Seventeen species have been assigned to the extinct ceratopsian dinosaur genus Psittacosaurus, although only nine to eleven are considered valid.[1][2][3][4] This is the greatest number of valid species assigned to any single dinosaur genus (not including birds).[5] By contrast, most dinosaur genera contain only a single species.
As some species are known only from skull material, species of Psittacosaurus are primarily distinguished by features of the skull and teeth. Several species can be recognized by features of the pelvis as well.[6] Overall size estimates of most species have not been published or are unavailable due to lack of fossil preservation. However, measurements of the skull or femur have been published for all well-established species and can be used as a basis for comparison. Species are listed in the order in which they were described, except for synonyms, which are described with the senior synonym.
Psittacosaurus mongoliensis is the type species of the genus, named by American paleontologist Henry Fairfield Osborn in 1923. Remains of this dinosaur were first discovered the year before, on the third American Museum of Natural History expedition to the Gobi Desert of Mongolia, when one of the expedition's drivers found the type specimen (AMNH 6254).[7] This same expedition turned up the remains of many other famous Mongolian dinosaurs, including Protoceratops, Oviraptor, and Velociraptor.[8] Many later expeditions by various combinations of Mongolian, Russian, Chinese, American, Polish, Japanese, and Canadian paleontologists also recovered specimens from throughout Mongolia and northern China. In these areas, Psittacosaurus mongoliensis fossils are found in most sedimentary strata dating to the Aptian to Albian stages of the Early Cretaceous Period, or approximately 125 to 100 Ma (million years ago). Fossil remains of over 75 individuals have been recovered, including nearly 20 complete skeletons with skulls.[9] Individuals of all ages are known, from hatchlings less than 13 centimeters (5 in) long, to very old adults reaching nearly 2 meters (6.5 ft) in length.[10]
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. mongoliensis is among the largest known species. The skull of the type specimen, which is probably a juvenile,[11] is 15.2 centimeters (6 in) long, and the associated femur is 16.2 centimeters (6.4 in) in length.[12] Other specimens are larger, with the largest documented femur measuring about 21 centimeters (8.25 in) long.[13]
When describing Psittacosaurus mongoliensis in 1923, Osborn also gave the name Protiguanodon mongoliense to another skeleton found nearby, believing it to represent an ancestor of the ornithopod Iguanodon.[7] When the skeleton was prepared further, it became clear that it was nearly identical to Psittacosaurus mongoliensis.[12] In 1958, Chinese paleontologist Yang Zhongjian (better known as C.C. Young) renamed the skeleton Psittacosaurus protiguanodonensis.[14] Today the specimen is generally referred to the species Psittacosaurus mongoliensis and the names Protiguanodon mongoliense and Psittacosaurus protiguanodonensis are considered junior synonyms of the name Psittacosaurus mongoliensis, which was coined first.[4][9]
In 1931, C.C. Young named a new species of Psittacosaurus for a partial skull discovered in Inner Mongolia, China.[15] The skull was named P. osborni after Henry Fairfield Osborn. The validity of this species is now considered equivocal. Sereno (1990) considered it a synonym of P. mongoliensis, which is found in nearby strata of the same age.[16] You and Dodson (2004) listed it as valid in a table, but not in their text.[9] In a 2010 review, Sereno again regarded P. osborni as a synonym of P. mongoliensis, but noted it was tentative because of the presence of multiple valid psittacosaur species in Inner Mongolia.[4]
Young described the species Psittacosaurus tingi in the same 1931 report which contained P. osborni. It is based on several skull fragments.[15] He later synonymized the two species under the name P. osborni.[14] You and Dodson (2004) followed this in a table,[9] but Sereno regarded both species as synonyms of P. mongoliensis;[4][16] a table in the latter reported P. tingi as a nomen dubium, however.[4]
The front half of a skull from Guyang County in the Inner Mongolian Autonomous Region of China was described as Psittacosaurus guyangensis in 1983. Disarticulated postcranial remains representing multiple individuals were found at the same locality and were assigned to the species.[17] While it differs from the type specimen of P. mongoliensis, it falls within the range of individual variation seen in other specimens of that species and is no longer recognized as a valid species.[16] You and Dodson (2004) included P. guyangensis in a table of valid taxa, but did not include it as such in their text.[9]
In the 1950s, a new Chinese species of Psittacosaurus was found in the Aptian-Albian Qingshan Formation of Shandong Province, southeast of Beijing. C.C. Young called it Psittacosaurus sinensis to differentiate it from P. mongoliensis, which had originally been found in Mongolia.[14] Fossils of more than twenty individuals have since been recovered, including several complete skulls and skeletons, making this the most well-known species after P. mongoliensis.[9]
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.[1][16] The skull of an adult P. sinensis can reach 11.5 centimeters (4.5 in) in length.[11]
Chinese paleontologist Zhao Xijin named a new species after his mentor, C.C. Young, in 1962.[18] However, the type specimen of Psittacosaurus youngi (a partial skeleton and skull) was discovered in the same rocks as P. sinensis and appears to be very similar, so P. youngi is generally considered a junior synonym of that better known species.[4][16] As with P. guyangensis and P. osborni, You and Dodson (2004) listed it as valid in a table, but not in their text.[9]
In 1988, Zhao and American paleontologist Paul Sereno described Psittacosaurus xinjiangensis, named after the Xinjiang Autonomous Region in which it was discovered.[19] Several individuals of different ages were discovered in the early 1970s by Chinese paleontologists and described by Sereno and Zhao, although the holotype and most complete skeleton belonged to a juvenile. An adult skeleton was later discovered at a different locality in Xinjiang.[6] These specimens come from the upper part of the Tugulu Group, which is regarded as Aptian-Albian in age.
