Psittacosaurus (/ˌsɪtəkəˈsɔːrəs/SIT-ə-kə-SOR-əs; "parrot lizard")[1] is agenus of extinctceratopsian dinosaur from the EarlyCretaceous of what is nowAsia, existing between 125 and 105million years ago. It is notable for being the mostspecies-rich non-avian dinosaur genus. Up to 13 species are known, from acrossChina,Mongolia,Russia, andThailand. The species ofPsittacosaurus wereobligate 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 ofTianyulong.Psittacosaurus probably had complex behaviours, based on the proportions and relative size of thebrain. 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 toTriceratops thanYinlong. Once in its own family, Psittacosauridae, with other genera likeHongshanosaurus, it is now considered to besenior synonym of the latter and an early offshoot of the branch that led to morederived forms. The genera closely related toPsittacosaurus are all from Asia, with the exception ofAquilops, fromNorth America. The first species was eitherP. lujiatunensis or closely related, and it may have given rise to later forms ofPsittacosaurus.
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, fromhatchling 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 labelling of Lower Cretaceous sediments ofeast Asia thePsittacosaurusbiochron.
In1922, American paleontologistHenry Fairfield Osborn took part in the Third Asiatic Expedition of theAmerican Museum of Natural History to discover fossils and geologic formations from theCretaceous andTertiary ofMongolia. In theOshih Formation of the Artsa Bogdo Basin, Wong, the Mongolian chauffeur, discovered a nearly completeskull,jaws, andskeleton of a dinosaur, which was given the nickname of "Red Mesa skeleton".[2][3] The location of discovery is also known as the Oshih locality of theKhukhtek Formation, ofEarly CretaceousAptian toAlbian age.[4] The specimen, catalogued as AMNH 6254, was described in1924 by Osborn, only partially prepared, who gave it the namePsittacosaurus mongoliensis, describing its parrot-like beak on the suggestion of fellow American paleontologistWilliam King Gregory. Osborn demonstrated the taxon was unique based on the short and deep snout, and the broad rear skull, as well as by lacking teeth in thepremaxilla.[2] In the same paper, Osborn also described another new taxon he considered similar toPsittacosaurus,Protiguanodon mongoliense, which was found in the same expedition but from theOndai Sair Formation. Theholotype ofProtiguanodon, AMNH 6253, included a nearly complete skeleton found articulated, and partial remains of the skull. While Osborn consideredProtiguanodon andPsittacosaurus separate based on the lack of horns on thejugal bones inProtiguanodon, a general dissimilarity in the skeletons, and wide geographic separation of the two specimens, Gregory suggested in correspondence that theProtiguanodon specimen could represent a juvenile ofPsittacosaurus, based on similarities in size, theparietal bones, and thequadrate bones. Osborn created the new familyPsittacosauridae forPsittacosaurus, which he considered possibly related toAnkylosauria, while he placedProtiguanodon within the familyIguanodontidae as the only member of the new subfamilyProtiguanodontinae.[2]
Osborn published an additional description of the specimens ofProtiguanodon andPsittacosaurus in1924, citing his previous study as naming both to be members of Psittacosauridae, and considering the separate status of Protiguanodontinae as uncertain. Further preparation of the skeleton of AMNH 6254 showed significant similarities in the skeletons ofPsittacosaurus andProtiguanodon, including the number of teeth, the number of pre-caudalvertebrae, and other details of the skull and skeleton. Osborn also referred the specimen AMNH 6261 from the Oshih Formation toPsittacosaurus, so the teeth of the two taxa could be compared.[3] It was mentioned in1932 by American paleontologistRoy Chapman Andrews that AMHN 6254 was the only good specimen that could be found at Oshih, with only one additional skull and jaws of an adult, and two hatchling skulls, having been found in a later revisit to the locality in 1923.[5]
Following the discovery of material of psittacosaurids inHaratologay inInner Mongolia,Yang Zhongjian described two additional species in 1932. Known from a crushed skull and fragmentary lower jaw, Young namedPsittacosaurus osborni, distinguished by its small size and lack of asagittal crest on the parietal. The second species,P. tingi, was named for partial lower jaws and teeth, which Young only tentatively referred toPsittacosaurus instead ofProtiguanodon.[6] Both specimens, stored in theInstitute of Vertebrate Paleontology and Paleoanthropology as IVPP RV31039 and IVPP RV31040 respectively, come from theXinpongnaobao Formation.[7] An additional tooth, partialhand, and fragments of vertebrae and limbs were found in the same locality, with the tooth being referred toProtiguanodon and the remainder of the material being uncertain.[6] AdditionalPsittacosaurus material from possibly the same locality was described later in1953 byBirger Bohlin, who considered the remains to likely belong toP. mongoliensis.[8][9]
