Tuesday, February 16, 2010

The Meaning of Feathered Dinosaurs

The past few weeks have witnessed a spate of remarkable announcements about feathered dinosaurs. First came the story of University of Kansas scientists reconstructing feathers on the four-winged Microraptor to test the gliding abilities of this little dinosaur (1); the authors concluded that Microraptor was fully arboreal, spending it’s lifetime largely in trees, and that avian flight likely evolved from the trees down rather than from the ground up. Next came stunning back-to-back news from two different teams that reconstructed color in the feathers of dinosaurs, giving us our first glimpses of dinosaur coloration (2, 3). Most recently, a study by David Hone and colleagues (4) clearly demonstrates that the feathers of Microraptor attached directly to the bone, as in birds living today, and that the feathers are preserved in life position, rather than being displaced after death, as some have argued.

Together, these announcements got me thinking about the larger meaning of feathered dinosaurs. Beyond the amazing (and seemingly unending) succession of discoveries, most based on exceptionally preserved fossils coming out of China, what larger lessons might we take away from these so-called “dino-birds”? Below I describe my top three. Other paleontologists would undoubtedly come up with different lists, and the items noted here would apply equally well to many non-dinosaurian phenomena. Yet I think that the burgeoning science of feathered dinosaurs admirably underscores each of these points.

1) Never say never.
Scientists like to talk about the limits of science—that is, the limits what’s knowable through scientific investigation. In doing so, we’re prone to make claims like, “Blah-blah-blah [insert topic here] will never be known with any degree of confidence.” Then along comes some bright, skeptical (typically young) investigator who finds a way to probe the supposedly intractable question from a new angle, making those earlier claims seem foolish. This is exactly what happened with dinosaur colors. Limited largely to fragmentary piles of bones and teeth, many paleontologists have lamented that we will never know the colors of dinosaurs. Then it was discovered that fossilized feathers preserved with some Chinese theropod dinosaurs preserve a color-bearing melanin pigment called melanosomes, and that these color-specific cues can be used to reconstruct feather coloration.

Anchiornis is the smallest known dinosaur, about 13 inches long and 100 grams (~4 oz), somewhere between the size of a robin and crow. It was also the animal for which Quanguo Li and colleagues(3) reconstructed coloration over the entire animal. The authors state that “[T]he body was gray and dark and the face had rufous speckles. The crown was rufous, and the long limb feathers were white with distal black spangles.” This claim is astonishing. Just five years ago, few would have imagined that we would ever have the hard data to make such a statement about any dinosaur. And keep in mind that Anchiornis lived more than 150 million years ago!

When it comes to paleontology, many of these surprising insights are possible only because of the application of new technologies. The discovery of dinosaur color depended on the ability of researchers to explore the shape and density of melanosomses in the microstructure of feathers. Similarly, Dave Hone and his colleagues(4) had to examine Microraptor fossils under ultraviolet light to see the detailed interface between bones and feathers.

The lesson here? Feathered dinosaurs teach us to never say never. Questions that appear impenetrable at the moment may well become answerable in the future with the advent of new technologies and new ways of thinking. At the moment, we have no way to determine the skin colors of non-feathered dinosaurs like T. rex or Triceratops. But don’t bet on this staying unknown. History tells us different.

2) Ignorance exceeds knowledge.
There is a major bias among nonscientists that we’ve pretty much figured out all the big stuff about nature. All we’re doing now, people seem to think, is making minor adjustments to our understanding—adding a few more grains of sand to the mountain of knowledge. This myth is perpetuated by school textbooks, which tend to give the implicit impression that the topic under consideration (e.g., biology, chemistry, physics, etc.) can be summarized as an accumulation of facts.

Feathered dinosaurs teach us differently. When I was a kid, dinosaurs were depicted as sluggish, dim-witted, swamp-dwelling, drab-colored behemoths sharing far more in common with Godzilla than with chickens. Thanks largely to the discovery of bird-like dinosaurs, sometimes called “raptors,” this view has been tossed on its head. Almost overnight it seemed, dinosaurs emerged from the swamps as complex, fast-moving, brightly colored creatures with a range of complex behaviors previously unimagined—for example, nest brooding, herding, and burrowing. Feathered dinosaurs showed us that not all of these ancient reptiles were giant; indeed some were no larger than mid-sized birds. To my knowledge, no one in the 1960s pictured little four-winged dinosaurs living in trees.