This species 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).[1] An adult femur has a published length of about 16 centimeters (6.3 in).[6]
A second species described in 1988 by Sereno and Zhao, along with two Chinese colleagues, was Psittacosaurus meileyingensis from the Jiufotang Formation, near the town of Meileyingzi, Liaoning Province, northeastern China. This species is known from four fossil skulls, one associated with some skeletal material, found in 1973 by Chinese scientists.[20] The age of the Jiufotang in Liaoning is unknown, but in the neighboring province of Inner Mongolia, it has been dated to about 110 Ma, in the Albian stage of the Early Cretaceous.[21]
P. meileyingensis has the shortest snout and neck frill of any species, making the skull is 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 P. major, and to a lesser degree in P. mongoliensis, P. lujiatunensis, P. sattayaraki, and P. sibiricus.[1][2] The complete type skull, probably adult, is 13.7 centimeters (5.5 in) long.[20]
French paleontologist Eric Buffetaut and a Thai colleague, Varavudh Suteethorn, described a partial upper and lower jaw from the Aptian-Albian Khok Kruat Formation of Thailand in 1992, giving it the name Psittacosaurus sattayaraki.[22] In 2000, Sereno questioned the validity of this species, citing its eroded and fragmentary nature, and noted an absence of features characteristic of the genus Psittacosaurus.[23] However, in 2002 the original authors published new images of the fossil which seem to show teeth in the lower jaw that exhibit the bulbous vertical ridge characteristic of psittacosaurs.[24] Other authors have also defended its validity,[2] while some continue to regard it as dubious.[1][4][9] Sereno (2010) proposed that the best assignment for the type material may be Ceratopsia incertae sedis.[4]
The dentary of P. sattayaraki has a flange similar to that found in P. mongoliensis, P. sibiricus, P. major, 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.[22] If this species is valid, it is by far the southernmost species in the genus.
Two new species of Psittacosaurus were described by Canadian Dale Russell and Zhao in 1996. The first was named Psittacosaurus neimongoliensis, after the Mandarin Chinese name for Inner Mongolia. It is based on a nearly complete fossil skeleton, including most of the skull, found in the Early Cretaceous Ejinhoro Formation with seven other individuals.[11]
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.[1] The type specimen has a skull length of 13.2 centimeters (5.2 in) and a femoral length of 13 centimeters (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.[11]
Russell and Zhao also named Psittacosaurus ordosensis in 1996, after the Ordos prefecture of the Inner Mongolia Autonomous Region. The type specimen is a nearly complete skeleton, including part of the skull. However, only the skull, lower jaw, and foot have been described. Three other specimens were referred to this species but remain undescribed. Like P. neimongoliensis, this species was discovered in the Eijnhoro Formation.[11]
P. ordosensis can be distinguished by numerous features of the jugals, which have very prominent 'horns.'[1] It is also the smallest known species. One adult skull measures only 9.5 centimeters (3.75 in) in length.[11] Sereno (2010) found the species as described to be indistinguishable from P. sinensis, another small species, but suggested that additional study of P. ordosensis might reveal diagnostic features. He provisionally designated P. ordosensis a nomen dubium.[4]
Xu Xing, another Chinese paleontologist, named a new species of Psittacosaurus in 1997, based on a complete skull with associated vertebrae and a forelimb. This material was recovered in Gansu Province, near the border with Inner Mongolia. This species is named Psittacosaurus mazongshanensis after the nearby mountain called Mazongshan (Horse Mane Mountain) and has been described in a preliminary manner.[25] Unfortunately, the skull was damaged while in the care of the Chinese Institute of Vertebrate Paleontology and Paleoanthropology (IVPP), and several fragments have been lost, including all of the teeth.[26] The remains were found in the Lower Xinminbao Formation, which have not been precisely dated, although there is some evidence that they were deposited in the late Barremian through Aptian stages.[27]
Sereno suggested in 2000 that P. mazongshanensis was a nomen dubium, with no unique features that separate it from any other species of Psittacosaurus.[23] However, more recent authors have noted that it can be distinguished by its proportionally long snout compared to other species of Psittacosaurus, as well as a prominent bony protuberance, pointing outwards and downwards, on the maxilla of the upper jaw.[2] The maxillary protuberance is also now missing.[4][26] Other features originally used to distinguish the species have been recognized as the results of the deformation of the skull after fossilization.[26] Sereno (2010) remained unconvinced of its validity.[4]