The Soviet Expeditions into Mongolia from1946 to1949 uncovered more material ofPsittacosaurus. In 1946 they discovered a new locality, Ulan Osh, where a disarticulated specimen ofPsittacosaurus mongoliensis was found, and in 1948 they revisited the sites of the American expeditions and excavated fragmentary postcrania from Oshih and Ondai Sair. The material from these expeditions was taken to thePaleontological Institute ofMoscow.[7] Soviet excavations nearKemerovo inSiberia also discovered a partial skull and skeleton of multiple individuals referrable toPsittacosaurus. This material was described by Soviet paleontologistAnatoly Rozhdestvensky in1955, who also proposed thatProtiguanodon mongoliense,Psittacosaurus osborni, andPsittacosaurus tingi werejunior synonyms ofPsittacosaurus mongoliensis.[10][7]
In1958, Yang published a paper on the dinosaurs ofLaiyang, in which he described multiple discoveries ofPsittacosaurus from a collection of localities of theQingshan Formation. Of this material, the nearly complete skeleton and skull IVPP V738 was described as the type of the new speciesPsittacosaurus sinensis, which was found in a red layer 10 mi (16 km) northwest of Rongyang City inShandong. Yang also assigned 11 other specimens to the taxon, considering it to be the most diversePsittacosaurus species known at the time. It was distinguished from the other known species by a shorter and wider snout, and an overall smaller size at 675 mm (26.6 in). Yang also revised the classifications of the other species ofPsittacosaurus. Following similar conclusions to Rozhdestvensky, Yang consideredProtiguanodon to be a junior synonym ofPsittacosaurus, but retained the species as separate giving formerProtiguanodon mongoliense the new species namePsittacosaurus protiguanodonensis, as otherwise both it andPsittacosaurus mongoliensis would have the same species name. Contrasting Rozhdestvensky, Yang retained the earlier Chinese speciesP. osborni andP. tingi as separate fromP. mongoliensis, but not separate from each other, makingP. tingi a junior synonym ofP. osborni. Following his new breakdown of species, Yang described the distribution of the genusPsittacosaurus:P. sinensis was the only species known from Shandong;P. osborni and possiblyP. mongoliensis were both known from Haratologay (also known as Tebch);P. mongoliensis andP. protiguanodonensis were both known form Oshih; andP. mongoliensis was possibly known from Kemerovo.[9]
Further discoveries in the Qingshan Formation of Laiyang in 1958 were described byZhao Xijin in1962, giving the new namePsittacosaurus youngi for the specimen BPV.149 in theBeijing Museum of Natural History. Known for a complete skull, partial vertebral series and partial pelvis,P. youngi was distinguished by Zhao by having the shortest skull of all species, vertebral and tooth counts, and various features of the skull and skeleton.P. youngi was considered to be most similar toP. sinensis, but separated them to bring the count of members of Psittacosauridae to one genus and five species.[11]
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 mostsedimentarystrata dating to theAptian toAlbianstages of the EarlyCretaceous Period, or approximately 125 to 100 mya. Fossil remains of over 75 individuals have been recovered, including nearly 20 complete skeletons with skulls.[12] Individuals of all ages are known, fromhatchlings less than 13 centimetres (5.1 in) long, to very old adults reaching nearly 2 metres (6 ft 7 in) in length.[13]
In a 2010 review, Sereno again regardedP. osborni as a synonym ofP. mongoliensis, but noted it was tentative because of the presence of multiple valid psittacosaur species in Inner Mongolia.[4] Young also described the speciesP. tingi in the same 1931 report which containedP. osborni. It is based on several skull fragments.[6] He later synonymised the two species under the nameP. osborni.[9] You and Dodson (2004) followed this in a table,[12] but Sereno regarded both species as synonyms ofP. mongoliensis;[14][4] a table in the latter reportedP. tingi as anomen dubium, however.[4] The front half of a skull fromGuyang County in Inner Mongolia was described asPsittacosaurus guyangensis in 1983. Disarticulated postcranial remains representing multiple individuals were found at the same locality and were assigned to the species.[15] While it differs from the type specimen ofP. mongoliensis, it falls within the range of individual variation seen in other specimens of that species and is no longer recognised as a valid species.[14] You and Dodson (2004) includedP. guyangensis in a table of valid taxa, but did not include it as such in their text.[12]
Seventeen species have been referred to the genusPsittacosaurus, although only nine to eleven are considered valid today.[16][17][4][18] This is the highest number of valid species currently assigned to any single non-avian dinosaur. In contrast, most other dinosaur genera aremonospecific, containing only a single known species. The difference is most likely due to artifacts of the fossilisation process. WhilePsittacosaurus 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 ofPsittacosaurus can be analysed more completely than that of most dinosaur genera, resulting in the recognition of more species. Mostextant 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 frombones and can only be evaluated from amorphological 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 behaviour, or colouration. Therefore, actual species diversity may be much higher than currently recognised in this and other dinosaur genera.[19] As some species are known only from skull material, species ofPsittacosaurus are primarily distinguished by features of the skull and teeth. Several species can be recognised by features of thepelvis as well.[20]
