Searching for key to bird flight amid tree-dwelling dinosaurs, leaping lizards
A small, lizard-like creature with wings like a bat and the life style of a flying squirrel may have been the first animal with the ''Right Stuff'' - the ancestor of all modern birds.
This proposal, put forward by Colin Pennycuick, an ornithologist from the University of Miami, is the latest wrinkle in the ongoing scientific debate over how and why the ancestors of modern birds earned their wings.
Through the millenia, a number of different species - everything from frogs to squirrels - have taken to the air in various fashions. But birds are the only ones that have done so from two, rather than four legs. This has meant that birds have been able to keep their feet - and the ability to move on the ground - at the same time that they developed the ability to soar among the clouds. This is one reason that birds have proved so successful. But it has also made it difficult to determine just how thebirds' ancestor made such a transition, an effort seriously hampered by the dearth of fossil records of early avians.
As a result, the scholarly debate on the evolution of flight has centered on birds. It dates back more than a century and involves two conflicting theories.
The current orthodoxy, the arboreal theory, holds that birds evolved from tree-dwelling dinosaurs that learned to fly by falling out of trees. Challenging this is the ''ground up,'' or cursorial theory that maintains that small, running dinosaurs developed wings and flight to control themselves while leaping off the ground to snag prey or escape from predators. Last Tuesday, this debate was updated and recapitulated at a symposium held at the California Academy of Sciences.
The ''ground up'' theory was in total eclipse until a decade ago, when John Ostrom of Yale University revived it. His argument is based primarily on the nature of the fossils of the earliest known bird, Archaeopteryx lithographica. A half-dozen marvelously preserved fossils of this 135 million-plus-year-old creature have been recovered from central Bavaria. These show not only the skeleton but also the imprint of feathers from a bird about the size of a magpie , with a long, feathered tail, sinuous neck, and a lizard-like jaw.
''Archaeopteryx has feet which look much more like those of ground birds than perching birds,'' Dr. Ostrom contends. The ancient bird's skeleton also suggests that it could not fly very well. In particular, the fossils show that it has three independent fingers on its wings. In modern birds these fingers have fused into a single bone.
Taken together, these features suggest that this bird may have been more like a road runner with a limited ability to glide rather than an arboreal bird, the scientist suggests.
It is generally agreed that feathers first evolved from scales as insulation when the bird's ancestors changed from cold-blooded to warm-blooded animals. Once feathered, these running dinosaurs found that they used feathered forelimbs to leap higher and longer until they gradually developed the ability to soar from cliffs, Ostrom suggests.
Last year three scientists from the University of Northern Arizona, led by ornithologist Russell Balda, took up this cause. They published a paper on the basic aerodynamics of leaping, feathered lizards which defends the possibility that flight could have developed in such a fashion. ''We're now in the process of analyzing the aerodynamics of proceeding from the trees down, and it looks more difficult,'' says group member Gerald Caple.
Walter Bock, a biologist from Columbia University, has assumed the mantle of defender of the arboreal theory. His defense relies primarily on the basic principles of evolution. Evolution, he explains, must proceed by a number of small, strictly chronological steps. For each adaptation, there must be a strong evolutionary rationale.
Dr. Bock sees the origin of flight proceeding along the following lines. First, there were ground-dwelling dinosaurs that adopted an upright, bipedal posture. These took to the trees, possibly for protection from predators. Because temperatures in the forest canopies are lower than those on the ground, they were forced to become warm-blooded and their scales evolved into insulating down. Leaping among the trees led to the development of rudimentary wings to act as parachutes in case these feathered saurians fell. Next, as their wings strengthened, they began to glide. Finally, soaring evolved into active flight.
Archaeopteryx, Bock envisions, was primarily a glider. It may have spent some of its time on the ground and some of its time in trees, climbing up the side using the claws on its wings as well as its hind feet.
Learning to fly from the trees down has a number of advantages, the scientist explains. Life in the trees would enable the bird's ancestors to develop the three-dimensional perception needed for flight. And working with gravity is much easier than working against it.
The main flaw in the ''ground up'' hypothesis, he contends, is its failure to explain the development of warm-bloodedness.
Dr. Pennycuick also favors the arboreal theory.
Looking at Archaeopteryx through the eyes of his studies with birds and bats, Pennycuick injected some fresh ideas into what has become an increasingly ritualized debate.
The long fingers on these ancient birds may not be for grasping as had been generally accepted, he proposed. Instead, they may have been joined by a membrane wing similar to that of a bat. At first, the feathers may have been attached to this skin membrane, rather than directly to the bone as is the case in modern birds. This idea is supported by the fact that in one of the fossils the feather imprints are separated from the bones but still remain precisely in order.
Putting membranes on Archaeopteryx's forelimbs would create a very nice set of gliding wings, the scientist says. They would be more efficient than those on today's flying squirrels. The only problem is that they would fly very fast, so perhaps feathers elongated as much to slow the primitive birds down as to aid in their flying.
The scientists involved in this longstanding debate agree that there is not enough information to resolve their differences. However, Dr. Ostrom appears to have achieved his main objective: ''At least, the arboreal theory is no longer accepted without question.''