Beginning in the 1950s, Russian paleontologists began excavating Psittacosaurus remains at a locality near the village of Shestakovo in the oblast of Kemerovo in Siberia. Two other nearby localities were explored in the 1990s, one of which produced several complete skeletons. This species was named Psittacosaurus sibiricus in 2000 in a scientific paper written by five Russian paleontologists, but credit for the name is officially given to two of those authors, Alexei Voronkevich and Alexander Averianov.[28] The remains were not completely described until 2006. Two nearly complete, articulated skeletons and a variety of disarticulated material from other individuals of all ages are known from the Ilek Formation of Siberia, which ranges from the Aptian to Albian stages of the Early Cretaceous.[2]
P. sibiricus is the largest known species of Psittacosaurus. The skull of the type specimen is 20.7 centimeters 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 its 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.[2]
Psittacosaurus lujiatunensis, named in 2006 by Chinese paleontologist Zhou Chang-Fu and 3 Chinese colleagues, is one of the oldest known species, based on four skulls from the lower beds of Yixian Formation of Liaoning Province, near the village of Lujiatun.[26] This bed has been dated differently by different authors, from 128 Ma in the Barremian stage,[29] to 125 Ma in the earliest Aptian.[30] P. lujiatunensis was contemporaneous with another psittacosaurid species, Hongshanosaurus houi, which was found in the same beds.[31] It is potentially synonymous with H. houi; Sereno (2010), who proposed that Hongshanosaurus is a synonym of Psittacosaurus, opted to leave P. lujiatunensis and H. houi separate species due to the inadequacies of the latter's type specimen.[4]
The type skull of P. lujiatunensis measures 19 cm (7.5 in) in length, while the largest known skull is 20.5 centimeters (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 and P. major. P. lujiatunensis lacks the postorbital horns seen in P. sinensis or P. major. Overall, this species is thought to exhibit several primitive characteristics compared to other species of Psittacosaurus, which is consistent with its greater geological age.[26]
Psittacosaurus major is from the lower beds of the Yixian Formation near Beipiao City in Liaoning Province. P. major, named for the large size of its skull, was described by Sereno, Zhao and two colleagues in 2007. Known from a nearly complete skeleton with a skull, it appears to be about the same age as P. lujiatunensis.[32] You and colleagues described an additional specimen and concurred that it was distinct from P. lujiatunensis.[3]
P. major is characterized by a proportionately large skull, which was 39% of the length of its torso, compared to 30% in P. mongoliensis. The type skull measures 20.3 centimeters (8 in), longer than the 19.7 centimeter (7.75 in) femur. These bones are larger than the type of P. mongoliensis but within the size range seen in that species. In addition to its large size, the skull is characterized by an extremely narrow snout and jugal 'horns' which are longer than in any species except P. sinensis, P. lujiatunensis and P. xinjiangensis. The 'horns' are angled slightly downwards instead of directly outwards as seen in those species. The flange on the dentary resembles that of P. meileyingensis but is even more prominent.[32] Although its contemporary P. lujiatunensis also had a large skull, P. major can be distinguished by general proportions: P. major had a long narrow skull, while P. lujiatunensis had a broad skull.[3]
The most recently described species is Psittacosaurus gobiensis from Inner Mongolia. P. gobiensis, named for the region it was found in 2001, is a small-bodied (1 metre (3.3 ft) long) dinosaur described by Sereno, Zhao and Lin in 2010. It is known from a skull and partial articulated skeleton with gastroliths. P. gobiensis 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.[33]
Many other specimens either cannot be determined to belong to any particular species, or have not yet been assigned to one. These specimens are generally all referred to as Psittacosaurus sp., although it is not assumed that they belong to the same species.[9]
More than 200 specimens of Psittacosaurus have been found in the Yixian Formation of Liaoning Province, which is famous for its fossils of feathered dinosaurs. The vast majority of these have not been assigned to any published species, although many are very well preserved and some have already been partially described.[34][35][36]
Nearly 100 Psittacosaurus skeletons were excavated in Mongolia during the summers of 2005 and 2006 by a team led by Mongolian paleontologist Bolortsetseg Minjin and American Jack Horner from the Museum of the Rockies in Montana. Although only P. mongoliensis has been described from Mongolia so far, these specimens are still in preparation and have not yet been assigned to a species.[37]