In the 1950s, a new Chinese species ofPsittacosaurus was found in the Aptian-AlbianQingshan Formation ofShandong Province, southeast ofBeijing. C. C. Young called itP. sinensis to differentiate it fromP. mongoliensis, which had originally been found in Mongolia.[9] Fossils of more than twenty individuals have since been recovered, including several complete skulls and skeletons, making this the most well-known species afterP. mongoliensis.[12] Chinese paleontologistZhao Xijin named a new species after his mentor, C. C. Young, in 1962.[11] However, the type specimen ofP. youngi (a partial skeleton and skull) was discovered in the same rocks asP. sinensis and appears to be very similar, soP. youngi is generally considered a junior synonym of that better-known species.[14][4] As withP. guyangensis andP. osborni, You and Dodson (2004) listed it as valid in a table, but not in their text.[12]
In 1988, Zhao and American paleontologistPaul Sereno describedP. xinjiangensis, named after theXinjiang Autonomous Region in which it was discovered.[21] 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.[20] These specimens come from the upper part of theTugulu Group, which is regarded as Aptian-Albian in age.[20]
A second species described in 1988 by Sereno and Zhao, along with two Chinese colleagues, wasP. meileyingensis from theJiufotang 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.[22] The age of the Jiufotang in Liaoning is unknown, but in the neighbouring province of Inner Mongolia, it has been dated to about 110 Ma, in the Albian stage of the Early Cretaceous.[23]
French paleontologistEric Buffetaut and a Thai colleague,Varavudh Suteethorn, described a partial upper and lower jaw from the Aptian-AlbianKhok Kruat Formation of Thailand in 1992, giving it the nameP. sattayaraki.[24] In 2000, Sereno questioned the validity of this species, citing its eroded and fragmentary nature, and noted an absence of features characteristic of the genusPsittacosaurus.[18] 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.[25] Other authors have also defended its validity,[26] while some continue to regard it as dubious.[12][17][4] Sereno (2010) proposed that the best assignment for the type material may be Ceratopsiaincertae sedis.[4]
Two new species ofPsittacosaurus were described by CanadianDale Russell and Zhao in 1996. The first was namedP. neimongoliensis, after theMandarin Chinese name for Inner Mongolia. It is based on a nearly complete fossil skeleton, including most of the skull, found in the Early CretaceousEjinhoro Formation with seven other individuals.[27] Russell and Zhao also namedP. ordosensis in 1996, after theOrdos 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. LikeP. neimongoliensis, this species was discovered in the Eijnhoro Formation.[27] Sereno (2010) found the species as described to be indistinguishable fromP. sinensis, another small species, but suggested that additional study ofP. ordosensis might reveal diagnostic features. He provisionally designatedP. ordosensis anomen dubium.[4]
Xu Xing, another Chinese paleontologist, named a new species ofPsittacosaurus in 1997, based on a complete skull with associatedvertebrae and a forelimb. This material was recovered inGansu Province, near the border with Inner Mongolia. This species is namedP. mazongshanensis after the nearby mountain called Mazongshan (Horse Mane Mountain) and has been described in a preliminary manner.[28] Unfortunately, the skull was damaged while in the care of the ChineseInstitute of Vertebrate Paleontology and Paleoanthropology (IVPP), and several fragments have been lost, including all of the teeth.[29] The remains were found in theLower Xinminbao Formation, which have not been precisely dated, although there is some evidence that they were deposited in the lateBarremian through Aptian stages.[30] Sereno suggested in 2000 thatP. mazongshanensis was anomen dubium, with no unique features that separate it from any other species ofPsittacosaurus.[18] However, more recent authors have noted that it can be distinguished by its proportionally long snout compared to other species ofPsittacosaurus, as well as a prominent bony protuberance, pointing outwards and downwards, on the maxilla of the upper jaw.[26] The maxillary protuberance is also now missing.[29][4] Other features originally used to distinguish the species have been recognised as the results of the deformation of the skull after fossilisation.[29] Sereno (2010) remained unconvinced of its validity.[4]
Beginning in the 1950s, Russian paleontologists began excavatingPsittacosaurus remains at a locality near Shestakovo village inKemerovo Oblast inWestern Siberia. Two other nearby localities were explored in the 1990s, one of which produced several complete skeletons. This species was namedP. 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 andAlexander Averianov.[31] 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 theIlek Formation of Siberia,[26] which ranges from the Barremian to Aptian stages of the Early Cretaceous.[32] Individuals of this species could grow up to 2.5 metres (8.2 ft) in length, making it one of the largest members of the genus.[33]
