An Ode to Archaeopteryx

Sometimes my timing is just plain awful. I had waited for years to see an authentic specimen of Archaeopteryx—the feather-covered fossil celebrated for 150 years as the first bird—but by time I finally got my chance, on the afternoon of July 27, news sources were trying to out-pun each other over the unceremonious demotion of the evolutionary icon. I scanned through the reports while sitting in the parking lot of the Wyoming Dinosaur Center, where the only Archaeopteryx in North American is on display. “Archaeopteryx Knocked From Roost as Original Bird” claimed WIRED Science, and the BBC played up the drama with “Feathers Fly in First Bird Debate.”

All this hubbub was stirred up by an article published a few hours before I rolled up to the museum in Thermopolis, Wyoming. In the issue of Nature published that day, paleontologist Xu Xing and colleagues described a previously unknown species of feathered dinosaur from the exceptionally fossil-rich beds of Liaoning, China. An interesting find, but given the number of feathered dinosaurs discovered during the past 15 years, not exactly something that newspapers would flip over. (As a freelance science writer, believe me that convincing some editors that dinosaurs are worth talking about is an uphill struggle.) What made all the difference was the way the new fossil was used to challenge the traditional position Archaeopteryx has held.

The backstory for the news goes back to 2009. In that year Xu and other paleontologists described a feather-covered creature they called Anchiornis. At first they thought it was an early bird, but a follow-up paper identified it as a feathered troodontid dinosaur. The newly described creature was very similar to Archaeopteryx—so much so that the discovery made me wonder if the beloved “urvogel” might eventually be stripped of that title, especially since Anchiornis might be even more ancient than the 150-million-year-old Archaeopteryx.

Now there’s Xiaotingia zhengi—another small theropod dinosaur draped in well-developed plumage. The holotype specimen which formed the basis of the new Nature paper exhibits the mostly complete skeleton on its side, and altogether the specimen looks like a tan and brown smudge of bones and feather impressions. It is said to date back to about 155 million years ago, but like many such fossils from China, the exact date is frustratingly uncertain because the fossil was purchased from a dealer and not scientifically excavated. In terms of the anatomical nitty-gritty, though, Xiaotingia looks quite similar to both Archaeopteryx and Anchiornis. Even though the skull was crushed, for example, Xiaotingia appears to have had a short skull fitted with small, peg-like teeth.

But the part of the study that garnered the most attention was the evolutionary analysis which removed Archaeopteryx and its closest kin from the base of the bird family tree. According to the paper, the dinosaurs Archaeopteryx, Anchiornis and Xiaotingia were united by several subtle characteristics, such as the lengths of the hand bones and the shape of the wishbone. The study places these dinosaurs closer to the sickle-clawed deinonychosaurs—the group which contains genera like Troodon and Deinonychus—than to the earliest birds.

Now here’s the part that was grossly underreported. “It should be noted,” the authors of the new paper wrote, “that our phylogenetic hypothesis is only weakly supported by the available data.” Headlines proclaimed the downfall of Archaeopteryx even though the actual evidence for such a change, as the authors of the study admitted, is not particularly strong. The uncertainty stems from the fact that some of the features seen in early birds may have appeared independently in more distantly related dinosaurs, so determining which traits are true signs of family ties and which evolved independently in different lineages is a difficult task. For example, the authors of the new study point out the similarity between the skulls of early birds such as Jeholornis and Sapeornis with oviraptorsaurs—all seem to have relatively deep and short skull profiles. But is this a real sign of close relationships, or a case of convergent evolution? There is no definite answer yet. When trying to tease out relationships, paleontologists must choose wisely or else features that evolved independently might be mistaken for common inheritance from a shared ancestor.

Likewise, previous studies by the same authors have frequently shifted the positions of feathered dinosaurs thought to be close to bird ancestry. The instability of the evolutionary trees being produced should make us proceed with caution. Take Anchiornis for example. It was originally described as a bird, then said to be a troodontid dinosaur, and is now cast as one of the closest relatives to Archaeopteryx in a lineage further removed from birds than previously thought. The patterns of relationships change from one publication to the next. It isn’t uncommon for relationships between dinosaurs to be unstable or uncertain, though. The relationships between dinosaur species are hypotheses that are subject to change with the addition of new information and context. Some hypotheses are stronger or better supported than others, but just because an evolutionary tree is published does not mean that it’s necessarily accurate or will remain the same as new discoveries are made.

