Mesozoic World

A 135 million year-old fossil dinosaur caught apparently taking a nap with its head tucked under its forearm was discovered by scientists in China in 2004. It was the earliest then-known example of an animal unearthed in a bird-like repose.

A report in the Nature journal stated the find suggested that 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 in which so many feathered dinosaurs have been discovered - astonishing finds which have fuelled the theory that modern birds can trace their lineage to the 'terrible lizards' which once ruled the planet.

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

'Peaceful' death

These same researchers were reporting in early October 2004 of the discovery of a fluffy-feathered cousin of the mighty Tyrannosaurus rex - another fossil unearthed in Liaoning. The scientists could tell from the mechanics of Mei long's skeleton that the posture which it had at death was one it would habitually take up - this was no accidental death pose.

Professor Xu speculated that the dinosaur may have been killed by poison gas from the volcanic eruption which then buried it in ash. Exactly how volcanic activity captured the life posture was not known at the time, with a number of options being available. 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 yet to be described in scientific literature also captured living behaviours. Mei long was found sitting squarely on its hind limbs. Its front legs were extended around the body, with the head tucked backwards on the left-hand side. This posture was identical to the stereotypical 'tuck-in' sleeping posture of many living birds. The big difference was that this animal did not have wings; its head was 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. Such small size may have been a necessary first step on the evolutionary path to flying birds. Several studies previously suggested that small size was crucial to the origin of flight, and that miniaturisation was responsible for many of the unique morphologies seen in avialans. M. long would therefore appear to be further evidence in support of this theory.

Scientists of the time believed that the tuck-in position in birds was 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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