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The notion that dinosaurs and birds are related dates back over a century. In 1861, quarry workers near Solnhofen, Germany, uncovered the fossil of a pigeon-size creature. Its bone structure and teeth were similar to those of dinosaurs. Yet along with the bones, the 150 million-year-old limestone in which it was trapped had also preserved the unmistakable impressions of feathers and wings. It was ultimately decided that Archaeopteryx, as it was named, was a transitional animal, related to dinosaurs but well along the evolutionary pathway to modern birds.
As happens so often in paleontology, though, the story has become much more muddled. The confusion began in 1964 with the discovery of a 13-ft.-long theropod called Deinonychus that was remarkably similar to Archaeopteryx, perhaps 50 million years more recent, but lacked wings and feathers. Apparently, the evolution from theropod to bird took many turns and detours.
Now comes Mononychus, one of the fruits of the first Western expeditions into the Mongolian Gobi in 60 years. "Central Asia probably has the greatest dinosaur-yielding potential of any area in the world," says Michael Novacek, dean of science at the American Museum of Natural History, who went to the Gobi in 1990 and has returned every year since. "There are areas the size of Montana that haven't even been prospected. You could spend a whole lifetime there."
Mononychus may be the discovery of a lifetime. The turkey-size predator, ! with its mouthful of sharp teeth and long tail, looked quite similar to the theropods. Even so, says paleontologist Mark Norell, it shares a number of features with modern birds. "In Archaeopteryx, for example," he explains, "the fibula ((the thin bone in the leg)) touches the ankle. In birds that doesn't happen, and the same is true of Mononychus. Birds have a keeled sternum ((or breastbone)), where the flight muscles attach. Mononychus also had a keeled sternum." Some of Mononychus' wristbones were fused together, which is another hallmark of adaptation for flight, suggesting that Mononychus may have evolved from a flying animal, just as ostriches and emus are descended from flying birds. That being the case, it was probably covered with feathers.
There are researchers skeptical, of course, about how Mononychus is labeled and about the larger question of how dinosaurs are related to birds. But since scientists cannot really decide for sure whether Mononychus should be considered a primitive flightless bird or a dinosaur, it seems plausible that there is really no essential distinction: it was both.
EXTINCTION BY COSMIC CATASTROPHE
The leading theory about what wiped out the dinosaurs used to be planetwide climate change; now it's something completely out of this world. Sixty-five million years ago, goes the story, at the very end of the Cretaceous period, an asteroid or comet smashed into the earth, throwing up a planetwide pall of dust. The sun was blotted out for months, killing most vegetation and starving the dinosaurs. The mammals, which had blown a chance during the last mass extinction, 150 million years earlier, rushed in to take over the suddenly vacant ecological slots.