Science, as a process, promotes an adversarial system. A scientist poses an hypothesis from an observation, then attempts to refute this hypothesis through further observations arrived at from experimentation and testing, and poses a further hypothesis from the results; if it stands, he can make a claim that a thing is, or isn’t. Another scientist comes along and attempts to refute that finding, and so on and so forth. We can presume that scientist A and scientist B are both using the same data or are merely increasing the data used to make observations, and that the same data is included by further authors, thus merely expanding the perspective. But it seems there are adversaries, and there are enemies. Some scientists, against seeming logic, will not even regard the same datasets offered, and use this as refutation of previous datasets or observations. Science, we presume, is not served when workers talk past one another, or make claims that a thing simply is, without any substantiation for why.
Last year, Drs. Ryan McKellar, Brian Chatterton and Alexander Wolfe from the University of Alberta joined Dr. Phil Currie, avant that he is, to describe fossilized amber from the late Campanian (Late Cretaceous) site of Grassy Lake in Alberta, Canada. This amber contained clear inclusions formerly living organisms, including insects, as well as the integumentary remains of various animals, from feathers, to hair, to some things that seemed an awful lot like primitive, stage I or II type feathers (as defined by Prum, 1999).
The structures found inside this amber were presumed by the authors to represent the first three-dimensionally preserved remains of the integumentary materials that had been presumed to cover the bodies of reptiles various, including Sinosauropteryx prima (Chen et al., 1998), then later the body of a juvenile referred to as Sinornithosaurus sp. (NGMC 91; Ji et al., 2001, Xu et al., 2001; however, Senter, 2010, recovered NGMC 91 as a sister taxon to a general “Microraptor” clade, which is still close to Sinornithosaurus), Beipiaosaurus inexpectatus (Xu et al., 1999; Xu et al., 2009), and the basal tyrannosauroid Dilong paradoxus (Xu et al., 2004). The authors described the material, also noting similarities to some of the preserved feathers that were found in modern Gaviidae (grebes), little nodes along the barbs that enabled the grebes to dive more efficiently; this, McKellar et al. argued, suggested that some of the Campanian animals that left these feathers behind in the amber would have developed modern-style adaptations for diving paralleled in modern birds. In order to differentiate the feathers from nonanimal remains, the authors subjected several of the specimens to confocal microscopy, either laser disk (LDCM) or spinning disk (SDCM) which bombards the objects being observed by various wavelengths of light, which excites the autofluorescence of the objects and cause them to react; different types of material react at different wavelengths, including keratin. Because the autofluorescence of amber tends to mask the autofluorescence of inclusions, depending on its composition, this analysis was inconclusive, although some objects appeared to react differently than others.
McKellar et al. appear to have run a gamut of tests in determining what the objects were. Some of them are clearly feathers, like the one shown above. Others, though, were more questionable, showing features seeming to relate to various forms of Prum’s feather developmental/evolutionary stages. The authors even compared the tendril diameters and apparent hollowness, where the confocal microscopy even supported that some of these questionable objects had hollow filaments, finding that they compare directly to barb and barbule diameters in bird feathers.
But in the end, the authors, concluding that many of these inclusions were feathers, they were careful to clearly argue where the deficiencies in their work lie. In despite of this, a response came from the Smithsonian Institution’s Dr. Carla Dove and Dr. Lorian Straker of the Universidade Federal do Rio de Janeiro. In it, they question several of McKellar et al.’s arguments. They repeat McKellar et al.’s concerns about LDCM/SDCM and its inconclusiveness, they question the hollowness of some of the fibres, the lack of comparisons to certain confirmed hair samples (which McKellar et al. do not illustrate but note is currently under study), and even the comparison of the diameter of the fibres to fungal hyphae or hair. Perhaps the most damning portion their argument highlights is that McKellar et al. did not perform the same series of tests and comparisons on each of the various studied specimens mentioned in the paper, instead opting for a few for each; but Dove & Straker (2012) focus much of their comments on the structure of the fibres themselves, either their form, twistiness, or hollowness.
There are a few reasons I am not very pleased with the response of Dove & Straker, but getting to those requires a tad bit of narrative.
BAD vs. BAND: The Eternal Struggle (I hope not)
After arguing the extreme minute size of these fibers in comparison to those structures preserved in specimens of Sinosauropteryx prima (Currie & Chen, 2001: NIGP 127586 and NIGP 127587) Dove & Straker (2012) write:
[O]ther researchers who examined the S. prima impressions reported measurements no thinner than 0.05 mm and declared the fibers to be collagen and not protofeathers (6).
