Mei long was a small troodontid theropod from the Early Cretaceous of Liaoning, China. It is known from two nearly complete skeletons preserved with the legs tucked beneath the body, the tail curled around the trunk and the head resting beside a bent forelimb. The fossils preserve a recurring resting posture that resembles one used by living birds.
The posture is real; the moment of death is not recorded. It is reasonable to infer that the animals were resting when rapidly buried, but no fossil can show that their eyes were closed or prove they were physiologically asleep. The pair belongs in the dinosaur catalogue as small troodontid theropods, while the sleep interpretation remains a hypothesis.
Quick facts
| Scientific name | Mei long Xu and Norell, 2004 |
|---|---|
| Group | Theropoda, Troodontidae |
| Age | Early Cretaceous |
| Locality | Lower Yixian Formation, Liaoning, China |
| Holotype | IVPP V12733 |
| Second specimen | DNHM D2154 |
| Size | Holotype about 53 cm in preserved length; second specimen larger |
| Diet | Unknown; small animals are plausible |
| Catalogue | Dinosaurs |
What two resting fossils establish
Two articulated skeletons preserve tucked legs, a curled tail and a head beside a forelimb. This supports a resting pose, not a proven moment of sleep.
The holotype was juvenile. Bone histology of DNHM D2154 records more than two years of growth and a slowing rate, but one open skull suture alone cannot establish age.
The skull, teeth, long hind limbs and enlarged second-toe claw support comparison with troodontids. They do not preserve a specific hunt, prey item or running speed.
Articulation implies rapid burial. The exact cause, including ash, sediment or shelter, is not determined by the posture.
Discovery and name
Xing Xu and Mark Norell named Mei long in 2004 from the holotype IVPP V12733. The Chinese name is usually translated as “sleeping dragon” and refers to the arrangement of the skeleton, not to a proven physiological state. The holotype is almost complete and articulated: the skull remains joined to the spine, the limbs lie beside the trunk and the tail loops around the body.
Open sutures in the skull and vertebrae show that this first individual was young. Its preserved length of about 53 centimetres describes that juvenile, not a known adult limit for the species. The shape of the skeleton is more informative than its small size alone, because it gives a rare view of a whole animal in a coherent position.
A second nearly complete individual, DNHM D2154, was described in 2012. It lies in a similar posture, though facing the opposite direction. Finding the same general arrangement twice makes it less likely that the first skeleton was assembled into that pose accidentally during transport. Still, two fossils cannot tell us every detail about the animals' final minutes.
What the posture can and cannot say
The legs are folded beneath the body, the tail bends around the trunk and the head rests close to a flexed forelimb. This arrangement resembles a resting posture in living birds. The repeated, articulated preservation supports the interpretation that both animals adopted the pose while alive. It is stronger evidence for rest than a set of scattered bones would be.
It does not preserve closed eyes, brain activity, body temperature or a sleep cycle. The idea that the tucked head helped conserve heat is plausible by comparison with birds, but the fossils do not preserve the soft covering needed to measure insulation. “Sleeping dragon” is a memorable name, not a direct observation of sleep.
Rapid burial is likely because decay, scavengers and water usually disrupt a skeleton before it becomes fossilised. The exact mechanism is unknown. Burial in sediment, ashfall, shelter or another event have been suggested, but none is demonstrated by the curled tail or the placement of the head. The second specimen's precise collecting horizon is also incompletely documented, which limits detailed reconstruction of the event.
Skull, teeth and movement
Mei had a short skull with an unusually large external nostril and many relatively small teeth. An enlarged, curved claw on the second toe is characteristic of troodontids. The hind limbs were long and the skeleton was lightly built, features consistent with bipedal movement. These anatomical observations do not give a measured running speed or identify one exact prey item.
The teeth could have taken small vertebrates or invertebrates, but the stomach contents of the two known skeletons have not been described. The foot claw may have played a role in prey handling, but its shape does not preserve the action itself. It is safest to describe a small predatory theropod with an uncertain menu rather than assign a detailed hunting routine.
The genus is placed in Troodontidae, a branch of small, bird-like theropods. In the analysis accompanying the second specimen, Mei occupied a relatively early position within that group. “Basal” here describes a branching order in a phylogenetic tree. It does not mean primitive in the sense of poorly adapted or unchanged.
Age and the Yixian landscape
Both skeletons came from the lower part of the Yixian Formation in Liaoning. Fine lake sediments and volcanic material in the Jehol Biota preserved many articulated animals. The region included lakes, streams and nearby land beside active volcanic centres. Those deposits are discussed in the overview of the Cretaceous Period.
The Yixian Formation accumulated over time, not in one simultaneous snapshot. Two animals from the same named formation need not have lived together, and a reconstruction placing every famous Jehol species beside Mei combines fossils that may be separated in age and location. A vivid landscape can be useful, but it is an illustration rather than a direct record of a single community.
Feathers and appearance
Troodontids belong to a theropod branch close to birds, and related animals preserve feather coverings. A feathered reconstruction of Mei is therefore better supported than a wholly bare, scaly model. Yet neither of the two known skeletons preserves a complete feather outline that maps the exact length and distribution of feathers on the arms, tail and torso.
Colour is unknown. Stripes, spots, fluffy cheeks and a particular facial expression are artistic choices. A profile in the dinosaur catalogue can show an informed silhouette, but it should not be mistaken for an anatomical measurement. The fossils are unusually valuable for posture, while offering far less direct information about the full external appearance.
Growth and what is known reliably
The holotype's unfused sutures indicate a juvenile. The second skeleton adds a different kind of evidence: microscopic structure in the tibia and fibula preserves lines of arrested growth. These lines show that the animal lived for more than two years and that its growth had begun to slow by the time it died. DNHM D2154 combines juvenile traits in the skull with more mature features elsewhere in the skeleton.
This mismatch is a reminder not to estimate age from one bone. Sutures can remain open for different lengths of time, and bones do not all mature in lockstep. Histology provides a growth record from the sampled limb bones, not an exact birthday or a complete life history.
The most secure account is modest but important: two small troodontids died in a recurring, tucked resting pose; the holotype was juvenile; and bone histology indicates that the second animal had lived for several years. Their exact cause of death, diet, colour, calls, speed and care of young are not fossilised. The value of Mei long is that it lets researchers discuss behaviour while keeping the observed posture separate from explanations of how it arose.
Frequently asked questions
Did Mei long really die while sleeping?
The fossils preserve a posture comparable to a resting bird, but sleep itself does not fossilise. A resting pose is supported; the exact state and cause of death remain unknown.
How many Mei long skeletons are known?
Two nearly complete articulated skeletons have been described: the holotype IVPP V12733 and the second specimen DNHM D2154.
Was Mei long an adult?
The holotype was juvenile. Histology shows that DNHM D2154 lived for more than two years and was growing more slowly, though age cannot be read from one unfused suture alone.
Why is Mei long compared with birds?
Both fossils have tucked legs, a tail curled around the body and the head beside a forelimb, a posture also seen in resting birds. The comparison concerns posture as well as its theropod relationships.