However, the new findings, if confirmed by additional research, will not undermine the prevailing theory that modern birds descended from dinosaurs.
In the 150 years since its discovery in Germany, Archaeopteryx has perched high on the avian family tree as the earliest and most primitive bird, somewhere near the evolutionary moment when some dinosaurs gave rise to birds. But recent fossil finds cast doubt on this interpretation: Archaeopteryx may be only a birdlike dinosaur rather than a dinosaur like true bird.
Chinese palaeontologists reported in the current issue of the journal Nature that a previously unknown chicken-size 155-million-year-old dinosaur with feathers, named Xiaotingia zhengi, “challenges the centrality of Archaeopteryx in the transition to birds.”
Like so many fossil dinosaurs and other life from the late Jurassic period, Xiaotingia was found in Liaoning Province, a happy hunting ground for palaeontologists. The skeleton was embedded in shale, along with the clear impressions of feathers. Scientists who studied the specimen said it was not as striking in appearance as several of the 10 known Archaeopteryx remains, but good enough apparently to contradict conventional wisdom about proto-birds.
The discovery team and other scientists emphasised that the new findings, if confirmed by additional research, would not undermine the prevailing theory that modern birds descended from dinosaurs. The question now is, if not Archaeopteryx, which of many feathered dinosaurs or dinosaur like birds being found is closest to the first bird? Other assumptions about the early evolution of birds, they said, would also need to be re-evaluated.
Xing Xu and his colleagues at the Chinese Academy of Sciences in Beijing said that their examination of Xiaotingia, in comparison with more recognisably bird skeletons from the same period as well as the 150-million-year-old Archaeopteryx, showed that the new fossils fell short of a place in the avian family. Several of its anatomical traits, like the long and robust forelimbs once thought to be diagnostic of birds, were actually common to a group of dinosaurs known as deinonychosaurs.
Dr. Xu's team concluded that “the most important result of our analysis” is that the anatomies of the Chinese specimen and Archaeopteryx were remarkably similar, meaning that both belonged to the lineage of the meat-eating deinonychosaurs, not the plant-eating early birds. In short, Archaeopteryx presumably was not an ancestral bird. The recent discovery of a tenth Archaeopteryx specimen “greatly improved our knowledge” of its similarities to the dinosaur group and its differences from birds, the palaeontologists said.
Several scientists who were not involved in the research said they were not especially surprised by the findings.
“It may seem heretical to say that Archaeopteryx isn't a bird, but this idea has surfaced occasionally since as far back as the 1940s,” Lawrence M. Witmer, a palaeontologist at Ohio University, wrote in a commentary accompanying the journal article. “Moreover, there has been growing unease about the avian status of Archaeopteryx as, one by one, its “avian” attributes (feathers, wishbone, three-fingered hand) started showing up in non-avian dinosaurs.”
Nor was this report likely to be the last word on the subject. The researchers themselves, among the leading dinosaur specialists in China, acknowledged that their interpretation was sure to be controversial. They conceded that some of their conclusions are “only weakly supported by the available data.” At such an early stage in the dinosaur-bird transition, distinctions among species were often subtle, or “rather messy affairs,” as Dr. Witmer said.
Scientists are expected to take another, deeper look at many feathered fossil animals that have been uncovered in China in the last 15 years. Several of these avian dinosaur species, including Epidexipteryx, Jeholornis and Sapeornis, may then take wing as the new early birds. And relentless fossil hunters are certain to turn up new species.
“This will be frustrating and exciting,” Dr. Witmer said in an interview, noting that — who knows? — the next discovery might tempt scientists to restore Archaeopteryx to its place in the proto-bird flock. “Some of these things may never be entirely conclusive,” he said. “It drives us nuts.”
Since “virtually all our notions about early avian evolution have previously been viewed through the lens of Archaeopteryx,” Dr. Witmer said, “the impact of losing Archaeopteryx from the avian clan is likely to rock the paleontological community for years to come.” — © New York Times News Service