyes, and looking
for one would certainly be a way of testing whether or not these structures,
whatever they are, are limited to theropods, but it is _not_ a means of
testing whether or not they are collagen.

And like everyone else, I would LOVE it if Mary Schweitzer et al could
get a few little scrapings off of these things to see if there is
beta-keratin present -- that would go a long way to determining whether or
not these things really do have anything to do with feathers. Some SEM shots
of the stuff would also be terrific, though in both cases, I think the
Chinese are presently loathe to let any parts of these specimens be subject
to destructive testing...

"Trying to estimate the divergence times
of fungal, algal or prokaryotic groups on
the basis of a partial reptilian fossil and
protein sequences from mice and humans
is like trying to decipher Demotic Egyptian with
the help of an odometer and the Oxford
English Dictionary."
-- D. Graur & W. Martin (_Trends
in Genetics_ 20[2], 2004)