Archaeopteryx, the fossil shows, had a hyperextendible second toe. Until now, the feature was thought to belong only to the species' close relatives, the deinonychosaurs. (The name means "fearsome claws." The deinonychosaur Velociraptor wielded switchblade-like examples of these talons in the movie Jurrasic Park.)
Contrary to all previous reconstructions of Archaeopteryx, the hind toe of the new specimen is not completely reversed to form a "perching" foot as it is in modern birds.
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The researchers believe that the fully reversed hind toe in other Archaeopteryx fossils shifted during preservation and never existed in the live animal.
In the new fossil, the foot looks more like that of the four-toed foot of Velociraptor and its other nonwinged theropod relatives. The specimen clearly lacks a reversed toe.
Because Archaeopteryx lacked this stabilizing toe, it almost certainly did not habitually perch in trees.
Land-Based Predator
Mayr, the study author, notes that the discovery that Archaeopteryx did not have a reversed toe "may also be important for future interpretations of its way of living."
Cracraft agrees. "The thing that's really nice about this paper is the whole discussion of the position of the first digit on the foot, which previous specimens suggested was reversed," he said.
"If their interpretation of that first digit is correct, that it's forward and not reversed, then that suggests that Archaeopteryx was much less arboreal [tree-dwelling] than previous interpretations, and that it was more a terrestrial predator."
The shape and articulation of other bones of the new specimen also help tie Archaeopteryx to the theropods.
The bones of its hind legs, for example, have played an important role in the dispute about bird ancestry. The new Archaeopteryx specimen shows a clearly visible hind leg bone structure that is identical to that of theropod dinosaurs.
Dispute Settled
Archaeopteryx, therefore, is closely related to the theropods. This in turn means that theropod dinosaurs are the ancestors of the modern birds that followed Archaeopteryx.
The find, according to Mayr, "not only provides further evidence for the theropod ancestry of birds, but it blurs the distinction between basal [the earliest] birds and basal deinonychosaurs," their fearsome-clawed ancestors.
"I do think that the question of a theropod ancestry of birds can now be considered settled once and forever," Mayr said.
A paper describing the fossil appears in tomorrow's issue of the journal Science.
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