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Birdlike Dinosaur Older Than Thought

Buitreraptor



BY JORGE GONZALEZ; COPYRIGHT THE FIELD MUSEUM
A rooster-size dinosaur named Buitreraptor gonzalezorum provides solid evidence that a group of theropods known as dromaeosaurs originated at least 20 million years earlier than previously thought. Not only does the find indicate that the group got its start on the supercontinent Pangaea before it split in two, but it suggests that birdlike flight may have evolved twice on two separate supercontinents.

"Buitreraptor is one of those special fossils that tells a bigger story about the earth's history and the timing of evolutionary events," says Peter Makovicky, curator of dinosaurs at the Field Museum in Chicago and lead author of a paper in the October 13 issue of Nature describing the species. Makovicky dated the new species--whose name comes from the Spanish word buitrera, for "vulture roost" and gonzalezorum, after the brothers Fabi¿n and Jorge Gonz¿lez, Argentinian archaeologists who discovered it--to 90 million years old. Related swift, bipedal, birdlike dinosaurs, including Utahraptor and Velociraptor, date to around this time, but their skeletal remains have all been found in the American West or China. These far-flung regions were once joined together as the supercontinent Laurasia 150 million years ago.

Until the discovery of Buitreraptor, it did not appear that dromaeosaurs existed on the other supercontinent to the south, Gondwana, which comprised modern-day South America, Africa, India, Antarctica and Australia. Although paleontologists had found a handful of ambiguous remains that hinted at the presence of dinosaurs similar to dromaeosaurs, they had more reasons to believe that the animals evolved on Laurasia long after Pangaea drifted apart.

But the unearthing of Buitreraptor changes everything. Makovicky's team found the near-complete skeleton in a 90-million-year-old sedimentary formation in the Patagonia region of Argentina, where paleontologists have unearthed the remains of other ancient creatures, including giant carnivorous dinosaurs, stocky herbivores, lizardlike reptiles, snakes and small mammals. After they compared it with dromaeosaurs from Laurasia and with primitive birdlike animals from Gondwana, they came to some startling conclusions.

Although many characteristics of the skeleton are similar to those found in other dromaeosaurs--including the shape of the skull, features of the spine, limbs, an enlarged claw on the second toe of each foot--other aspects set it apart. It has a longer, more slender snout, and small, widely spaced teeth that lack the serrations found in other theropods. What's more, Buitreraptor has a long tail like that seen in the primitive bird Archaeopteryx, found in Europe and it shares some pelvic and hind limb characteristics of a birdlike creature known as Rahonavis, which was discovered in modern-day Madagascar. Although experts have suggested that Rahonavis is a primitive bird, its clawed second toe--more characteristic of dromaeosaurs--has been a source of confusion. Buitreraptor indicates that Rahonavis wasn't a primitive bird at all, but a related dromaeosaur.

What it all suggests is that dromaeosaurs originated on Pangaea and then after the mega-continent split, the separated populations each evolved into creatures that exhibited birdlike flight. On Gondwana, they gave way to Rahonavis and on Laurasia, they gave way to Archaeopteryx. To date, the team has collected bones belonging to four Buitreraptor individuals and is hoping to find additional remains to piece together the origin of birds and flight.

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