Examples from Dinosaurs
Timothy Rowe, University of Texas at Austin
For the Symposium Presented by the Society for the Study of Evolution
"Building the Web of Life: Evolution in Action" : NABT Ft. Worth, 10/99
Examples from the Evolutionary History of Dinosaurs
Department of Geological Sciences and
Vertebrate Paleontology Laboratory
The University of Texas at Austin
Introduction: Macroevolution describes the pattern of evolutionary history as it has played out over the grand scale of geological time. The origins of the major living lineages, the emergence of key adaptations, and the exploitation of novel adaptive niches are among the issues considered under the umbrella of macroevolution.
One of the major problems in understanding macroevolution involves mapping historical patterns. Although a lot is known about biological processes operating today, such as genetic mutation and speciation, only after the historical pattern of evolution has been mapped can we begin to determine what processes have occurred historically. Most debate about evolutionary process is a result of underlying uncertainty on pattern. So, how can we know the pattern of evolutionary events that occurred millions of years ago?
Historical Background: One of the oldest and most persistent problems of macroevolution involves the origin of birds and the evolution of flight. This was one of the first challenges thrown at the theory of evolution when Darwin published his revolutionary book On the Origin of Species in 1859. At that time it seemed an intractable problem because no transitional forms were known between birds and other vertebrates. Critics challenged that Darwin's mechanistic theory of natural selection couldn't explain how complex new organs like feathers or complex new functions like flight could have evolved gradually and via transitional stages. The thought of a transitional, partly flying bird, was like the thought of being partly pregnant. How could flight have evolved from non-flying ancestors without the force of gravity killing all the transitional species? The origin of birds and the evolution of flight provided one of the first great battle grounds for the theory of evolution by natural selection.
Dinosaurs provided the first key insights into understanding of how birds and avian flight might have evolved. When Darwin's book hit the news stands, the scientific image of dinosaurs was that they were all "fearfully great" saurians, that they were all gigantic extinct behemoths. Then, in 1860, a tiny dinosaur named Compsognathus was discovered in the famous Solnhofen limestones of Bavaria. A developmental biologist and early evolutionist named Carl Gegenbaur had been studying the development of the skeleton in modern birds. When he learned about the structure of the ankle in Compsognathus, he saw features that are present in embryonic stages of modern birds and was he first to make the connection that birds are descendants of dinosaurs. That same year, the discovery of the primitive toothed bird Archaeopteryx showed a transitional stage between Compsognathus and flying birds. Archaeopteryx preserved feathers like modern birds, along with a bony tail and other skeletal features more characteristic of reptiles. By 1870, scientists had discovered many transitional features between birds and dinosaurs.
The Problem in Mapping Macroevolutionary Pattern: So, if scientists in 1870 found evidence linking living birds to extinct dinosaurs, why does a controversy still persist? Newspapers are full of accounts of disagreement on the ancestry of birds. The answer to this question is the idea of convergent evolution or homoplasy. Different, unrelated species can independently evolve similar solutions to common environmental challenges. For example, both birds and bats have wings, but no one today believes that they inherited wings from a common ancestor that could fly. Although similarities are generally indications of relationship, we can be fooled by homoplasy.
At this point it is instructive to ask, "How do we know that the wing in birds and bats evolved convergently?" To answer, we believe that bats are mammals and unrelated to birds because bats have hair, mammary glands, a placenta, and many, many other features that indicate their relationship to other mammals. Birds, on the other hand, have molecular and anatomical structures that indicate they are related to reptiles. There are also many transitional fossils that indicate bats to be branches of the mammalian family tree whereas birds are branches on the reptilian family tree. When all the evidence is considered, it is far simpler to believe that wings evolved convergently than to believe that birds and bats belong to the same flying lineage. We tend to prefer explanations that take into account all of the evidence over explanations that account for only some o the evidence.
Today's Perspective on the Origin of Birds: Modern computer assisted techniques enable biologists to map historical patterns by comparing huge numbers of anatomical and molecular details. In short, we try to find the simplest hierarchical pattern that explains all of our observations. In the case of the origin of birds, our evidence comes from the anatomy of living species along with the anatomy of fossils. Hundreds of characters have been observed and compared, and there is one overwhelming pattern. Although there are also homoplastic characters in this pattern, the majority of the evidence indicates that birds lie on a branch of the dinosaurian family tree. In a biological sense, birds are dinosaurs and only some dinosaurs became extinct.
One reason that this has been controversial is that Mesozoic dinosaurs were highly diversified and many look nothing like birds. Stegosaurs, ceratopsians, the giant sauropods, and many other groups look nothing like birds. But there is one group ñ the carnivorous theropod dinosaurs ñ that has many detailed resemblances to modern birds. Even the theropods are very diverse and some of their members, like Tyrannosaurus rex, are off on their own extinct side branches. However, one lineage of persistently small theropods manifests a hierarchy of unique features that are still present in modern birds.
Since the discovery of Compsognathus and Archaeopteryx a century and a half ago, many new theropod fossils have been discovered that fill the gap between modern birds and their extinct dinosaurian relatives. John Ostrom's discovery of Deinonychus is the most famous, but there is a great deal of other evidence that points in the same direction. Details of the head, neck, arms, hands, pelvis, hind limb, and tail all point to a dinosaurian ancestry for birds. Some recent discoveries from China preserve soft tissues including proto-feathers. Additional evidence comes from embryology and the development of modern birds.
Based on this pattern of relationships, it now appears that feathers originated before the ability to fly, and that feathers were only secondarily adapted for flight. In a similar fashion, the feathers of birds develop first for insulation and only later in development do flight feathers and the ability to fly emerge. The first dinosaurs to have feathers were fast-running predators who probably used their arms to grab prey items. The bones of the wrist constrain the movements of their hands to the same pattern of movement that we see in the flight stroke of birds. Flight feathers and the ability of powered flight were superimposed upon an anatomical pattern involving predatorial grasping and short proto-feathers.
The Radiation of Living Dinosaurs - the Birds: The macroevolutionary pattern underlying the origin of birds is based on observations from the entire skeleton and integument in living birds and fossils that extend across 230 million years of the fossil record. While the non-avian dinosaurs all became extinct at the end of the Mesozoic, birds speciated in one of the greatest adaptive radiations of all time. Many birds flew out to the islands of Pacifica and thousands of new species emerged in these isolated places. Another fountain of avian species was the circum-Pacific mountains of North and South America.
When the entire macroevolutioanry pattern of dinosaurian history is considered, living birds must be considered along with their extinct relatives. From this perspective, it appears that comparatively few dinosaurian species were affected by the great extinction event at the end of the Mesozoic. Far more profound have been the effects of human occupation of the islands of the world. Perhaps as many as 8,000 species of birds became extinct as humans inhabited these islands. A next wave of extinction is moving onto the continents as the human population soars to unprecedented levels.
Today, scientists studying macroevolution are attempting to measure the complete pattern of speciation for an entire evolving lineage. The pattern that we see today indicates that birds are deeply internested within the hierarchy of dinosaurian relationships, and that humans may be the most severe source of extinction of dinosaurian species.
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Ostrom, J. H. 1979. Bird Flight: How did it begin? American Scientist 67:46-56.
Rowe, T., K. Kishi, J. W. Merck, Jr., and M. W. Colbert 1998.
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