Psittacosaurus

Psittacosaurus  is a genus of psittacosaurid ceratopsian dinosaur from the Early Cretaceous Period of what is now Asia, between 123 to 100 million years ago. It is notable for being the most species-rich dinosaur genus. Nine to eleven species are recognized from fossils found in different regions of modern-day China, Mongolia and Russia, with a possible additional species from Thailand.

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, possibly serving a display function. Psittacosaurs were extremely early ceratopsians. Although they developed many novel adaptations, they shared many anatomical features with later ceratopsians such as Protoceratops and Triceratops.

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. Fossils of over 400 individuals have been collected so far, including many complete skeletons. Most different age classes are represented, from hatchling through to adult, which has allowed several detailed studies ofPsittacosaurus growth rates and reproductive biology. The abundance of this dinosaur in the fossil record has led to the creation of the Psittacosaurus biochron for Lower Cretaceous sediments of east Asia.

Description
Different species of Psittacosaurus varied in size and specific features of the skull and skeleton, but shared the same overall body shape. The best-known species, P. mongoliensis, reached 2 meters (6.5 ft) in length. The maximum adult body weight was most likely over 20 kilograms (44 lb) inP. mongoliensis. Several species approached P. mongoliensis in size (P. lujiatunensis, 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 skull was extremely tall in height and short in length, 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 croppingplant material. As the generic name suggests, the short skull and beak superficially resembled those of modern parrots. Psittacosaurus skulls shared several adaptations with more derivedceratopsians, such as the unique rostral bone at the tip of the upper jaw, and the flared jugal (cheek) bones. There was still no sign of the bony neck frill or prominent facial horns which would develop in later ceratopsians. Bony horns did protrude from the skull of P. sibiricus, but these are thought to be an example of convergent evolution.

Diet
Psittacosaurs had self-sharpening teeth that would have been useful for cropping and slicing tough plant material. Unlike later ceratopsians, they did not have teeth suitable for grinding or chewing their food. Instead, they used gastroliths, stones swallowed to wear down food as it passed through the digestive system. Gastroliths, sometimes numbering more than fifty, are occasionally found in the abdominal cavities of psittacosaurs, and may have been stored in a gizzard, as in modern birds.

Integument
The integument, or body covering, of Psittacosaurus is known from a Chinese specimen, which most likely comes from the Yixian Formation. 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 a German museum and arrangements are being made to return the specimen to China. This specimen likely represents P. sibiricus or P. mongoliensis, due to the cheek horns. 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 bristles, approximately 16 centimeters (6.4 in) long, were also preserved, arranged in a row down the dorsal (upper) surface of the tail. According to some scientists "at present, there is no convincing evidence which shows these structures to be homologous to the structurally different feathers and protofeathers of theropod dinosaurs." As the structures are only found in a single row on the tail, it is unlikely that they were used for thermoregulation, but they may have been useful for communication through some sort of display.

Provenance
Psittacosaurus fossil from China preserved on a slab alongsideOrdosemys liaoxiensis, displayed inShanghai Ocean Aquarium

Psittacosaurus is known from over 400 individual specimens, of which over 75 have been assigned to the type species, P. mongoliensis. All Psittacosaurus fossils discovered so far have been found in Early Cretaceous sediments in Asia, from southern Siberia to northern China, or possibly as far south as Thailand. The most common age of geologic formations bearingPsittacosaurus fossils is from the late Barremian through Albian stages of the Early Cretaceous, or approximately 125 to 100 Ma (million years ago). Many terrestrial sedimentary formations of this age in Mongolia and northern China have produced fossils of Psittacosaurus, leading its use as in defining the Psittacosaurus biochron for this time period in the region. The earliest known species is P. lujiatunensis, found in the lowest beds of the Yixian Formation. Over 200 specimens attributed to this genus have been recovered from these and other beds of the Yixian, the age of which is the subject of much debate. Although many early studies using radiometric dating put the Yixian in the Jurassic Period, tens of millions of years outside of the expected temporal range of Psittacosaurus, most recent work dates it to the Early Cretaceous. Using argon-argon dating, a team of Chinese scientists dated the lowest beds in the formation to about 128 Ma, and the highest to approximately 122 Ma. A more recent Chinese study, using uranium-lead dating, suggests that the lower beds are younger, approximately 125 Ma, while agreeing with an age of 122 Ma for the upper beds.

Taxonomy
Seventeen species have been referred to the genus Psittacosaurus, although only nine to eleven are considered valid today. This is the highest number of valid species currently assigned to any single dinosaur genus (not including birds). In contrast, most other dinosaur genera are monospecific, containing only a single known species. The difference is most likely due to quirks of the fossil record. While Psittacosaurus is known from hundreds of fossil specimens, most other dinosaur species are known from far fewer, and many are represented by only a single specimen. With a very high sample size, the diversity of Psittacosaurus can be analyzed more completely than that of most dinosaur genera, resulting in the recognition of more species. Most extant animal genera are represented by multiple species, suggesting that this may have been the case for extinct dinosaur genera as well, although most of these species may not have been preserved. In addition, most dinosaurs are known solely from bones and can only be evaluated from a morphological standpoint, whereas extant species often have very similar skeletal morphology but differ in other ways which would not normally be preserved in the fossil record, such as behavior, or coloration. Therefore actual species diversity may be much higher than currently recognized in this and other dinosaur genera. Psittacosaurus is the type genus of the family Psittacosauridae, which was also named by Osborn in 1923. While Psittacosauridae was an early branch of the ceratopsian family tree, Psittacosaurus itself was probably not directly ancestral to any other groups of ceratopsians. All other ceratopsians retained the fifth digit of the hand, a plesiomorphy or primitive trait, whereas all species of Psittacosaurus had only four digits on the hand. In addition, the antorbital fenestra, an opening in the skull between the eye socket and nostril, was lost during the evolution of Psittacosauridae, but is still found in most other ceratopsians and in fact most other archosaurs. It is considered highly unlikely that the fifth digit or antorbital fenestra would evolve a second time. It has been suggested that P. lujiatunensis is the most primitive species, as it appears earlier in the fossil record and appears to show more basal characteristics.