This isn’t the first time that the avian relationships of Archaeopteryx have been challenged. General doubts have percolated through the paleontological community about Archaeopteryx for decades. Back when the first recognized Archaeopteryx specimens were found—a feather in 1860 and the first body fossil in 1861—nothing like it had been found before. Sites of exceptional preservation—where feather and body impressions could be found along with preserved bone—were rare, and Western naturalists had no idea that China held a rich store of feather-covered dinosaurs waiting to be discovered. Under these conditions Archaeopteryx seemed to be a dead ringer for the earliest known bird: After all, only birds had feathers. Not everyone was entirely agreed that Archaeopteryx was important to the origin of the first birds. Thomas Henry Huxley proposed that birds were derived from a dinosaur-like ancestor—something akin to Compsognathus—and had gone through a flightless, ostrich-like stage before taking to the air. This would make Archaeopteryx an aberrant side branch, Huxley proposed, not part of the direct line of descent.

The general consensus, despite Huxley’s work, became that Archaeopteryx really was the first bird. The trouble was that there was not much connecting it to its ancestral stock or later fossil birds. It sat right in the middle of everything—a key part of the transition without the appropriate bookends. Eventually, in the late 20th century, the discovery of dinosaurs like Deinonychus provided an appropriate rootstock for birds. In fact, the work of John Ostrom, the chief describer of Deinonychus, on Archaeopteryx solidified a connection that students of paleontology now take for granted. The deinonychosaurs (or the “raptors”) were the closest to birds given the close resemblance between them an Archaeopteryx.

Additional fossil finds have complicated the picture. Dinosaurs such as the four-winged Microraptor looked generally similar to Archaeopteryx, yet remained classified in the non-avian dinosaur group. More than that, the discovery of so many feathered dinosaurs brought previous lines of reasoning into question. Feathers, bird-like nesting behavior, bones infiltrated by air-filled sacs, and other features kept moving “avian” traits further back down the family tree. Many traits only seen among birds today appeared at a much earlier date among dinosaurs—Archaeopteryx was not nearly as unique as had originally been thought.

Unfamiliar dinosaurs also have their role to play in this shake-up. Paleontologists are still discovering and delineating dinosaur groups, and one of the most latest is a collection of small, strange creatures called scansoriopteryids. Little is known about these dinosaurs. Known from a handful of little-studied specimens, these unusual dinosaurs appear to be closely related to some of the first unequivocal birds. If this is true then the deinonychosaurs were not nearly as close to bird ancestry as was previously thought, although the scansoriopteryids have been so poorly studied that they are among the most enigmatic of all known dinosaurs.

At this point, how closely Archaeopteryx is related to the first birds is an open question which requires more detailed study. Xu and colleagues conclude that it may not have belonged to the official bird group and was just a very bird-like, non-avian dinosaur. This is not a major categorical difference—remember, the bird lineage is just a subgroup of the coelurosaurian dinosaurs—but represents the distinction among a few minor, tell-tale features near the base of a transition. Teasing out the details of such relationships keep paleontologists quite busy. As you approach the base of a group it becomes increasingly difficult to differentiate between the first members of a novel lineage and their ancestral stock. If you were to compare a modern bird to the dinosaurs which gave rise to birds the differences would be relatively obvious and distinct, but at the point of transition, the evolutionary picture is difficult to resolve. Rather than being an embarrassment, this wonderful frustration emphasizes the truth of evolutionary change.

There’s a lot of tradition and academic inertia behind calling Archaeopteryx the earliest known bird, but that’s something we can no longer take for granted. I think that’s a good thing. The question of what Archaeopteryx is provides a gauge of how much we have learned about bird origins and opens up the field for new debates. Creationists and other members of the anti-science crowd may try to turn the news to their advantage, but in fact, the uncertainty about Archaeopteryx highlights the fact that scientists are beginning to resolve a transition that we have only had the outlines of previously. And Archaeopteryx remains a beautiful example of how transitional features can be detected in the fossil record. Paleontologists only rarely detect direct lines of descent, but creatures that possess intermediate or transitional features help flesh out the way in which great transformations happened. Even if Archaeopteryx falls on the non-avian rather than avian side of the dinosaur family tree, it is still a feathered dinosaur with many traits once thought to be unique to birds. That, alone, is a powerful illustration of evolution, and I have no doubt that Archaeopteryx will remain a classic symbol of how life has drastically transformed.