This citation (“6″) is to Lingham-Soliar et al. (2007), who dismissed the fibers as those of collagen (I mention Theagarten Lingham Soliar (hereafter, TLS) here and here). Of the authors, the second — Alan Feduccia — is rather infamous as a promoter of the opposition view that “birds are not dinosaurs,” a position lovingly termed BAND. In a fairly well-known book (The Origin and Evolution of Birds, 1996, updated 1999), Feduccia laid out what is the most essential position of BAND, that birds are not, nor CAN they be, dinosaurs, largely predicated on a variety of papers attempting to claim that the pelvis or tarsal structure cannot be compared, that “dinosaurian” physiology cannot give rise to bird physiology, etc. Paul’s Dinosaurs of the Air(2002) pretty much tackles and dismantles these arguments in turn, citing most of them. TLS, turning his skills to detailed microscopic examination of the dermis of extant animals, has attempted to compare this directly to fossil animals. To date, none of TLS’s study specimens have affirmed the presence of “feathers” arising from the skin of the variety of animals discussed, including ichthyosaurs, sharks, frogs, etc. TLS has yet to assess whether birds or mammals preserved at Liaoning do, in fact, have feathers or hair, and that the fibers found forming a carbonized halo around their bodies are NOT comprised of collagen. We await with baited breath for this ground-breaking study.
edit: In the above figure, I use the phrase “pennaceous, w/” and “w/o barbs” — what I meant was “barbules,” as the barbs, those branching off from the main rachis, are clearly present in these taxa regardless. I will emend the figure at a future point to clarify.
I spend far too much time, I think, belaboring under the illusion that TLS’s skills will eventually be turned to questioning the conclusions of his last few papers. I think that when TLS cautions other authors of rhetoric and use of fallacies (Lingham-Soliar, 2011), he should use this as an opportunity not to indulge in those remarks which he takes others to task for. His closing paragraph is, in fact, instructive [posted in its entirety]:
Finally, while Zhang et al.’s (2010) study was an apparently sincere attempt to investigate potential phaeomelanosomes in the integumental structures of Sinosauropteryx and settle in their view the question of whether they were early feathers or collagen fibres, the methodology and results are critically flawed. SEM of fossilised melanosomes as shown earlier (Vinther and Briggs 2008) may be an extraordinarily useful tool in authenticating ambiguous fossilised integumental structures, when objectively applied, but one should be mindful that rarely if ever do we find a “magic bullet” in science as a universal solution. However, poor use of relevant technology, as shown in Zhang et al.’s SEM (2010, figure 3c), and abandonment of scientific procedure, give many reasons to be sceptical of how future ultrastructural studies of other non-avian dinosaurs, e.g. Psittacosaurus and Tianyulong (Zhang et al. 2010, p. 2) will be applied. Witmer (2009, p. 294), a supporter of the dinosaur protofeather hypothesis, recently intimated the dangers of rhetoric and concocting “complicated scenarios for feather evolution” based on what was all along thought by most workers to be a “seemingly simple question” of whether the filaments are epidermal or dermal, acknowledging that it “is surprisingly hard to answer.” Yet, there were criticisms (e.g. Ruben and Jones 2000; Lingham-Soliar 2003; Feduccia et al. 2005; Lingham-Soliar et al. 2007, and references therein) of precisely such rhetoric and more especially of a lack of scientific rigor, which were spurned in articles and dismissed as minority views (Sues 2001, p. 1036; Prum and Brush 2002, p. 4). If a lesson has been learnt, as implied by Witmer (2009), I am skeptical given the report on Witmer’s comments in The New York Times (Zimmer 2010), specifically concerning Sinosauropteryx (Zhang et al. 2010), “the study decisively closes the case on whether the whiskers are feathers or collagen.” His denouncement of rhetoric is unconvincing given that this comment appeared before any chance of potential counter-studies. The problem of this “surprisingly hard” question to answer has far from diminished as demonstrated here by Zhang et al.’s (2010) simplistic treatment of melanosomes in Sinosauropteryx, underscoring that, while the application of technology and interpretation of novel ideas are welcomed, they can nevertheless become self-serving to preconceived notions that continue to fuel the rhetoric of “complicated scenarios of feather evolution” and now feather colour patterns. As important as establishing the legitimacy of a hypothesis is the regard for the due processes of science (otherwise science may just as well be relegated to the casting of a die where the law of averages states that some of the time the call will be correct) and the avoidance of Machiavellian politics—i.e. the end justifies the means. A blanket view that all sinuous structures in the Chinese dinosaurs were “protofeathers,” is now being compounded by a blanket view that all micro-particles found in the same integumental structures are melanosomes. Is it possible that most workers will so soon be lulled once more into repeating the former complacency that it is a “seemingly simple question” to answer? While feathers in birds and some non-avian dinosaurs may be correctly identified by melanosomes, in others, specifically in Sinosauropteryx, they are almost certainly not. Zhang et al.’s (2010) study does nothing to detract from previous conclusions based on a number of lines of evidence (Lingham-Soliar et al. 2007, p. 1826) that the filamentous structures were in all probability collagenous.
