Caudipteryx zoui
Age
Cretaceous（Late Barremian）
125.6-124.6 Ma
Taxonomy
Kingdom: Animalia
Phylum: Chordata
Class: Sauropsida
Superorder: Dinosauria
Order: Saurischia
Suborder: Theropoda
Clade: Oviraptorosauria
Family: Caudipteridae
Genus: Caudipteryx
Species: Caudipteryx zoui
Morphological description
Caudipteryx zoui was a small, feather-covered theropod dinosaur. The holotype, NGMC 97-4-A, described in the original study measured approximately 89 cm in total length, whereas the paratype, NGMC 97-9-A, measured about 72.5 cm. The skull was relatively short, primarily because of the reduced antorbital region. The premaxilla was large and formed most of the margin of the large external naris, whereas the maxilla and nasal were short and the frontal and jugal were relatively long. The symphysis of the dentary had a broad, beak-like anterior margin, and a well-developed sliding intramandibular joint was present between the dentary and suprangular. Teeth were restricted to the premaxilla. The original description reported four elongate, slightly procumbent, hooked teeth on each side, with nearly needle-like crowns and very broad roots; both the maxilla and dentary were edentulous. Later specimens likewise confirmed that the teeth were confined to the premaxilla and showed a distinct constriction at the base of the crown.
The 1998 description identified 10 amphicoelous cervical vertebrae and five sacral vertebrae. In 2000, the more completely preserved specimens BPM 0001 and IVPP V12430 were estimated to have approximately 12 cervical and nine dorsal vertebrae, resulting in a revision of the reconstructed cervical count. The tail contained only about 22 unfused caudal vertebrae and was approximately one-quarter of the body length. The anterior chevrons were elongate and rod-like, whereas the more posterior chevrons became lower and expanded both anteriorly and posteriorly. Uncinate processes were present on the dorsal ribs.
The pectoral girdle included paired sternal plates. The scapula was longer than the humerus, and the clavicles were fused to form a broad U-shaped furcula. Specimens described in 2000 showed that the distal end of the scapula was slightly expanded and that the coracoid bore a prominent biceps tubercle and a simple supracoracoid foramen. The forelimbs were relatively short, and the metacarpals remained unfused. The manual phalangeal formula was initially reconstructed as the typical theropod condition of 2-3-4, but two more complete specimens later showed that the third digit was strongly reduced and contained only two short phalanges, leading researchers to revise the formula to 2-3-2. The third digit lay closely appressed to the second digit, and its terminal ungual was also extremely reduced.
The pelvic bones were unfused, and the acetabulum was large, measuring approximately one-quarter of the length of the ilium. The ischium possessed a large obturator process. Material described in 1998 was interpreted as showing some features of an opisthopubic pelvis, whereas the more complete specimens described in 2000 were interpreted as having a pubis directed anteroventrally, more closely resembling the condition in other oviraptorosaurs. The hindlimbs were substantially longer than the forelimbs. The fibula extended to the ankle and contacted the calcaneum. The astragalus, calcaneum, and tibia remained unfused, as did the metatarsals, with metatarsal III being the longest. Metatarsal I articulated with the posteromedial surface of metatarsal II, indicating that the first digit was at least partially reversed.
The plumage is exceptionally well preserved. The forelimb of the holotype preserves at least 14 remiges attached to metacarpal II, the first phalanx of digit II, and the base of the second phalanx of digit II. The fourth distal primary measured approximately 9.5 cm long, slightly exceeding the length of the humerus. These feathers had clearly defined rachises and vanes, with the two sides of each vane being approximately symmetrical. A fan of rectrices was present at the tip of the tail. The holotype preserves 10 complete and two incomplete rectrices, with 11 identifiable on the left side. Because these were presumably paired with corresponding feathers on the right side, the original tail fan may have consisted of approximately 11 pairs. The rectrices were restricted to the last five or six caudal vertebrae. The body was also covered with short, down-like feathers, particularly well preserved around the hips and base of the tail, reaching lengths of approximately 1.4 cm. The fossilized tail feathers also preserve banded differences in coloration.
Etymology
The genus name Caudipteryx means "tail feather," referring to the well-developed feathers at the end of the tail.
The specific name zoui honors Zou Jiahua, then Vice Premier of the State Council of China, who was a long-standing supporter of scientific research in Liaoning Province.
Biological description
Caudipteryx zoui was formally described in 1998. Its original stratigraphic occurrence was reported from the Jiulongsong Member of the Chaomidianzi Formation in the Sihetun area of Liaoning Province. Subsequent studies have generally discussed these fossils within the context of the Early Cretaceous Yixian Formation and the Jehol Biota. The original phylogenetic analysis placed Caudipteryx zoui outside Avialae and regarded it as the sister taxon to avialans. Later studies more commonly recovered Caudipteryx within Oviraptorosauria, a maniraptoran lineage closely related to Oviraptor and its relatives.
