CHICAGO--The recent discovery of a90-million-year-old dinosaur in Patagonia demonstrates thatdromaeosaurs, a group of carnivorous theropods that includesVelociraptor and is closely related to birds, originated much earlierthan previously thought. Rather than originating during the Cretaceous,dromaeosaurs can now be traced back to the Jurassic, possibly as farback as 180 million years ago. Meanwhile, the new dinosaur's birdlikefeatures--its huge, hollow wishbone; long, winglike forelimbs; andbird-like pelvis--provide more evidence linking dinosaurs to birds.
Buitreraptor gonzalezorum is described in the cover story of NatureOctober 13. It was excavated last year by a team of Argentine andAmerican paleontologists, including Peter Makovicky, curator ofdinosaurs at The Field Museum.
"Buitreraptor is one of those special fossils that tells a bigger storyabout the Earth's history and the timing of evolutionary events," saysMakovicky, lead author of the Nature paper. "It not only providesdefinitive evidence for a more global distribution and a longer historyfor dromaeosaurs than was previously known, but also suggests thatdromaeosaurs on northern and southern continents took differentevolutionary routes after the landmasses they occupied drifted apart."
The paleontological field team was led by Argentine paleontologistSebastián Apesteguía. He and Federico Agnolin, both affiliated with theMuseo Argentino de Ciencias Naturales and the Fundación Félix de Azara,are co-authors of the Nature paper.
"The preservation of Buitreraptor is superb,and the rock layer it comes from represents the oldest interval of theLate Cretaceous,"Apesteguíasays. "The rich fauna of this area, known as La Buitrera, includesother carnivorous dinosaurs, such as mid-sized abelisaurs and thegigantic Giganotosaurus. However, the most common animals are bulkyherbivorous sphenodontids, snakes, terrestrial crocodiles and mammals.
"Except for its faunal composition, La Buitrera resembles the Gobidesert in its abundance of fossils and their exquisite state ofpreservation,"Apesteguía adds.
A cast of the unusual dinosaur is on public display on the upper floorof The Field Museum in the MacDonald's Preparation Lab. It will be partof Evolving Planet, a permanent exhibition that opens March 10, 2006.
About 200 million years ago, all ofthe Earth's land was amassed in one supercontinent called Pangaea.During the Middle and Late Jurassic, Pangaea split into two landmasses.Laurasia, composed of North America, Asia and Europe, drifted to thenorth; Gondwana, composed of the southern hemisphere continents plusIndia, drifted to the south.
Until recently, dromaeosaurs (swift-running, bi-pedal, birdlikedinosaurs) have been found only in Cretaceous rocks of Asia and NorthAmerica, northern continents that were part of Laurasia. (Laurasiandromaeosaurs include the famous Velociraptor from the Gobi Desert, thelarge Utahraptor from the American West, and the recently discoveredMicroraptor and Sinornithosaurus from China, both of which preserveamazing traces of bird-like plumage.) This distribution led scientiststo believe that dromaeosaurs originated in Laurasia after it driftedapart from Gondwana.
In the last few years, however, a handful ofspecimens of possible dromaeosaurs or early birds have been discoveredon southern continents. Nevertheless, their incomplete preservation ledto some ambiguity and debate regarding their identities.
The new discovery provides definitive evidence that dromaeosaurs alsolived in South America, which was part of Gondwana. As a result,dromaeosaurs must have originated when all of the continents were stillassembled in a single landmass during the Jurassic as far back as 180million years ago--much earlier than previously thought.
Results of an analysis of evolutionary relationships of advancedtheropods undertaken as part of this research indicate that theGondwanan dromaeosaurs and Rahonavis, an animal previously consideredto be a very primitive bird, actually constitute a separate branch ofthe dromaeosaurid family tree. This branch is distinct fromVelociraptor and other Laurasian dromaeosaurids, including some of thefamous feathered dinosaurs from China.
Because Rahonavis has long and wing-like forelimbs, this finding couldimply that flight may have evolved twice, once in birds and once amongthis group of Gondwanan dromaeosaurs. This evolutionary research ispart of a larger, ongoing project to assemble the evolutionary familytree of dinosaurs (including birds), and their relatives, by a team ofscientists that includes Makovicky and is funded by the NationalScience Foundation.
Remarkably different dinosaur
Buitreraptor(bwee-tree-rap-tor) is about the size of a very large rooster, but witha long head and very long tail. It is the most complete small theropod(carnivorous dinosaur) ever discovered in South America.
Buitreraptor is remarkably different than other domaeosaurs, mostobviously due to its long, slender snout and relatively small, widelyspaced teeth. Unlike most other theropods, the teeth of Buitrearaptorlack the steak-knife-like serrations along their edges. Althoughscientist are unsure why the animal evolved such peculiar headproportions and unusual dentition, it may have been an adaptation tohunt small prey, such as the abundant burrowing snakes, mammals, andlizards that have been discovered alongside Buitreraptor.
The remarkable dinosaur was discovered in northwestern Patagonia about700 miles southwest of Buenos Aires. Although Buitreraptor is rathersmall, the paleontological team needed 10 days to chisel out the800-pound slab of rock containing the fossil, which was subsequentlyprepared at the Museo Argentino de Ciencias Naturales in Buenos Airesand The Field Museum. The fieldwork, in rugged terrain, was supportedby funding from NASA, the Jurassic Foundation, and Michael andJacqueline Ferro of Chicago. The Agencia Cultura ofRío Negro Province loaned the fossil to The Field Museum.
The holotype, or definitive, fossil of this adult dinosaur is inexcellent condition: articulated and nearly complete. An additionalpartial skeleton discovered on an earlier expedition led byApesteguíahelped fill in missing bones. Since discovering the first twoBuitreraptor fossils, the scientists have discovered at least two morein the same area during fieldwork conducted in January 2005.
"Although Buitreraptor is a recent discovery, we already have a verygood sample of this remarkable dinosaur," Makovicky says. "A growthseries of Buitreraptor individuals would allow us to study how theanimal grew and how its proportions may have changed with growth --information that may be useful to understanding the changes in bodyproportions that led to the origin of birds and flight."
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