During the Jurassic period, between about 200 million and 145 million years ago, some meat-eating dinosaurs began evolving birdlike skeletons and sprouting feathers on their bodies. One group of these creatures eventually split off to become birds, although researchers have long debated which one it was and when it actually happened. Now, a team of scientists claims to have found the earliest known bird, a discovery that could finally put these questions to rest. But critics question whether it really is a bird, and some are not entirely convinced that it's an authentic specimen.
About 30 species of feathered dinosaurs have been discovered in the past dozen years or so, mostly from geological formations in China's northeastern Liaoning province. But up until now, few paleontologists have argued that any of them qualified as the earliest known bird. That honor has been held for 150 years by Archaeopteryx, a 150-million-year-old creature discovered in Germany, several well-preserved specimens of which have been found over the past century and a half. Nevertheless, 2 years ago, China's most famous fossil hunter, Xing Xu of the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing, argued in a paper in Science that Archaeopteryx was not really a bird, although many researchers did not agree.
But even if Archaeopteryx is a bird, the new specimen, reported by a team led by Pascal Godefroit, a paleontologist at the Royal Belgian Institute of Natural Sciences in Brussels, could push it off its perch as the earliest one known. The new fossil, which the team has named Aurornis xui ( Aurornis meaning "dawn bird," and xui in honor of Xing Xu's contributions to bird origins), was found in Liaoning province's Tiaojishan Formation, in sediments dated about 160 million years ago, according to the Nature report; that's around the time that dinosaurs are believed to have started evolving into birds. The fossil (see photo), reported online today in Nature, is so complete that Godefroit and his colleagues were able to compare nearly 1000 different features of its skeleton with those of about 100 other dinosaurs and birds, resulting in a computer-generated evolutionary tree with Aurornis at the very bottom of the bird branch. The evolutionary tree, called a phylogenetic analysis, also restored Archaeopteryx's avian status and reclassified some other feathered dinosaurs as early birds—including the 150-million- to 160-million-year-old Anchiornis, also from the Tiaojishan Formation and first identified by Xu in 2009.
Early bird experts agree that the discovery of Aurornis is potentially big news. "The specimen is very exciting," says Lawrence Witmer, a paleontologist at Ohio University in Athens. Stephen Brusatte, a paleontologist at the University of Edinburgh in the United Kingdom, agrees: "If it is the most primitive bird, then it is a huge discovery."
But Luis Chiappe, a paleontologist at the Natural History Museum of Los Angeles County in California, says that Aurornis must fly through at least one important hoop before it can claim to be the first bird: Its authenticity must be proved. That's because the specimen was not found during excavations by Godefroit's team, but—like most early birds and feathered dinosaurs from Liaoning province, including many studied by Xu—it was supposedly found by farmers and acquired from a Chinese fossil dealer, who sold it to the province's Yizhou Fossil and Geology Park. China's museums and research institutions have been plagued in recent years by the circulation of fake or altered fossils, including the infamous "Archaeoraptor" fake of the late 1990s, a claimed "missing link" between dinosaurs and birds published by National Geographic.
"I would take a skeptical look at this specimen," Chiappe says. "The fact that it is so neatly arranged and so complete makes it suspicious." Chiappe acknowledges, however, that the authors considered the possibility that the fossil might be a fake in supplementary information published to accompany the main paper. In the supplementary materials, the team relates that the skeleton is preserved in a shale slab, which the fossil dealer claims was found near the site where a specimen of Anchiornis was also discovered. The shale seems from its appearance to come from Liaoning, and the fossil was examined by experts at the Yizhou park as well as two Chinese co-authors on the paper, all of whom concluded that the likelihood that the fossil is a fake was "accordingly low."
Godefroit takes umbrage at the suggestion that the fossil might be a fake or that specimens acquired from dealers should be dismissed. "If farmers did not start collecting specimens in Liaoning, nothing would be known about feathered dinosaurs and bird evolution in China," he says. Witmer agrees that researchers are "absolutely correct" to question the fossil's provenance but says he is "satisfied" with the team's conclusions that it is authentic. "The alternative is to ignore it, and that seems less justified," he says.
Yet, even if Aurornis is indeed authentic, it's not necessarily the first bird, researchers say. Brusatte says that the team's evolutionary analysis is only one of several that are possible. "We know that birds are dinosaurs, but we still don't know the exact genealogy of the first birds" despite the new discovery, he says. He adds that Aurornis and Anchiornis could be either adult or juvenile versions of the same species, rather than separate species; if so, that could lead to very different conclusions about their evolutionary status, including the definite possibility that they are not birds at all.
And Xu says that while Godefroit and colleagues' phylogenetic analysis should be "seriously considered," especially because it uses the largest known dinosaur database, "their analysis also produced several results inconsistent with those of most recent studies," such as putting still other species considered to be feathered dinosaurs into the bird category. "I don't consider this piece of work as a final solution," he says.
Witmer says that the debate over "all these little dino-birds" may be "maddening," but it's necessary. Untangling these evolutionary strands, he says, "is gonna be messy."