Artist's reconstruction of Gigantoraptor with a much smaller feathered ornithomimid.

Credit: Zhao Chuang and Xing Lida/IVPP.

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SYDNEY: The fossil of an enormous "chicken-like" dinosaur has been discovered in China. The one-and-a-half-tonne beaked behemoth is the largest bird-related dinosaur yet discovered.

Several times the height of an average human, the five-metre-tall Gigantoraptor presents a conundrum; some of its features have never been seen before in any dinosaur, said scientists from Beijing, China, who describe the find today in the U.K. journal Nature.

The find adds further complexity to the family tree of the theropods, a group of primarily carnivorous dinosaurs, including Tyrannosaurus rex and the Velociraptor. Although the researchers have grouped the new species with oviraptorosaurs, a subgroup of the bipedal theropods, Gigantoraptor was at least 35 times heavier than other dinosaurs in this family.

Teen angst

"If you saw a mouse as big as a pig you would be very surprised – it is the same when we found the Gigantoraptor," said Xing Xu, co-author of the study and a palaeontologist at the Chinese Academy of Sciences in Beijing. "It is an unexpected discovery."

The remains of the 85 million-year-old dinosaur were found in the Gobi desert of northern China, and seem to belong to an adolescent, said the researchers, indicating that the full-grown creature could have been even bigger.

Like living birds, Gigantoraptor was bipedal, had a beak but no teeth, long arms and thin legs. Its close relationship to other feathered dinosaurs hints that it would also have been feathered, if only on its arms and tail.

Weird features of Gigantoraptor, never seen in other dinosaurs, include a strangely bowed humerus (the bone running from shoulder to elbow), an oddly-shaped lower jaw and holes in its tail vertebrae. The function of these holes remains unknown, said Xing.

Another interesting feature is its slim "chicken-like" legs. The researchers noted that large dinosaurs – particularly bipedal ones – normally have short, stout lower legs. Gigantoraptor, in contrast, had more slender, longer legs.

This suggests that it could also have been the fastest dinosaur on two legs, said Xing.

"Truly scary beast"

"Gigantoraptor is truly a scary beast," commented Patricia Vickers-Rich, a palaeontologist at Monash University in Melbourne, Australia. "It was likely an open pursuit predator that would have been difficult to escape," she said.

"This [find] shows that while much has been learned about dinosaurs in recent decades, much more is still no doubt out there to be found," commented Tom Rich, a palaeontologist from Museum Victoria also in Melbourne.

Skeletal reconstruction showing preserved elements of Gigantoraptor (in white) with a 175cm-tall man for a scale (Credit: Li Rongshan/IVPP). Download File

Xing discovered the first femur of the Gigantoraptor by accident in 2005 during the shooting of a documentary about one of his previous finds. The crew asked him to demonstrate how he finds fossils and during the shot, he chanced upon a section of the femur, or thighbone.

"We randomly picked up a bone on the surface," said Xing. "We initially thought it was from the same species as we had discovered before, but minutes later we realised it was from a meat-eating dinosaur… this big size is very unusual for a meat-eater and only a Tyrannosaur is of similar size."

Xing is one of the world's leading fossil hunters and has discovered more than 20 new species at various sites across China.