Psittacosaurus fossils

Psittacosaurus

Psittacosaurus is a genus of psittacosaurid ceratopsian dinosaur from the Early Cretaceous Period of what is now Asia. It is notable for being the most species-rich dinosaur genus. At least ten extinct species are recognized from dinosaur fossils found in different regions of modern-day China, Mongolia and Russia.

All species of Psittacosaurus were gazelle-sized bipedal herbivores 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, however recent research suggests these may not be quills or bristles but rather collagen fibrils. Psittacosaurs were extremely early ceratopsians and, while they developed many novel adaptations of their own, they also shared many anatomical features with later ceratopsians, such as Protoceratops and Triceratops dinosaurs.

Psittacosaurus is not as familiar to the general public as its distant relative Triceratops but it is one of the most completely known dinosaur genera. Dinosaur bones of over 400 individual dinosaur skeletons have been collected so far, including many complete fossil skeletons. Most different age classes are represented, from hatchling through to adult, which has allowed several detailed studies of Psittacosaurus growth rates 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.
Different species of Psittacosaurus varied in size and specific features of the dinosaur skull and dinosaur skeleton, but shared the same overall body shape. The best-known species, P. mongoliensis, reached 2 metres (6.5 ft) in length. The maximum adult body weight was most likely over 20 kilograms (44 lb) in P. mongoliensis. Several species approached P. mongoliensis in size (P. major, P. neimongoliensis, P. xinjiangensis), while others were somewhat smaller (P. sinensis, P. meileyingensis). P. ordosensis was the smallest known species, 30% smaller than P. mongoliensis. The largest were P. lujiatunensis and P. sibiricus, although neither was significantly larger than P. mongoliensis.

The skull of Psittacosaurus was highly modified compared to other ornithischian dinosaurs of its time. The dinosaurs skull was extremely tall and short, with an almost round profile in some species. The portion in front of the orbit (eye socket) was only 40% of total skull length, shorter than any other known ornithischian. The lower jaws of psittacosaurs are characterized by a bulbous vertical ridge down the center of each tooth. Both upper and lower jaws sported a pronounced beak, formed from the rostral and predentary bones, respectively. The bony core of the beak may have been sheathed in keratin to provide a sharp cutting surface for cropping plant material. As the generic name suggests, the short skull and beak superficially resembled those of modern parrots. Psittacosaurus skulls shared several adaptations with more derived ceratopsians, such as the unique rostral bone at the tip of the upper jaw, and the flared jugal (cheek) bones. However, there was still no sign of the bony neck frill or prominent facial horns which would develop in later ceratopsians.

Psittacosaurus postcranial dinosaur skeletons were more typical of a 'generic' bipedal ornithischian. In P. mongoliensis, similarly to other species, the forelimbs were only 58% as long as the hindlimbs, indicating that these animals were almost totally bipedal in life. There were only four digits on the manus ('hand'), as opposed to the five found in most other ornithischians (including all other ceratopsians). Overall, the four-toed hindfoot was very similar to many other small ornithischians.

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