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No one knows what the very first true dinosaur looked like, but a young paleontologist named Paul Sereno of the University of Chicago has come closer than anyone else to finding out. In 1991, working with Argentine scientists in Ischigualasto Provincial Park at the edge of the Andes, he unearthed one of the oldest dinosaur fossils ever found. The animal, now known as Eoraptor, was a carnivore that dates from 230 million years ago. Like the much later Tyrannosaurus, the Eoraptor belonged to the saurischian, or lizard-hipped, category of dinosaurs. (The name refers to the arrangement of its pelvic bones; the other category of dinosaurs, which includes Stegosaurus and other herbivores, is labeled ornithischian, or bird-hipped. Ironically, birds are descended from the lizard-hipped class.)
Eoraptor has so many primitive features, including an exceptionally simple jaw, that Sereno thinks it probably originated just a short time after the ornithischians and saurischians diverged. Says Sereno: "Fifteen years ago, it was a radical idea to think that dinosaurs came from a common stem. Now we are just inches away from finding that stem."
Sereno is even more interested in the question of how dinosaurs managed to take over the world. One thing is clear from his Argentine excavations: it happened quickly. In Eoraptor's day, dinosaurs were rare. Ten million years later, however -- the blink of an eye in geologic terms -- many reptiles and crocodilians were in steep decline, while dinosaurs were headed toward dominance.
The reason, he and many colleagues believe, may have been a mass extinction of many of the planet's species late in the Triassic period. It could have been caused by the impact of a massive asteroid or comet, perhaps, or by dramatic climate changes triggered as Pangaea separated to form distinct continents. As other animals disappeared wholesale, the dinosaurs evolved rapidly to fill vacant ecological niches. Says Sereno: "It's very difficult to argue that the dinosaurs had something the others didn't. Instead of evolving because they were better, maybe they evolved because there was a sudden vacuum." For whatever reason, the early mammals, although they arose at about the same period, remained bit players for the next 150 million years. "Mammals during this time," says Hans-Dieter Sues of the Royal Ontario Museum in Toronto, "were nothing more than small, insect-eating organisms."
BONES THAT SPEAK VOLUMES
The traditional way to understand dinosaurs is through their bones, the only body parts that are preserved and converted into rock by the process of fossilization. The way bones fit together can reveal how an animal's joints worked, how its limbs moved, what kind of food it ate and how agile it was. Comparisons with living animals are also invaluable. "To understand dinosaur bones, you must take apart living animals," asserts paleontologist David Weishampel, who teaches anatomy at the Johns Hopkins University School of Medicine. "Fossils don't come with instruction kits."