Letters to Nature

Nature 431, 838-841 (14 October 2004) | doi:10.1038/nature02898; Received 16 April 2004; Accepted 26 July 2004

A new troodontid dinosaur from China with avian-like sleeping posture

Xing Xu1,2 & Mark A. Norell2

  1. Institute of Vertebrate Paleontology & Paleoanthropology, Chinese Academy of Sciences, PO Box 643, Beijing 100044, China
  2. American Museum of Natural History, Central Park West at 79th Street, New York City, New York 10024, USA

Correspondence to: Xing Xu1,2 Email: xu@amnh.org or xingxu@vip.sina.com


Discovering evidence of behaviour in fossilized vertebrates is rare. Even rarer is evidence of behaviour in non-avialan dinosaurs that directly relates to stereotypical behaviour seen in extant birds (avians) and not previously predicted in non-avialan dinosaurs1, 2. Here we report the discovery of a new troodontid taxon from the Early Cretaceous Yixian Formation of western Liaoning, China. Numerous other three-dimensionally preserved vertebrate fossils have been recovered recently at this locality, including some specimens preserving behavioural information3. The new troodontid preserves several features that have been implicated in avialan origins. Notably, the specimen is preserved in the stereotypical sleeping or resting posture found in extant Aves4. Evidence of this behaviour outside of the crown group Aves further demonstrates that many bird features occurred early in dinosaurian evolution5, 6.

Theropoda Marsh, 1881
Maniraptora Gauthier, 1986
Troodontidae Gilmore, 1924
Mei long gen. et sp. nov.
Etymology. Mei from Chinese, meaning to sleep soundly; long from Chinese, meaning dragon.
Holotype. IVPP (Institute of Vertebrate Paleontology and Paleoanthropology, Beijing) V12733, a nearly complete, fully articulated skeleton (Fig. 1).
Locality and horizon. Lujiatun, Shangyuan, Beipiao City, western Liaoning, China; lowest more fluvial, volcaniclastic beds of Yixian Formation, older than 128 and younger than 139 million years7.
Diagnosis. Mei long is distinguishable from all other troodontids on the basis of extremely large nares extending posteriorly over one-half of the maxillary tooth row; closely packed middle maxillary teeth; maxillary tooth row extending posteriorly to the level of the preorbital bar; a robust, sub-'U'-shaped furcula; presence of a lateral process on distal tarsal IV; and the most proximal end of the pubic shaft is significantly compressed anteroposteriorly, and extends laterally just ventral to the articulation with the ilium.

Figure 1: Holotype of Mei long (IVPP V12733).
Figure 1 : Holotype of Mei long (IVPP V12733). Unfortunately we are unable to provide accessible alternative text for this. If you require assistance to access this image, or to obtain a text description, please contact npg@nature.com

ac, Photographs of the skeleton in dorsal (a), ventral (b) and dorsolateral (c) views. d, Line drawing of the skeleton in dorsolateral view. cev, cervical vertebrae; cv, caudal vertebrae; dv, dorsal vertebrae; fu, furcula; lac, left astragalus–calcaneum; lc, left coracoid; lf, left femur; lh, left humerus; li, left ilium; lm, left manus; lp, left pubis; lpe, left pes; lr, left radius; ls, left scapula; lt, left tibia; lu, left ulna; pg, pelvic girdle; rac, right astragalus–calcaneum; rc, right coracoid; rf, right femur; rh, right humerus; ri, right ilium; rm, right manus; rp, right pubis; rpe, right pes; rr, right radius; rs, right scapula; ru, right ulna; sk, skull. Scale bar, 2 cm.

High resolution image and legend (84K)

Mei long is about 53 cm in length, similar in size to the basal dromaeosaurid Microraptor zhaoianus and the basal avialan Archaeopteryx lithographica5, 8, 9. IVPP V12733 is not an adult, as several cranial sutures are unfused, and although fused, sutures between the neural arches and centra are still apparent on the dorsal vertebrae. However, the holotype is also not a hatchling as caudal and sacral vertebral sutures are not apparent, the parietal is formed from a single element and there is complete fusion of astragalus and calcaneum, indicating that the individual is approaching maturity.

As in Sinovenator changii and basal dromaeosaurs5, the skull is proportionally small (about 69% of the femoral length), the trunk short and the hindlimbs very long. Long hindlimbs relative to the trunk is a feature correlated with a knee-based avian running mechanism10; it is also present in the basal troodontids Sinovenator and Sinornithoides11, 12, the basal dromaeosaurid Microraptor, and the basal oviraptorosaurian Caudipteryx5, 10. As in other troodontids13, 14 the numerous maxillary teeth (approximately 24) are tightly packed anteriorly (Fig. 2a, b). Unlike other troodontids, even the middle teeth lack inter-crown space. Posteriorly, the teeth are more stout and re-curved. As with other troodontids5, the internarial bar is strap-like and 'T'-shaped. Additionally, a pneumatopore lies on the posterior quadrate surface; a prominent convexity lies lateral to the foramen magnum on the exoccipital/opisthotic; and the distally expanded pendulous paroccipital processes are vertical and apneumatic. The dentary nutrient foramina lie in a horizontal groove on the labial surface of the dentary as in other troodontids13, 15.

