November 28, 2012
How did feathered dinosaurs take to the air? Paleontologists have been investigating and debating this essential aspect of avian evolution for over a century. Indeed, there have been almost as many ideas as they have been experts, envisioning scenarios of dinosaurs gliding through trees, theropods trapping insects with their feathery wings and even aquatic Iguanodon flapping primitive flippers as flight precursors (I didn’t say that all the ideas were good ones). The biomechanical abilities of bird ancestors and their natural history has always been at the center of the debate, and a new Current Biology paper adds more fuel to the long-running discussion.
At present, hypotheses for the origin of avian flight typically fall into one of two categories. Either bird ancestors accrued the adaptations necessary for flight on the ground and, through evolutionary happenstance, were eventually able to take off, or small tree-dwelling dinosaurs used their feathery coats to glide between trees and, eventually, flapped their way into a flying lifestyle. There are variations on both themes, but feathers and the characteristic avian flight stroke are at the core of any such scenario. In the case of the new paper, Yale University paleontologist Nicholas Longrich and colleagues draw from the plumage of early bird Archaeopteryx and the troodontid Anchiornis to examine how feathers changed as dinosaurs started to fly.
In modern flying birds, Longrich and coauthors point out, the wing arrangement typically consists of “long, asymmetrical flight feathers overlain by short covert feathers.” This pattern creates a stable airfoil but also lets the flight feathers separate a little during the upstroke of a wing beat, therefore reducing drag. When the paleontologists examined the fossilized wings of Archaeopteryx and Anchiornis, they found different feather arrangements that would have constrained the flight abilities of the Jurassic dinosaurs.
Both prehistoric creatures had long covert feathers layered on top of the flight feathers. Anchiornis, in particular, appeared to have an archaic wing form characterized by layers of short, symmetrical flight feathers and similarly shaped coverts. Archaeopteryx showed more specialization between the flight feathers and the coverts but still did not have a wing just like that of a modern bird. As a result, Longrich and collaborators hypothesize, both arrangements would have stabilized the wing at the cost of increased drag at low speeds, making it especially difficult for Anchiornis and Archaeopteryx to take off. As an alternative, the researchers suggest that these dinosaurs might have been parachuters who jumped into the air from trees, which might hint that “powered flight was preceded by arboreal parachuting and gliding.”
The trick is determining whether Anchiornis and Archaeopteryx actually represent the form of bird ancestors, or whether the dinosaurs, like Microraptor, were independent experiments in flight evolution. At the Society of Vertebrate Paleontology conference in Raleigh, North Carolina last month, flight expert Michael Habib quipped that all that was needed to make dromaeosaurs aerially competent was the addition of feathers. If Habib is right, and I think he is, then there could have been multiple evolutionary experiments in flying, gliding, wing-assisted-incline-running and other such activities. There’s no reason to think that flight evolved only once in a neat, clean march of ever-increasing aerodynamic perfection. Evolution is messy, and who knows how many ultimately failed variations there were among flight-capable dinosaurs?
The three-step Anchiornis-Archaeopteryx-modern bird scenario of wing evolution fits our expectations of what a stepwise evolutionary pattern would look like, but, as the authors of the new paper point out, shifting evolutionary trees currently confound our ability to know what represents the ancestral bird condition and what characterized a more distant branch of the feathered dinosaur family tree. We need more feathery fossils to further investigate and test this hypothesis, as well as additional biomechanical and paleoecological information to determine whether such dinosaurs really took off from trees. We must take great care in distinguishing between what an organism could do and what it actually did, and with so much up in the air, the debate on the origin of flight will undoubtedly continue for decades to come.
Longrich, N., Vinther, J., Meng, Q., Li, Q., Russell, A. 2012. Primitive wing feather arrangement in Archaeopteryx lithographica and Anchiornis huxleyi. Current Biology DOI: 10.1016/j.cub.2012.09.052
March 9, 2012
Microraptor was an exquisitely feathered dinosaur. The small, sickle-clawed predator, which lived about 120 million years ago, was covered in well-developed plumage, including long feathers on its arms and legs. But we now know that Microraptor was not only beautiful in an anatomical structure sense. A detailed new study has painted this dinosaur in a glossy black sheen.
The range of the dinosaur palette has been one of the most mysterious aspects of dinosaur biology. For most species, we just don’t know—bones and teeth can’t tell us anything about skin color. But feathered dinosaurs contain evidence of their hues within their feathers. Microscopic organelles called melanosomes are the key. In fossil creatures—just as in living ones—the size, shape, density and distribution of these tiny, pigment-filled blobs created different colors. By studying the characteristics of melanosomes in feathered dinosaurs and comparing the patterns with those that create the colors of modern birds, paleontologists can reconstruct dinosaur feather colors.
Several dinosaurs have already received a color treatment. After establishing that fossil melanosomes are faithful indicators of prehistoric color in ancient birds, paleontologist Jakob Vinther and colleagues restored the full-body coloring of the feathered, non-avian dinosaur Anchiornis. This small dinosaur looked something like a magpie with a bright red splash of feathers on top of its head. Earlier this year, Vinther, Ryan Carney and co-authors determined that the famous feather used to name the earliest known bird—Archaeopteryx—was black. And a different team of researchers, led by paleontologist Fucheng Zhang, hypothesized that the fuzzy Sinosauropteryx had a candy-cane tail ringed in white and rusty red. Paper by paper, dinosaurs are being colored in.
