On the Origin of Birds

Xiaotingia zhengi skeletonXU ET AL. NATURE This year marks the 150th anniversary of the discovery of one of the most iconic dinosaur fossils of all time: Archaeopteryx. Unearthed in 1861 in southern Germany, it has long been considered the missing link between dinosaurs and birds: it has the skeleton of a reptile—including a long bony tail and teeth—combined with what are considered hallmarks of modern birds—feathers and a wishbone. But the discovery of a new bird-like dinosaur, Xiaotingia zhengi, in China threatens to dethrone Archaeopteryx as the most primitive of birds. The inclusion of the new fossil in a comprehensive phylogenetic analysis of dinosaurs and early birds, published today (July 27) in Nature, resulted in a rearrangement of evolutionary relationships—removing Archaeopteryx from the avian family and placing it in a closely related family of non-avian dinosaurs. “For the past 150 years, Archaeopteryx has been this filter through which we’ve run every idea about the early evolution of birds,” said Lawrence Witmer, a paleontologist at Ohio University who was not involved with the study. The new fossil will force researchers to re-evaluate avian origins from a different perspective, he added. Roughly the size of a chicken, Xiaotingia zhengi is morphologically very similar to Archaeopteryx, sporting long arms and (most likely) four wings. Like Archaeopteryx, it also walked, or possibly glided, across the Earth during the Late Jurassic Period around 150 million years ago—right at the genesis of birds. “It’s an amazing fossil,” said lead author Xing Xu, a paleontologist at the Institute of Vertebrate Paleontology and Paleoanthropology in China. “The information recovered from this fossil will have a tremendous impact on our understanding of bird origins.” Artist’s impression of Xiaotingia zhengi XING LIDA AMD LIU YA Xu first stumbled upon this new specimen in 2009 during a visit to the Shandong Tianyu Museum of Nature, where it was mistakenly classified as another bird-like dinosaur species, Anchiornis (first described by Xu a few years earlier). But Xu immediately realized this one was different, and represented yet another feathered dinosaur from the Jurassic Period. A phylogenetic analysis of 374 morphological characters from 89 species of dinosaurs and primitive birds, including Xiaotingia, revealed that Xiaotingia, Archaeopteryx, and Anchiornis all belonged to a small family right at the base of the Deinonychosauria lineage—a group of non-avian, yet bird-like dinosaurs that includes the famous velociraptor featured in the 1993 movie Jurassic Park. When the researchers repeated the analysis excluding Xiaotingia from the dataset, however, Archaeopteryx reverted right back to the avian group, as previous analyses had predicted. “We are so close to the evolutionary origin of all of these different groups that one fossil can tip the balance,” Witmer said. “These results suggest that Xiaotingia is the linchpin.” The new phylogeny could have several implications about avian evolution, including what ancient birds may have eaten. While recent studies have suggested that early birds were herbivores, the skull of Archaeopteryx was more suited for a meat-eating diet, leading scientists to believe that birds had started out as meat-eaters and then evolved an herbivorous diet. But removing Archaeopteryx as a direct ancestor of the group suggests that the first birds could have in fact been plant-eaters. This is not the first time Archaeopteryx’s place at the base of the avian family tree has come under fire. Over the past two decades, scientists have discovered a new crop of dinosaur skeletons with bird-like features that have called into question Archaeopteryx’s missing link status. “A lot of the uniquely avian features that Archaeopteryx has have turned out to be not so unique after all,” Witmer said. As he wrote in Nature’s accompanying editorial, “perhaps the time has come to finally accept that Archaeopteryx was just another small, feathered, bird-like theropod fluttering around in the Jurassic.” Of course, a new fossil may rearrange the evolutionary relationships once again, he added. “In some respects the major story here is just how tangled the knot near the origin of birds and bird-like dinosaurs really was.” X. Xu et. al., "An Archaeopteryx-like theropod from China and the origin of Avialae," Nature, doi:10.1038/nature10288, 2011.

On the Origin of Birds

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