P. lujiatunensis, named in 2006 by Chinese paleontologistZhou Chang-Fu and three Chinese and Canadian colleagues, is one of the oldest-known species, based on four skulls from the lower beds of theYixian Formation, near the village of Lujiatun.[29] While this bed has been dated differently by different authors, from 128 Ma in the Barremian stage,[34] to 125 Ma in the earliest Aptian,[35] revised dating methods have shown them to be about 123 million years old.[36]P. lujiatunensis was contemporaneous with another psittacosaurid species,Hongshanosaurus houi, which was found in the same beds.[37] It is potentially synonymous withH. houi; Sereno (2010), who proposed thatHongshanosaurus is a synonym ofPsittacosaurus, opted to leaveP. lujiatunensis andH. houi separate species due to the inadequacies of the latter's type specimen.[4]
One nearly complete skeleton ofP. lujiatunensis from the same lower beds of the Yixian Formation had previously been classified in its own species,Psittacosaurus major, named for the large size of its skull by Sereno, Zhao and two colleagues in 2007.[16] You and colleagues described an additional specimen and concurred that it was distinct fromP. lujiatunensis.[38]P. major was originally characterised by a proportionately large skull, which was 39% of the length of its torso, compared to 30% inP. mongoliensis, and other features. However, a 2013 study utilising morphometric analysis showed that the supposed differences betweenP. lujiatunensis andP. major were due to differences in preservation and crushing. The study concluded that both represented a single species.[36]
A third species of Lujiatun psittacosaur, the first to be named, was described asHongshanosaurus houi in 2003. The generic nameHongshanosaurus was derived from theMandarin Chinese words 紅 (hóng: "red") and 山 (shān: "hill"), as well as theGreek wordsauros ("lizard"). This name refers to the ancientHongshan culture of northeastern China, who lived in the same general area in which the fossil skull ofHongshanosaurus was found. Thetype and only named species,H. houi, honoursHou Lianhai, a professor at the IVPP in Beijing, whocurated the specimen. Genus and species were both named by Chinese paleontologistsYou Hailu, Xu Xing, andWang Xiaolin in 2003. Sereno (2010) regarded its distinct proportions as due to crushing and compression of theHongshanosaurus skulls.[4] He regardedHongshanosaurus as ajunior synonym ofPsittacosaurus, and potentially the same asP. lujiatunensis. He did not synonymise the two species because of difficulties with the holotype skull ofH. houi, instead considering new combinationP. houi anomen dubium withinPsittacosaurus.[4] Sereno's hypothesis was supported by a morphometric study in 2013, which foundP. houi andP. lujiatunensis to be synonymous. WhileP. houi is the oldest available name, the researchers argued that because the type specimen ofP. lujiatunensis was better preserved, the correct name for this species should beP. lujiatunensis rather thanP. houi, which would normally have priority.[36] A new skull ofP. houi reported in 2025, however, supported thatP. lujiatunensis andP. houi are separate species based on previously unknown features and phylogenetic analyses.[39]
P. gobiensis is named for the region it was found in 2001, and first described by Sereno, Zhao and Lin in 2010. It is known from a skull and partial articulated skeleton with gastroliths.[40] 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 asPsittacosaurus sp., although it is not assumed that they belong to the same species.[12] More than 200 specimens ofPsittacosaurus have been found in the Yixian Formation, which is famous for its fossils offeathered 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.[41][42][43] Nearly 100Psittacosaurus skeletons were excavated in Mongolia during the summers of 2005 and 2006 by a team led by Mongolian paleontologistBolortsetseg Minjin and AmericanJack Horner from theMuseum of the Rockies inMontana. Although onlyP. mongoliensis has been described from Mongolia so far, these specimens are still in preparation and have not yet been assigned to a species.[44]
P. amitabha was named by Napoliet al. in 2019 from a complete skull and partial skeleton. recovered in the BarremianAndakhuduk Formation of Mongolia. It is named afterAmitabha Buddha.[45]
The species ofPsittacosaurus vary in size and specific features of theskull and skeleton, but share the same overall body shape. The best-known—P. mongoliensis—can reach 2 metres (6.5 ft) in length.[46] The maximum adult body weight was most likely over 20 kilogrammes (44 lb) inP. mongoliensis.[47] Several species approachP. mongoliensis in size (P. lujiatunensis,P. neimongoliensis,P. xinjiangensis),[16][27][20] while others are somewhat smaller (P. sinensis,P. meileyingensis).[22] The smallest known species,P. ordosensis, is 30% smaller thanP. mongoliensis.[27] The largest areP. lujiatunensis andP. sibiricus, although neither is significantly larger thanP. mongoliensis.[29][26]Psittacosaurus postcranial skeletons are more typical of a 'generic' bipedal ornithischian.[48] There are only four digits on themanus ('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.[12]
