Archaeopteryx lithographica
Archaeopteryx lithographica was, for more than a century, the oldest and most primitive known bird. Archaeopteryx lived during the Late Jurassic period, about 145 million years ago. Its home was a reef-fringed tropical archipelago, which millions of years later would become Germany.
The fact that the limestones were laid down in layers in still water meant that, when quarried, they split. This made them useful for roofing slates. Then, in the 19th century, the invention of lithographic printing- which requires the use of large, flat blocks of stone as printing plates- led to extensive quarrying of the limestones. In the process, workers often found fossils in the limestones, which they kept as souvenirs, and occasionally sold to scientists. Eventually, one of these, a fossil of a bird, was bought by the British Museum in London. In 1861 it was described as Archaeopteryx lithographica (ancient wing from the lithographic stones) by the famous (and infamous) Sir Richard Owen, one of the greatest comparative anatomists of this age, or any other.
Archaeopteryx was a transtional fossil; it is arguably the transitional fossil. It combined classic avian characters such as feathers, wings, and a wishbone, so it's clearly a bird. And yet unlike a bird, it has ancestral characters seen in reptiles- clawed fingers, a long bony tail, jaws full of teeth. The fact that Owen described it has a wonderful irony to it. In mapping out the underlying similarities of various vertebrate groups, Owen provided powerful evidence for Charles Darwin's theory of evolution. Now, Owen provided another powerful piece of evidence that Darwin was right. One of the main shortcomings of Darwin's theory was the absence of transitional forms. Darwin argued that the transitional forms linking one group of animals to another had long since disappeared, but they must once have existed. Here, we had an animal combining features seen in birds with features seen in lizards and crocodilians . You couldn't get much more transitional than that.
Archaeopteryx later played a critical role in shedding light on the origins of birds from dinosaurs. Following his 1969 description of the dromaeosaur Deinonychus antirrhopus, palaeontologist John Ostrom decided to take a break from dinosaurs to study pterosaurs. While looking at a 'pterosaur' fossil from the Solnhofen, he spotted traces of feathers and realized that, instead, he held a long-overlooked specimen of Archaeopteryx. What was more surprising was that the wrist and hand structure of this Archaeopteryx was virtually identical to what he'd just finished describing in Deinonychus. Ostrom realized that the similarities were too extensive to be the result of chance; the similarities could only be the result of a close relationship between Deinonychus and birds.
The Archaeopteryx specimens come from the Solnholfen lithographic limestones. The Solnhofen limestones formed in the oxygen-starved waters at the bottom of a lagoon. The absence of oxygen meant that there were few scavengers there, so if a bird happened to die in the lagoon and then sink, it would lay there at the bottom without being disturbed. They would be slowly buried in an ooze of tiny limestone grains, which were produced by plankton called coccolithophores. As the limestone entombed the bird, layer by layer it formed a mold around the feathers and hardened, preserving minute details of feather structure and arrangement.
My work on Archaeopteryx goes back to 2002. Norell et al.described a remarkable specimen of the dromaeosaurid Microraptor with long, vaned feathers. Looking at the specimen, I remember thinking, "that's really odd. It looks like there are long feathers on the legs." I promptly dismissed the idea, because, of course, everybody knows that feathers on birds attach to the arms, *not* the legs. In 2003, Xu et al. described a remarkable new specimen of this animal, the 'four-winged' Microraptor gui. They showed that long flight feathers attached to both the forelimbs and the hindlimbs. What's more, they argued that Microraptor represented the ancestral form. I found this a little hard to buy. After all, everybody knew Archaeopteryx didn't show anything similar. If no other fossil forms showed this four-winged design, Microraptor was best interpreted as an odd side-branch in the tree. Not an ancestral form, more the crazy uncle in the family.
I've slowly continued my research on Archaeopteryx. It hasn't progressed as quickly as I would have liked: trying to advance my four-winged idea was an uphill battle, and for the next paper I wanted to have more evidence. But working with Jakob Vinther, I studied the feather arrangement in Archaeopteryx and Anchiornis. We recently put out a paper looking at this project. We showed that instead of having a modern wing arrangment, the feather arrangement in Archaeopteryx was actually much more primitive.
