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University of Maryland paleontologist Thomas R. Holtz Jr. looks over the fossil-rich strata at Dinosaur Provincial Park in Alberta. Holtz served as a consultant on "Walking With Dinosaurs" and "When Dinosaurs Roamed America," and wrote a "Jurassic Park" dinosaur guide.
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It’s no longer any mystery that birds and dinosaurs are related. But how close is that relationship? How could the “fearfully great lizards” transform themselves into feathered fliers? The debate continues, but paleontologists are converging on solutions to these mysteries.

At first glance, few creatures could seem more different than birds and dinosaurs. However, as far back as the 1860s, scientists began to notice many similarities in the skeletons of birds and even the largest of these fearfully great lizards - the literal translation of “Dinosauria.”

The gulf between giant dinosaurs and modern birds was first bridged with the discovery of Archaeopteryx. This famous fossil creature, dating from rocks about 150 million years old, was endowed with feathers and all but universally recognized as an ancient bird. In 1861, when the first Archaeopteryx skeleton was discovered, there was nothing else quite like it in the fossil record. Since then, however, we have found more and more small, advanced, birdlike dinosaurs ... and more and more primitive birds.

Now we’ve reached a point where there are no huge jumps in the evolutionary development of birds from other dinosaurs - and not just any dinosaurs, but specifically the “psycho killers” of the dinosaur world, coelurosaurian theropods like “raptors” and tyrannosaurs. Under the modern scheme of classification, birds would be considered coelurosaurian dinosaurs, just as bats are still considered mammals, even though unlike other mammals they can fly and have sonar.

The lines once used to distinguish birds from all other animals have become blurred. Many characteristics unique to living birds are now known to have occurred in a variety of dinosaur types — and indeed all are present in the coelurosaurian dinosaurs interpreted as closest to birds: wishbones, a backward-pointing pubis, the halfmoon-shaped wrist bone and so on. Even feathers have now been found in dinosaurs other than birds, due to spectacular discoveries in China.

Furthermore, other features used to distinguish modern birds from other living animals are absent in Archaeopteryx and other very primitive birds: toothless beaks, tail vertebrae fused into a short stump, a big-keeled breastbone. These clearly evolved after the origin of Archaeopteryx, within the bird lineage itself.

So, given the anatomical evidence, why is there still a debate? A small minority of paleontologists say that the similarities between birds and dinosaurs are the result of convergence - the fact that different evolutionary lines can develop the same features due to similar life habits. These paleontologists argue that birds might be distant cousins of the dinosaurs, but not direct descendants. The basis for this debate focuses on several questions, which should be examined based on the evidence.

As in other areas of paleontology, biology, geology and other historical sciences, there will always be debate and some conflict over the bird-dinosaur connection. But there comes a point where arguing against a particular theory - say, the idea that continental plates drift against each other - is simply a sign of stubbornness. For many paleontologists, the dinosaurian origin of birds approaches that level of support.

That is not to say there aren’t additional questions that interest paleontologists within the framework of the dinosaurian hypothesis of bird origins. For example, were any of the dinosaurs considered closest to birds feathered? So far none of these dinosaurs - such as raptors, troodonts or oviraptorosaurs - have been found in the type of fine-grained sedimentary rocks that preserves either feather or skin impressions: what their body covering was like is unknown.

Were any of these forms actually descendants of flying Archaeopteryx relatives, making raptors or troodonts “secondarily flightless” like the ostrich or kiwi of today? When in the history of birds did warm-bloodedness arise: Before Archaeopteryx? After it? At the base of the dinosaur family tree? These and other questions still intrigue paleontologists, and are actively researched today.

In a broader context, why should we care about the answers to these mysteries? Scientifically, the bird-dinosaur connection is significant in that it involves questions about major transitions. How quickly can organisms adapt to a changing environment? How quickly, and when, did the many species of bird arise? Why did birds alone among the dinosaurs survive the great extinction at the end of the Cretaceous period, 65 million years ago? Understanding the ancient sources of that diversity, adaptability and success may help us better understand today’s ecosystems and avert tomorrow’s extinctions.

Thomas R. Holtz Jr. is a vertebrate paleontologist at the University of Maryland.

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