You see, I think TLS could turn this entire game on its ear, because he is uniquely interested in examining the structure of the soft-tissue of various animals. He should, in fact, have started this entire process by examining the arguments and processes by which integument can be differentiated from epidermal structures (hair, feathers, etc.). Instead, we get arguments avoiding this point, and it is severely disappointing. TLS is, by far, one of the few scientists who takes their work seriously enough to do this, yet it seems he will not go far enough, not seriously enough. Papers he produces pop up less than or up to a year after the work he is replying to does, performing yet newer work, and yet it never goes beyond discussion of collagen degradation. This has become schtick, although it seems TLS is the only one not playing along. He even plugged that paper with a reconstruction of Sinosauropteryx prima as a fin-tailed reptile who died in the throes of opisthotonic convulsions, apparently responsible for the displacement of sections of its continuous dorsal fin. Let’s ignore the absolute uniqueness of that “dorsal fin,” proposed by TLS before (et al., 2007) and the lack of any comparison save to marine reptiles, or that he proposed this fin in part from presuming that its foundering at the anoxic bottom of a body of water meant that it must have been aquatic. The real issues involved in this discussion are the tissue comparisons, and how TLS refuses to look at birds or mammals in that environment.
Even more shocking is the absolute obstinacy and bile that comes from some authors, such as Storrs Olson’s now-infamous open letter to Peter Raven (on the Committee for Exploration and Research at the National Geographic Society, who has recently published on the then-unknown hooplah now known as “Archaeoraptor” by Christopher Sloan in the November, 1999 issue of National Geographic), and his response to a response by Henry Gee (managing editor at Nature), which had been shared openly as a response to Gee’s open comments to Raven. Olson, an ornithologist and current colleague of Dove’s at the Smithsonian Institution’s Department of Birds in the National Museum of Natural History (NMNH), ended his rant against Raven thusly:
The idea of feathered dinosaurs and the theropod origin of birds is being actively promulgated by a cadre of zealous scientists acting in concert with certain editors at Nature and National Geographic who themselves have become outspoken and highly biased proselytizers of the faith. Truth and careful scientific weighing of evidence have been among the first casualties in their program, which is now fast becoming one of the grander scientific hoaxes of our age—the paleontological equivalent of cold fusion. If Sloan’s article is not the crescendo of this fantasia, it is difficult to imagine to what heights it can next be taken. But it is certain that when the folly has run its course and has been fully exposed, National Geographic will unfortunately play a prominent but unenviable role in the book that summarizes the whole sorry episode.
This vitriolic comment in one of two public letters certainly brooks no feeling of trust in the observations of such authors or the work they perform. They are clearly devoted. Olson’s argument, deriding Raven for his “faith” in this ideal, is firmly entrenched when, as in other papers (cited in Martin, 2008), he derides Gee for being partial to the argument for a dinosaurian origin of birds:
I am not much concerned, however, about NATURE having rejected good papers that should have been published and I am prepared to accept your word that this has not happened. Of much more concern is the publication of bad papers that should NOT have been accepted. Obvious dinosaurs (Mononykus) have been described as birds (NATURE 1993, 362:623). Obvious birds (Protarchaeopteryx and Caudipteryx) have been described as dinosaurs (NATURE 1998, 393:753). Thus it is little wonder that the “birds-are-dinosaurs “movement is thriving.
