Mesozoic World

A 135-million-year-old fossil dinosaur caught apparently taking a nap with its head tucked under its forearm has been discovered by scientists in China. It is the earliest known example of an animal unearthed in a bird-like repose.

A Nature magazine report says the find suggests the characteristic sleeping posture probably first arose in the dinosaur ancestors of modern birds.

Mei long, which means "soundly sleeping dragon", was pulled out of the famous fossil beds of Liaoning Province. This is the location in China where so many feathered dinosaurs have been discovered - astonishing finds that have fuelled the theory that modern birds can trace their lineage to the "terrible lizards" that once ruled the Earth.

Mei long is described in Nature by Xing Xu, from the Chinese Academy of Sciences in Beijing, and Mark Norell, of the American Museum of Natural History.

'Peaceful' death

Only last week, these researchers were reporting the discovery of a fluffy-feathered cousin of the mighty Tyrannosaurus rex - another fossil unearthed in Liaoning. The scientists can tell from the mechanics of Mei long's skeleton that the posture it died in was one it would habitually take up - this was no accidental death pose.

Professor Xu speculates that the dinosaur may have been killed by poison gas from the volcanic eruption that then buried it in ash. "Exactly how volcanic activity captured the life posture, we don't know," Professor Xu said. "There are many possibilities. For example, volcanic gas cut off the oxygen and the animal died sleeping, peacefully. Then, later, the body was covered quickly by ash."

Whatever the cause of death, it must have been quite sudden. Other dinosaurs discovered in the vicinity but not yet described in scientific literature also capture living behaviours, Professor Xu says. Mei long was found sitting squarely on its hind limbs. Its front legs are extended around the body, with the head tucked backwards on the left side.

"This posture is identical to the stereotypical 'tuck-in' sleeping posture of many living birds," Xu and Norell write in Nature. The big difference is this animal did not have wings; its head is covered by a leg.

Cool heads

The dinosaur was a young animal approaching maturity. It had a proportionally small head, a short trunk, and very long hind limbs, indicating that it was good at running on two legs. One of Mei long's toes was armed with an unusually large claw.

Xu and Norell remark in their journal paper that small size might have been a necessary first step on the evolutionary path to flying birds. "Several studies have suggested that small size is crucial to the origin of flight, and that miniaturisation was responsible for many of the unique morphologies seen in avialans. Thus, M. long provides further evidence for this theory," they write.

Scientists believe the tuck-in position in birds is related to the control of body heat - the position reduces the animal's surface area and prevents heat loss from the head in particular.

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