Following his own advice, Xu and his team found Anchiornis huxleyi by delving below the Yixian shales to the little-known Jurassic rocks of the Haifangou Formation. Like Archaeopteryx, Anchiornis sported fully feathered wings and may have been capable of gliding flight, but this creature lived more than 160 million years ago, at least 10 million years before the “first bird.” It was a small dinosaur, only a foot long, and resembled Microraptor, with downy body feathers as well as vaned feathers on wings, legs, and feet. Anchiornis also sported a distinct feathered head crest, not unlike a modern Cardinal or a Steller’s Jay. Named in honor of Thomas Huxley, this fossil was presented by Xu at the 2009 Society of Vertebrate Paleontology meeting, neatly tying up one of the last serious criticisms of the theropod-bird link that Huxley suggested nearly 150 years before.
Taken together, these feathered dinosaurs (and the twenty other species uncovered to date) make a pretty compelling case for Richard Prum’s developmental theory of feather evolution, which lays out five evolutionary steps that must have occurred, in order, to produce the complex structure of a modern plume. They all bear Stage II filaments; Beipiaosaurus has Stage I quills. Caudipteryx covers Stage III (and possibly IV), while Microraptor and Anchiornis take care of Stages IV and V. “We can’t see everything,” Prum admits. “Barbules are pretty small to fossilize, and it’s hard to tell when things are hollow. But overall I’m pretty pleased with what the fossil evidence tells us.”
Developmental model of feathers by Nicholas Judson
The fact that all five feather stages can be found in theropods as well as in modern birds underscores the close relationship between these groups. It adds crucial evidence to what may be the first real scientific consensus on avian and feather origins. Plenty of details remain to haggle over, but the vast majority of paleontologists and ornithologists now find the theropod-to-bird framework convincing. Kevin Padian, an evolutionary biologist from the University of California at Berkeley, put it this way: “The earth is round, the sun doesn’t go around it, the continents move, and birds evolved from dinosaurs.”
No two scientists have done more to advance current ideas of feather evolution and avian origins than Prum and Xu, but even they don’t agree on everything. In conversation, both struck me as intensely curious, constantly questioning, refining, and challenging their own ideas as well as each other’s. Though I never talked with the two together, or even on the same continent, I kept a sort of conversation going through phone calls and e-mails and when I met Prum in his office at Yale’s Peabody Museum of Natural History. The points on which they agree frame our understanding of feather origins, but their differences taught me something about the nuances of evolution itself.
A few years after Prum’s theory came out, Xu published his own model of feather development. It’s similar but allows for quills and filaments (Prum’s Stages I and II) to develop before the invention of the follicle. Xu feels this modification better accounts for what he sees in the fossils, where evidence of integumentary filaments, or “dino fuzz,” is becoming more and more widespread. “This year we published a new species of Ornithischian [a group of dinosaurs that includes Triceratops and other armored species only distantly related to theropods] with long filaments that could be protofeathers,” he told me. Something resembling dino fuzz has even shown up on pterosaurs, and Xu thinks that filaments may have originated before theropods, or may have evolved independently many times.
“That’s possible,” Prum admitted, when I asked him about Xu’s idea. “Our own research more or less came to the same conclusion. But you wouldn’t have had any structural complexity until the follicle— no barbs, no rachis. Without the follicle, a feather would basically be like a wart.”
In Xu’s thinking, the early stages of feather evolution were extremely unstable. Quills, filaments, or possibly even more advanced forms could have appeared and winked out again many times in different lineages. “Novel structures can easily disappear and reappear,” he explained. “Only after they stabilized could feathers diversify in form and function. Maybe that stabilization only occurred sometime around Anchiornis or Archaeopteryx.”
Prum and Xu both agreed that feather evolution was iterative, an accumulation of novel morphological traits. Then when, I asked, do you call a feather a feather?
“If it’s a hollow tube, it’s a feather,” Prum replied immediately. “One thing I keep saying again and again is that there’s no such thing as a ‘protofeather.’ No one talks about a ‘protolimb.’ You either have a limb or you don’t. Why should feathers be any different? If it’s a tube, it’s a feather. Period.”