The issue has long been akin to the ancient arguments about the “simian origin of Man,” and the rather interesting and often-public debates between proponents of human evolution (such as Huxley) against its detractors who argued for something more akin to “special creation.” In this case, all of the participants “believe” in evolution, including TLS and Olson: but the “dogma” of the dinosaurian origin of birds, entrenched essentially since 1969 with the description of Deinonychus antirrhopus, has seemingly sparked a war of ideals: That the data can be explained with a clean-cut linear progression of skeletal form, but for which integumental data is sparse, versus a largely typological position where there is a “clear” feather archetype present (as in Longisquama‘s … things [n1]) and one in which there isn’t (Sinosauropteryx prima). This has a lot, I think, to do with the current inability to absolutely track the progression of a modern-like feather — pennaceous or plumed, or at least clearly with a rachis — in a theropod dinosaur with “reptilian” style body design. Data, pure Scientific data, supports through a variety of biological and phylogenetic tests that birds arose from within dinosaurs — moreover, that they are dinosaurs. It may seem jarring to hear people talk about those dinosaurs flying outside their windows, or the jarring calls of dinosaurs hanging out on telephone wires and power lines, or the dinosaurs partaking of the bird baths and bird feeders.
The weight of this evidence has become so profound, in fact, that former opponents have shifted sides (or rather, shifted the goal posts): Larry Martin, along with Stephan Czerkas, now advocate that many so-called non-avian dinosaurs are, in fact, birds! This largely sprouted in 2002 with the publication of “The Dinosaur Museum Journal,” a book (promising to be the first of many in a new “journal” not seen for 10 years) which included a large number of papers with Stephan A. Czerkas as the primary author on each of them. One paper, Czerkas et al. (2002), is titled “Flying dromaeosaurs,” and describes a specimen as Cryptovolans pauli. This specimen (LPM 0020), previously mentioned by Norell et al. (2002), is probably another specimen of Microraptor zhaoianus.
The authors sought to affirm their position by using the analysis of Maryańska et al. (2002), which purported a different arrangement of basal paravian dinosaurs, including placing oviraptorosaurs closer to birds than were dromaeosaurs or troodontids. Rather than assess the lack of quality in the analysis or compare its results with other, larger analyses (e.g., Xu et al., 2001), the authors used the analysis to presume that there was doubt or lack of consensus on the matter of the origin of birds; simply switching two near-avian ancestors of birds does not, however, support this argument, but it was used nonetheless to sow the seeds of doubt to the cladistic analysis, which none of the principle authors supporting BAND favor the use of. At such a point, the case becomes shut, end of discussion. This way, the presence of fully-developed pennaceous feathers need never be questioned. The weight of the data is so compelling that Feduccia, Martin, and others then just pulled the goal posts for the extent to which others had to prove their case; they agreed, suddenly, with all of the authors with whom they had argued against on this position. But instead of taking up the cause, against all logic, they then decided that NO OTHER theropod dinosaur could be antecedent to birds, just those. Moreover, they posit that the origin of birds, from Feduccia’s book to Martin’s 2008 piece, continues to emplace the bizarre reptile Longisquama insignis at its root [n2], to explain the “proavis” concept envisioned by people like Gerhard Heilmann and C. William Beebe.
It doesn’t matter that some of these “birdy” theropod specimens possess the same type of filamentous “fibres” as Sinosauropteryx prima (for example, “Dave” – NGMC 91). Because these animals are “clearly” not bird-like, they cannot be related, and their integumental preservation cannot be sourced to feathers. My bias is that I “buy” the so-far consensus view that phylogenetic data supports a theropod origin of birds from a dinosaurian origin of theropods, from an archosaurian origin of dinosaurs, from a reptilian origin of archosaurs. It matters not what terms or taxonomic labels apply to these categories. Moreover, I “buy” consensus data that shows that various feather-like structures observed on the bodies of various mammals, birds, and non-avian dinosaurs at the Late Jurassic/Early Cretaceous Daohugou, Yixian and Jiufotang faunas represent epidermal, rather than dermal, structures. I am not set in this, as I take the opportunity for people like TLS to further the conversation and much enjoy reading the papers he writes; I am willing to have my mind changed when I find the consensus broken by good, solid and clear data.
The Non-Science of Not Doing Science
There are several forms of discussion by which scientists may engage the arguments of others. One of the primary methods is by actually repeating the experiments of those they seek to question, or by performing additional, new experiments, and comparing the results. These results stand on their own, but scientists often must “put them in context.” This can often end up being about how the context sets the tone of the piece in total, and thus “color” it. This was my primary argument against the manner by which Jack Horner and John Scanella presented their analysis sinking Torosaurus into Triceratops (although I admit they have much more work to demonstrate this in the wings, and their process is simply unorthodox, but nonetheless scientific). I’m somewhat of a science purist: I like keeping my Science apart from my context, so that I may be aware of what is “real” and what is “supposed.” I do not like papers being presented as reviewing the scientific arguments of another that do not, in themselves, do science. Are there cases where context trumps the Science that is purported? Yes, although in many of these it is based on the non-Science that is being done in those papers, such as arguing a squid is creating a self-portrait of itself, or that inorganic matter is, itself, “alive.” TLS does Science, but many of his conclusions do not flow as he explicitly refuses to test the argument he makes against birds, while Martin’s 2008 review piece cannot ever be used as Science.
