A team of scientists led by James M. Clark, Ronald B. Weintraub Associate Professor of Biology at The George Washington University, and Xu Xing of the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) in Beijing, have discovered a new genus and species of dinosaur that is the oldest known and most primitive tyrannosauroid. The new basal tyrannosauroid, named Guanlong wucaii, sheds light on the early evolution and geographical distribution of coelurosaurs, small theropod dinosaurs that include the closest relatives of birds. The discovery is announced by Clark, Xu, and six other colleagues in a paper titled "A Basal Tyrannosauroid Dinosaur from the Late Jurassic of China," that appears in the Feb. 9, 2006, edition of the science journal Nature.
Guanlong's most striking feature is a large, complex, fragile cranial crest formed by the dinosaur's fused nasal bones. The delicate, 1.5 mm thick crest runs from its nose to the back of its head and is larger and more elaborate than any yet reported for a non-avian theropod dinosaur. Guanlong is the only species of tyrannosauroid known to have a cranial crest. The team is unsure about the function of this exaggerated ornamental trait, but they suggest that it may be sexually selected and useful in display or species recognition.
"It seems paradoxical that a presumably predatory dinosaur like Guanlong would possess such a delicate, air-filled cranial crest," said Clark. "More specimens need to be analyzed, but based on similar ornaments found in extinct vertebrates we hypothesize that the primary function of the crest was to make the animal more noticeable or attractive to other members of its species."
The team found two specimens of Guanlong wucaii preserved with three specimens of other carnivorous dinosaurs. One specimen of Guanlong, which was determined from growth rings in the bone to have died in its 12th year of life, is a partially articulated skeleton preserving most of the elements. The other specimen, which was six years old when it expired, is smaller, but fully articulated and nearly complete. A histological analysis suggests Guanlong reached full-adult size at age seven.
"The discovery of two nearly complete basal tyrannosaur skeletons is amazing considering how fragmentary most known specimens of derived predatory dinosaurs in Jurassic time are," said Xu. "This is an unbelievable discovery with tremendous new information on the evolution of the tyrannosaurs and of an even bigger group, the coelurosaurs, the group giving rise to birds."
Guanlong wucaii was not as physically imposing as the gigantic Tyrannosaurus rex that stars in the movie Jurassic Park, which stood 10-feet tall at the hips and was 40-feet long. The medium-sized Guanlong stood 1.1-meters (3.6 feet) tall at the hips and was approximately 3-meters (9.8 feet) long from nose to tail. Its small size is one of the many features in which Guanlong is more like its coelurosaurian relatives and lacks the specializations of later tyrannosaurs. Thus, unlike T. rex it has a long and shallow snout and relatively long forelimbs. However, Guanlong is identified as a tyrannosauroid based on seven features it shares with this group, including the shape of its teeth, the shape of openings in its skull, and features of its pelvis. The two Guanlong specimens date back to the beginning of the Late Jurassic, approximately 160 million years ago.
"The features that make Guanlong wucaii a member of the Tyrannosauroidea are very subtle," said Clark. "Guanlong represents a specialized lineage very early in the evolution of tyrannosauroids, so it has only a few features of this group."
Tyrannosaurs are best known from the end of the Cretaceous period, about 65-70 million years ago. Fragmentary specimens from the middle part of the Upper Jurassic had been suggested as tyrannosaur relatives, but were too incompletely known to be sure. The new fossils, in which nearly all parts of the skeleton are represented, offer a glimpse at what tyrannosaurs looked like shortly after they branched from other coelurosaurs, 95 million years before T. rex died out at the end of the Cretaceous.
The specimens were found in the Junggar Basin of Xinjiang, China, in the far western reaches of the Gobi Desert, near the old silk road. The rugged badlands in this area were the setting for many scenes in the movie Crouching Tiger, Hidden Dragon. Clark and Xu have led five separate cooperative expeditions into Xinjiang (pronounced Shin Jang) since 2001. The dinosaur beds are in the upper Middle to lower Upper Jurassic Shishugou Formation, one of the few fossil deposits preserving dinosaurs and their contemporaries from the time when dinosaurs had just begun to reach enormous sizes and dominate the world's terrestrial ecosystems. In Chinese, Guanlong means crowned dragon and wucai refers to the rich colors of rocks that produced the specimens at the locality of Wucaiwan.
"This area of China was of particular interest to us because it has only really been explored in recent years and it contains specimens that date back to a very poorly known time period, the Mid- to Late Jurassic, when the continents were first starting to break apart into their modern configurations," said Clark.
Tyrannosaurs are a member of the group Coelurosauria that includes birds and their closest relatives among dinosaurs. This close relationship is supported by, among other things, the discovery of Dilong paradoxus, a primitive tyrannosaur from the Early Cretaceous of China preserved with simple feathers, described by Xu, Norell and others in 2004. Paleontologists hypothesized that tyrannosaurs existed in the Late Jurassic because the oldest bird, Archaeopteryx, is from Late Jurassic deposits, and its coelurosaurian relatives must have diversified before then. However, the fossil record of coelurosaurs includes few Jurassic forms, and Guanlong is one of the oldest and most completely known coelurosaurian dinosaurs yet discovered. The discovery of a tyrannosaur older than Archaeopteryx fills in a large hole in the early coelurosaurian fossil record and confirms that the coelurosaurian radiation was well under way by the time of Archaeopteryx.
The field work and study of the specimens that led to the discovery of Guanlong was supported by the National Geographic Society, the National Natural Science Foundation of China, the Jurassic Foundation, the Hilmar Tharp Sallee Charitable Trust, The George Washington University, the Chinese Academy of Sciences, the National Science Foundation Division of Earth Sciences, and the American Museum of Natural History. Co-authors include Clark, Xu, Catherine A. Forster of Stony Brook University, Mark Norell of the American Museum of Natural History, Gregory M. Erickson of Florida State University, David A. Eberth of the Royal Tyrrell Museum, Chengkai Jia of IVPP, and Qi Zhao of IVPP.
Some of Clark and Xu's other fossil findings in China include the most complete skeleton found of a land-based crocodilian from the Middle Jurassic period. This new species -named Junggarsuchus sloani - proved to be an important link in the evolution of crocodilians. The new species confirmed that the crocodilians? skull and crushing jaws developed when the reptiles lived on land, before they evolved into the semi-aquatic creatures we see today. Clark, Xu, Forster, and Yuan Wang of IVPP, published these findings in the August 2004 edition of Nature.
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