Earliest Bird Had Feet Like Dinosaur, Fossil Shows
for National Geographic News
|December 1, 2005|
A 150-million-year-old fossil of Archaeopteryx, long considered the oldest bird, may put to rest any scientific doubt that dinosaursspecifically the group of two-legged meat-eaters known as theropodsgave rise to modern birds.
Until recently, the crow-size specimen was housed in a private collection. It is now owned by the Wyoming Dinosaur Center in Thermopolis.
The fossil is the ninth example of Archaeopteryx known to science. (A tenth specimen of a winged dinosaur from a closely related genus also exists.) All ten fossils were found in a limestone deposit near Solnhofen, Germany.
The latest specimen is among the best preserved. It is a slightly broken skeleton in a single slab of pure limestone, showing clear wing- and tail-feather impressions.
The skull is the only Archaeopteryx specimen that reveals a bird's-eye view of the species' upper head surface.
"The skull of the new specimen is the best preserved one of an archaeopterygid," said Gerald Mayr, a lead study author and prehistoric bird expert at the Senckenberg Institute for Research in Frankfurt, Germany.
"[It] presents important new details of the skull morphology [shape and function] of the earliest known bird," he said, "showing also that the skull of Archaeopteryx is much more similar to that of nonavian theropod dinosaurs than previously thought."
Joel Cracraft, curator of birds at the American Museum of Natural History and adjunct professor at Columbia University in New York, believes the paper presents a very convincing case.
"This pretty much puts the final nail in the coffin for all those people resisting the idea that birds are related to theropod dinosaurs," he said. Cracraft was not involved in the study.
Of Feet and Toes
The animal's feet, both of them perfectly preserved, attracted the researchers' particular attention.
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.
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.
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.
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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