Just as Charles Darwin was proposing his radical theory of evolution, paleontologists discovered a curious fossil specimen in modern-day Germany: Archaeopteryx. The feathered specimen, pegged by many as the first bird, helped provide further evidence for the theory of evolution and the idea that modern birds evolved from dinosaurs.

Archaeopteryx's status as the first prehistoric bird has held for more than a century, but a new high-tech analysis of a rare sample of bones from a German specimen challenge that long-held presumption. It turns out that the 150-million-year-old Archaeopteryx appears to have matured to adulthood slowly, more like a dinosaur than a bird, pushing the appearance of more modern bird characteristics forward to the more recent past.

"Our research shows us that this is much more like a dinosaur than a modern bird," says Gregory Erickson of the Department of Biological Science at Florida State University in Tallahassee, and a lead author on the new report, which was published online yesterday in the journal PLoS ONE. By looking at the growth patterns in bone, Erickson and his colleagues were able to discover that Archaeopteryx had a slow and stilted growing period like other dinosaurs, and unlike modern birds, which "just explode into adult size," he explains.

The new data were obtained by studying tiny, 250-micron-long chips from the fossilized leg bones of Archaeopteryx and more than a dozen other species of birds and dinosaurs using polarizing microscopy. The researchers found features in Archaeopteryx's bones, including annual growth lines, small blood vessels and parallel bone cells, shared by other similar-size dinosaurs, such as the Jeholornis prima. Fragments from a more birdlike animal, the Ichthyornis dispar, which lived about 94 million years ago, revealed no annual growth lines, substantial vasculature and a more random distribution of bone cells—all characteristic of quick growth.

Researchers now estimate that Archaeopteryx would have taken a little more than two and a half years to mature—compared with just a few months for modern birds of similar size. The findings confirm the previous assumption that all 10 known Archaeopteryx specimens are, in fact, juveniles. The news also bumps the estimated size of an adult Archaeopteryx up to about the size of a raven, larger than had previously been estimated.

Despite the general similarities to modern birds—feathers, a beak and a wishbone—that led to its avian categorization in the first place, the Archaeopteryx "would be very foreign to bird-watchers today," Erickson says.

The distinction, however, that many casual observers  have drawn between non-avian dinosaurs and modern birds is perhaps an exaggerated one. As Erickson points out, "We know that birds are dinosaurs," that is, even modern birds belong to the same lineage as dinosaurs. "Everybody would agree now that birds are really a kind of dinosaur, just like humans are a kind of primate," adds Mark Norell, chair of the Division of Paleontology at the American Museum of Natural History in New York City. Modern birds—or avian dinosaurs—share a different set of traits, such as hollow bones and rapid growth, than those exhibited by their prehistoric predecessors.

Research published by one of the co-authors (Dongyu Hu, of the Paleontological Institute at Shenyang Normal University in China) last month in Nature also moved a once-supposed avian species more firmly back into the dinosaur clan. (Scientific American is part of the Nature Publishing Group.) The well-feathered, 155-million-year-old Anchiornis huxleyi was shown to share more skeletal similarities with bona fide dinosaurs, despite its heavy plumage.

The Archaeopteryx's new qualification as a more dinolike creature does not change its status as what Erickson calls a "poster child for evolution." It does, however, underscore the extent to which many of the early birds still looked—and acted—a lot like dinosaurs. "It's not just a simple story," Norell says. "These things didn't evolve overnight…. The assembly of modern birds is a very complex process—there's intermediate stages in metabolism, there's intermediate stages in morphology."

The new findings further augment the argument Darwin's adherent Thomas Henry Huxley made nearly 150 years ago that birds did evolve from dinosaurs. In fact, modern analysis techniques and a heap of new specimens have helped to fill in many of the gaps that 19th-century scientists still faced. "It's no longer a missing link by any stretch," says Erickson of the bird–dinosaur connection.

Not all of the blanks have been filled in, however. The next step will be to pin down more specifically when modern bird physiology really started to appear, Erickson says. That hunt will certainly be helped along by new fossil finds and improved technology. But like this study, future discoveries may also be illuminated by reexamining well-known fossils. "The field is moving quickly enough that a lot of people are asking new questions about the same old specimens," Norell says. Progress is not only made through finding new animals and better examining equipment, he adds, but can also be made simply by uncovering "new ways of looking at old materials."