A turkey-size, carnivorous dinosaur that looked something like a diminutive but malevolent version of Big Bird from “Sesame Street” is changing scientists’ thinking about when and where some of the closest relatives to birds evolved.
Buitreraptor is named for La Buitrera, an area of the Patagonian desert in Argentina where paleontologists found an almost complete skeleton of the 90-million-year-old, probably feathered creature in January 2004. It is described in Thursday’s issue of the science journal Nature.
The fossil buitreraptor (pronounced bwee-trey-rap-ter) conclusively proves that a family of small, swift dinosaurs called dromaeosaurs evolved tens of millions of years earlier than previously believed, said Field Museum paleontologist Peter Makovicky, the article’s lead author.
Until now, scientists believed that dromaeosaurs–meat-eaters that include the famous velociraptor–evolved in the Northern Hemisphere after the world’s land mass split into two supercontinents, Gondwanaland and Laurasia, 135 million years ago.
The presence of buitreraptor in South America proves dromaeosaurs evolved before that split–140 million to 145 million years ago–and spread throughout the world.
“We can now push the history of dromaeosaurs as far back as the first birds, like archaeopteryx,” Makovicky said.
Close relation to birds
Buitreraptor and dinosaurs like it are not the ancestors of birds–they lived simultaneously–but they are closely related.
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“Dromaeosaurs are really interesting because they are so close to birds in evolution, having feathers and other, similar attributes,” said American Museum paleontologist Mark Norell, who called the work by Makovicky and two Argentine colleagues an important discovery.
Dromaeosaurs are a subgroup of theropods, two-legged carnivorous dinosaurs that also include Tyrannosaurus rex. Buitreraptor has the characteristic wishbone, birdlike pelvis and long, winglike forelimbs of a dromaeosaur.
“It is the most complete small theropod ever found in South America,” said Makovicky.
Yet buitreraptor is also something of an odd duck. Its skull and jaws are more slender and less powerful than other known dromaeosaurs, and its relatively short teeth lack the sharp, serrated edges of most theropod teeth.
“Instead of powerful jaws, it relied more on being able to very quickly open and close its jaws,” Makovicky said. “Other dromaeosaurs and theropods tended to feed on large prey, but this one seems to have eaten small prey, like rat-sized mammals that ran around then, and lizards and the primitive snakes of that time.”
The fossil presents “a very neat evolutionary picture,” Makovicky said, because the presence of dromaeosaurs shows how old that family of animals is, “how it was distributed, how the north and south branches of the family evolved differently in isolation.”
Similar to China fossils
Though the fossil was located in a type of rock formation that could not preserve evidence of feathers, Makovicky said he believed it almost certainly was feathered because of its similarities to two dromaeosaur fossils found in China, microraptor and sinornithosaurus. Those specimens were found in extremely fine limestone beds that preserved impressions of the animals’ extensive plumage.
“It could not fly,” Makovicky said of buitreraptor, “but it may have had some aerodynamic capability,” perhaps being able to glide.
Prior to buitreraptor’s discovery, paleontologists had found bone fragments of animals that some suspected were dromaeosaurs, but the finds were too incomplete to confirm it.
In 2003, Argentine paleontologist Sebastian Apesteguia found a bone sticking out of a rock cliff in Patagonia that he thought might be from a dromaeosaur. In 2004 he invited Makovicky, who specializes in carnivorous dinosaurs, to collaborate with him and another Argentine colleague, Federico Agnolin.
The fossil was embedded in rock so hard that they had to remove a 4-foot-long, half-foot-thick slab from the cliff so the bones could be chipped out in a laboratory. After preliminary work in Argentina, the final preparation was done at the Field Museum in Chicago.
The skeleton is back in Argentina, which owns it, but a replica is at the Field.
The Patagonian region in which it was found is rich with dinosaur fossils. Buitreraptor lived in a forested plain where it and other small animals shared the ecosystem with huge plant-eaters and T. rex-style predators, including the 45-foot-long giganotosaurus, a fossil of which was found 60 miles from the buitreraptor site in 1993.
Buried during flooding
Buitreraptor and other small animals found near it probably died and were buried during seasonal flooding, said Makovicky.
“Those flood events would barely wash the ankles of the big dinosaurs in the forest,” he said. But big animals like giganotosaurus could get caught and killed in deep, raging rivers in the region, with flood debris eventually preserving their bones in rock.
“This area of Patagonia is one of the only places in the world where the fossil record has both big and small fossil animals,” said Makovicky. “They just aren’t together. Each was fossilized in a different depositional mechanism that tended to sort out the size of animals.
“In terms of figuring out the ecosystem it is pretty nice.”
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Dinosaur discovery shifts thoughts on origins
The remains of a small 90-million-year dinosaur discovered in South America indicates that its raptor family originated much earlier than thought. It also provides more evidence linking dinosaurs to birds.
A BIRDLIKE RAPTOR
Name: Buitreraptor gonzalezorum
Unique features: Feathers; large, hollow wishbone; long, winglike
forelimbs; birdlike pelvis; small, widely spaced and unserrated teeth
Diet: Small prey, such as burrowing snakes, mammals and lizards
Where to see it: A cast of the nearly complete skeleton will be at the Field Museum’s McDonald’s Preparation Lab beginning March 10.
DISCOVERY SHEDS LIGHT ON ORIGINS
PERIODS:
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TRIASSIC / JURASSIC / CRETACEOUS
MILLIONS OF YEARS AGO
Previously: Raptors traced to Cretaceous
Raptors now: As far back as 180 million years ago
SIZE COMPARISON
Similar to a very large rooster.
Raptors almost always are found in the northern continents. But because the raptor was found in the Southern Hemisphere, scientists think the dinosaurs were present before the Earth’s land mass divided.
Argentina: Site of discovery
Illustration by Jorge Gonzalez, Field Museum
Sources: Nature, Field Museum, ESRI
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– See microfilm for complete graphic.
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