Will St. John Detroit Free Press Taking Wing gives its readers a splendid view of both birds and brains in action.
Taking Wing: Archaeopteryx and the Evolution of Bird Flightby Pat Shipman
In 1861, just a few years after the publication of Charles Darwin's On the Origin of Species, a scientist named Hermann von Meyer made an amazing discovery. Hidden in the Bavarian region of Germany was a fossil skeleton so exquisitely preserved that its wings and feathers were as obvious as its reptilian jaws and tail. This transitional creature offered/i>
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In 1861, just a few years after the publication of Charles Darwin's On the Origin of Species, a scientist named Hermann von Meyer made an amazing discovery. Hidden in the Bavarian region of Germany was a fossil skeleton so exquisitely preserved that its wings and feathers were as obvious as its reptilian jaws and tail. This transitional creature offered tangible proof of Darwin's theory of evolution.
Hailed as the First Bird, Archaeopteryx has remained the subject of heated debates for the last 140 years. Are birds actually living dinosaurs? Where does the fossil record really lead? Did flight originate from the "ground up" or "trees down"? Pat Shipman traces the age-old human desire to soar above the earth and to understand what has come before us. Taking Wing is science as adventure story, told with all the drama by which scientific understanding unfolds.
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Prologue: A Flight of Fancy
There are seven specimens, and a feather.
It is not much to document the origin of bird flight. Strictly speaking, these specimens are not the entire body of evidence. There are, of course, other fossil birds, not to mention bats, pterodactyls, and insects that have a few things to say about bird flight. And there are living creatures, mathematical models, and aerodynamic theories to help us understand this amazing evolutionary accomplishment. Nonetheless, these seven specimens are crucial. They lie at the heart of complex debates that began with the discovery of the first specimen of Archaeopteryx more than 130 years ago and continue up until today. These few, special fossils have served as the basis for brilliant deductions, wild speculations, penetrating analyses, and amazing insights. They have revealed and continue to reveal not only the pathway through which birds and bird flight may have originated, but they also tell us much about the strengths and weaknesses of science and scientists.
Only seven precious specimens: as I have learned of them, I have been struck by their paradoxical nature. Seven specimens seem paltry evidence to tell the world everything about a lost animal, yet few extinct species are so well-known. Remarkably, among these seven is perhaps the most beautiful fossil in the world. It is the Berlin Archaeopteryx, an exquisite slab and counterslab that capture an extraordinary moment in evolution, when reptiles were turning into birds. No special training is required to see what the Berlin Archaeopteryx is; its wings and feathers are as obvious as the teeth in its reptilian jaws and the long, bony tail. The importance of the Berlin specimen cannot be overstated. It is more than a stony record of an extinct species. It is an icon a holy relic of the past that has become a powerful symbol of the evolutionary process itself. It is the First Bird. Do not think of the specimen as a skeleton in the round, such as the dinosaurs you see mounted in museums. Archaeopteryx is more nearly a bas-relief, a two-dimensional sculpture of a fabulous chimera, half-bird, half-reptile. It is a skeleton, plus ligaments, tendons, skin, and feathers, pressed almost flat between the rocks of the ages. Its mode of preservation makes Archaeopteryx as much objet d'art as scientific specimen. The difference is that the artist whose metaphorical hand carved this work of art was Time itself.
Time's display is beautifully laid out and daintily detailed. The grinning, toothy skull speaks of death, of birds fused with reptiles, of creatures that no longer exist. The skull is set on a gracefully curved neck, arched strongly backward to show the relentless tightening of tendons in death. On the right and left sides of the slab are delicate feathered wings, evoking a sense of angelic innocence that is disrupted by the three wickedly clawed fingers that adorn each wing. Lower on the slab lie two strong legs with clawed feet, three toes pointing forward on each and a fourth opposed to them, for grasping. To the left lower corner is the impression of the tail, familiar in shape and yet oddly wrong, with symmetrically placed pairs of feathers arising from each vertebra in the peculiarly long tail. While all known specimens of Archaeopteryx are remarkable in some way, it is the Berlin specimen that exemplifies them all, combining fragility and grace with enormous emotional and intellectual power.
