"Sleeping Dragon" Fossil May Link Dinosaurs, Birds

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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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