Rewriting Evolution

The skeleton of Buitreraptor gonzalezorum in the field. A string of vertebrae and the right shoulder (lower left) and femur are showing. The match-stick shows scale.
Credit: Peter Makovicky, Courtesy of The Field Museum

The recent discovery of a 90-million-year-old dinosaur in Patagonia demonstrates that dromaeosaurs, a group of carnivorous theropods that includes Velociraptor and is closely related to birds, originated much earlier than previously thought. Rather than originating during the Cretaceous, dromaeosaurs can now be traced back to the Jurassic, possibly as far back as 180 million years ago. Meanwhile, the new dinosaur’s birdlike features–its huge, hollow wishbone; long, winglike forelimbs; and bird-like pelvis–provide more evidence linking dinosaurs to birds.

Buitreraptor gonzalezorum is described in the cover story of Nature October 13. It was excavated last year by a team of Argentine and American paleontologists, including Peter Makovicky, curator of dinosaurs at The Field Museum.

"Buitreraptor is one of those special fossils that tells a bigger story about the Earth’s history and the timing of evolutionary events," says Makovicky, lead author of the Nature paper. "It not only provides definitive evidence for a more global distribution and a longer history for dromaeosaurs than was previously known, but also suggests that dromaeosaurs on northern and southern continents took different evolutionary routes after the landmasses they occupied drifted apart."

The paleontological field team was led by Argentine paleontologist Sebastián Apesteguía. He and Federico Agnolin, both affiliated with the Museo 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 the Late Cretaceous," Apesteguía says. "The rich fauna of this area, known as La Buitrera, includes other carnivorous dinosaurs, such as mid-sized abelisaurs and the gigantic Giganotosaurus. However, the most common animals are bulky herbivorous sphenodontids, snakes, terrestrial crocodiles and mammals.

"Except for its faunal composition, La Buitrera resembles the Gobi desert in its abundance of fossils and their exquisite state of preservation," Apesteguía adds.

A cast of the unusual dinosaur is on public display on the upper floor of The Field Museum in the MacDonald’s Preparation Lab. It will be part of Evolving Planet, a permanent exhibition that opens March 10, 2006.

Photo of reconstructed skeleton of Buitreraptor gonzalezorum. The long hindlimbs indicate that the animal was a fast runner. The elongated arms and massive shoulder girdle indicate powerful prey-grasping abilities. Like all other dromaeosaurs, Buitreraptor was armed with an enlarged claw on the second toe of each foot. Credit: © 2005 The Field Museum, Photo by John Weinstein, image# GEO86430_30d

Drifting continents

About 200 million years ago, all of the 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 the north; Gondwana, composed of the southern hemisphere continents plus India, drifted to the south.

Until recently, dromaeosaurs (swift-running, bi-pedal, birdlike dinosaurs) have been found only in Cretaceous rocks of Asia and North America, northern continents that were part of Laurasia. (Laurasian dromaeosaurs include the famous Velociraptor from the Gobi Desert, the large Utahraptor from the American West, and the recently discovered Microraptor and Sinornithosaurus from China, both of which preserve amazing traces of bird-like plumage.) This distribution led scientists to believe that dromaeosaurs originated in Laurasia after it drifted apart from Gondwana.

In the last few years, however, a handful of specimens of possible dromaeosaurs or early birds have been discovered on southern continents. Nevertheless, their incomplete preservation led to some ambiguity and debate regarding their identities.

The new discovery provides definitive evidence that dromaeosaurs also lived in South America, which was part of Gondwana. As a result, dromaeosaurs must have originated when all of the continents were still assembled in a single landmass during the Jurassic as far back as 180 million years ago–much earlier than previously thought.

Results of an analysis of evolutionary relationships of advanced theropods undertaken as part of this research indicate that the Gondwanan dromaeosaurs and Rahonavis, an animal previously considered to be a very primitive bird, actually constitute a separate branch of the dromaeosaurid family tree. This branch is distinct from Velociraptor and other Laurasian dromaeosaurids, including some of the famous feathered dinosaurs from China.

Because Rahonavis has long and wing-like forelimbs, this finding could imply that flight may have evolved twice, once in birds and once among this group of Gondwanan dromaeosaurs. This evolutionary research is part of a larger, ongoing project to assemble the evolutionary family tree of dinosaurs (including birds), and their relatives, by a team of scientists that includes Makovicky and is funded by the National Science Foundation.

Remarkably different dinosaur

Buitreraptor (bwee-tree-rap-tor) is about the size of a very large rooster, but with a long head and very long tail. It is the most complete small theropod (carnivorous dinosaur) ever discovered in South America.

The Buitreraptor gonzalezorum quarry. The holotype specimen was extracted in an 800-pound block of sandstone that took 10 days to extract using a rock-saw and chisels. The picture shows Field Museum preparator Jim Holstein (left) and Argentine paleontologist Pablo Gallina at work. Temperatures soared above 100 F on most days. Fieldwork was supported by NASA, The Jurassic Foundation, and Michael and Jacqueline Ferro of Chicago.
Credit: Peter Makovicky, Courtesy of The Field Museum

Buitreraptor is remarkably different than other domaeosaurs, most obviously due to its long, slender snout and relatively small, widely spaced teeth. Unlike most other theropods, the teeth of Buitrearaptor lack the steak-knife-like serrations along their edges. Although scientist are unsure why the animal evolved such peculiar head proportions and unusual dentition, it may have been an adaptation to hunt small prey, such as the abundant burrowing snakes, mammals, and lizards that have been discovered alongside Buitreraptor.

The remarkable dinosaur was discovered in northwestern Patagonia about 700 miles southwest of Buenos Aires. Although Buitreraptor is rather small, the paleontological team needed 10 days to chisel out the 800-pound slab of rock containing the fossil, which was subsequently prepared at the Museo Argentino de Ciencias Naturales in Buenos Aires and The Field Museum. The fieldwork, in rugged terrain, was supported by funding from NASA, the Jurassic Foundation, and Michael and Jacqueline Ferro of Chicago. The Agencia Cultura of Río Negro Province loaned the fossil to The Field Museum.

The holotype, or definitive, fossil of this adult dinosaur is in excellent condition: articulated and nearly complete. An additional partial skeleton discovered on an earlier expedition led by Apesteguía helped fill in missing bones. Since discovering the first two Buitreraptor fossils, the scientists have discovered at least two more in the same area during fieldwork conducted in January 2005.

"Although Buitreraptor is a recent discovery, we already have a very good sample of this remarkable dinosaur," Makovicky says. "A growth series of Buitreraptor individuals would allow us to study how the animal grew and how its proportions may have changed with growth information that may be useful to understanding the changes in body proportions that led to the origin of birds and flight."