What Archaeopteryx was and its significance in bird evolution is obviously a very complicated matter, but nuance is not exactly something that news reports do well. I think a number of reports boiled down a complex debate into oversimplified statements. In a video supplement to their story, the Guardian reported, “’Oldest Bird’ Archaeopteryx was a dinosaur, say scientists.” “Of course it was!” I thought—all birds are the descendants of dinosaurs and therefore can be called dinosaurs themselves. Whether Archaeopteryx is a bird or not, it’s still a feathered dinosaur—the headline is equivalent to saying, “Early human Australopithecus afarensis was a mammal, say scientists.” Equally frustrating was the Christian Science Monitor’s “Archaeopteryx may not have been a bird, but just a feathery dinosaur.” JUST a feathery dinosaur? As if feather-covered dinosaurs have suddenly become mundane. More than that, the significance of Archaeopteryx and the many other fuzzy and feathery dinosaurs that have been discovered is that they blur the boundary between what were thought to be two distinct groups and help inform one of the most magnificent evolutionary transformations in the history of life.

But the worst headlines came from news services that just went straight for the most sensationalist spin possible. “Newly discovered dinosaur could disprove ‘earliest bird’ theory” said the Telegraph, though the article itself included only the ambiguous conclusion that the new research “would force experts to reassess current assumptions about how modern birds evolved.” What assumptions? What is being questioned and what are the alternative ideas? The article does not give readers any context, and the headline has just enough of a creationist gloss to make me cringe. Likewise, in what may have been the worst coverage of the story, the Herald Sun asserted “Charles Darwin may have just lost Evolution Exhibit A, otherwise known as Archaeopteryx.” Not only does the story wrongly assert that Charles Darwin used Archaeopteryx as his favorite example of evolution—something I debunked in my book Written in Stone—but the entire piece presents paleontologists as stubborn cranks who are making things up as they go along, or that the change in perspective on Archaeopteryx somehow undercuts what Darwin proposed about evolution. Nonsense. New discoveries are changing our understanding of the natural world every day, and a slight change in perspective acts as a referendum on Darwin’s evolutionary theory only to those with only a superficial understanding of how science actually works.

We will probably continue to see similar headlines and articles as the discussion about Archaeopteryx continues. Paleontologists should question the place and relevance of Archaeopteryx in bird evolution—we should be wary of the pull 150 years of tradition might have as we sift through new finds—but the new study only offers a weakly supported hypothesis that requires a great deal of additional study to test. Archaeopteryx, despite the title given to a summary of the new Nature paper by paleontologist Lawrence Witmer in the same issue, is not yet “An icon knocked from its perch.” As Witmer says in his News & Views piece, the discovery of dinosaurs that are now competing with Archaeopteryx for the title of Earliest Bird means that “we’ve got some fresh work to do,” especially since “Just as Xiaotingia moved Archaeopteryx out of the birds, the next find could move it back in—or to somewhere else within this fuzzy tangled knot that makes up the origins of birds and bird-like dinosaurs.”

So what if Archaeopteryx turns out to be a feathered dinosaur more closely related to Deinonychus than the earliest true birds? Even if this turns out to be the case, the creature will still have played a major part in the history of evolutionary through and helped confirm the connection between dinosaurs and birds. The exact transitional series could turn out to be different, but Archaeopteryx will remain significant in terms of how feathers, and perhaps even flight, evolved. We have a tendency to cherish creatures that slot in neatly within patterns of major evolutionary change—the famous transitional forms of life’s major transformations—but in order to understand those changes we need many other fossils to provide background and context. As far as the evolution of birds is concerned, I have no doubt that Archaeopteryx will remain an important part of that context.

But I wasn’t thinking about all that as I stood in front of the glass case containing the Thermopolis Archaeopteryx in the Wyoming Dinosaur Center. I had not seen the paper at that point, after all, and I pushed the blare of the headlines out of my mind so that I might just stand there and appreciate something beautiful. Call it a bird, a feathered dinosaur, or whatever you like, Archaeopteryx was a gorgeous animal which combined the sleek and deadly anatomy of a predatory dinosaur with the exquisite plumage we so admire in its modern-day cousins. Archaeopteryx was a mosaic of the archaic and what we have thought of as modern—a 150-million-year-old portent of fantastic transformations that, through our understanding of them, have altered the way we see our place in this constantly evolving world.

References:

Witmer LM (2011). Palaeontology: An icon knocked from its perch. Nature, 475 (7357), 458-9 PMID: 21796198

Xu, X.; You, H.; Du, K.; Han, F. (2011). An Archaeopteryx-like theropod from China and the origin of Avialae Nature, 475, 465-470 DOI: 10.1038/nature10288

Riley Black | | READ MORE

Riley Black is a freelance science writer specializing in evolution, paleontology and natural history who blogs regularly for Scientific American.