Early studies interpreted Caudipteryx zoui as primarily a terrestrial, bipedal runner. In 1998, researchers noted its relatively elongate hindlimbs and the elevated, anteromedially directed position of the first toe. This configuration lacked the highly specialized grasping foot of modern perching birds and was considered more consistent with terrestrial locomotion. The forelimbs of Caudipteryx zoui were shorter than those of most long-armed non-avian coelurosaurs. Its remiges were only slightly longer than the humerus, and the most distal feathers were shorter than the more proximal ones. In addition, the vanes were approximately symmetrical, in marked contrast to the asymmetrical flight feathers associated with efficient powered flight in birds. These features led researchers to conclude that Caudipteryx was probably incapable of true flight.
Large numbers of semiplumes and down-like feathers are preserved around the body of Caudipteryx zoui. Based on this evidence, the researchers who described the animal in 1998 suggested that most of the body was probably feather-covered and proposed insulation as an important early function of feathers. Caudipteryx zoui already possessed true pennaceous feathers, including remiges and rectrices, despite lacking the ability to fly. Its anatomy therefore provides evidence that the evolution of feathers and the evolution of flight did not necessarily occur simultaneously.
A 2018 aerodynamic study directly tested what the wings of Caudipteryx might have been capable of doing and estimated a representative body mass of approximately 5 kg. Each reconstructed wing measured about 24 cm in mediolateral length and had an area of approximately 0.01797 m², while the estimated maximum running speed was about 8 m/s. Even when the animal was modeled running at approximately 8 m/s with both wings fully extended laterally, classical aerodynamic calculations produced only about 1.4 N of total lift and 0.6 N of total drag, compared with a body weight of approximately 49 N. Other fixed-wing models using different angles of attack still generated less than about 4 N of total aerodynamic force at 8 m/s, equivalent to only around 8% of body weight. The feathered forelimbs were therefore incapable of lifting the body from the ground and could not have produced sufficient lift to support gliding from an elevated position.
The most direct fossil evidence concerning diet comes from the abdominal cavity. Both NGMC 97-4-A and NGMC 97-9-A preserve concentrations of numerous small, smooth, rounded stones in the stomach region. The largest are approximately 0.45 cm in diameter, although most are smaller than 0.4 cm, and they were identified as gastroliths. Later specimens likewise preserve masses of gastroliths within the abdominal cavity. Gastroliths alone cannot establish the precise diet of the animal. However, when discussing the possible function of the feathered forelimbs, the 2018 study followed the prevailing interpretation of basal oviraptorosaurs at the time and regarded them as probably having a predominantly herbivorous diet.
STM4-3, described in a 2021 study, is a well-preserved, partially articulated specimen collected from the Yixian Formation near Dapingfang Town, Chaoyang City, western Liaoning. This specimen was identified only to the genus Caudipteryx. Portions of the feathers and body outline remain preserved. Researchers sampled cartilage from the distal articular surface of the right femur for palaeohistological analysis. After demineralization, large numbers of exceptionally well-preserved chondrocyte structures were revealed. The cells could broadly be divided into transparent alumino-silicified cells and brown, striated ironized cells, while the transparent cells could themselves be separated into smooth-surfaced and porous forms. The researchers proposed that the porous cells may have been undergoing cell death at the time mineralization occurred. The same specimen also preserves a small fragment of alumino-silicified muscle tissue without obvious iron infiltration.
H&E histological staining revealed a sharply defined purple circular structure inside one of the transparent chondrocytes, with dark-purple thread-like structures visible within it. When compared with modern chicken chondrocytes processed using the same technique, its position, morphology, and staining pattern were similar. The researchers interpreted these structures as preserved cytoplasm, nuclear membrane, nucleus, and condensed chromatin threads. The chromatin-like threads remained visible under high magnification and polarized light. However, the study did not demonstrate the preservation of sequenceable ancient DNA in Caudipteryx. Although H&E staining reacts with components associated with nucleic acids in the nuclei of living cells, H&E staining alone is insufficient to demonstrate that Mesozoic DNA remains within the fossil. The evidence instead supports the conclusion that at least some of the original chemical differentiation between the nucleus and cytoplasm of the cartilage cells was preserved, together with fossilized nuclear material displaying the morphology and staining characteristics of chromatin.
（Author: Shui-Ye You）
References
Dyke GJ et al. (2005). Caudipteryx as a non-avialan theropod rather than a flightless bird. Acta Palaeontologica Polonica.
Qiang J et al. (1998). Two feathered dinosaurs from northeastern China. Nature.
Talori YS. (2018). Winged forelimbs of the small theropod dinosaur Caudipteryx could have generated small aerodynamic forces during rapid terrestrial locomotion. Scientific Reports.
Zheng X et al. (2021). Nuclear preservation in the cartilage of the Jehol dinosaur Caudipteryx. Communications Biology.
Zhou ZH et al. (2000). Important features of caudipteryx-evidence from two nearly complete new specimens. Vertebrata Palasiatica.
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