Xu was more equivocal. “I’ve been thinking a lot about this. How do you define a structure? These days everyone agrees that taxonomic names are arbitrary. What is a dinosaur and what is a bird— if one evolved gradually into the other, where do you draw the line? I argue it’s the same with structures.” He paused for breath, and then more ideas tumbled out. “Feathers have unique morphology, chemistry, keratins, structural features, et cetera. It’s very likely that those complex features evolved in a steplike way, sequentially. So where do you draw the line between feathers and nonfeathers? I don’t know.”
Alan Feduccia has a different perspective. In spite of the growing theropod consensus (or perhaps in some ways because of it), the remaining members of the Birds Are Not Dinosaurs “BAND” continue to publish critiques, questioning methods and results and looking for holes in the evidence. Feduccia doesn’t deny that fossils like Caudipteryx or Microraptor have feathers, but he prefers to consider them secondarily flightless birds, like modern ostriches, rheas, or emus. “You can boil my whole position down to one statement,” he told me: “If it has bird feathers, it’s a bird.”
“That’s their latest ploy!” Prum exclaimed when I told him this. “If they can’t deny that the dinosaur’s feathers are real, then they just call it a bird. And that’s after saying for years that these same fossils weren’t even related to birds!” He admits a certain exasperation with Feduccia and company, who criticize his ideas without offering any clear, testable alternatives. In fact, continual reporting on the BAND’s minority viewpoint (in the interest of journalistic fairness) may be perpetuating a controversy that most scientists consider over and done with. But when I pressed him, Prum did admit that certain criticisms had helped refine his thinking, whatever the cost to his blood pressure. “This whole debate is really overblown,” Feduccia told me. “Everyone agrees that birds and dinosaurs are related. Prum’s group thinks they evolved from theropods. My group just thinks they evolved much earlier and remained distinct from dinosaurs.”
Feduccia has long argued that birds evolved from an as yet undiscovered archosaur, one of the ancient reptiles that predated and gave rise to the dinosaurs (as well as the pterosaurs and the crocodilians). In the BAND’s version of the story, birds and theropods are more like distant second cousins. Their host of similar traits came about less through relatedness than convergent evolution— similar adaptations to suit similar lifestyles. He illustrates his point with a skeletal feature of birds and theropods that doesn’t appear to match. It’s one of the last lingering questions in the theropod-bird theory, an apparent inconsistency in which digits make up their three-fingered hands. Since both evolved from a five-fingered condition, the question is, which two digits were lost? In birds it looks like one and five, while in theropods it’s four and five.
Prum and Xu are not convinced. They counter that developmental and molecular studies show that digit identity is malleable and that various developmental patterns could be tweaked to a three-fingered result. Xu allowed that “settling the digit issue is definitely a research priority” and pointed to a promising new theropod fossil whose first digit was dramatically reduced, a possible analog to the pattern in birds. Either way, he and Prum consider digits a minor snag in the face of overwhelming evidence.
Feduccia is fond of countering the theropod “orthodoxy” with a line from H. L. Mencken: “For every complex problem there is a solution, which is simple, neat, and wrong.” As fossils, studies, and expert opinions line up against him, however, his own position is starting to sound like the simple one. At the end of our conversation, I asked him what it felt like to be constantly swimming against the tide. “Look, I don’t know how all of this will play out,” he answered with a sort of weary good cheer, “but I do know that the orthodox viewpoint will be challenged on all kinds of levels.”
Feduccia is certainly right that the orthodoxy will be challenged, altered, and refined as new fossils come to light— that’s just how science is supposed to work. But perhaps for the first time, the theoretical framework appears robust, and the arguing will now be about the details. When I asked Xu what the weak points were in the theropod and feather development models, he had to stop and think. It was the longest period of silence in our conversation. “No, there is no real weakness,” he said finally. “The only thing we need is more evidence. We have this big framework, but there are so many details to fill in.” It sounds like it’s still a good time to be a fossil hunter.
Rick Prum gave me a similar response. “If I had to alter anything from my original paper, I might de-emphasize the importance of the follicle,” he said, but otherwise the model was working well and leading in all kinds of interesting directions. He compared feather research to stumbling across a new, unexplored valley of science. “You top a rise and suddenly there’s this beautiful place with a braided river running down the middle, and you hike into it thinking, ‘No one has ever been here before!’”
Thor Hanson is an author and biologist based in the Pacific Northwest of the USA. Visit his website at http://www.thorhanson.net