Psittacosaurus

Psittacosaurus (/ˌsɪtəkəˈsɔːrəs/ SIT-ə-kə-SOR-əs; 'parrot lizard') 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 Mongolia, Siberia, China, and possibly Thailand. 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. The genera closely related to Psittacosaurus are all from Asia, with the exception of Aquilops, from North America. The first species was either P. lujiatunensis or closely related, and it may have given rise to later forms of Psittacosaurus. Psittacosaurus is one of the most completely known dinosaur genera. Fossils of hundreds of individuals have been collected so far, including many complete skeletons. Most 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 the labelling of Lower Cretaceous sediments of east Asia the Psittacosaurus biochron. The species of Psittacosaurus vary in size and specific features of the skull and skeleton, but share the same overall body shape. The best-known—P. mongoliensis—can reach 2 metres (6.5 ft) in length. The maximum adult body weight was most likely over 20 kilogrammes (44 lb) in P. mongoliensis. Several species approach P. mongoliensis in size (P. lujiatunensis, P. neimongoliensis, P. xinjiangensis), while others are somewhat smaller (P. sinensis, P. meileyingensis). The smallest known species, P. ordosensis, is 30% smaller than P. mongoliensis. The largest are P. lujiatunensis and P. sibiricus, although neither is significantly larger than P. mongoliensis. Psittacosaurus postcranial skeletons are more typical of a 'generic' bipedal ornithischian. There are only four digits on the manus ('hand'), as opposed to the five found in most other ornithischians (including all other ceratopsians), while the four-toed hindfoot is very similar to many other small ornithischians. The skull of Psittacosaurus is highly modified compared to other ornithischian dinosaurs of its time. Extremely tall in height and short in length, the skull has an almost round profile in some species. The portion in front of the orbit (eye socket) is only 40% of total skull length, shorter than any other known ornithischian. The lower jaws of psittacosaurs are characterised by a bulbous vertical ridge down the centre of each tooth. Both upper and lower jaws sport 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 resemble those of modern parrots. Psittacosaurus skulls share 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. There is still no sign of the bony neck frill or prominent facial horns which would develop in later ceratopsians. Bony horns protrude from the skull of P. sibiricus, but these are thought to be an example of convergent evolution. The integument, or body covering, of Psittacosaurus is known from a Chinese specimen, which most likely comes from the Yixian Formation of Liaoning Province, China. The specimen, which is not yet assigned to any particular species, was illegally exported from China, in violation of Chinese law, but was purchased by the Senckenberg Museum in Germany. It was described while awaiting repatriation. Most of the body was covered in scales. Larger scales were arranged in irregular patterns, with numerous smaller scales occupying the spaces between them, similarly to skin impressions known from other ceratopsians, such as Chasmosaurus. A series of what appear to be hollow, tubular bristle-like structures, approximately 16 centimetres (6.3 in) long, were also preserved, arranged in a row down the dorsal (upper) surface of the tail. These were confirmed by the authors, as well as an independent scientist, to not represent plant material. The bristle-like integumentary structures extend into the skin nearly to the vertebrae, and were likely circular or tubular before being preserved. Under ultraviolet light, they gave off the same fluorescence as scales, providing the possibility they were keratinized. The study stated that, 'at present, there is no convincing evidence which shows these structures to be homologous to the structurally different integumentary filaments of theropod dinosaurs'. However, they found that all other feather-like integument from the Yixian Formation could be identified as feathers. In 2008, another study was published describing the integument and dermis of Psittacosaurus sp., from two different specimens. The skin remains could be observed by a natural cross-section to compare them to modern animals, showing that dinosaurian dermal layers evolved in parallel to those in many other large vertebrates. The collagen tissue fibres in Psittacosaurus are complex, virtually identical to all other vertebrates in structure but having an exceptional thickness of about forty layers. As the sections of dermis were collected from the abdomen, where the scales were eroded, the tissue may have assisted with the musculature of the stomach and intestines and offered protection against predators.