All known specimens of Archaeopteryx come from one special place, in the Bavarian region of Germany. It is a small window through which we can peer at an ancient world. Some 150 million years ago, a few individuals of Archaeopteryx died and fell into the still waters of ancient Solnhofen, a shallow lagoon fringed with mud flats and bottomed in finegrained muds that had been deposited for tens of millions of years. When an Archaeopteryx died of accident, injury, or simple old age its carcass sank beneath the waters, becoming buried deep in these calcareous sediments, as did the remains of the insects, pterosaurs, fish, crustaceans, nautiloids, and other creatures that lived in or near the lagoon including a small theropod dinosaur, Compsognathus, whose presence proved fateful. The mud sealed it from further decay and destruction, protecting and preserving each anatomical detail as the body was infiltrated and replaced by stone. In time, the sediments consolidated into a fine-grained limestone. Still later, in the nineteenth century, these limestones were prized by humans as the perfect medium for detailed lithographic printing. Solnhofen limestone is so smooth that it will take the sharpest lines, conveying the subtlest textures that can be created by the artist's hand. This happenstance the value of Solnhofen limestone for printing was an essential ingredient in the strange story of Archaeopteryx. Stone, of course, has many potential uses, and the Solnhofen limestone has been quarried for paving-stones since Roman times. Only in the later nineteenth century did lithography become so important that these wonderfully fine-grained stones were rendered valuable rather than simply useful, As a result, a painstaking process of hand-quarrying (still practiced today) began that led directly to the discovery of Archaeopteryx and many other fine fossils. Each slab of Solnhofen limestone is chiseled out by hand, split, inspected for flaws, sorted, and then often trimmed further to the exact dimensions required. From start to finish, sometimes as many as a dozen skilled quarrymen examine each surface of each slab with care, so fossils are not missed even though they are not intentionally sought. Only the coincidence of the needs of the artist and of Archaeopteryx accounts for this fact. More mechanized quarrying the rule elsewhere would have certainly destroyed the fossils.
But Archaeopteryx is more than the world's most beautiful fossil. Its status is singular, despite the seven specimens. It has been a celebrity among fossils almost from the first moment of its discovery. Archaeopteryx holds a place in the heart and minds of the public, and of paleontologists, that is unparalleled by any other species. Children and grown-ups alike stare openmouthed at the specimens on exhibit, wondering at their completeness and strangeness. The same awe is experienced by paleontologists confronted with the original specimens, too. To understand this phenomenon, you have to know a little bit about paleontology, the study of ancient organisms. It is a science practiced mostly by the impassioned, who are not discouraged by the difficulties of dealing with specimens that are rare and all-too-often fragmentary. Paleontology is not a career that is particularly prestigious, nor is it easy; it is certainly not lucrative. But those who choose it can rarely imagine another life that would be as interesting or as much fun.
A paleontologist's fondest hope is to find a complete skeleton of all extinct species in which he or she is interested: an "earliest" or a "best." Careers are made and theories are built on such finds. To find a complete skeleton with an impression of the soft tissues that once surrounded it is a romantic dream that no paleontologist could reasonably hope to fulfill. To have seven such specimens simply defies all probability. Yet that is the case with Archaeopteryx. All specimens of Archaeopteryx (save the lone feather) are partial skeletons, and a few are nearly complete. Most show feather impressions, for indeed it is the feathers themselves that have been crucial in the history of discovery and recognition. It is the feathers that, from the beginning, have identified Archaeopteryx as the earliest bird (although the species has been assigned a number of other taxonomic positions as well). In reality, the chance that one of the very few fossil birds to be so exquisitely preserved was literally the first bird is diminishingly small. Nonetheless, deposing Archaeopteryx from its throne as First Bird would be a difficult task. Other bird fossils that have been found, and whatever fossils may yet be found, have to vie with Archaeopteryx for that title. They will have to rival its completeness and beauty; they will have to be both birdlike and reptilian, without question; they must be undoubtedly ancient as well. It is a daunting challenge. What's more, paleontologists rarely take kindly to the few heretics who have tried to challenge the nearly sacred status of Archaeopteryx.
Hovering in the background of any discussion of this species is a scurrilous charge that recurs periodically: that Archaeopteryx is a hoax. The accusation, made as long ago as the first discoveries and as recently as the mid-1980s, is that Archaeopteryx is not a feathered reptile but a small dinosaur skeleton falsely fitted out with feather impressions. The recent charges have accused putative Victorian forgers, who may have wanted either to prop up evolutionary theory at a time when its acceptance was hotly contested or simply to increase the value of a specimen up for sale. The charge has long been answered and dismissed: the feather impressions are microscopically intricate, showing details of structure of the flight feathers that cannot have been faked in the nineteenth century and, indeed, would be immensely difficult to fake today. Too, recently discovered specimens have been cleaned and prepared under the light of scientific scrutiny, with no opportunity for illicit modification. Now the forgery charge is rarely spoken of in the literature nor is the entire imbroglio surrounding the charges discussed in scholarly papers or at learned conferences. But the charge of imposture is not forgotten; it casts a pale shadow over the entire topic of Archaeopteryx and the origin of bird flight.
The superb preservation of Archaeopteryx enabled paleontologists to disprove this dangerous slander. But, ironically, it is also this superb preservation that poses one of the most serious problems to the curators entrusted with these precious skeletons and to the scientists who would analyze them. These specimens simply cannot be treated as any ordinary fossil would be. Preparing the bones fully that is, freeing them completely from the surrounding rocky matrix is a standard procedure that would make some aspects of anatomy clearer. But in this case, the bones cannot be fully prepared, for it would destroy the invaluable feather impressions, not to mention the evidence of anatomical articulations and relationships. So the skeletal anatomy of Archaeopteryx is tantalizingly complete, yet remains partially veiled. Isolating the bones from one another by removing all of the rocky matrix would destroy the little direct evidence we have of Archaeopteryx's position in life. It would also be tantamount to dismantling a masterpiece, an abomination comparable to cutting the Mona Lisa out of its background: unthinkable, too dangerous, too destructive.