My disappointment with the comments of Dove & Straker (2012) does not just end at the citation of a scientist who not only denies the prevailing bird-dinosaur link, but also refuses to extend his research in actually testing his hypothesis on other types of animals rather than just using the same ones (all of them not “furry”), thus “confirming” his previously asserted arguments. Dove & Straker (2012) had plenty of opportunity to compare and examine the actual structures they were comparing their works to, especially in light of the comments made by McKellar et al. (2011) in comparing their material favorably to some birds (especially grebes, citing Chandler, 1916). Remarking on the same material, Dove & Straker simply pose the comparison to a non-feather biological, seeds of the cottonwood, Populus trichocarpa. This comparison poses the twisting tendrils as potentially non-animal in nature, while offering little in the manner of positive experiment. Certainly, they bring an interesting comparison to the supposedly flat rachis with twisted tendrils springing forth in the lower image (center set, top left) and its general similarities to cottonwood seeds. But this comparison is superficial, and tells us nothing about how they can be differentiated explicitly from McKellar et al.’s argument that they represent stage II type feathers.
In this way, I feel Dove & Straker approached the topic with narrative alone, and their choice of citation suggests this was the purpose: to throw doubt at the problem, rather than solve it or offer solutions. I may be VERY unfair to Dove and Straker, as I do not know them; I am making this observation from a very, very small set of assumptions and very little else to back it up. But I do know that a comment in a scientific journal should, in some ways, purport to do Science. Dove & Straker (2012) seem merely to question and offer no positive argument, something that arises merely from commentary and context, and not process and experimentation. I think part of this is actually McKeller et al.’s fault, although not maliciously: they chose to submit their work to a journal which imposes severe length constraints, and much of the Science they do is in the supplemental data, material that in a typical research paper should appear in the main manuscript. Review comments should arise as part of the overall context or conversation which surrounds the science itself; the opposite, performing experiments to conform to a given context, seems backwards. But the non-work being pushed, and I’m afraid the “comment” from Dove and Straker counts as this, doesn’t necessarily further this conversation, permitting new data to enter the mix. That work was merely to make a few comments, toss everything into the air, then walk away. Now, does this further the conversation? Yes. It sets Dove & Straker in a sort of opposition to McKellar et al., but there doesn’t seem to be a position advocated other than “doubt, doubt, doubt,” and that seems so familiar to the position of various pseudoscientific opposition arguments, including those of the “intelligent design” and creationism movements, portions of both sides of the anthropogenic global warming debate, and so forth. I think Science functions better with positive arguments, presentation of new data, or more inclusive sets, rather than just questioning the previous argument. Now, there is merit in pointing out the flaws of previous arguments, but when one does so regardless of the similar arguments made by the authors you are objecting to, or invoking the tiresome, oft-repeated and untested statements of non-ideal analyses, one does Science no favors.
(I am, of course, aware of the irony of not adding new information to this discussion while talking about it in this post.)
I fear, of course, that my familiarity with the technique and materials so weak as to offer nothing constructive toward Dove & Straker’s particular points in and of themselves, merely to comment on the style of argument and the almost de facto assumption that TLS’s rejection of dinosaurian feathers as collagen is a useful argument. I thus leave this discussion no less enlightened than I was when I entered it.
[n1] Various authors have attempted to weigh in on interpreting the structures of Longisquama insignis. This is a topic I will not belabor in this post to much of any degree right now, as doing so would take up a lot of my writing. Suffice it to say that this topic bears heavily against the BAND position and not for it, as they claim, without defying a good deal of the development of feathers themselves, especially how they do not appear to arise from scale-like structures at all.
[n2] I have not read Alan Feduccia’s latest book, Riddle of the Feathered Dragons, but in it I am told that Feduccia argues that, rather than with Longisquama, the origin of birds lies somewhere with Marasuchus, a near-dinosaur dinosauriform archosaur. The idiocy, though, is that it is still not a dinosaur, but that it is getting closer. The ancestor of birds can be anything, as long as it’s not a dinosaur, or theropod, you see.
