Xu Xing of the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing, China, is the lead author of the new paper. He said his discovery is significant because it sheds light on the evolution of tyrannosaurids, which include the giant carnivore T. rex.
Tyrannosaurids belong to a broader, diverse group of dinosaurs known as the coelurosaurs. Most paleontologists believe this group gave rise to birds. How this evolutionary change occurred is complex, Xu said. Large tyrannosaurids like T. rex were reportedly covered in scales instead of feathers, suggesting they were distantly related to birds.
"With this new find, we can see a perfect evolutionary transition from typical coelurosaurians to highly specialized large tyrannosaurids and clarify a number of questions," Xu said.
In particular, the new specimen's head shares many characteristics with advanced tyrannosaurids, Xu said, whereas its body is of a more primitive, unspecific coelursaurian shape.
Hans-Dieter Sues, the associate director for research and collections at the Smithsonain Institution's National Museum of Natural History in Washington, D.C., agrees. "The new small tyrannosaur definitely shows an interesting mosaic of primitive and derived features; its skull is already more like that of other, later tyrannosaurs than the rest of its skeleton," said Sues, who is a member of the National Geographic Society's Committee for Research and Exploration.
Advanced tyrannosaurid skull features observed in Dilong include teeth and a single nasal bone. (More primitive dinosaurs featured a pair of nasal bones.)
But unlike more advanced tyrannosaurids, Dilong had relatively long forearms with three fingers, the researchers report.
According to Holtz, the University of Maryland paleontologist, the combination of an advanced tyrannosaurid head with a more generalized coelurosaur body is consistent within paleontology.
"For example, the primitive ceratopsian Psittacosaurusa contemporary, and perhaps prey, of Dilongshows a head with the initial changes toward the feeding specializations of later horned dinosaurs. Bits [of] its body is a generic herbivore's body," he said.
The description of Dilong paradoxus is based on the fossils of four specimens, including a fragmented one with evidence of protofeathersprecursors to the feathers found on modern birds.
The fragmented fossil went unidentified until a more complete fossil of the same creature was studied and found to match the morphology, or form and structure, of that found in the earlier fragments. The fossils come from a geologic feature in northeastern China known as the Yixian formation, which has yielded several other feathered dinosaurs.
According to Norell, the American Museum of Natural History paleontologist, large adult tyrannosaurs like T. rex probably lacked primitive feathers, an indication that the hairlike structures evolved to insulate warm-blooded dinosaurs rather than to enable flight.
Small animals, Norell said, need insulation to keep warm. Bigger animals no longer need the insulationa probable reason why animals such as elephants are bald.
"It's doubtful a 40-foot-long [12-meter-long] tyrannosaur was covered with this stuff, if we are right that [the feathers] were needed for insulation. As the dinosaurs get bigger, they need to dump heat, not hold onto it," Norell said.
Xu added that even large dinosaurs like T. rex may have had feathers when they were young. "They are not likely to be completely featherless animals for [their] whole life," he said."
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