Psittacosaurus

Psittacosaurus (/ˌsɪtəkəˈsɔrəs/ SIT-ə-kə-SOR-əs; from the Greek for "parrot lizard") is a genus of psittacosaurid ceratopsian dinosaur from the Early Cretaceous Period of what is now Asia, between 123 to 100 million years ago.[2] 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 of Psittacosaurus 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.[3] The maximum adult body weight was most likely over 20 kilograms (44 lb) in P. mongoliensis.[4] Several species approached P. mongoliensis in size (P. lujiatunensis, P. neimongoliensis, P. xinjiangensis),[5][6][7] while others were somewhat smaller (P. sinensis, P. meileyingensis).[8] P. ordosensis was the smallest known species, 30% smaller than P. mongoliensis.[6] The largest were P. lujiatunensis and P. sibiricus, although neither was significantly larger than ''P. mongoliensis'.[9][10]

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 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. There was still no sign of the bony neck frill or prominent facial horns which would develop in later ceratopsians.[11] Bony horns did protrude from the skull of P. sibiricus, but these are thought to be an example of convergent evolution.

Psittacosaurus postcranial 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 and their range of motion indicates that the hands could neither be rotated at the forearm (pronated) nor used in scrambling fashion, to generate propulsive force, suggesting that these animals were totally bipedal in life.[12] 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.

Daily activity patterns
Comparisons between the scleral rings of Psittacosaurus and modern birds and reptiles suggest that it may have been cathemeral, active throughout the day and night at short intervals.

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.[3]

Unlike many other dinosaurs, psittacosaurs had akinetic skulls: that is to say, the upper and lower jaws each behaved as a single unit, without internal joints. The only joint was the jaw joint itself, and psittacosaurs could slide their lower jaws forward and backward on the joint, permitting a shearing action. Unlike most ceratopsians, their beaks did not form curved tips, but were instead rounded and flattened. If the jaws were aligned, the beaks could be used to crop objects, but if the lower jaw was retracted so that the lower beak was inside the upper beak, the jaws may have served a nutcracking function. A nut- or seed-rich diet would also match well with the gastroliths often seen in well-preserved psittacosaur skeletons.

Integument
The integument, or body covering, of Psittacosaurus is known from a Chinese specimen, which most likely comes from the Yixian Formation of Liaoning. 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.[15]

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 "[a]t present, there is no convincing evidence which shows these structures to be homologous to the structurally different feathers and protofeathers of theropod dinosaurs."[15] 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.

Growth rate
Several juvenile Psittacosaurus have been found. The smallest is a P. mongoliensis hatchling conserved in the American Museum of Natural History (AMNH), which is only 11 to 13 centimeters (4–5 inches) long, with a skull 2.8 centimeters (1 in) in length. Another hatchling skull at the AMNH is only 4.6 centimeters (1.8 inches) long. Both specimens are from Mongolia.[16] Juveniles discovered in the Yixian Formation are approximately the same age as the larger AMNH specimen.[17]

A histological examination of P. mongoliensis has determined the growth rate of these animals. The smallest specimens in the study were estimated at three years old and less than 1 kilogram (2.2 lb), while the largest were nine years old and weighed almost 20 kilograms (44 lb). This indicates relatively rapid growth compared to most reptiles and marsupial mammals, but slower than modern birds and placental mammals.[4] An age determination study performed on the fossilized remains of Psittacosaurus mongoliensis by using growth ring counts suggest that the longevity of the basal ceratopsian was 10 to 11 years.