Figure 2: Holotype of Mei long (IVPP V12733).
Figure 2 : Holotype of Mei long (IVPP V12733). Unfortunately we are unable to provide accessible alternative text for this. If you require assistance to access this image, or to obtain a text description, please contact npg@nature.com

a, b, Photograph (a) and line drawing (b) of skull in right lateral view. Note that the right external naris is much smaller than its actual size due to the anterior displacement of the right nasal. c, Furcula in anteroventral view. af, antorbital fossa; en, external naris; jde, dorsal extension of posterior jugal; map, anterior projection of the maxilla; p, parietal; qp, pterygoid process of quadrate; ra, right angular; rd, right dentary; rfr, right frontal; rj, right jugal; rl, right lacrimal; rml, right maxilla; rn, right nasal; rpm, right premaxilla; rq, right quadrate; rqj, right quadratojugal; rsa, right surangular; sc, sclerotic plates. Scale bar, 1 cm.

High resolution image and legend (176K)

As in Sinovenator changii the dorsal vertebrae have fan-shaped neural spines and slender transverse processes, but lack pneumatic foramina6. The distal caudals are elongate, with a reduced centrum and a sulcus on their dorsal surface as in other troodontids12. The ilium is short and tapers posteriorly. The pubis is long and proximally thick, but is not mediolaterally compressed as in other troodontids including Sinovenator changii. The ischium is fairly short with a distally positioned obturator process and two small processes on the dorsal edge. The tibia has an anteroposteriorly wide proximal end, and a distal end with a thick lateral margin. As in basal dromaeosaurids and Sinovenator16, 17 the feet are subarctometatarsalian, and metatarsal III is reduced but still visible on the plantar surface (Fig. 1b). Metatarsal II bears a prominent medial flange and metatarsal IV a lateral one on the ventral surface, producing a longitudinal metatarsal palmar excavation. As in other deinonychosaurs, the second pedal digit is specialized with a hypertrophied claw, but is not developed to the same degree as seen in derived dromaeosaurids5, 18.

Mei long differs with respect to a number of features compared with most other troodontids. Many of these differences are similar to conditions seen in avialans and dromaeosaurids5, 17, 18, 19. Laterally, the skull has an antorbital fossa that is much smaller than that of other non-avialan theropods (except for oviraptorosaurians5) and a large orbit that is apparently confluent with the infra-temporal fenestra (Fig. 2a, b). No postorbital is preserved, but if present it was undoubtedly small, as in Archaeopteryx9. As in avialans19 and mononykines20, there is no corresponding ascending postorbital process of the thin jugal to form a complete postorbital bar; however, there is a small dorsal expansion of the jugal at its posterior end (as in Archaeopteryx19), near where it contacts the quadrate, which is buttressed by an extremely small quadratojugal (Fig. 2a, b). The squamosal is also reduced and does not contact the quadratojugal, and the quadrate has a sharp and ventrally located pterygoid ramus. The snout is very short (Fig. 2a, b). The premaxilla has a large pre-naris portion and a long ascending process that accounts for about one-half of the snout length. The anterior projection of the maxilla is very long, and shallow, and forms the entire ventral border of the extremely large external nares that extend posteriorly over one-half of the length of the maxillary tooth row. Consequently, the nasals are short and consistently wide along their entire length. Comparatively, the frontal is elongate, about 45% the total length of the skull, and is blunt with a slight transverse expansion anteriorly. A sagittal crest is absent on the parietal. The braincase is expanded dorsoventrally and transversely (much wider than the minimum interorbital region). The supraoccipital crest is prominent, divided by an unclosed suture, and is hourglass-shaped, bordering distinct epiotic/opisthotic ossifications. The teeth are unserrated and distally re-curved as in Byronosaurus14. As in dromaeosaurids, the posterior dorsal vertebrae bear distinct parapophyses; however, they are not as stalk-like as those in advanced dromaeosaurs (for example, Velociraptor). The radius is thin and proportionally much longer than in other troodontids (see Supplementary Information), about 95% of the humeral length. A prominent proximally projected process lies on the lateral margin of distal tarsal IV, a feature otherwise only seen Microraptor gui. As in dromaeosaurids18, pedal phalanges II-1 and II-2 both have a medially centred ventral heel.

The holotype preserves and clarifies several unknown features of troodontids. The cervicals are incipiently heterocoelous as in some dromaeosaurids. The first and second dorsals bear prominent ventral processes on the anterior half of the centra, which are distally ball-like. Two small fragments of flat bones may represent sternal rib fragments as found in other maniraptorans18, 21. A large furcula is present in loose articulation with the acromion process of the scapula. It is robust and flat in cross section, with an incipient hypocleidium and expanded distal articular ends (Fig. 2c), and is more 'U'-shaped and robust as in avialans rather than in dromaeosaurids5, 17. As in dromaeosaurids and avialans, the scapulocoracoid is 'L'-shaped and the scapula is positioned close to the neural spines of the dorsals and is almost parallel to the vertebral column. The coracoid has a large coracoid tubercle, a sub-glenoid fossa and is divided into an anterodorsal and an anteroventral surface by a low ridge, as in many other maniraptorans5. The humerus projects laterally and the forearm and the manus fold in an avian mode (Fig. 1a, b), as in other maniraptorans5.