The skull ofPsittacosaurus is highly modified compared to otherornithischian dinosaurs of its time. Extremely tall in height and short in length, the skull has an almost roundprofile in some species. The portion in front of theorbit (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 therostral andpredentary bones, respectively. The bony core of the beak may have been sheathed inkeratin to provide a sharp cutting surface for croppingplant material. As the generic name suggests, the short skull and beak superficially resemble those of modern parrots.Psittacosaurus skulls share several adaptations with morederived ceratopsians, such as the unique rostral bone at the tip of the upper jaw, and the flaredjugal (cheek) bones. There is still no sign of the bony neck frill or prominent facial horns which would develop in later ceratopsians.[12] Bony horns protrude from the skull ofP. sibiricus, but these are thought to be an example ofconvergent evolution.[26]
Theintegument, or body covering, ofPsittacosaurus is known from a Chinese specimen, SMF R 4970, which most likely comes from theYixian Formation ofLiaoning Province, China. The specimen, which is not yet assigned to any particular species, was likely illegally exported from China and was purchased in 2001 by theSenckenberg Museum inGermany.[49][50] It was described while awaiting repatriation; previous repatriation attempts were unsuccessful.[50][51]
Most of the body was covered inscales. 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 asChasmosaurus. 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 thedorsal (upper) surface of the tail. These were confirmed by the authors, as well as an independent scientist, to not represent plant material.[50] The bristle-like integumentary structures extend into the skin nearly to the vertebrae, and were likely circular or tubular before being preserved. Underultraviolet light, they gave off the same fluorescence as scales, providing the possibility they werekeratinized. The study stated that, "at present, there is no convincing evidence which shows these structures to behomologous to the structurally different integumentary filaments oftheropod dinosaurs". However, they found that all other feather-like integument from the Yixian Formation could be identified as feathers.[50]
In 2008, another study was published describing the integument anddermis ofPsittacosaurus sp., from a different specimen. 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. Thecollagen tissue fibres inPsittacosaurus 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.[52]
As described in a 2016 study, examination of melanosomes preserved in the specimen ofPsittacosaurus preserved with integument indicated that the animal wascountershaded, likely related to living in a dense forest habitat with little light, much like many modern species of forest-dwelling deer and antelope; stripes and spots on the limbs may representdisruptive coloration. The specimen also had dense clusters of pigment on its shoulders, face (possibly for display), andcloaca (which may have had an antimicrobial function, though this has been disputed[53]), as well as largepatagia 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.[54][55][56] The authors were unable to determine which species ofJehol FormationPsittacosaurus the specimen belonged to due to the way the skull is preserved, but ruled outP. mongoliensis, based on hip features.[57]
Another 2016 study usedlaser-stimulated fluorescence imaging to analyze the internal structure of the bristles. The highly cornified bristles were arranged in tight clusters of three to six individual bristles, with each bristle being filled with pulp. The authors considered the bristles as being most similar to the quills ofTianyulong, and the sparsely distributed elongated broad filamentous feathers (EBFFs) ofBeipiaosaurus. Similar, non-feather-derived bristles are found in a few extant birds such as the "horn" on thehorned screamer and the "beards" ofturkeys; these structures differ from feathers in that they are unbranched, heavily cornified and do not develop from afollicle, but instead arise from discrete cell populations that exhibit continuous growth.[58] A 2016 study by Ji Qiang and colleagues was published in the Journal of Geology. Their conclusion was that these were actually highly modified scales because the morphology and anatomy did not resemble feathers.[59] A darkened soft-tissue structure was also found near the jugal horn; this may represent a keratinous sheath or a skin flap.[60]
A 2021 study of SMF R 4970 examined itscloaca, the first one known from a non-avian dinosaur. The positioning of the individual when it died is oriented obliquely, so the structure can be seen better in the right side.Psittacosaurus' cloaca is comparable to those of crocodilians, with discrete lateral lips that converge anteriorly, giving the cloaca a v-shape anatomy. It also shows resemblance to that of birds, with the dorsal lobe being homologous to the birds' cloacal protuberance.[53] A 2022 study of SMF R 4970 identified it as an approximately 6–7 year old subadult by comparing its femoral length to that of similarly-aged specimens ofP. lujiatunensis, and found that it preserves the firstumbilicus (belly button) known from a non-avian dinosaur (the oldest known from anamniote). Because the specimen is close to sexual maturity, it is likely that the umbilicus probably retained throughout this individual's life and thatPsittacosaurus had its umbilicus at least until sexual maturity. It is uncertain whether the umbilicus is present in mature or nearly mature individuals of all non-avian dinosaurs.[61]
