Raptor Line: Fossil finds push back dinosaur ancestry

October 12, 2005 at 9:38 am

Fossils of a newly discovered dinosaur species unearthed in Argentina suggest that the reptile’s lineage is older and more widespread than previously suspected. The finding might require scientists to remodel parts of the dinosaur family tree.

SNAPPY SNOUT. The long, slender snout of Buitreraptor gonzalezorum hints that the creature preyed on creatures such as lizards and small mammals. J. González © The Field Museum

The creature, dubbed Buitreraptor gonzalezorum, belonged to a group of bipedal, meat-eating dinosaurs called dromaeosaurids, the clan of raptors that includes the darting, humansize velociraptors made famous in the 1993 film Jurassic Park. An adult B. gonzalezorum would have been slender and measured about 1.5 meters from the tip of its snout to the tip of its tail—”like a big rooster with long legs and a long tail,” says Peter J. Makovicky, a vertebrate paleontologist at the Field Museum in Chicago. He and his colleagues describe the dinosaur in the Oct. 13 Nature.

Brothers Jorge and Fabián González, Argentine paleontologists for whom the species was named, found the original remains of the small dinosaur. Overall, researchers have found four specimens, all far south of where any indisputable dromaeosaurid remains have ever been discovered. One particularly well-preserved skeleton is missing only some hand, foot, and tail bones.

Previously unearthed fossils suggested that dromaeosaurids lived in the Southern Hemisphere, but the new find seals the case, says Makovicky. If the earlier finds were dromaeosaurids, B. gonzalezorum is older, by millions of years, than any of this group found south of the equator. The newfound fossils were excavated from rocks that lie beneath, and so must be older than, a layer of volcanic material laid down about 90 million years ago.

B. gonzalezorum differs from its kin found in the Northern Hemisphere. For instance, its skull is between 25 and 33 percent longer than its femur, the upper leg bone, whereas its Asian and North American cousins typically have skull and femur measurements that approximately match. The long, slim snout suggests that B. gonzalezorum preyed on small animals that didn’t put up much of a fight. Fossils of potential victims such as lizards and small mammals are abundant where B. gonzalezorum was unearthed, says Makovicky.

When the dinosaur strolled across the South American landscape, that continent already had long since split from the landmasses that became Asia and North America. Dromaeosaurids’ presence on all of those continents suggests that the group’s common ancestor lived on the Gondwana supercontinent when it was still intact, at least 145 million years ago.

The new find indicates that dromaeosaurids were far more widespread than previously recognized, says Mark A. Norell, a paleontologist at the American Museum of Natural History in New York.

The discovery thus could require researchers to rearrange the dinosaur family tree. Many features of B. gonzalezorum are similar to those of Rahonavis ostromi, a small, birdlike dinosaur from Madagascar. Those commonalities might prompt paleontologists to move Rahonavis from the primitive-bird branch of the dinosaur family tree into the dromaeosaurid group, says Thomas R. Holtz Jr. of the University of Maryland, College Park.

Dromaeosaurids had the same joint arrangement as did early birds such as Archaeopteryx, Holtz adds. Although B. gonzalezorum probably couldn’t have sustained powered flight, “it’s not out of the realm [of possibility] that a common ancestor of Archaeopteryx and dromaeosaurs could have been a flyer,” he notes.