Size comparison of P. mongoliensis
to a human. Each grid segment represents one square metre.
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.
Skeletal restoration of the P. mongoliensis
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.
Integument and pigmentation
sp. specimen with preserved tail integument and pigmentation, Senckenberg Museum
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.
Model based on SMF R 4970
As described in a 2016 study, examination of melanosomes preserved in the specimen of Psittacosaurus preserved with integument indicated that the animal was countershaded, likely due to preferring a habitat in dense forests with little light, much like many modern species of forest-dwelling deer and antelope; stripes and spots on the limbs may represent disruptive coloration. The specimen also had dense clusters of pigment on its shoulders, face (possibly for display), and cloaca (which may have had an antimicrobial function), as well as large patagia on its hind legs that connected to the base of the tail. Its large eyes indicate that it also likely had good vision, which would have been useful in finding food or avoiding predators. The authors pointed out that there might have been variation in coloration across the range of the animal, depending on differences in the light environment. The authors were unable to determine which species of Jehol Formation Psittacosaurus the specimen belonged to due to the way the skull is preserved, but ruled out P. mongoliensis, based on hip features.
Another 2016 study used laser-stimulated fluorescence imaging to analyze the internal structure of the bristles. The highly cornified bristles are arranged in tight clusters of three to six individual bristles, with each bristle being filled with pulp. The authors considered the bristles as being homologous to the quills of Tianyulong, the elongated broad filamentous feathers (EBFFs) of Beipiaosaurus, and possibly the feathers of birds as well. Similar bristles are found in extant birds such as the horned screamer; these structures differ from feathers in that they do not develop from a follicle, but both arise from discrete cell populations. A darkened soft-tissue structure was also found near the jugal horn; this may represent a keratinous sheath or a skin flap.
Restored heads of eight species, to scale
Skulls of P. mongoliensis are flat on top, especially over the back of the skull, with a triangular depression, the antorbital fossa, on the outside surface of the maxilla (an upper jaw bone). A flange is present on the lower edge of the dentary (the tooth-bearing bone of the lower jaw), although it is not as prominent as in P. meileyingensis or P. major (=P. lujiatunensis). P. mongoliensis is among the largest known species. The skull of the type specimen, which is probably a juvenile, is 15.2 centimetres (6 in) long, and the associated femur is 16.2 centimetres (6.4 in) in length. Other specimens are larger, with the largest documented femur measuring about 21 centimetres (8.25 in) long.
P. sinensis is readily distinguished from all other species by numerous features of the skull. Adult skulls are smaller than those of P. mongoliensis and have less teeth. Uniquely, the premaxillary bone contacts the jugal (cheek) bone on the outside of the skull. The jugals flare out sideways, forming 'horns' proportionally wider than in any other known Psittacosaurus species except P. sibiricus and P. lujiatunensis. Because of the flared cheeks, the skull is actually wider than it is long. A smaller 'horn' is present behind the eye, at the contact of the jugal and postorbital bones, a feature also seen in P. sibiricus. The mandible (lower jaw) lacks the hollow opening, or fenestra, seen in other species, and the entire lower jaw is bowed outwards, giving the animal the appearance of an underbite. The skull of an adult P. sinensis can reach 11.5 centimeters (4.5 in) in length.
P. sibiricus is the largest-known species of Psittacosaurus. The skull of the type specimen is 20.7 centimetres long (8.25 in), and the femur is 22.3 cm (8.75 in) in length. It is also distinguished by its neck frill, which is longer than any other species, at 15 to 18% of skull length. A very striking feature of P. sibiricus is the number of 'horns' around the eyes, with three prominences on each postorbital, and one in front of each eye, on the palpebral bones. Similar horns found on the postorbital of P. sinensis are not as pronounced but may be homologous. The jugal has extremely prominent 'horns' and may contact the premaxilla, both features also seen in the possibly related P. sinensis. There is a flange on the dentary of the lower jaw, similar to P. mongoliensis, P. meileyingensis, and P. sattayaraki. It can be told apart from the other species of Psittacosaurus by a combination of 32 anatomical features, including six that are unique to the species. Most of these are skull details, but one unusual feature is the presence of 23 vertebrae between the skull and pelvis, unlike the 21 or 22 in the other species where the vertebrae are known.
P. xinjiangensis is distinguished by a prominent jugal 'horn' that is flattened on the front end, as well as some features of the teeth. The ilium, one of the three bones of the pelvis, also bears a characteristically long bony process behind the acetabulum (hip socket). An adult femur has a published length of about 16 centimetres (6.3 in). P. meileyingensis has the shortest snout and neck frill of any species, making the skull nearly circular in profile. The orbit (eye socket) is roughly triangular, and there is a prominent flange on the lower edge of the dentary, a feature also seen in specimens of P. lujiatunensis, and to a lesser degree in P. mongoliensis, P. sattayaraki, and P. sibiricus. The complete type skull, probably adult, is 13.7 centimetres (5.5 in) long. The dentary of P. sattayaraki has a flange similar to that found in P. mongoliensis, P. sibiricus, P. lujiatunensis and P. meileyingensis, although it is less pronounced than in those species. The material appears to be roughly the same size as P. sinensis. The frontal bone of P. neimongoliensis is distinctly narrow compared to that of other species, resulting in a narrower skull overall. The ischium bone of the pelvis is also longer than the femur, which differs from other species in which these bones are known. The type specimen has a skull length of 13.2 centimetres (5.2 in) and a femoral length of 13 centimetres (5.1 in), but is not fully grown. An adult P. neimongoliensis was probably smaller than P. mongoliensis, with a proportionately longer skull and tail. P. ordosensis can be distinguished by numerous features of the jugals, which have very prominent 'horns'. It is also the smallest known species. One adult skull measures only 9.5 centimeters (3.75 in) in length.
The type skull of P. lujiatunensis measures 19 cm (7.5 in) in length, while the largest-known skull is 20.5 centimetres (8 in) long, so this species was similar in size to P. mongoliensis and P. sibiricus. There is a fossa in front of the eye, as in P. mongoliensis. The jugal bones flare outwards widely, making the skull wider than it is long, as seen in P. sinensis. Widely flared jugals are also found in P. sibiricus. Overall, this species is thought to exhibit several primitive characteristics compared to other species of Psittacosaurus, which is consistent with its greater geological age. P. gobiensis was small-bodied (one metre (3 ft 3 in) long) and differs from other species of Psittacosaurus by "significant, but structurally minor, details." These include the presence of a pyramidal horn on the postorbital, a depression on the postorbital-jugal contact, and enamel thickness. P. mongoliensis was a contemporary.