Skulls ofP. mongoliensis are flat on top, especially over the back of the skull, with a triangular depression, theantorbital fossa, on the outside surface of themaxilla (an upper jaw bone). A flange is present on the lower edge of thedentary (the tooth-bearing bone of the lower jaw), although it is not as prominent as inP. meileyingensis orP. major (=P. lujiatunensis).P. mongoliensis is among the largest known species. The skull of the type specimen, which is probably a juvenile,[27] is 15.2 centimetres (6 in) long, and the associatedfemur is 16.2 centimetres (6.4 in) in length.[62] Other specimens are larger, with the largest documented femur measuring about 21 centimetres (8.25 in) long.[63]
P. sinensis is readily distinguished from all other species by numerous features of the skull. Adult skulls are smaller than those ofP. mongoliensis and have less teeth. Uniquely, thepremaxillary bone contacts thejugal (cheek) bone on the outside of the skull. The jugals flare out sideways, forming 'horns' proportionally wider than in any other knownPsittacosaurus species exceptP. sibiricus andP. lujiatunensis. Because of the flared cheeks, the skull is actually wider than it is long. A smaller 'horn' is present behind theeye, at the contact of the jugal andpostorbital bones, a feature also seen inP. sibiricus. Themandible (lower jaw) lacks the hollow opening, orfenestra, seen in other species, and the entire lower jaw is bowed outwards, giving the animal the appearance of anunderbite.[17][14] The skull of an adultP. sinensis can reach 11.5 centimeters (4.5 in) in length.[27]
P. sibiricus is the largest-known species ofPsittacosaurus. 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 itsneck frill, which is longer than any other species, at 15 to 18% of skull length. A very striking feature ofP. sibiricus is the number of 'horns' around the eyes, with three prominences on each postorbital, and one in front of each eye, on thepalpebral bones. Similar horns found on the postorbital ofP. sinensis are not as pronounced but may behomologous. The jugal has extremely prominent 'horns' and may contact the premaxilla, both features also seen in the possibly relatedP. sinensis. There is a flange on the dentary of the lower jaw, similar toP. mongoliensis,P. meileyingensis, andP. sattayaraki. It can be told apart from the other species ofPsittacosaurus 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.[26]
P. xinjiangensis is distinguished by a prominent jugal 'horn' that is flattened on the front end, as well as some features of the teeth. Theilium, one of the three bones of the pelvis, also bears a characteristically long bony process behind theacetabulum (hip socket).[17] An adult femur has a published length of about 16 centimetres (6.3 in).[20]P. meileyingensis has the shortest snout andneck frill of any species, making the skull nearly circular in profile. Theorbit (eye socket) is roughly triangular, and there is a prominent flange on the lower edge of the dentary, a feature also seen in specimens ofP. lujiatunensis, and to a lesser degree inP. mongoliensis,P. sattayaraki, andP. sibiricus.[26][17] The complete type skull, probably adult, is 13.7 centimetres (5.5 in) long.[22] The dentary ofP. sattayaraki has a flange similar to that found inP. mongoliensis,P. sibiricus,P. lujiatunensis andP. meileyingensis, although it is less pronounced than in those species. The material appears to be roughly the same size asP. sinensis.[24] Thefrontal bone ofP. neimongoliensis is distinctly narrow compared to that of other species, resulting in a narrower skull overall. Theischium bone of the pelvis is also longer than thefemur, which differs from other species in which these bones are known.[17] 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 adultP. neimongoliensis was probably smaller thanP. mongoliensis, with a proportionately longer skull and tail.[27]P. ordosensis can be distinguished by numerous features of the jugals, which have very prominent 'horns'.[17] It is also the smallest known species. One adult skull measures only 9.5 centimeters (3.75 in) in length.[27]
The type skull ofP. 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 toP. mongoliensis andP. sibiricus. There is a fossa in front of the eye, as inP. mongoliensis. The jugal bones flare outwards widely, making the skull wider than it is long, as seen inP. sinensis. Widely flared jugals are also found inP. sibiricus. Overall, this species is thought to exhibit several primitive characteristics compared to other species ofPsittacosaurus, which is consistent with its greater geological age.[29]P. gobiensis was small-bodied (one metre (3 ft 3 in) long) and differs from other species ofPsittacosaurus by "significant, but structurally minor, details." These include the presence of a pyramidal horn on thepostorbital, a depression on the postorbital-jugal contact, and enamel thickness.P. mongoliensis was a contemporary.[40]