Science has only scratched the surface of what might be known. To use dinosaurs as an example, more “new” varieties of dinosaurs have been named in the past 25 years than in all prior history, with no signs that the well of discovery is running dry. Similar kinds of statements might be made for virtually all areas of science. Even in disciplines such as physics with a much longer track record than paleontology, ground-breaking, innovative work remains to be done. Science has tended to create artificially walled “silos” around each discipline, walls that today are being torn down. As a result, some of the most exciting work done today occurs at the interfaces of once separate disciplines, often with appropriately hybrid names like “geobiochemistry.” I am excited about 21st Century paleontology, which is becoming increasingly integrated, synthesizing the traditional study of fossils with knowledge and tools from such disparate fields as geochemistry, histology, molecular biology, and paleoclimatology. There’s no question—countless exciting discoveries await talented, hard-working folks with access to as yet unimagined tools. Expect the unexpected.

3) Evolution happens.
Despite the fact that we just celebrated the 150th anniversary of Darwin’s famous treatise on biological evolution, and even though virtually all practicing biologists regard evolution as the abundantly documented centerpiece of their field, only about one half of Americans accept the veracity of this idea. Anti-evolutionists frequently make the claim that we paleontologists have no evidence of intermediate forms in the fossil record. Feathered dinosaurs tell a different story.

When I was a kid, life was a lot simpler. Even kids knew that reptiles (including dinosaurs) had scales and birds had feathers. This na├»ve categorization has been roundly defeated by not one or two but well over a dozen kinds of dinosaurs with feathers or feather-like structures (but see note below). Indeed the line between dinosaur and bird has blurred to the point that paleontologists frequently describe a new Mesozoic beast as a bird only to have others show that it’s a dinosaur, or vice versa.

Back in the old days (i.e., when I was a kid), biologists often spoke of missing links. In one sense, feathered dinosaurs are exemplary missing links. In another sense, these “dino-birds” underscore the double inaccuracy of the term; missing links are neither missing nor links. Most trivially, intermediate fossil forms are not missing because we’ve found them. More profoundly, they are not links, because evolution rarely follows a simple, ladder progression, with one form evolving into another and so on. Instead, evolution typically branches like a bush, generating not one but numerous lineages that spawn variable numbers of descendents and persist for differing amounts of time. Feathered dinosaurs are an ideal example of this kind of arborescent evolution. Microraptor and Anchiornis are not directly ancestral to modern day birds, but their lineages branched off the dinosaur family tree around the same time as birds did. That’s why they share so many features with living avians, including true feathers.

Dinosaurs aren’t extinct. They fly around above our heads today, charm us with their songs, and adorn our dinner tables. In that sense, all birds are feathered dinosaurs. And they’re not the only well documented example of transitional fossil forms. We have similarly robust evidence for the origin of amphibians, whales, and many other groups—including hominids. Evolutionary biologists need to do an even better job of taking their message public. Evolution happens.

References
1) Alexander, D. E., E. Gong, L. D. Martin, D. A. Burham, and A. R. Falk. 2010. Model tests of gliding with different hindwing configurations in the four-winged dromaeosaurid Microraptor gui. Proceedings of the National Academy of Science. doi: 10.1073/pnas.0911852107.
2) Zhang, F., S. L. Kearns, P. J. Orr, M. J. Benton, Zhou, Z., D. Johnson, Xu. X & Xiaolin W.. 2010. Fossilized melanosomes and the colour of Cretaceous dinosaurs and birds. Nature, doi:10.1038/nature08740.
3) Li, Q., Gao, K.-Q., Vinther, J., Shawkey, M.D., Clarke, J.A., D'Alba, L., Meng, Q., Briggs, D.E.G. and Prum, R.O. "Plumage color patterns of an extinct dinosaur." Science,. doi:10.1126/science.1186290.
4) Hone, D. W. E., H. Tischlinger, Xu X., Zhang, F. 2010. The Extent of the Preserved Feathers on the Four-Winged Dinosaur Microraptor gui under Ultraviolet Light. PLoS ONE, 5(2): e9223. doi:10.1371/journal.pone.0009223.

(Note: One of the hallmarks of science is that every claim always remains open to questioning. Although the vast majority of vertebrate paleontologists now agree that birds are descended from small-bodied feathered dinosaurs, it is worth noting that a small contingent of naysayers remains. Among them is John Ruben of Oregon State University, who has a brand new paper out making this claim. I have not yet read this publication, but think that it is appropriate to mention it here: Ruben, J. 2010. Paleobiology and the Origins of Avian Flight. Proceedings of the National Academy of Sciences, doi:10.1073/pnas.0915099107.)

12 comments:

  1. Hi, Scott Sampson.

    I'm a University of Chicago graduate working with Paul Sereno at the moment before going to graduate school in the fall. I've been enjoying reading your insightful discussions since the beginning of this blog and I just finished your book Dinosaur Odyssey, which I plan to write a very positive review on my blog soon (http://akiopteryx.blogspot.com/). Regarding your statement, "To my knowledge, no one in the 1960s pictured little four-winged dinosaurs living in trees," I just wanted to point out that William Beebe did hypothesize an intermediate, four-winged, gliding stage in the origin of birds in 1915!