When I began to follow the Archaeopteryx debates closely, I could not help but muse: how much better could the specimens be? It seemed absurd to me, for my background is in paleoanthropology. Many of the questions in paleoanthropology are similar to those that surround Archaeopteryx How did this form move? What ecological niche was it adapted for? Who were its ancestors? and similar techniques are often applied. In the course of my research into human origins, I have worked on many collections of fossils pertaining to the record of human evolution and I know from firsthand experience that paleoanthropologists cannot boast of a single fossil as excellent as the Berlin or London Archaeopteryx. Despite being in a field often labeled as "contentious," paleoanthropologists can agree on many facets of the anatomy and evolution of the earliest members of our lineage. Thus my background gave me an unusual perspective on the debates over the origins of birds and bird flight. How many more skeletons could paleontologists and ornithologists possibly need before they were able to resolve the basic issues of Archaeopteryx's anatomy and capability? But now I understand the complexities and ambiguities better and I know why Archaeopteryx will always remain enigmatic.
The scientists involved with Archaeopteryx are not stupid people or inept analysts. To the contrary, they are some of the brightest minds on several continents: keen, dedicated, and clever. But they are handicapped by the very nature of the specimens, by the importance of the specimens' position, and by the singularity of the fossils' preservation. These paleontologists may never be able to see these bones in the round, except perhaps indirectly by sophisticated radiography, because they are too well preserved and what cannot be seen will always be the focus of argument. The paradox is this: the excellence of preservation makes Archaeopteryx especially important by showing the admixture of avian and reptilian features, but that very excellence is also the source of endless frustration and confusion.
And so debate revolves around the positions of the bones in life, how they functioned and how they moved. Did the legs support the body in this position, or were they angled like that? Was the pelvis backwardly tilted, like a bird's, or more forwardly oriented in a more primitive reptilian position? How were the back and neck curved in its habitual posture? Are its feet and toes adapted for perching and grasping tree limbs, or for something else? What are the clawed hands designed for?
Articulation is peculiarly important to understanding the function of avian and reptilian fossils. Where two bones meet each other to make a joint at which movement can occur, the end of each bone has an articular surface. In adult mammals, only a superficial layer of a special cartilage, known as articular cartilage, covers the ends of the bones; the bony tissue itself makes the detailed shape of the joint surfaces. But in birds and reptiles, not only the articular surface but also the entire end of the bone does not ossify and remains cartilaginous. Cartilage, as a soft tissue, does not fossilize in the normal course of events, and so the exact shape of the two surfaces that fit together to form a joint is not preserved in a fossil bird or reptile. Since the shape of those two opposing articular surfaces dictates how the bones that participate in the joint can move, putting the bones of a fossilized bird together into a skeleton and predicting how that skeleton could move are difficult. Without knowing the exact geometry of the joint itself because the soft cartilage has decayed, disarticulated bird or reptile bones can be articulated in a wide range of positions, some of which were surely impossible in life. A fossilized bird skeleton simply provides much less anatomical evidence of that bird's habitual posture than does a skeleton of a fossil mammal. Thus the fact that Archaeopteryx is fossilized in a death position (if not in life) is immensely valuable because the bones are unlikely to lie in positions relative to one another that were impossible in life, at least so long as the skin and feathers remain intact. Absurdly, many body parts of Archaeopteryx have never been seen in their entirety, even though they are preserved in several specimens. The three-dimensional shape and relationship of some of its bones to others are a constant topic of debate.
Then there are the myriad questions about the abilities of Archaeopteryx. Did its wing fold up fully, like a bird's, or only partially? Could Archaeopteryx perform a wing flip, a tricky maneuver essential for taking flight from the ground, or not? In fact, could Archaeopteryx fly at all? Ironically, there is too much information in the slabs to dismantle them, yet there is too little information to answer these questions definitively.
There is more than this: there is the history of the discovery of Archaeopteryx, a tale that cannot be separated from the interpretations of this species. At issue are a series of accidents literal and intellectual discoveries that have forever colored the perception of this species. Three specimens of the seven are of special importance: the London Archaeopteryx, because it was the first skeleton; the Berlin Archaeopteryx, because it is the most complete; and the Haarlem Archaeopteryx, because of the extraordinary circumstances and emotional impact of its discovery.
Here is the story of the discovery of a special fossil known as Archaeopteryx and the arguments it has inspired.
Copyright © 1998 by Pat Shipman
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Meet the Author
Pat Shipman, an anthropologist at Pennsylvania State University, is the author of The Evolution of Racism and, with Alan Walker, The Wisdom of the Bones. She has written extensively on evolution and anthropology for such magazines as Discover, Natural History, New Scientist, and Focus. She lives in State College, Pennsylvania.
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