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Psittacosaurus is thetype genus of the family Psittacosauridae, which was also named by Osborn in 1923.[2][64] Psittacosaurids werebasal to almost all known ceratopsians exceptYinlong and perhaps theChaoyangsauridae.[12][65] 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, aplesiomorphy or primitive trait, whereas all species ofPsittacosaurus had only four digits on the hand. In addition, theantorbital fenestra, an opening in the skull between the eye socket andnostril, was lost during theevolution of Psittacosauridae, but is still found in most other ceratopsians and in fact most otherarchosaurs. It is considered highly unlikely that the fifth digit or antorbital fenestra wouldevolve a second time.[12]
In 2014, the describers of a new taxon of basal ceratopsian published a phylogenetic analysis encompassingPsittacosaurus. The below cladogram is from their analysis, placing the genus as one of the most primitive ceratopsians. The authors (Farkeet al.) noted that all taxa outside ofLeptoceratopsidae andCoronosauria with the exception of their genusAquilops are from Asia, meaning the group likely originated there.[66]
Although many species ofPsittacosaurus have been named, their relationships to each other have not yet been fully explored and no scientific consensus exists on the subject.[27][14][28] Severalphylogenetic analyses have been published, with the most detailed being those byAlexander Averianov and colleagues in 2006,[26]Hai-Lu You and colleagues in 2008,[38] andPaul Sereno in 2010.[4] In 2005, Zhou and colleagues suggested thatP. lujiatunensis is basal to all other species. This would be consistent with its earlier appearance in the fossil record.[29] In 2025, Asato Ishikawa and colleagues published a re-evaluation ofHongshanosaurus and concluded that it belonged toPsittacosaurus (the speciesP. houi). An abbreviated version of the analysis they published is shown below.[39]
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The brain ofP. lujiatunensis is well known; a study on the anatomy and functionality of three specimens was published in 2007. Until the study, it was generally thought the brain ofPsittacosaurus would have been similar to other ceratopsians with lowencephalization quotients. Russell and Zhao (1996) believed "the small brain size of psittacosaurs implies a very restrictive behavioural repertoire relative to that of modern mammals of similar body size". However, the 2007 study dispelled this theory when it found the brain to be more advanced. There is generally negativeallometry for brain size with development in vertebrates, but this was shown not to be true inPsittacosaurus. The EQ score forP. lujiatunensis is 0.31, significantly higher than genera such asTriceratops. A higher EQ correlates with more complex behaviour, and various dinosaurs have high EQs, similar to birds, which range from 0.36 to 2.98. Thus,Psittacosaurus behaviour could have been as complex as that inTyrannosaurus, whose EQ ranges from 0.30 to 0.38. Behaviours influenced by high EQs include nest-building, parental care, and bird-like sleeping, some of which have been shown to be present inPsittacosaurus.[67]
The senses ofPsittacosaurus can be inferred from theendocast. Largeolfactory bulbs are present, indicating the genus had an acute sense of smell. The size of these bulbs are comparable to large predatory theropods, although they likely evolved to avoid predators instead of to seek out prey. Thesclerotic rings in reptiles directly show the size of the eyeball. The rings are not well preserved inPsittacosaurus, with one individual preserving them likely contracted postmortem, but if they are similar to those ofProtoceratops,Psittacosaurus would have had large eyes and acute vision. The curvature of thesemicircular canals is related to the agility of reptiles, and the large curved canals inPsittacosaurus show that the genus was much more agile than later ceratopsians.[67] Comparisons between thescleral rings ofPsittacosaurus and modern birds and reptiles suggest that it may have beencathemeral, active throughout the day and for short intervals at night.[68]
Ford and Martin (2010) proposed thatPsittacosaurus 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 longchevrons (and with the bristles on the tail interpreted as possibly skin-covered, forming a fin), providing a propulsive surface; and the presence ofgastroliths, interpreted as ballast. They further suggested that some species ofPsittacosaurus were more terrestrial than others.[69]
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. Sometimes numbering more than fifty, these stones are occasionally found in the abdominal cavities of psittacosaurs, and may have been stored in agizzard, as in modern birds.[46]
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.[40]
