By Matt Kaplan of Nature magazine
Analysis of fossil traits suggests that Archaeopteryx is not a bird at all. The latest discovery of a fossil that treads the line between birds and non-avian dinosaurs is leading paleontologists to reassess the creature that has been considered the evolutionary link between the two.
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Archaeopteryx has long been placed at the base of the bird evolutionary tree. It has traits that have helped to define what it is to be a bird, such as long and robust forelimbs. Yet in recent years, the discoveries of numerous small, feathery dinosaurs have created a conundrum for paleontologists, raising questions about which animals are the ancestors of modern birds and which are just closely related cousins.
The fossil that is driving the latest Archaeopteryx rethink is called Xiaotingia zhengi, and is described in Nature today by Xing Xu, a paleontologist at the Institute of Vertebrate Paleontology and Paleoanthropology 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 analyzing 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.
Out of first place
The first Archaeopteryx specimen was discovered in 1861, just a few years after the publication of Charles Darwin's On the Origin of Species. Its combination of lizard-like and avian features made it the ideal 'missing link' with which to demonstrate evolution from non-avian dinosaurs to birds. But the latest rearrangement knocks it from its position as the earliest bird. "I think Archaeopteryx's placement was the result of both history and relatively poor sampling at the dinosaur-bird transition," explains Xu.
Even so, he acknowledges that the move is bold. "Because it has held the position as the most primitive bird for such a long time, I am kind of nervous about presenting this result," says Xu. But immediate responses from others in the field suggest that the decision will be widely embraced.
Archaeopteryx was a bird because it had feathers and nothing else had them. But then other animals started being found that had wishbones, three-fingered hands and feathers. Heck, even T. rex had a wishbone. So one by one we've learned Archaeopteryx 's uniquely avian traits weren't so unique. The writing was really on the wall," says Lawrence Witmer, a paleontologist at Ohio University in Athens.
Whether this change will be permanent depends on what other animals are discovered in the future, says Thomas Holtz, a paleontologist 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."
This article is reproduced with permission from the magazine Nature. The article was first published on July 27, 2011.