Scientists studying the evolutionary process of birds are in a flap after discovering the skeletal remains of a giant, bird-like dinosaur that stood as tall as the Tyrannosaurus rex.
Skeletal remains of the feathered but flightless big bird were found in the Gobi desert in Inner Mongolia, China, officials announced on Wednesday.
The fossilized bones suggest the new species, believed to be the biggest toothless dinosaur ever found, was about eight metres long, stood five metres tall and weighed about 1,400 kilograms.
Called Gigantoraptor elrianensis, the bird had a beak and slender legs, making it 35 times larger than its likely close relation, the Caudiperyx, a small, feathered dinosaur species.
The Gigantoraptor lived about 85 million years ago. Scientists haven't determined if the dinosaur was a herbivore, which had small heads and long necks, or a carnivore, which had sharp claws. The dinosaur has both, said Xu Xing, a paleontologist at the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing.
The discovery challenges the evolutionary theory that dinosaurs became smaller as they evolved into birds and bigger dinosaurs, Xu said. Scientists also thought bigger dinosaurs had less bird-like characteristics.
"This is like having a mouse that is the size of a horse or cow," said Xu, who co-authored a paper on the finding published Thursday in the journal Nature.
"It is very important information for us in our efforts to trace the evolution process of dinosaurs to birds. It's more complicated than we imagined."
The Gigantoraptor and Caudiperyx belong to a group of dinosaurs called oviraptors, which tend to be human-sized or smaller.
Paleontologists have found turkey-sized, feathered representatives of the group in recent years, but they've never found anything close to the scale of Gigantoraptor.
"It's one of the last groups of dinosaurs that we would expect to be that big," said Mark Norell, curator of paleontology at the American Museum of Natural History in New York City.
A paleontologist at the University of Alberta, however, said the size of the Gigantoraptor would be a natural step in the evolutionary process of the oviraptors.
"Almost every group that has evolved has tended to evolve giant forms," said Philip Currie.
Animals tend to become bigger with evolution because larger creatures have an easier time getting food. Their size allows them to avoid predators and helps them attracts more potential mates.
But size has disadvantages, too, as bigger animals need more food and territory. Large species have fewer offspring and reproduce less frequently than smaller animals.
Large animals are also particularly vulnerable when environmental conditions change. Just a few million years after Gigantoraptor evolved, every dinosaur species on Earth became extinct.
Scientists who made the discovery originally thought the fossils belonged to a tyrannosaur because of its size, but a closer look revealed it was an oviraptor.
"It was an unexpected finding," Xu said.
The area where the bones were found has attracted paleontologists from around the world for a long time because of the richness of its fossils.
With files from The Associated Press