Studies by Phil Senter in 2007 conducted onP. neimongoliensis andP. mongoliensis concluded that the forelimbs of these taxa (and likely those of otherPsittacosaurus species) were too short (only about 58% as long as the hindlimbs) to reach the ground, and their range of motion indicates they could neither bepronated nor generate propulsive force for locomotion, suggesting thatPsittacosaurus was entirely bipedal. The forelimbs were also too short to be used in digging or bringing food to the mouth, and Senter suggested that ifPsittacosaurus found it necessary 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.[48]
However,Psittacosaurus may not have been entirely bipedal for its entire lifespan. Taking sections from the limb bones of 16 specimens ofPsittacosaurus, ranging in age from less than a year old to ten-year-old adults, Qi Zhao from the University of Bristol found thatPsittacosaurus was probably secondarily bipedal. The infants' front limbs grew at faster rates than the hind limbs at between hatching 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 ofPsittacosaurus 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.[70][71]
Several juvenilePsittacosaurus have been found. The smallest is aP. mongoliensis hatchling conserved in theAmerican Museum of Natural History (AMNH), which is only 11 to 13 centimetres (4–5 inches) long, with a skull 2.4 centimetres (0.94 in) in length. Another hatchling skull at the AMNH is only 4.6 centimetres (1.8 in) long. Both specimens are from Mongolia.[13] Juveniles discovered in the Yixian Formation are approximately the same age as the larger AMNH specimen.[41]
Ahistological examination ofP. 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 andmarsupial mammals, but slower than modernbirds andplacental mammals.[47] Anage determination study performed on thefossilized remains ofP. mongoliensis by using growth ring counts suggest that the longevity of thebasal ceratopsian was 10 to 11 years.[47]
The find of a herd of sixPsittacosaurus 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 andgregariousness extending beyond the nest; the earliest such evidence for any ceratopsian.[72] Even very young psittacosaur teeth appear worn, indicating they chewed their own food and may have beenprecocial.[13] 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.[73] There is no evidence for parental care.[74]
In 2004, a specimen found in the Yixian Formation was claimed as evidence for parental care in dinosaurs. The specimen DNHM D2156 consists of 34 articulated juvenilePsittacosaurus 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 aburrow.[41] 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 shownP. 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.[74]
A 2014 analysis of the same specimen supported the association and 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.[75][76]
Out of the hundreds of knownPsittacosaurus specimens, only one has been described to possess any sort ofpathology. The specimen in question, consisting of a complete adult skeleton and tentatively assigned toP. 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 rightfibula. The bone exhibits a large round pit, evidence ofnecrosis 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 most likely have been fatal. Unlike the femur andtibia, 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.[42]
Another fossil from the Yixian Formation provides direct evidence ofPsittacosaurus as aprey animal. One skeleton ofRepenomamus robustus, a largetriconodont mammal, is preserved with the remains of a juvenilePsittacosaurus in its abdominal cavity. Several of the juvenile's bones are still articulated, indicating that thecarnivorous mammal swallowed its prey in large chunks. This specimen is notable in that it is the first-known example ofMesozoic mammals preying on live dinosaurs.[77] Heavy predation on juvenilePsittacosaurus may have resulted inR-selection, the production of more numerous offspring to counteract this loss.[78]
Psittacosaurus is known from hundreds of individual specimens, of which over 75 have been assigned to the type species,P. mongoliensis.[12][42][44] AllPsittacosaurus fossils discovered so far have been found inEarly Cretaceous sediments in Asia, from southernSiberia to northern China, and possibly as far south as Thailand. The most common age ofgeologic formations bearingPsittacosaurus fossils is from the lateBarremian throughAlbian stages of the Early Cretaceous, or approximately 125 to 105 mya (million years ago).[79] Many terrestrialsedimentary formations of this age in Mongolia and northern China have produced fossils ofPsittacosaurus, leading to the definition of this time period in the region as thePsittacosaurus biochron.[79][80]
The earliest known species isP. lujiatunensis, found in the lowest beds of the Yixian Formation.[29] 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.[42] Although many early studies usingradiometric dating put the Yixian in theJurassic Period, tens of millions of years outside of the expected temporal range ofPsittacosaurus, most recent work dates it to the Early Cretaceous. Usingargon–argon dating, a team of Chinese scientists dated the lowestbeds in the formation to about 128 mya, and the highest to approximately 122 mya.[34] A more recent Chinese study, usinguranium–lead dating, suggests that the lower beds are younger, approximately 123.2 mya, while agreeing with an age of 122 mya for the upper beds.[35]
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