Psittacosaurus mongoliensis
Age
Cretaceous（Aptian-Albian）
121.4-100.5 Ma
Taxonomy
Kingdom: Animalia
Phylum: Chordata
Class: Sauropsida
Superorder: Dinosauria
Order: Ornithischia
Clade: Ceratopsia
Family: Psittacosauridae
Genus: Psittacosaurus
Species: Psittacosaurus mongoliensis
Morphological description
Psittacosaurus mongoliensis was a small- to medium-sized ceratopsian dinosaur characterized by a short, deep skull, a parrot-like keratinous beak, and a robust lower jaw. The snout possessed a rostral bone, a distinctive feature of ceratopsians, while the external nares were small and dorsally positioned. The jugal projected ventrolaterally to form a horn-like process, and the mandible bore a prominent triangular coronoid process. Compared with other psittacosaurs, P. mongoliensis had a proportionally longer snout, a greater number of teeth with relatively smaller crowns, an upturned prefrontal margin, a more primitive postorbital morphology and ornamentation, and a relatively straight retroarticular process. Some larger specimens retained incompletely fused cranial sutures. The jaw joint had a relatively flat articular surface, allowing anteroposterior sliding movements of the lower jaw.
Juveniles had proportionally larger orbits and braincases, an arched skull roof, and a shorter snout, with neural arches that had not yet fused to the vertebral centra. The basicranial lengths of AMNH 6536 and AMNH 6535 were only approximately 42 and 28 mm, respectively. Their postcranial anatomy indicates a gracile body, with elongate scapulae, curved femora, and tridactyl hind feet, consistent with bipedal locomotion.
Etymology
Genus name, Psittacosaurus: Derived from psittaco-, meaning "parrot," and -saurus, meaning "lizard," referring to its distinctive parrot-like, hooked beak.
Species name, mongoliensis: Refers to Mongolia, the region where the fossils were discovered.
Biological description
Psittacosaurus mongoliensis was a herbivorous ceratopsian dinosaur that inhabited central Asia during the Cretaceous Period. The holotype specimen, AMNH 6254, was discovered in the Oshih Formation of the Artsa Bogdo Basin in Mongolia. Two exceptionally small juvenile specimens, AMNH 6535 and AMNH 6536, were subsequently recovered from the same locality. Additional specimens representing developmental stages from juveniles to larger individuals were collected from the Khukhteeg Formation at Khamrin-Us, Mongolia. These include specimens PIN 1369/1-11/1948, PIN 698/1977, PIN 698/15/1977, and PIN 698/5-3/1946; femora PIN 698/4-22/1946, PIN 698/7-4/1946, PIN 698/1977, PIN 698/15/1977, and PIN 698/2-2/1977-15; and tibiae PIN 698/4-22/1946, PIN 698/1946, PIN 698/1977, PIN 698/2-2/1977-15, and PIN 698/5-9/2/1946. Taphonomic evidence suggests that these animals were buried in deltaic sands deposited across a tropical floodplain. The same environment supported tyrannosaurs, stegosaurs, ankylosaurs, turtles, lizards, and triconodont mammals (Triconodonta). A partial skull of P. mongoliensis, designated LH PV3, was also discovered in the Bayan Gobi Formation of Inner Mongolia, where it occurred alongside the smaller-bodied Psittacosaurus gobiensis. This association suggests that two psittacosaur species differing substantially in body size may have coexisted in at least some regions.
The total body length of the juvenile AMNH 6536 was estimated at approximately 39.5 cm by scaling down the proportions of an adult skeleton, while AMNH 6535 was estimated to measure approximately 26.5 cm. The latter was even smaller than a common pigeon (Columba livia) and was among the smallest dinosaur specimens known when it was described in 1980. Both juveniles exhibited clear evidence of tooth wear, indicating that they had already begun processing abrasive vegetation at an early age. Their well-ossified limb bones further suggest that they were precocial, capable of relatively independent locomotion and feeding soon after hatching.
These exceptionally small juveniles were estimated to have weighed only about 0.7% as much as adults. Researchers therefore suggested that direct feeding and parental care after hatching would have been difficult, given the enormous size difference between adults and their offspring. The juveniles may also have faced a considerable risk of being accidentally crushed by larger individuals. Consequently, some researchers proposed that hatchling P. mongoliensis may have begun foraging independently soon after emergence from their eggs. Their extremely high surface-area-to-body-mass ratio would also have caused them to lose heat much more rapidly than adults, leading to the hypothesis that such tiny juveniles were unlikely to possess fully developed avian- or mammalian-like endothermic physiology.
Psittacosaurs possessed relatively rigid upper and lower jaws that lacked significant cranial kinesis, yet their teeth developed oblique shearing wear facets. To explain this unusual combination, researchers proposed a jaw mechanism termed clinolineal shear. During the power stroke of occlusion, the lower jaw moved posterodorsally, while the posterior divergence of the tooth rows accommodated oblique contact between opposing teeth without requiring transverse expansion of either jaw. Following each shearing stroke, the lower jaw was depressed and repositioned anteriorly in preparation for the next occlusal cycle.
Well-preserved psittacosaur skeletons are frequently associated with numerous relatively large gastroliths. Large gastroliths are often associated with the processing of coarse, hard food materials, suggesting that psittacosaurs may have specialized in plant resources with high fibre and energy content, such as hard-shelled seeds and nuts. Their powerful bite, specialized oblique shearing mechanism, and apparent ability to process mechanically resistant plant material together provide evidence for the probable feeding ecology of P. mongoliensis.
Histological studies have reconstructed the growth history of P. mongoliensis using individuals representing successive developmental stages. Estimated body masses increased from approximately 0.94 kg at three years of age to 2.03 kg at four years, 3.02 kg at five years, 6.96 kg at six years, and 8.79 kg at seven years. Growth then accelerated markedly, with body mass reaching approximately 17.16 kg at eight years and 19.87 kg at nine years. The largest nine-year-old specimen approached the maximum size known for the species at the time. However, because the reconstructed growth curve had not yet reached a complete plateau, the estimated mass of 19.87 kg may underestimate the true maximum adult body mass.
Psittacosaurus mongoliensis also exhibited an unusual pattern of skeletal growth. As individuals aged, portions of their long bones became progressively more highly vascularized, with some of the largest specimens developing radially vascularized bone tissue. This pattern contrasts with the more typical condition in many dinosaurs, in which bone formation gradually slows as individuals approach skeletal maturity, suggesting that localized osteoblastic activity remained unusually high during later development. Several explanations have been proposed, including changes in skeletal loading, shifts in muscle attachment sites during growth, or a possible transition from facultative bipedality toward more frequent quadrupedal locomotion. Pathological bone formation was also considered, although no evidence of callus formation was observed. The available evidence remains insufficient to distinguish among these hypotheses.
（Author: Shui-Ye You）
References
Coombs Jr WP et al. (1980). Juvenile ceratopsians from Mongolia—the smallest known dinosaur specimens. Nature.
Erickson GM et al. (2000). Growth curve of Psittacosaurus mongoliensis Osborn (Ceratopsia: Psittacosauridae) inferred from long bone histology. Zoological Journal of the Linnean Society.
Mayr G et al. (2016). Structure and homology of Psittacosaurus tail bristles. Palaeontology.
Sereno PC et al. (2010). A new psittacosaur from Inner Mongolia and the parrot-like structure and function of the psittacosaur skull. Proc Biol Sci.
Zhao Q et al. (2014). Juvenile-only clusters and behaviour of the Early Cretaceous dinosaur Psittacosaurus. Acta Palaeontologica Polonica.
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