The fossil of a raven-sized bird with a slashing claw like that found on meat-eating velociraptors provides the most compelling evidence yet of the highly controversial argument that modern-day birds are descended from dinosaurs, researchers said Tuesday.
The 65-million-year-old fossil, dating from the end of the age of the dinosaurs, was found on the island of Madagascar, where it represented probably the last of its breed. It has long forearms with evidence of well-developed feathers, indicating that it was a capable flier.
But unlike modern birds, it also had a long, bony tail and the sickle-like killing claw at the end of its second toe, both of which link it to the group of dinosaurs called therapods.
"This new fossil is one of the strongest last nails in the coffin of those who doubt that dinosaurs had anything to do with the origin of birds," said paleontologist Catherine Forster of the State University of New York at Stony Brook, lead author of a paper that will appear in the journal Science on Friday.
The discovery is "nothing short of amazing," said paleontologist Paul Sereno of the University of Chicago, because the find so closely resembles the flying dinosaur archaeopteryx, which lived 80 million years earlier. "To find something so close to archaeopteryx surviving . . . on a lonely island millions of years after its kind has disappeared [elsewhere] is simply remarkable."
In effect, the creature would have been a living fossil in its own time, experts said, much as the primitive deep-sea fish called the coelacanth is today considered a living fossil.
A different team of scientists is expected to report Thursday in the journal Nature another fossil find that also places dinosaurs directly in the evolutionary history of birds.
The two papers taken together, Sereno said, "provide a very convincing argument" that birds are descended from dinosaurs. "Now it is going to be up to other scholars in the field to prove them wrong."
Critics, however, think that the Forster team has inadvertently combined pieces from two different fossils--one from a dinosaur, one from a bird--to make an erroneous identification.
"I think it's a chimera--a little dinosaur hindquarter with a bird's forelimbs," paleontologist John Ruben of Oregon State University in Corvallis told Science.
Forster's group, however, argues that the hind limbs are clearly bird legs, possessing such birdlike traits as an opposable big toe (which allowed it to grasp onto tree branches) and a small fibula, or lower leg bone.
The heated debate about the link between birds and dinosaurs has gone on for nearly a decade. Proponents base their claim on the many similarities between the bone structures of dinosaurs, such as deinonychus, which stand on two feet, and those of modern birds. Some dinosaurs have been found to be feathered.
Evidence reported three years ago also showed that dinosaurs sat on their nests to keep their eggs warm, a forerunner of the brooding behavior exhibited by birds today.
But other researchers, such as Ruben and biologist Alan Feduccia of the University of North Carolina, argue that other evidence supports the independent evolution of birds. Feduccia has found that birds lack the embryonic thumb that was present in dinosaurs, making it unlikely that the two species are related.
Researchers have therefore been looking for fossils that might settle the argument by exhibiting traits of both dinosaurs and birds. The new results are "a wonderful example of how the fossil record provides the basic data for formulating, testing, and revising ideas about life through time," said Chris Maples of the National Science Foundation.
The fossil was discovered in 1995 by Forster and paleontologist Scott Sampson of the New York College of Osteopathic Medicine in Old Westbury. They found a long, slender, lower wing bone with quill knobs for the attachment of feathers. It was near, but not attached to, several hind limb bones similar to those of archeopteryx.
The finding looked so promising that they chopped out a 60-pound block of stone containing the fossil and shipped it home to New York. It was there that they discovered the slashing claw so characteristic of the meat-eating therapods.
Paleontologists have long suspected that therapods gave rise to birds, Forster said, and the presence of this dinosaur-like claw and toe "really clinches it for us. This discovery lends a lot of weight to the idea that birds are a side-branch of the therapod family tree."
The team named the fossil Rahona ostromi. Rahona is the Malagasy word for menacing cloud, while ostromi honors paleontologist John Ostrom of Yale, one of the earliest proponents of the idea that birds descended from dinosaurs.
The bird's pelvic and pubic bones strongly resembled those of archeopteryx and other very early birds, making the new find appear very primitive for its time in the late Cretaceous, when more modern birds had already begun to appear.
Researchers attribute this primitiveness to the bird's isolation on Madagascar, where it was preserved out of the direct evolutionary line of birds. While other bird ancestors on the mainland continued to evolve, Rahona must have remained stable in its safe biological niche, Forster speculated.