A computer-generated picture that restores the image of Gigantoraptor erlianensis, a new species of bird-like dinosaur living about 85 million years ago. Illustration by Zhao Chuang and Xing Lida.
It had a small head, a beak, slim hind legs, a massive body, the size of a Tyrannosaurus, and was covered with feathers. This is the new species of dinosaur that lived on the plains of North China 85 million years ago. It looked more like a bird, but it could not fly. But it could run like the wind.
The fossil specimens of Gigantoraptor erlianensis, which were unearthed in the Inner Mongolia Autonomous Region two years ago, are adding a new piece to the evolution puzzle of birds, according to Chinese paleontologists who reported their groundbreaking discovery in Nature magazine today.
"It is unique because the dinosaur has a bird's features, but a much larger body than other known bird-like dinosaur species living earlier or later than it," said Xu Xing, a leading paleontologist at the Institute of Vertebrate Paleontology and Paleonanthropology, of the Chinese Academy of Sciences. Xu and his fellow researchers at the Longhao Institute of Geology and Paleontology in Inner Mongolia conducted the research.
The discovery of Gigantoraptor is a vital clue to better understanding how dinosaurs evolved into birds, according to Xu, a famous dinosaur fossil hunter.
Fossil specimens suggest the giant beast was 8 meters long, 5 meters high, and weighed about 1.4 tons.
"We thought it could be a type of Tyrannosaurus, which is known for its huge size," recalled Xu.
But further comparative research revealed that Gigantoraptor actually belonged to Oviraptorosauria, a type of dinosaur living in the Late Cretaceous (about 65.5 to 99 million years ago). Featuring a beak-like mouth and sharp claws, Oviraptorosauria was a close relative of the bird family.
Though the paleontologists found no direct evidence of feathers on the fossil specimen, they infer from the dinosaur's close relationship to the other feathered species, such as Caudipteryx, that Gigantoraptor probably had feathers on its body, at least on its arms and tail.
"The feathers could have been used for ostentation, or temperature conservation while it hatched eggs," Xu said. He added fossilized feathers would not have been able to be preserved in the alluvial environment, where fossils were found.
However, previously known members of Oviraptorosauria were usually small and light and some weighed less than than 1 kilogram. While Gigantoraptor was nearly 300 times as heavy as the Caudipteryx and Protarchaeopteryx, species of the Oviraptorosauria family, both of which had feathers. Gigantoraptor's huge size challenges old theories.
It is a unanimously held theory that birds evolved from dinosaurs. Paleontologists used to believe that the evolution of the bird's features was related to the size of dinosaurs - a dinosaur of a smaller size must have had more features of a bird; while a larger dinosaur would have lost many of its bird features.
"But this new dinosaur is an exception. It has more features than other smaller dinosaurs of the same family, such as longer forelimbs," Xu said.
He added that Gigantoraptor lived between the time when Oviraptorosauria emerged about 130 million years ago and when it reached its population peak about 70 million years ago. However the earliest birds emerged around 150 million years ago.
The discovery means the evolutionary path leading dinosaurs towards birds was actually more varied and complex than previously thought. The evolutionary path was a spiral shape rather than a straight line. The discovery also convinced paleontologists they could find other lost links in the puzzle - bird-like dinosaurs of different sizes.
Gigantoraptor is also unique in other ways compared with ordinary dinosaurs. The paleontologists cannot explain functions of small holes found in the caudal vertebrae, located on near the tail joint. This feature has never been seen on other dinosaurs before.
Its unusual limb proportions are also baffling. Usually, when dinosaurs grow larger, the limbs become larger. But Gigantoraptor had slimmer hind limbs and more slender calves compared with dinosaurs of similar size. It indicated an ability to run quickly, like an African ostrich or an Australian emu.
The research team uncovered about 50 fossil samples, equivalent to almost 80 percent of the dinosaur, in sedimentary rocks of the Late Cretaceous in Erlian Basin, which is famous for its abundance of dinosaur fossils. They estimate the dinosaur was 11 years old and had just matured. An adult Gigantoraptor could grow much heavier than this one, they said.
After two years of analyses, Xu and his colleagues made a preliminary conclusion that Gigantoraptor should have had a growth rate faster than most other theropods, including the large North American Tyrannosaurs made famous in the Jurrasic Park films.
X-ray scanning also showed that the posterior caudal centra of this dinosaur had spongy internal structures, similar to those of the sauropod group Titanosauria, probably related to saving weight.
Further research is still needed to reveal the diet of Gigantoraptor. "Its small head and long neck indicate it is a vegetarian, however it belongs to a family of carnivore," Xu said.
The function of the beak of Gigantoraptor is also not fully understood. It could be used to cut plants, knock out eggshells, squeeze open clams or pull seeds, the paleontologist said.
Before the discovery of Gigantoraptor, the largest feathered animal was Stirton's Thunder Bird (Dromornis stirtoni) that weighed 500 kilograms and lived in Australia 6 to 8 million years ago.
But Gigantoraptor has broken the record, being three times as heavy as Thunder Bird.
Now Inner Mongolia's Erenhot, the city where the dinosaur fossils were discovered, is planning to build a dinosaur museum covering an area of 15,000 square meters to display the evolution of dinosaurs.