Limb function
Studies by Phil Senter in 2007 conducted on Psittacosaurus neimongoliensis and Psittacosaurus mongoliensis concluded that the forelimbs of these taxa (and likely those of other Psittacosaurus species) were too short to reach the ground and could neither be pronated nor generate propulsive force for locomotion, suggesting that Psittacosaurus was entirely bipedal. The forelimbs were also too short to be used in digging or bringing food to the mouth, and Senter suggested that if Psittacosaurus needed to dig depressions in the ground it may have used its hindlimbs instead. The forelimbs could be used for two-handed grasping of objects or scratching the body, but due to their extremely limited flexibility and reach, they could have only been used to grasp objects very close to the belly or sides of the animal and could have scratched only the belly, flank and knees. Even though the hands could not reach the mouth, Psittacosaurus could have still used them to carry nesting material or food to a desired location.[12] New findings may dispute the hypothesis of Psittacosaurus being entirely bipedal thanks to the studies of Qi Zhao from the University of Bristol. Taking sections from the limb bones from 16 specimens of Psittacosaurus, ranging in age from less than a year old to ten-year old adults, Zhao found that Psittacosaurus was probably secondarily bipedal. The infants' front limbs grew at faster rates than the hind limbs at between birth and three years of age. At the age of between four and six years, arm growth slowed and leg growth accelerated as the animal became mature. At this stage, Psittacosaurs would switch to a bipedal stance. These findings further reveal that the ancestor of Psittacosaurus was likely quadrupedal and eventually gained the ability to become bipedal as it evolved, with the young retaining the quadrupedal gait of the ancestor in question. These findings also lead to the hypothesis that many such dinosaur families may have evolved along this path at some point in their evolution.

Gregarious juveniles
The find of a herd of six Psittacosaurus individuals killed and buried by a volcanic mudflow indicates the presence of at least two age groups from two distinct clutches gathered together. This find has been taken as evidence for group fidelity and gregariousness extending beyond the nest; the earliest such evidence for any ceratopsian.[20] Even very young psittacosaur teeth appear worn, indicating they chewed their own food and may have been precocial.[16] Another juvenile-only cluster shows that specimens of different ages grouped together. These juveniles may have associated together as a close knit, mixed-age herd either for protection, to enhance their foraging, or as putative helpers at the parental nest.[21] There is no evidence for parental care.[22]

In 2004, a specimen found in the Yixian Formation of Liaoning Province, China was claimed as evidence for parental care in dinosaurs. The specimen DNHM D2156 consists of 34 articulated juvenile Psittacosaurus skeletons, closely associated with the skull of an adult. The juveniles, all approximately the same age, are intertwined in a group underneath the adult, although all 34 skulls are positioned above the mass of bodies, as they would have been in life. This suggests that the animals were alive at the time of burial, which must have been extremely rapid, perhaps due to the collapse of a burrow.[17] However, a 2013 paper pointed out that the adult specimen did not belong with the nest, its skull having no sedimentary connection to the main slab where the juveniles occurred, but had been glued onto it. This artificial association led to the inference that the skull belonged to an individual, possibly a "mother", that was providing parental care for the 34 juveniles - a claim that is unfounded. Furthermore, the adult was also shown to be six years old, whereas histological studies have shown P. mongoliensis was unable to breed until it reached ten years of age. It is also unlikely that a single female would have so many offspring at one time.[22]

A 2014 analysis of the same specimen concluded that the proximity of the six-year old specimen to the post-hatchlings may indicate post-hatchling cooperation, making the six-year old specimen a possible caretaker.[23][24] Such behavior is also found in cooperative breeding modern birds.

Possible aquatic behavior
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.

Predation
Another fossil from the Yixian Formation provides direct evidence of Psittacosaurus as a prey animal. One skeleton of Repenomamus robustus, a large triconodont mammal, is preserved with the remains of a juvenile Psittacosaurus in its abdominal cavity. Several of the juvenile's bones are still articulated, indicating that the carnivorous mammal swallowed its prey in large chunks. This specimen is notable in that it is the first known example of Mesozoic mammals preying on live dinosaurs.[27] Heavy predation on juvenile Psittacosaurus may have resulted in R-selection, the production of more numerous offspring to counteract this loss.

Pathology
Out of over 400 known Psittacosaurus specimens, only one has been published with any sort of pathology. The specimen in question, consisting of a complete adult skeleton and tentatively assigned to P. mongoliensis, was found in the lower beds of the Yixian Formation. There is no sign of a bone fracture, but very clear signs of an infection can be seen near the midpoint of the right fibula. The bone exhibits a large round pit, evidence of necrosis due to a lack of blood supply to the region. The pit is surrounded by a massive amount of swelling along the lower third of the bone. This large amount of bone deposited around the injury indicates that the animal survived for quite a while despite the injury and subsequent infection. As psittacosaurids were bipedal animals, a similar injury to a weight bearing bone in the leg would likely have been fatal. Unlike the femur and tibia, the fibula is not a weight-bearing bone, so this animal would still have been able to walk to some extent. The source of the injury remains unknown.