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"Sleeping Dragon" Fossil May Link Dinosaurs, Birds

John Roach
for National Geographic News
October 13, 2004
 
Paleontologists have discovered a duck-size dinosaur they believe died while catching some z's. Researchers found the creature's fossilized remains curled up with its head tucked under a forelimb, a pose that today is unique to sleeping and resting birds.

"I never expected we'd find a sleeping dinosaur in general, let alone with the tuck-in position," said Xu Xing, a curator at the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing, China.



Mark Norell, chairman of the division of paleontology at the American Museum of Natural History in New York, said the discovery further strengthens the chain linking dinosaurs and birds, suggesting this birdlike sleeping posture first evolved in dinosaurs.

"This is another stereotypical bird behavior in another nonavian theropod," he said. Theropods are meat-eating dinosaurs characterized by short forelimbs and powerful hind legs. Many scientists believe small theropods are ancestors to the first birds.

Xu and Norell describe the new dinosaur in tomorrow's issue of the science journal Nature. The pair named the 21-inch-long (53-centimeter-long) creature Mei long, Chinese for "soundly sleeping dragon."

Mei long is a troodontid, one of the most birdlike types of theropods. Several other features of Mei long support theories that nonavian dinosaurs were warm-blooded and that small size was a prerequisite for flight, according to the paleontologists.

The research was supported, in part, by a grant from the National Geographic Society's Committee for Research and Exploration.

Hans-Dieter Sues, the associate director for research and collections at the Smithsonian Institution's National Museum of Natural History in Washington, D.C., said the fossil does appear to be preserved in a resting or sleeping posture.

"One should keep in mind, however, that this could have been a death posture as well," said Sues, who is a member of the National Geographic Society's Committee for Research and Exploration.

Sues and Philip Currie, curator of dinosaurs at the Royal Tyrell Museum in Alberta, Canada, point out that an incomplete troodontid skeleton from Mongolia, Sinornithoides, was discovered in 1994 in a similar pose.

"Overall, I think [Mei long] is a very remarkable find and is especially amazing because it is the second small troodontid in this pose," Currie said. "Not much doubt that this is the way they slept."

Rare Discovery

Researchers discovered Mei long in Liaoning Province in northeastern China. The region is fast becoming known as a treasure trove for dinosaur finds and is yielding fossils that provide unprecedented details about dinosaur behavior and body covering.

Several dinosaur fossils from the region include evidence that they were covered in hairlike feathers, including an early cousin of Tyrannosaurus rex named Dilong paradoxus (see news story and photos). Xu, Norell, and colleagues described the find in last week's issue of Nature.

While feathered-dinosaur discoveries are becoming almost routine, the discovery of a sleeping dinosaur is a rare surprise, Norell said. "There are so very few fossils of animals which are basically buried alive, preserving behavior that's interesting," he said.

The new fossil specimen is an almost fully grown adult. It sits on long, folded hind limbs. Its forelimbs are folded birdlike next to its body and its neck curves to the left, so that its relatively small head lies between the left elbow and body. The posture is identical to the "tuck-in" posture of many living birds, according to Xu and Norell.

Mei long was found in layers of volcanic and riverbed sediment that have been dated to about 130 million years ago. At that time, Liaoning Province was a volcanically active, forested region filled with lakes and streams.

"It is kind of difficult to imagine how a fossil can be preserved in such a posture. It must be like it instantly died and was buried," Xu said. Scientists are uncertain as to exactly how the fossil was preserved.

According to Xu, one possible scenario is that exposure to a volcanic gas killed the creature during its sleep and subsequent flows of mud and ash buried it. Another theory is that the dinosaur was in a cave or burrow and covered instantly in a thick ashfall.

Dinosaur-Bird Link

Living four-legged creatures rest and sleep in various postures, but only birds and a subset of mammals rest on folded limbs. And only birds, with their long, flexible necks, tuck their heads behind a forelimb or wing to rest, according to Xu and Norell.

Scientists think birds curl up in this manner to conserve heat; tucking their heads under a feathered wing keeps them warm. According to Xu and Norell, observation of this behavior in dinosaurs supports the theory that dinosaurs were warm-blooded animals like birds.

"It's another highly suggestive feature that these animals were exhibiting some form of thermic behavior," Norell said. "It's not enough evidence to say it was as advanced as modern birds, but certainly a lot different than a crocodile."

Sues, the Smithsonian Institution paleontologist, expressed caution about inferring a warm-blooded physiology from this posture alone: "Most land vertebrates adopt body postures to minimize the loss of body heat, regardless of their basic physiology," he said.

Currie, of the Royal Tyrell Museum, said the "tucked-in sleeping posture probably supports the idea that these little theropods were warm-blooded. The presence of feathers on related forms is stronger evidence, however."

Xu and Norell believe that Mei long, like many of its contemporaries from this region, was covered in primitive feathers. However, researchers found no direct evidence for this theory.

Distinguishing it from most previously discovered troodontids, Mei long has large nostrils, a relatively small skull, long hind limbs, numerous closely packed teeth in the middle of its jawbone, and a large, U-shaped wishbone.

The fossil shares many features with dromaeosaurs (small, meat-eating dinosaurs with large heads, sharp teeth, and clawed hands) and avialans, the group that includes living birds.

Examples of these features include a short snout, a long forehead, a large eye socket, a long and thin forearm, an L-shaped bone at the shoulder joint, and a shoulder blade close to the spine, among others.

Xu and Norell said that Mei long's shared features with birds and its small size also support the theory that miniaturization was crucial to the development of flight and other characteristic bird traits.

"You find that small size is not only critical to flight but also is very important to developing other bird features, bird characteristics," Xu said.

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