Chicago Tribune (MCT)
The species, discovered by fossil hunters in Inner Mongolia, could not fly but had avian characteristics including a beak and lightweight, hollow bones. Roughly 25 feet long and more than 16 feet tall, the dinosaur would have towered over its turkey-sized relatives.
The discovery of such a large hybrid adds complexity to the story of how birds evolved from dinosaurs, a gradual shrinking process by which the huge thunder lizards gave way to small, light and feathered creatures. Finding a dinosaur 300 times heavier than similar species underscores the twists and turns that evolution can take.
Reported by a group of Chinese paleontologists in this week's issue of Nature, the discovery also greatly expands the view of diversity during the Late Cretaceous period, when dinosaurs were at their most varied and bird populations began to rise.
"This species is the largest of its kind, and a lot bigger than anyone ever expected these animals to get," said Peter Makovicky, assistant curator of dinosaurs at the Field Museum.
"When animals get big they tend to look less birdlike; for instance, we have no birds that reach a ton or more in body size," said Makovicky, who was not involved in the discovery. "But here, some of the birdlike traits are actually retained or developed even though the animal is very large."
Given the Latin name Gigantoraptor erlianensis, the species roamed the prehistoric continent of Asiamerica roughly 75 to 95 million years ago, late in the dinosaur era.
Xing Xu of the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing and colleagues found the relatively complete fossil used to characterize Gigantoraptor in the Erlian Basin in northeastern China.
The region has attracted paleontologists' attention since the 1920s, Xu said, but several interesting discoveries have been made in the area recently, including the four-winged Microraptor and several other feathered dinosaurs.
By analyzing the state of growth in the fossil's calf bone, researchers determined the dinosaur was relatively young when it died, about 11 years old. That suggests that a full-size Gigantoraptor may be even larger than this specimen indicates.
Gigantoraptor shared gigantic size with dinosaurs of the tyrannosaur family — it is about two-thirds the size of a Tyrannosaurus rex — but it weighed much less, in part due to differences in bone structure.
The T. rex was able to balance on two hind limbs in part because it had bones filled with air pockets in the front of its body. Gigantoraptor, however, possessed these light, hollow bones throughout its body-the same modification that later allowed birds to develop the ability to fly.
The Gigantoraptor fossil also had longer, more slender limbs than is usual for an animal of its size, as well as a toothless "beak" rather than the tooth-filled mouth characteristic of large carnivore dinosaurs. The beak suggests to experts that the species may have had an omnivorous diet of plants, eggs and meat, similar to other dinosaurs with avian features.
While the fossil cannot tell scientists directly whether Gigantosaurus had feathers, researchers agree that it likely did. Recent fossil discoveries revealed that several bird-like dinosaurs in the same family, the Dromaeosaurids, were feathered.
"One would assume Gigantoraptor is feathered since its ancestry lies with a group of animals that we know also had feathers," said Makovicky.
However, Xu said, these feathers would definitely not be used for flight. "Being 3,000 pounds heavy, no one would expect this species to fly," said Xu.
Feathers likely evolved at first as a form of insulation for animals, similar to fur in mammals. But because it is easier for large animals to retain heat, researchers suggest the Gigantoraptor's feathers served a more colorful purpose: to communicate.
"Dinosaurs were clearly animals that liked to display to each other; that's why they had crests, horns and bumps," said Philip J. Currie, professor of dinosaur paleobiology at the University of Alberta. "Feathers almost certainly developed first as insulation in small species, but other dinosaurs surely started using them as display. A species might have crown feathers at the top of its head or feathers at the end of its tail."
Whether Gigantoraptor truly used feathers in this peacock-like way, as well as why such a large animal retained its bird-like features, remain mysteries. In any case, the discovery underlines the bumpy and indecisive road of evolution, reflecting a particularly strange offshoot during the transition from dinosaurs to birds.
"One identified pattern is that large-sized dinosaurs are less bird-like than their small-sized relatives," Xu said. "The discovery of Gigantoraptor complicates this pattern: It is much larger, but has more bird-like features, than its small relatives. This implies the presence of multiple underlying mechanisms in the evolution of avian characters."
(c) 2007, Chicago Tribune.
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