Inclusion of M. long into a recent phylogenetic analysis22 places M. long as the sister group to Sinovenator at the base of troodontids, and supports a monophyletic Deinonychosauria, which in turn is the sister group to Avialae (see Supplementary Information). For the included taxa this consensus solution is identical to that proposed by ref. 22.

Several studies have suggested that small size is crucial to the origin of flight and that miniaturization was responsible for many of the unique morphologies seen in avialans6, 16, 17; thus, M. long provides further evidence for this theory. For instance, M. long and Archaeopteryx lack a jugal–postorbital and a quadratojugal–squamosal contact9, 19, and so might basal dromaeosaurids5, 19. The nasal–frontal articulation is weak in M. long and possibly also in basal dromaeosaurs. These features suggest that some kind of cranial kinesis (probably prokinesis) evolved in the early evolution of eumaniraptorans. Notably, these features are also found in mononykines20. Derived larger members of the two deinonychosaurian groups often have the primitive states of these characters5, 23 and show a non-avialan akinetic skull.

This specimen displays the earliest recorded occurrence of the stereotypical sleeping or resting behaviour found in living birds. The body sits on folded symmetrical hindlimbs. The forelimbs are also symmetrical and extend laterally, folded avian-like next to the body, with the elbows slightly displaced laterally relative to the trunk (the left elbow more than the right, with the distal-most mani underneath the knees). The neck curves posteriorly on the left side of the body so that part of the head lies medial to the left elbow at the side of the trunk. This posture is identical to the stereotypical 'tuck-in' sleeping posture of many living birds. In the M. long holotype, we propose that this posture represents a life posture for the animal captured when it was sleeping or resting on the ground, based on the sedimentary context and comparative pose data of most other dinosaur specimens (see Methods).

Living tetrapods display diverse sleeping and resting postures, but only avians and some mammals rest on their folded limbs. Avians, owing to their long and very flexible necks, differ from mammals in tucking their heads between one of their anterior appendages and their torso. The M. long holotype is preserved in a remarkable life pose, capturing this tuck-in sleeping (or resting) posture. In birds, the tuck-in posture reduces surface area and conserves heat in the head, a major region of heat loss in these animals24, 25. It is therefore usually associated with heat conservation24, 25. From the phylogenetic position of the fossil we can tell that the tuck-in behaviour originated in the non-avialan precursors to modern birds. Although we are unable to measure it directly, the physiological/thermal implications of this and other fossil evidence (for example, brooding and feathers) are highly suggestive that these animals shared a homeothermic physiology with modern avians.



Life posture inference

The holotype of M. long is inferred to be buried in deposits when the animal was sleeping or resting, and thus preserves a life posture. Exceptionally well-preserved vertebrate specimens have been collected from the Lujiatun beds over the last few years, and it has been proposed that some of them provide information for dinosaur behaviour3. The Lujiatun beds are alluvial deposits mainly comprising tuffaceous conglomerate debris flows, sandstones and mudstones26. Some of the most fossiliferous locations are considered to be the result of instant catastrophic mass mortality events26, 27 preserved in tuffaceous ashes up to 3 m in thickness. Although not all are volcanic in origin, such Pompeii-like depositional conditions are present in other localities that preserve behavioural characters in fossil vertebrates1, 28. Other dinosaur specimens from the lacustrine beds of the Yixian formation26 and specimens from the more classically fluvial lower sandstone beds (such as Shenzhousaurus29) are not preserved in life posture, but in more typical death poses with heads bent back above the spine and the hind appendages and tails extended30. Both the specimen itself and the sedimentology of the deposits from which this specimen was collected indicate that the preserved posture represents a life posture for the animal. Such a posture can also be inferred from the Early Cretaceous troodontid Sinornithoides from the Ordos basin and perhaps for the nesting oviraptorid Citipati (IGM 100/979); however, these specimens are incomplete and more distorted1, 11.



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Supplementary Information

Supplementary information accompanies this paper.



We thank J. Meng, X.-J. Ni, Y.-M. Hu, J. Clarke and R. Prum for discussions, J.-M. Yang and H.-J. Wang for preparing the specimen, and M. Ellison for illustrations. We also thank Z.-H. Zhou and other members of the Liaoxi expedition team of the IVPP. This work was supported by grants from the National Natural Science Foundation of China, Special Funds for Major State Basic Research Projects of China, National Geographic Society, American Museum of Natural History, Chinese Academy of Sciences and National Science Foundation of USA.


Competing interests statement

The authors declare no competing financial interests.


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