Rather than having a single layer of wing feathers making up the wing (as in modern birds), Archaeopteryx actually had multiple layers of wing feathers. This meant that even though the individual feathers were quite weak (their central shafts were quite slender), they could have functioned as an airfoil because many layers were stacked together, creating a strong airfoil. A similar, but somewhat more primitive arrangement is seen in Anchiornis.
The third reason is that the feathers were once far more extensive, extending back from the leg and ankle to cover the knee. Unfortunately, scientists prepared them away to expose the foot. Before the specimen was prepared, many scientists, including William Beebe, were struck by how long the feathers were. Beebe was the one who suggested the idea of a four-winged, parachuting ancestor for birds, the "tetrapteryx". Paleontologists have told the story that Beebe was speculating wildly and just happened to be right... but if you read the original paper, you'll see that he wasn't speculating at all. He was making a reasonable hypothesis based on his observations of Archaeopteryx.
In 2006, I did a paper (Longrich 2006, Paleobiology) looking at the function of these feathers. Some simple calculations showed that these feathers could have contributed to the flight performance of the bird, allowing the legs to act as airfoils, and decreasing stall speed and increasing turning radius modestly. The more important implication is that the presence of hind limb wings on Archaeopteryx meant that this was the ancestral condition. Microraptor wasn't some weird offshoot, early birds really did have a four-winged design. The discovery of long feathers on the legs and toes of a new species, the even more primitive Anchiornis, backs up this idea.
There are several answers. The first is that people see what they expect to see. Birds only have two wings, so nobody expects to see four. When I went to Berlin to see the specimen for the first time, I pored over the wings, glanced at the tail, observed the toes, studied the fingers... I saw the leg feathers but didn't recognize them for what they were. They weren't interesting. We 'knew' Archaeopteryx had two wings, not four, we 'knew' the leg feathers were only for insulation. I can speak to the power your preconceptions have over your observations, because until Microraptor forced me to reconsider everything I thought I knew, I assumed Archaeopteryx was a fairly standard bird.
The second has to do with the history of science and how it interplays with world history. The specimen that preserves the leg feathers is located in the Humboldt Museum fur Naturkunde in Berlin, Germany. Germany used to be a world leader in the sciences, but they found themselves caught up first in World War I, then dealing with reparations and the Great Depression; then World War II came along, followed by separation of the country into Soviet and Allied occupied East and West Germany. The Humboldt Museum ended up to the east of the Berlin Wall, and Archaeopteryx was on the East. People in East Germany tended to have bigger problems than paleontology. And Germany was still engaged in rebuilding half a century later. When I first visited the Humboldt Museum, I remember seeing a bombed-out wing, three unroofed walls rising up several stories, with grass and a small tree growing up in the middle. To this day, there are still pockmarks from shrapnel and bullets on many of the old facades in east Berlin.
This is when I took a look at a cast of Archaeopteryx and noticed a huge series of vaned feathers sticking off the shin. I couldn't believe that I had missed this, not least because I'd seen the actual specimen and never noticed them. Archaeopteryx did, in fact, have flight feathers on the hind limbs. How was it possible that this could be overlooked?
The implication here is that it's not just the skeleton that is evolving. Early birds had very different feather structure and feather arrangement. This also implies that the wings couldn't function in the same way that they do in living birds. Modern birds can separate their feathers on the upstroke, twisting the individual feathers apart like the slats on a venetian blind so the wind slips through as they whip the wings up. It's not possible to do this if you stack many layers of feathers together. This feather separation mechanism is particularly important at low speeds, where you need to beat the wings rapidly and steeply up through the air to get any lift. The implication is that taking off and landing at low speeds would have been difficult, which implies high speed takeoffs from the trees- trees-down origin of flight, not ground up. There is probably still a lot more to learn about Archaeopteryx and other feathered dinosaurs. The traditional approach has been to use modern birds as a model for understanding early forms. This leads, unsurprisingly, to the conclusion that early forms were fairly modern. But when you dig into the details of the anatomy, surprises emerge. The more we study these things, I imagine, the more primitive they will turn out to be.