    Beebe C. W. 1915. A tetrapteryx stage in the ancestry of birds. Zoologica 2:38-52.

    An illustration from the original paper was featured in a recent article by John Ruben in PNAS (www.pnas.org/cgi/doi/10.1073/pnas.0915099107)

    Cheers!

    -Aki Watanabe

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  2. Scott,

    I really love reading your blog entries so far. You hit the ball home when it comes to "never say never" in science. I just wonder what new discoveries await in paleontology and other sciences in the next 25-50 years from now. I'm still in awe after finding out about the color of feathered dinosaurs. There wont be an excuse for paleoartists to get the colors of Anchiornis or Sinosauropteryx wrong. I haven't read your book yet, so I better order a copy soon, because I have heard only good things about it.

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  3. Steven,

    I'd argue that there's still lots of scope for palaeoartists and colours. After all, we have jsut sampled one individual in each case. There would likely have been more than one species of each of these genera runnign around at various times, males and females might have been different colours or patterns, juveniles and adults might be different, or there could be regional or seasonal variations.

    So I'd not call any colour pattern 'wrong' as such, just speculative, even if it does contradict those that have been published.

    Dave

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  4. Very true indeed, thanks for pointing that out Dave.

    It would be okay to speculate that all these feathered dinosaurs could of adorn all types of elaborate displays of various colors, wich were no different than there modern decendants and could this also open the door on more speculation on their behavior perhaps as well?

    Steven

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  5. Hi Steven,

    well palaeontolgoists are always speculating about behaviour. Obviously tying this to what we have in the fossil record is the tricky bit, but certainly the fact that these colours (in agian these two lone examples) are clearly not cryptic is interesting and thus you could, perhaps, tentatively, maybe, suggest that disaply might have been more important in some aspects than say crypsis or thermal regualtion. Ish.

    However, as it happens I've just had a paper accpeted (as a co-author) talking about feather origins and flight with some commentary on behaviour so there is more to come on this in the short term in addition of course to all the other researchers who are doubtless out there and working on these very questions.

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  6. "At the moment, we have no way to determine the skin colors of non-feathered dinosaurs like T. rex or Triceratops."

    Who knows if T.rex or Triceratops weren't feathered (or at-least "fuzzy")? Never say never...

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  7. I heard a poster was presented at an SVP meeting while back about Triceratops having psittacosaur-like quills on its back.

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  8. Aki,

    Thanks for the kind remarks, and for the reminder about Beebe's paper. I had forgotten about that!

    Best wishes,
    Scott

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  9. Dear Andrea,

    Although it's true that a number of non-theropod dinosaurs may have had some kind of protofeather-like structures (the jury is still out on this), we do have skin impressions from a range of dinosaurs, from large-bodied theropods like Allosaurus and Carnotaurus to big herbivores like hadrosaurs and even horned dinosaurs. These finds seem to indicate that, if present at all, protofeathers had an extremely limited distribution on the body in many large dinosaurs (not surprising given that feathers make good insulators, and big animals have a problem with dumping heat rather than trying to hold onto it). So that still leaves us with the problem (perhaps solvable) of determining skin coloration. We shall see. Thanks for the comment!

    Cheers, Scott

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  10. Hi Scott,
    A question (I hope not too ingenuous):
    Is it possible that protofeathers and tubercolate scales co-existed not only in different areas of the body but in the same areas, as overlapping structures or for example, with the filamentous "feathers" emerging between adjacent scales? I saw some bird foot showing this pattern, so, perhaps, the absence of protofeathers in scaly fossils could be a taphonomic artifact that allowed only scaly impressions to preserve. In the holotype of Scipionyx, for example, we see preserved the corneous claws but no the feather-like structures that (following the "Extinct" Phylogenetic Bracketing) it probably displayed in life.
    It's only an hypothesis (perhaps forced by my will to consider the tegumentary structures of Tianyulong homologous to coelurosaurian ones), but I think many people often interprete the (very limited) fossil record of dinosaur tegument too literally and/or into a simple dicotomy "scales vs feathers".

    Cheers

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  11. Andrea,

    Yes, while not necessarily likely, I think it's possible that feathers or plume-like structures existed on animals for which we have (apparently featherless) skin impressions. For one thing, we have few examples of skin impressions that encompass most of the body, so feathers may be present in the areas for which no integument is preserved. For another, as you note, it is possible that feathers were present but their traces are not preserved (or at least noted thus far). Finally, as is often depicted, giant dinosaurs may have had feathered offspring, which then lost these structures during growth. No doubt, more integumentary surprises await discovery!

    Scott

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  12. Now that I was taking a look at that bird I wonder whether there were dinosaurs similar as wolves. I remember I love that movie called an American werewolf in London, this is not about dinosaurs but the film is great. I get very amused by watching the commercials about Viagra Online.

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