March 14, 2006

Uncovering the Past

James M. Clark Co-Leads Discovery of Oldest Known Tyrannosaur


Few tourists rush to the Junggar Basin in China’s Xinjiang Province for a summer getaway, but for James M. Clark, GW’s Ronald Weintraub Associate Professor of Biology, the area has become a paleontologist’s paradise. Clark has spent every summer since 2001 criss-crossing the badlands of the Gobi Desert in search of ever-more impressive fossil finds. Now those long summers have paid off. Clark and his colleagues recently unveiled the discovery of a new species of dinosaur dubbed Guanlong wucaii, an early ancestor of the infamous Tyrannosaurus rex and the oldest known and most primitive tyrannosaur, in the February edition of the science journal Nature.

The international 20-member team credited with the discovery was co-led by Clark and Xu Xing (pronounced Shoo Shing) of the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) in Beijing, China. Clark emphasized the importance of team work when taking on a project of this magnitude.

“No individual can make these discoveries without the help of other team members,” Clark said. “We even enlist the help of locals to assist in the search for new fossils.”

Teetering on the edge of the Gobi Desert in northwestern China, the Junggar Basin is reminiscent of a barren landscape on a desolate, uninhabited foreign planet. Both winters and summers are extreme in the Junggar, with temperatures dipping as low as minus four degrees Fahrenheit and summer temperatures soaring to 120 degrees. Clark knew the area had potential to provide great discoveries during an erarlier trip in 2000.

Despite the rugged conditions, the Junggar Basin serves as a vast, untapped reserve of evolutionary history that is beginning to fill in the gaps of tyrannosaur genealogy and sharpening the link between dinosaurs and birds. One hundred and sixty million years ago, Junggar’s warm environment consisted of lakes and marshes similar to the Pantanal seasonal wetlands of Brazil, which provided fertile ground for many different species to thrive.

“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.

In 2002, Clark and his team were alerted to the Guanlong specimens by T. Yu, a local guide hired to help the team spot fossils. Two skeletons were unearthed, one lying on top of the other. The top dinosaur is thought to be approximately in its 12th year of life indicated by growth rings in the bones. Most of the bones were preserved separatly from each other. The lower skeleton is thought to be about seven years of age and is smaller, but is a nearly complete skeleton in which the bones are articulated. But Guanlong wucaii would have to wait nearly four years to be introduced to the rest of the world.

The process in sharing this type of discovery is usually long and arduous. Clark noted in addition to conducting the research and writing the paper, extracting Guanlong wucaii’s remains was especially difficult because of the multiple specimens preserved together and the type of rock where the specimen was found.
“When you have a significant discovery, you do get a little anxious to share it, but getting all the facts, cleaning the fossils, and submitting a good paper is well worth the wait,” said Clark.

The fossils were analyzed thousands of miles away at the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing. Although Clark made several research trips to China in the summer, his responsibilities as chair of GW’s biology department, kept him half a world away from Guanlong for months at a time.

“As my colleagues were unveiling the fossils, I would receive ‘Christmas gift’ e-mails with exciting updates on the progress,” said Clark. “Xu would constantly send new pictures and would keep me posted on the latest developments so I felt a part of the process.”

The name of the new discovery is derived from the Chinese guan (crown) long (dragon) and wucaii (five colors), which represents the more colorful rocks of Wucaiwan, an area about 10 kilometers from the site where the fossils were found.

A fragile 1.5 mm crest running along the skull represents Guanlong’s most unique and distinct feature. Such a trait is not characteristic of a predatory dinosaur, Clark noted.

“The delicate, air-filled cranial crest was by far the biggest find,” said Clark. “More specimens need to be analyzed, but, based on similar ornaments found in extinct vertebrates, we hypothesize the primary function of the crest was to make the animal more noticeable or attractive to other members of its species.”

Guanlong wucaii was significantly smaller than its descendant, T. rex, but shared similar traits linking the relatives together. Measuring approximately 10 feet from its head to the tip of its tail, Guanlong wucaii was one-fourth the size of T. Rex, which measured 40 feet from head to tail. However, both tyrannosaurs had similarly shaped teeth, the same shape of openings in the skull, and similar features of the pelvis.

On the other hand, T. Rex’s arms were much shorter in comparison to Guanlong wucaii’s arms, and had only two fingers compared to Guanlong’s three fingers. Because of Guanlong’s smaller size, it was probably much more fleet of foot than the T. Rex. Roaming the earth 160 million years ago, Guanlong wucaii came 90 million years before T. Rex, which lived 65–70 million years ago, at the end of the Cretaceous Period.

These differences shed important light on the evolution not only of tyrannosaurs but also of birds, which descended from theropod dinosaurs. Although the fossil remains did not contain any evidence of feathers, Guanlong wucaii displayed bird-like features, including its three clawed fingers. Guanlong wucaii is thought to be the link between tyrannosaurs and coelurosaurs, the group of dinosaurs giving rise to birds.

“This discovery is not only significant for understanding the early evolution of the tyrannosaurs, but for the origin of birds,” emphasized team co-leader Xu.

When asked if any more discoveries are forthcoming, a sly grin flashes across Clark’s face.

“We have just scratched the surface,” said Clark. “The team has enough work to keep us busy for decades, and we still are planning to explore the area further. Stay tuned, because there is plenty more to come.”

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