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Re: Featherless Sinosauropteryx



I want to see an analysis of whether it's collagen or feathers.

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...

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Jerry D. Harris
Director of Paleontology
Dixie State College
Science Building
225 South 700 East
St. George, UT  84770   USA
Phone: (435) 652-7758
Fax: (435) 656-4022
E-mail: jharris@dixie.edu
and     dinogami@gmail.com
http://cactus.dixie.edu/jharris/

"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)