Scientists Discover Oldest-Known and Most-Primitive Tyrannosaur
Fossil find sheds new light on origin of Tyrannosaurs, establishes new dino genus and species
Guanlong wucaii is a small theropod dinosaur called a coelurosaur, like T. rex, which is most closely related to birds.
According to a report in this week's issue of the journal Nature, several of the skeleton's cranial features and long and shallow snout differs from other tyrannosaurs. Still, Guanlong is identified as a tyrannosaur based on the shape of its teeth, the shape of openings in its skull, and its pelvic features, said James Clark of George Washington University in Washington, D.C.
Clark led the scientific team, along with paleontologist Xu Xing of the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing, China. The animal's forelimbs are relatively large compared to its hind limbs and slightly bowed, similar to other coelurosaurs, the scientists say.
"The features that make Guanlong wucaii a member of the tyrannosaur family are very subtle," said Clark. "Guanlong represents a specialized lineage in the early evolution of tyrannosaurs."
In Chinese, the word guanlong means "crested dinosaur." Wucai refers to the rich colors of the rocks where the specimens were found. The dinosaur was unearthed in the Junggar Basin of Xinjiang (pronounced shin jang), China, near the Gobi Desert. Dinosaur beds there lie in the Middle to Late Jurassic Shishugou Formation, one of the few fossil deposits preserving dinosaurs from the time when they had begun to reach enormous sizes and dominate the world's terrestrial ecosystems.
"This discovery is significantly revising our understanding of tyrannosaurs," said Richard Lane, program director in the National Science Foundation (NSF)'s division of earth sciences, which funded the research. "This smallest and oldest known tyrannosaur has much to tell us about the early evolution and geographic distribution of coelurosaurs."
The discovery is "a breakthrough in going deep in time to trace the evolution of coelurosaurs, a process culminating in the origin of birds," said Xing.
Guanlong's most striking feature is a large, complex, fragile cranial crest that runs from its nose to the back of its head. Guanlong is the only species of tyrannosaur known to have this crest. Scientists are unsure of the function of the exaggerated ornament, but suggest it may be useful in display or species recognition.
"Based on similar ornaments found in extinct vertebrates, we think the primary function of the crest was to make the animal more attractive to other members of its species," said Clark.
The team found two specimens of Guanlong wucaii with five different carnivorous dinosaurs in an apparent mud pit. "This area was of particular interest because it has only been explored in recent years, and it contains specimens that date back to a poorly known time period, the Jurassic, when the continents were first starting to break apart," said Clark.
One of the two new specimens, which died at age 12, is a mostly preserved skeleton. The other specimen, which died at six, is smaller but nearly complete. Guanlong likely reached adult size at seven. The two specimens date back to the Upper Jurassic, approximately 150 million years ago.
John Francis, vice president for research, conservation and exploration at the National Geographic Society (NGS), which also supported the research, said the society "is thrilled to have played a part in the discovery of this important new tyrannosaur. This fossil adds significant new knowledge about a little-known prehistoric period."
Other team members include Catherine Forster of the State University of New York at Stony Brook, Mark Norell of the American Museum of Natural History, Gregory Erickson of Florida State University, David Eberth of the Royal Tyrrell Museum, and Chengkai Jia and Qi Zhao of IVPP.
In addition to NSF and NGS, the research was supported by the National Natural Science Foundation of China, Jurassic Foundation, Hilmar Tharp Sallee Charitable Trust, George Washington University, Chinese Academy of Sciences and American Museum of Natural History.
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