September 24, 2009 | 4
Non-avian dinosaurs are long extinct, but paleontological thinking about them, especially the dino–bird specimens, clearly continues to evolve long after they are discovered. For instance, the Anchiornis huxleyi, a small, feathered dinosaur, was described last December and assumed to be a transitional species that existed between dinosaurs and birds. But new evidence—and a much better specimen—has revealed that this ambiguous animal actually belongs to the dinosaur clan.
Described from a partial specimen in the Chinese Science Bulletin, A. huxleyi was proposed to be an "intermediate…between non-avian and avian dinosaurs," wrote Xing Xu, a paleontologist at the Institute of Vertebrate Paleontology and Paleoanthropology at the Chinese Academy of Sciences in Beijing, and his colleagues. But as a dinosaur—now proposed to be a troodontid, a birdlike group of theropods—it sets the clock back for bird evolution.
The dinosaur–bird transition has been the subject of debate for more than a century, and some researchers are still arguing that other birdlike dinos are too recent to be the ancestors of birds. The quandary, known in the paleontology field as the temporal paradox, has been dealt another blow by the reassessment of the A. huxleyi, which is dated to about 155 million years old—about 30 million years before the feathered dinosaur Microraptor and about five million years before the oldest known bird, Archaeopteryx. This earlier date for the emergence of feathered dinosaurs undermines claims that birds lacked enough time to evolve from dinosaurs.
In addition, the research "sheds new light on the early evolution of feathers and demonstrates the complex distribution of skeletal…features close to the dinosaur–bird transition," the paper authors write [pdf] in the letters section of Nature this week (Scientific American is part of the Nature Publishing Group).
Adding lift to the theory that these animals once flew or glided with all four wings, the A. huxleyi appears to have had several contour features (aka pennaceous feathers)—found on modern birds—on its hind legs. The authors of the most recent paper, to which Xu also contributed, note that this supports the idea that feathers developed first on the tail region of dinosaurs and spread later to the forewings, before disappearing from the legs of contemporary birds.
Both specimens were unearthed in the Tiaojishan Formation in Liaoning, China. The small dinosaur originally described in the Chinese Science Bulletin measures about 34 centimeters long and weighed about 110 grams. Although it might have played a part in ushering in a brave new world for birds, the A. huxleyi is named not for the famed science-fiction author, but for Thomas Henry Huxley, an early advocate of evolution and one of the first people to suggest that birds may indeed be descended from dinosaurs.
Images courtesy of Xing Xu et al.