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The fossil that is driving the latest Archaeopteryx rethink is called Xiaotingia zhengi, and is described in Nature today1 by Xing Xu, a palaeontologist at the Institute of Vertebrate Palaeontology and Palaeoanthropology in Beijing, and his colleagues. It was found in western Liaoning, China, in rocks dating to the Late Jurassic epoch, 161 million–145 million years ago. Like many similar fossils, it is surrounded by feather impressions in the rock, but has claws on the ends of its forelimbs and sharp teeth.
These traits by themselves do little to help place the fossil in the dinosaur–bird transition, but Xu reports that it also has extremely long middle and last finger bones and a wishbone with an L-shaped cross-section at one end. These characteristics, Xu argues, identify Xiaotingia as very closely related to Archaeopteryx and another feathery relative, Anchiornis.
After analysing the traits present in Xiaotingia and its relations, Xu and his colleagues are suggesting that the creatures bear more resemblance to the dinosaurs Velociraptor and Microraptor than to early birds, and so belong in the dinosaur group Deinonychosauria rather than in the bird group, Avialae. Many features led the team to this decision, but the most immediately noticeable are that Xiaotingia, Archaeopteryx and Anchiornis have shallow snouts and expanded regions behind their eye sockets. Microraptor has similar traits, but the early birds in Avialae have very different skulls.
However, the article (written in 2011) also warns that we should not be quick to make snap judgments about these sorts of discoveries:
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Whether this change will be permanent depends on what other animals are discovered in the future, says Thomas Holtz, a palaeontologist at the University of Maryland in College Park. "I don't think this is going to be the last word on this subject. You take this new Chinese species out of the mix and the argument falls apart, so the new placement is precarious at best until further evidence is dug up."
It's very interesting, don't get me wrong, but the issue is far from settled. Hence, my original claim that there is "spirited debate" amongst the community regarding the origins of Avialae.