In the case of Microraptor, the dinosaur did not turn out quite like any of the restorations that artists had previously composed. Many Microraptor illustrations envisioned the dinosaur in shades of brown, white and blue. But when Vinther, Quanguo Li and collaborators studied the melanosomes sampled from 26 different locations on a Microraptor specimen designated BMNHC PH881, they didn’t find those colors. Microraptor feathers were iridescent blue-black. In appearance, Vinther said via email, Microraptor would have looked similar to “grackles or a magpie, or indeed a crow.”
Black was apparently quite fashionable among feathered dinosaurs. Anchiornis, while overall more colorful, was also predominantly black, and the lone Archaeopteryx feather was also black. Why black was so common for dinosaurs with complex, specialized feathers isn’t clear. Vinther pointed out that the small sample size might be creating this pattern, especially since other, unpublished specimens show different colors. Then again, black and other dark shades might have had something to do with where the animals lived. Citing a phenomenon called Gloger’s rule, Vinther explained that mammals and birds that live in hot, humid environments near the equator have more of the pigment melanin, and therefore appear darker, than those living closer to the poles, though “sample size needs to be increased to make any generalizations like these,” he cautioned.
Vinther is confident that further studies will increase the number of dinosaurs for comparison. “The material is clearly there,” he said. It is only a matter of time before paleontologists can start to understand how color varied between individuals, and possibly even between the sexes. For the moment, though, the handful of dinosaurs that have been restored in color have shown that intricate avian traits existed far back in the past. “We were speculating about how deep iridescent colors might be and we were pretty excited when we realized that Microraptor indeed is iridescent,” Vinther said, and this discovery can tell us something about how feathers and even behaviors evolved among early birds and their dinosaurian kin.
“We can see that the paravian clade,” the group that contains birds and non-avian dinosaurs more closely related to birds than dinosaurs , “has complex feather morphologies and exhibit colors and color patterns for display and even iridescence like in modern birds, so these features are ancient and indeed suggest that at least the derived theropod dinosaurs were more similar in ecology and behavior to birds,” Vinther said. And, as research continues on feathered dinosaurs more distantly related to birds, Vinther suspects that many characteristics of modern birds will be pulled “deep down” the dinosaurian tree. The more we learn about feathered dinosaurs, the further back we can draw traits seen among birds today.
And there are still things to learn about the anatomy of feathered dinosaur plumage. While the iridescent hues of Microraptor are the major finding of the new paper, the study also pointed out that specimen BMNHC PH881 had a specialized set of paired feathers at the end of the tail. Similar feathers had been noted in other Microraptor specimens before, but this fossil had an especially lovely set. The structures are “simply too small and the feathers too spaced to create any lift,” Vinther said, so it’s unlikely that they aided the dinosaur in gliding or flying. Instead, citing the assessment of co-author Julia Clarke, Vinther said that the feathers might have been a display structure. Combined with the flashy feathers, these structures might be another clue that display and visual communication were very important factors in the early evolution of feather anatomy and color.
For most of my life, I was told that we would never know what colors dinosaurs were. Now, amazingly, there is a way to restore the appearances of some dinosaurs with a fidelity never thought possible. But I had to wonder if paleo-artists have felt any aggravation about such discoveries. As new studies establish feather colors for dinosaurs, the realistic palettes for those dinosaurs are constrained. I asked Vinther if he has received any irritated comments from artists about his work. He replied that, to the contrary, his research has been greeted with excitement. And while defining dinosaur colors “might take some of the imagination from the artists,” Vinther said, “I think that their fascination with these beasts gives them a desire to make them more scientifically correct.” The colorfully restored dinosaurs seem to be a hit. “I am struck by awe when I google-image Anchiornis and see forty plus versions of Anchiornis by various artists all over the world and even tattoos of it,” Vinther said. With any luck, the new glossy Microraptor will be just as popular.
Carney, R., Vinther, J., Shawkey, M., D’Alba, L., & Ackermann, J. (2012). New evidence on the colour and nature of the isolated Archaeopteryx feather Nature Communications, 3 DOI: 10.1038/ncomms1642
Li, Q., Gao, K., Vinther, J., Shawkey, M., Clarke, J., D’Alba, L., Meng, Q., Briggs, D., & Prum, R. (2010). Plumage Color Patterns of an Extinct Dinosaur Science, 327 (5971), 1369-1372 DOI: 10.1126/science.1186290
Li, Q., Gao, K., Meng, Q., Clarke, J., Shawkey, M., D’Alba, L., Pei, R., Ellison, M., Norell, M., & Vinther, J. (2012). Reconstruction of Microraptor and the Evolution of Iridescent Plumage Science, 335 (6073), 1215-1219 DOI: 10.1126/science.1213780
Zhang, F., Kearns, S., Orr, P., Benton, M., Zhou, Z., Johnson, D., Xu, X., & Wang, X. (2010). Fossilized melanosomes and the colour of Cretaceous dinosaurs and birds Nature, 463 (7284), 1075-1078 DOI: 10.1038/nature08740