Psittacosaurus, meaning “parrot lizard,” is a genus of ceratopsian dinosaur from the Early Cretaceous Period of what is now Asia (130 to 100 million years ago). It is the most species-rich dinosaur genus with nine to eleven recognized species from fossils found in different regions of modern-day China, Mongolia and Russia. There is also a possible species from Thailand.

Psittacosaurus was named by Henry Fairfield Osborn, paleontologist and president of the American Museum of Natural History (AMNH). He published his nomenclature in a paper on October 19, 1923. The generic name is suggested by the superficially parrot-like beak of the animal, and of course, their reptilian nature.

Psittacosaurus was considered a semi-aquatic dinosaur, swimming through the use of its tail similar to crocodiles, and through paddling and kicking. Findings of this was based on interpretation of evidence including: the lake setting of many specimens, the position of the nostril and eyes, interpretations of the arms and legs, and a fin-like tail, providing a propulsive surface. Presence of gastroliths also gives rise to a semi-aquatic nature, as they would provide some ballast.

Psittacosaurus was a bipedal herbivore characterized by a high, powerful beak on the upper jaw. At least one species had long, quill-like structures on its tail and lower back, possibly serving as a display function. It was an extremely early ceratopsian, and while it developed many novel adaptations, it also shared many anatomical features with later ceratopsian dinosaurs, such as Protoceratops and Triceratops.

While not as famous to the general public as its distant relative Triceratops, it is one of the most completely known dinosaur genera. More than 400 individuals have been collected so far, including many complete skeletons. The collected specimens represent many different age classes, from hatchling through old age, which had allowed for several detailed studies of its growth rate and reproductive biology. The abundance of this dinosaur in the fossil record has led to its use as an index fossil for Early Cretaceous sediments of central Asia.

Species of Psittacosaurus varied in size and specific features of the skull and skeleton, but all shared the same overall body shape. The average size for all specimens was around 6.5 feet in length and weighing about 44 pounds. Recognized species include: P. mongoliensis, P. major, P. neimongoliensis, P. xinjiangensis, P. sinensis, P. meileyingensis, P. lujiatunensis and P. sibiricus.

The skull of Psittacosaurus was highly modified compared to other ornithischian dinosaurs of its time. The skull was extremely tall in height and short in length, with a nearly round profile in some species. The portion in front of the eye socket was only 40 percent of total skull length, shorter than any other known ornithischian. The lower jaws of Psittacosaurus are characterized by a bulbous vertical ridge down the center of each tooth. Both upper and lower jaws sported a pronounced beak. The bony core of the beak may have been sheathed in keratin to provide a sharp cutting surface for cropping plant material.

The forelimbs of Psittacosaurus were a little more than half as long as the hind limbs and their range of motion indicates that the hands could neither be pronated nor used to generate propulsive force, indicating it was totally bipedal. There were only four digits on the hand, as opposed to the five found in most other ornithischians. Overall, the four-toed hind foot was very similar to many other small ornithischians.