Guanlong wucaii
Age
Jurassic（Oxfordian）
161.5-154.8 Ma
Taxonomy
Kingdom: Animalia
Phylum: Chordata
Class: Sauropsida
Superorder: Dinosauria
Order: Saurischia
Suborder: Theropoda
Family: Proceratosauridae
Genus: Guanlong
Species: Guanlong wucaii
Morphological description
Guanlong wucaii was an early-diverging tyrannosauroid dinosaur. The holotype, IVPP V14531, had a total body length of approximately 3 m. It possessed several cranial features characteristic of Tyrannosauroidea, including a tall premaxillary body, fused nasals, a large frontal contribution to the supratemporal fossa, a pneumatic foramen in the jugal, and a short retroarticular process of the mandible. The premaxillary teeth were relatively small and arranged more transversely, with U-shaped crowns that were thick labiolingually; the maxillary and dentary teeth were likewise relatively thick. The mandible had a comparatively large external mandibular fenestra, an extremely weak surangular ridge, and apparently lacked a surangular foramen.
The nasals of Guanlong formed a large cranial crest. Its central portion consisted of a highly pneumatic, thin bony plate that was only about 1.5 mm thick along most of its length and was supported laterally by four laminae, producing a large, complex, yet delicate crest. Other diagnostic features included a deep, narrow groove along the anterior margin of the premaxilla, a distinct opening near the premaxilla–maxilla contact, a low rugose ridge along the midline of the frontals, and a dorsally flattened parietal bearing two parallel sagittal crests.
The forelimbs were proportionally long, reaching nearly 60% of hindlimb length. The humerus exceeded 60% of femoral length, while the manus was approximately 140% of humeral length. The ulna was bowed posteriorly, the manual phalangeal formula was 2-3-4, and the wrist possessed a semilunate carpal. The pelvis retained several relatively primitive features. The ilia were inclined medially but did not contact one another, and the ischium bore a thin sheet of bone pierced by a large foramen. Pedal digit I was attached to the posterior margin of metatarsal II, producing a partially reversed hallux. This combination of primitive and more derived features is one of the most distinctive aspects of the anatomy of Guanlong.
The juvenile specimen IVPP V14532 differed markedly from the adult holotype in several ontogenetic features. The juvenile had a proportionally shorter snout, a larger orbit, shorter teeth, a longer manus relative to the humerus, a longer pubis with a less-developed anterior expansion of the pubic boot, and relatively longer distal hindlimb segments. Several of these juvenile features were closer to the conditions seen in more derived coelurosaurs.
Etymology
The generic name Guanlong is derived from the Chinese words guan, meaning "crown," and long, meaning "dragon," referring to its prominent cranial crest.
The specific name wucaii is derived from the Chinese wucai, meaning "five colours," in reference to the richly coloured rocks from which the specimens were recovered.
Biological description
Guanlong wucaii lived during the Oxfordian Stage of the Late Jurassic and is known from the Shishugou Formation in the Wucaiwan area of the Junggar Basin, Xinjiang, China. When first described in 2006, it was the oldest known tyrannosauroid, extending the reliable fossil record of the group substantially back into the Jurassic. The holotype, IVPP V14531, preserves most of the skeleton but is only partly articulated. A second specimen, IVPP V14532, is considerably smaller and consists of a nearly complete, largely articulated skeleton.
Both specimens were recovered from a bed of finely laminated to massive, tan-coloured tuffaceous mudstone whose sedimentological characteristics indicate a paludal, or wetland, environment. Their preservation records different postmortem histories. The smaller individual, IVPP V14532, probably died in situ and may later have been trampled by the larger IVPP V14531. It was buried before prolonged subaerial exposure could cause extensive disarticulation. In contrast, IVPP V14531 appears to have remained exposed at the surface for a considerable period after death, accounting for its lower degree of articulation.
Histological analysis allowed researchers to reconstruct the ages and developmental stages of both individuals. The holotype IVPP V14531 appears to have reached full adult size at about seven years of age and died during its twelfth year of life, when it had entered a relatively old, late stationary stage of growth. The juvenile IVPP V14532 died during its sixth year and was still in the exponential stage of development. Its bone cortices remained highly vascularized, growth lines near the periosteal surface were broadly spaced, only a thin single lamella of endosteal bone was present, and only a few Haversian canals had developed. These features indicate that Guanlong was still growing rapidly during its later juvenile stage.
Phylogenetic analysis placed Guanlong near the base of Tyrannosauroidea. In all of the most parsimonious trees recovered in the original analysis, it fell outside the clade formed by Tyrannosauridae and Dilong. Guanlong combined characteristic tyrannosauroid features with primitive tetanuran traits and anatomical features more typical of derived coelurosaurs, indicating a complex mosaic pattern of morphological evolution during the early diversification of Coelurosauria. Guanlong was also larger than Dilong, which did not conform to the simple evolutionary trend toward progressively increasing body size in more derived tyrannosauroids proposed by earlier studies. Together with its numerous autapomorphies, these features led the authors to interpret Guanlong as a specialized lineage in the early evolution of Tyrannosauroidea.
The enormous cranial crest of Guanlong may have functioned in visual display, species recognition, or courtship. Its size and complexity exceeded those of any other non-avian theropod crest known at the time of its description, leading researchers to suggest that it may have been a sexually selected ornament. Such a large and delicate structure could have imposed a viability cost on its bearer while providing advantages in courtship or social signalling.
The predominant tooth-replacement pattern in Guanlong involved replacement teeth developing on the lingual side of the functional teeth and gradually resorbing their predecessors. Only a small proportion of replacement teeth developed directly beneath the functional teeth. In the right dentary of the juvenile specimen, for example, such teeth accounted for only about 5% of the dentition. Most replacement teeth therefore spent the majority of their development outside the pulp cavities of the preceding functional teeth.
（Author: Shui-Ye You）
References
Ke YH et al. (2024). High-resolution CT-scan data reveals the tooth replacement pattern of the Late Jurassic tyrannosauroid Guanlong wucaii (Dinosauria, Theropoda). Vertebrata Palasiatica.
Xu X et al. (2006). A basal tyrannosauroid dinosaur from the Late Jurassic of China. Nature.
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