Its long back legs suggest it was a fast runner. It likely used its muscled shoulders and the enlarged claw on the second toe of each forelimb to grasp its prey. The animal did not fly, but its back legs almost certainly gave it the ability to leap several yards.
Feathers rarely survive fossilization, and the fossil of Buitreraptor showed no evidence of them.
But Sebastian Apesteguía, a professor at Maimonides University in Buenos Aires and a co-author of the paper, is certain that the animal was feathered like its northern cousins.
Buitreraptor's evolutionary relationships, he said, virtually assure this. "If you find the bones of a monkey," he said, "you would not dare to imagine it was bald, because you know that monkeys and their relatives are hairy."
Since Buitreraptor's close relatives had feathers, it certainly did as well, he said.
The find was reported in the October 13 issue of Nature, which includes an artist's rendering of a very birdlike animal covered with small, almost hairlike feathers.
Longer feathers on the forelimbs make them look like stubby wings.
"The artistic reconstruction is based on accurate dimensions of the skeleton of Buitreraptor," said Peter J. Makovicky, the paper's lead author.
"We also chose a fairly drab color scheme, so the poetic license was kept to a minimum," Makovicky, a geologist with Chicago's Field Museum of Natural History, said.
According to Apesteguía of Maimonides University, fossil evidence suggests two theories about the evolution of flight.
The first is that wings may have developed in the common ancestor of birds and raptors.
"So their presence in southern raptors is not a novelty, but an atavisman old feature that persisted in the evolution of the species, like the hind limbs in pythons," he said.
Alternatively, he said, some believe that wings evolved twiceonce in birds and once in the southern raptors, which developed wings from their extremely long forelimbs.
Analysis of the relationship between Buitreraptor and its close cousins favors the hypothesis that wings evolved twice, Apesteguía said, because true wings are not present in Buitreraptor, the oldest and most primitive of the southern dromaeosaurs.
Paul Sereno, professor of paleontology at the University of Chicago, believes the new fossil "is very interesting and highly significant" because it might help answer questions about the origins of flight.
"Flight, or at least some flight capacity, may have arisen more than once," he said.
"This [find] adds to evidence from fossils and recent biomechanical studies on birds that the transition to flight was not night and day, but that there exists lots of useful and adaptive states for what might be termed poor fliersthis new species may have been one of them."
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