Chandler, A. C. 1916. A study of the structure of feathers, with reference to their taxonomic significance. University of California, Publications in Zoology 13(11):243-446.
Chen P.-j., Dong Z.-m. & Zhen S.-n. 1998. An exceptionally well-preserved theropod dinosaur from the Yixian Formation of China. Nature 391:147-152.
Currie, P. J. and Chen P.-j. 2001. Anatomy of Sinosauropteryx prima from Liaoning, northeastern China. Canadian Journal of Earth Sciences — Revue canadienne des sciences de la Terre 38(4):1705-1727.
Czerkas, S. A., Zhang D.-s., Li J.-l. & Li Y.-x. 2002. Flying Dromaeosaurs. pg.97-126 in Czerkas, S. J. (ed.) Feathered Dinosaurs and the Origin of Flight. The Dinosaur Museum Journal 1:97-126.
Dove, C. J. & Straker, L. C. 2012. Comment on “A diverse assemblage of Late Cretaceous dinosaur and bird feathers from Canadian amber”. Science 335:796b.
Feduccia, A. 1996. The Origin and Evolution of Birds. Yale University Press (New Haven).
Feduccia, A. 2011. Riddle of the Feathered Dragons: Hidden Birds of China. Yale University Press (New Haven, Connecticut).
Hou L.-h., Martin, L. D., Zhou Z.-h. & Feduccia, A. 1996. Early adaptive radiation of birds: Evidence from fossils from northeastern China. Science 274:1164-1167.
Ji Q., Norell, M. A., Gao K.-q., Ji S.-a. & Ren D. 2001. The distribution of integumentary structures in a feathered dinosaur. Nature 410:1084-1087.
Kurochkin, E. N. 1996. A new enantiornithid of the Mongolian Late Cretaceous, and a general appraisal of the infraclass Enantiornithes (Aves). Russian Academy of Sciences, Special Issue: 1-50.
Lingham-Soliar, T. 2011. The evolution of the feather: Sinosauropteryx, a colourful tail. Journal of Ornithology 152(3):567–577.
Lingham-Soliar, T., Feduccia, A. & Wang X.-l. 2007. A new Chinese specimen indicates that ‘protofeathers’ the Early Cetaceous theropod dinosaur Sinosauropteryx are degraded collagen fibres. Proceedings of the Royal Society, B: Biological Sciences 274:1823-1829.
Martin, L. D. 2008. Origins of avian flight – a new perspective. Oryctos 7:45-54.
Maryańska, T., Osmólska, H. & Wolsan, M. 2002. Avialan status for Oviraptorosauria. Acta Palaeontologica Polonica 47(1):97-116.
McKellar, R. C., Chatterton, B. D. E., Wolfe, A. P. & Currie, P. J. 2011. A diverse assemblage of Late Cretaceous dinosaur and bird feathers from Canadian amber. Science 333:1619-1622.
Norell, M. A., Ji Q., Gao K.-g., Yuan C.-x., Zhao Y. & Wang L.-h. 2002. “Modern” feathers on a non-avian dinosaur. Nature 416:36.
Olson, S. L. 1999. Open letter to Dr. Peter Raven, Committee for Research and Exploration, National Geographic Society. and Olson, S. L. 1999. Response to Henry Gee, managing editor of Nature, Nature Publishing Group.
Prum, R. O. 1999. Development and evolutionary origin of feathers. Journal of Experimental Zoology 285:291-306.
Senter, P. 2010. Using creation science to demonstrate evolution: application of a creationist method for visualizing gaps in the fossil record to a phylogenetic study of coelurosaurian dinosaurs. Journal of Evolutionary Biology 23(8):1732-1743.
Xu X., Norell, M. A., Kuang X.-w., Wang X.-l., Zhao Q. & Jia C.-k. 2004. Basal tyrannosauroids from China and evidence for protofeathers in tyrannosauroids. Nature 431:680-684.
Xu X., Tang Z.-l. & Wang X.-l. 1999. A therizinosauroid dinosaur with integumentary structures from China. Nature 399:350–354.
Xu X., Zheng, Z.-t. & You H.-l. 2009. A new feather type in a nonavian theropod and the early evolution of feathers. Proceedings of the National Academy of Sciences, Philadelphia 106(3):832-834.
Xu X., Zhou Z.-h. & Prum, R. O. 2001. Branched integumental structures in Sinornithosaurus and the origin of feathers. Nature 410:200–204.
This work is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License (the photos are the product of McKellar et al., and of Lingham-Soliar; I own only the main text).