page_title: Psittacosaurus
section_title: Paleobiology
hierarchical_section_title: Psittacosaurus / Paleobiology
caption_attribution_description: Skull of Psittacosaurus lujiatunensis (formerly major), LHPV 1, Near Beipiao City, Liaoning Province, China, lower portion of the Lower Cretaceous Yixian Forma− tion, in lateral (A) and dorsal (B) views.
context_page_description: Psittacosaurus is a genus of extinct ceratopsian dinosaur from the Early Cretaceous of what is now Asia, existing between 126 and 101 million years ago. It is notable for being the most species-rich dinosaur genus. Up to 12 species are known, from across China, Mongolia, Siberia, and possibly Thailand and Laos. The species of Psittacosaurus were obligate bipeds at adulthood, with a high skull and a robust beak. One individual was found preserved with long filaments on the tail, similar to those of Tianyulong, and scales across the rest of the animal. Psittacosaurus probably had complex behaviours, based on the proportions and relative size of the brain. It may have been active for short periods of time during the day and night, and had well-developed senses of smell and vision.
  Psittacosaurus was one of the earliest ceratopsians, but closer to Triceratops than Yinlong. Once in its own family, Psittacosauridae, with other genera like Hongshanosaurus, it is now considered to be senior synonym of the latter and an early offshoot of the branch that led to more derived forms.
context_section_description: The brain of P. lujiatunensis is well known; a study on the anatomy and functionality of three specimens was published in 2007. Until the study, it was generally thought the brain of Psittacosaurus would have been similar to other ceratopsians with low Encephalisation Quotients. Russell and Zhao (1996) believed "the small brain size of psittacosaurs implies a very restrictive behavioural repertoire relative to that of modern mammals of similar body size". However, the 2007 study dispelled this theory when it found the brain to be more advanced. There is generally negative allometry for brain size with development in vertebrates, but this was shown not to be true in Psittacosaurus. The EQ score for P. lujiatunensis is 0.31, significantly higher than genera such as Triceratops. A higher EQ correlates with more complex behaviour, and various dinosaurs have high EQs, similar to birds, which range from 0.36 to 2.98. Thus, Psittacosaurus behaviour could have been as complex as that in Tyrannosaurus, whose EQ ranges from 0.30 to 0.38. Behaviours influenced by high EQs include nest-building, parental care, and bird-like sleeping, some of which have been shown to be present in Psittacosaurus.
  The senses of Psittacosaurus can be inferred from the endocast. Large olfactory bulbs are present, indicating the genus had an acute sense of smell. The size of these bulbs are comparable to large predatory theropods, although they likely evolved to avoid predators instead of to seek out prey. The sclerotic rings in reptiles directly show the size of the eyeball. The rings are not well preserved in Psittacosaurus, with one individual preserving them likely contracted postmortem, but if they are similar to those of Protoceratops, Psittacosaurus would have had large eyes and acute vision. The curvature of the semicircular canals is related to the agility of reptiles, and the large curved canals in Psittacosaurus show that the genus was much more agile than later ceratopsians. Comparisons between the scleral rings of Psittacosaurus and modern birds and reptiles suggest that it may have been cathemeral, active throughout the day and for short intervals at night.
  Ford and Martin (2010) proposed that Psittacosaurus was semi-aquatic, swimming with its tail like a crocodile, and paddling and kicking. They based their interpretation on evidence including: the lacustrine (lake) depositional setting of many specimens; the position of the nostrils and eyes; interpretations of the motions of the arms and legs; tails with long chevrons (and with the bristles on the tail interpreted as possibly skin-covered, forming a fin), providing a propulsive surface; and the presence of gastroliths, interpreted as ballast. They further suggested that some species of Psittacosaurus were more terrestrial than others.
caption: P. lujiatunensis skull LHPV 1 from the left and above
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