Popo the Protoceratops

## Popo

Popo the Protoceratops

Popo Hes a Protoceratops a Color is Bown Belly is Tan

**Popo** is a Protoceratops animated feature film, *the Mesozoic Kingdom* and the deuteragonist of the 2003 film, *Dinosaur Planet*. He is a Protoceratops and the best friend of Gary a Gallimimus. Most animals are dismissive of Popo due to his repugnant odor, but he remains happy-go-lucky by following a philosophy called Walk the Dinosaur

## Information

***Protoceratops***first horned face') is a genus of small protoceratopsid dinosaurs that lived in Asia during the Late Cretaceous, around 75 to 71 million years ago. The genus *Protoceratops* includes two species: ***P. andrewsi*** and the larger ***P. hellenikorhinus***. The former was described in 1923 with fossils from the Mongolian Djadokhta Formation, and the latter in 2001 with fossils from the Chinese Bayan Mandahu Formation. *Protoceratops* was initially believed to be an ancestor of ankylosaurians and larger ceratopsians, such as *Triceratops* and relatives, until the discoveries of other protoceratopsids. Populations of *P. andrewsi* may have evolved into *Bagaceratops* through anagenesis.

*Protoceratops* were small ceratopsians, up to 2–2.5 m (6.6–8.2 ft) long and around 62–104 kg (137–229 lb) in body mass. While adults were largely quadrupedal, juveniles had the capacity to walk around bipedally if necessary. They were characterized by a proportionally large skull, short and stiff neck, and neck frill. The frill was likely used for display or intraspecific combat, as well as protection of the neck and anchoring of jaw muscles. A horn-like structure was present over the nose, which varied from a single structure in *P. andrewsi* to a double, paired structure in *P. hellenikorhinus*. The "horn" and frill were highly variable in shape and size across individuals of the same species, but there is no evidence of sexual dimorphism. They had a prominent parrot-like beak at the tip of the jaws. *P. andrewsi* had a pair of cylindrical, blunt teeth near the tip of the upper jaw. The forelimbs had five fingers of which only the first three bore wide and flat unguals. The feet were wide and had four toes with flattened, shovel-like unguals, which would have been useful for digging through the sand. The hindlimbs were longer than the forelimbs. The tail was long and had an enigmatic sail-like structure, which may have been used for display, swimming, or metabolic reasons.

*Protoceratops*, like many other ceratopsians, were herbivores equipped with prominent jaws and teeth suited for chopping foliage and other plant material. They are thought to have lived in highly sociable groups of mixed ages. They appear to have cared for their young. They laid soft-shelled eggs, a rare occurrence in dinosaurs. During maturation, the skull and neck frill underwent rapid growth. *Protoceratops* were hunted by *Velociraptor*, and one particularly famous specimen (the Fighting Dinosaurs) preserves a pair of them locked in combat. *Protoceratops* used to be characterized as nocturnal because of the large sclerotic ring around the eye, but they are now thought to have been cathemeral (active at dawn and dusk).

## History of discovery[edit]

In 1900 Henry Fairfield Osborn suggested that Central Asia may have been the center of origin of most animal species, including humans, which caught the attention of explorer and zoologist Roy Chapman Andrews. This idea later gave rise to the First (1916 to 1917), Second (1919) and Third (1921 to 1930) Central Asiatic Expeditions to China and Mongolia, organized by the American Museum of Natural History under the direction of Osborn and field leadership of Andrews. The team of the third expedition arrived in Beijing in 1921 for the final preparations and started working in the field in 1922. During late 1922 the expedition explored the famous Flaming Cliffs of the Shabarakh Usu region of the Djadokhta Formation, Gobi Desert, now known as the Bayn Dzak region. On September 2, the photographer James B. Shackelford discovered a partial juvenile skull—which would become the holotype specimen (AMNH 6251) of *Protoceratops*—in reddish sandstones. It was subsequently analyzed by the paleontologist Walter W. Granger who identified it as reptilian. On September 21, the expedition returned to Beijing, and even though it was set up to look for remains of human ancestors, the team collected numerous dinosaur fossils and thus provided insights into the rich fossil record of Asia. Back in Beijing, the skull Shackelford had found was sent back to the American Museum of Natural History for further study, after which Osborn reached out to Andrews and team via cable, notifying them about the importance of the specimen.[1][2]

In 1923 the expedition prospected the Flaming Cliffs again, this time discovering even more specimens of *Protoceratops* and also the first remains of *Oviraptor*, *Saurornithoides* and *Velociraptor*. Most notably, the team discovered the first fossilized dinosaur eggs near the holotype of *Oviraptor* and given how abundant *Protoceratops* was, the nest was attributed to this taxon.[2] This would later result in the interpretation of *Oviraptor* as an egg-thief.[3] In the same year, Granger and William K. Gregory formally described the new genus and species *Protoceratops andrewsi* based on the holotype skull. The specific name, *andrewsi*, is in honor of Andrews for his prominent leadership during the expeditions. They identified *Protoceratops* as an ornithischian dinosaur closely related to ceratopsians representing a possible common ancestor between ankylosaurs and ceratopsians. Since *Protoceratops* was more primitive than any other known ceratopsian at that time, Granger and Gregory coined the new family Protoceratopsidae, mostly characterized by the lack of horns. The co-authors also agreed with Osborn in that Asia, if more explored, could solve many major evolutionary gaps in the fossil record.[1] Although not stated in the original description, the generic name, *Protoceratops*, is intended to mean "first horned face" as it was believed that *Protoceratops* represented an early ancestor of ceratopsids.[4] Other researchers immediately noted the importance of the *Protoceratops* finds, and the genus was hailed as the "long-sought ancestor of *Triceratops*". Most fossils were in an excellent state of preservation with even sclerotic rings (delicate ocular bones) preserved in some specimens, quickly making *Protoceratops* one of the best-known dinosaurs from Asia.[2][5]

After spending much of 1924 making plans for the next fieldwork seasons, in 1925 Andrews and team explored the Flaming Cliffs yet again. During this year more eggs and nests were collected, alongside well-preserved and complete specimens of *Protoceratops*. By this time, *Protoceratops* had become one of the most abundant dinosaurs of the region with more than 100 specimens known, including skulls and skeletons of multiple individuals at different growth stages. Though more remains of *Protoceratops* were collected in later years of the expeditions, they were most abundant in the 1922 to 1925 seasons.[2][5] Gregory and Charles C. Mook published another description of *Protoceratops* in 1925, discussing its anatomy and relationships. Thanks to the large collection of skulls found in the expeditions, they concluded that *Protoceratops* represented a ceratopsian more primitive than ceratopsids and not an ankylosaur-ceratopsian ancestor.[6] In 1940, Barnum Brown and Erich Maren Schlaikjer described the anatomy of *P. andrewsi* in extensive detail using newly prepared specimens from the Asiatic expeditions.[5]

In 1963, the Mongolian paleontologist Demberelyin Dashzeveg reported the discovery of a new fossiliferous locality of the Djadokhta Formation: Tugriken Shireh. Like the neighbouring Bayn Dzak, this new locality contained an abundance of *Protoceratops* fossils.[7] During the 1960s to 1970s, Polish-Mongolian and Russian-Mongolian paleontological expeditions collected new, partial to complete specimens of *Protoceratops* at this locality, making this dinosaur species a common occurrence in Tugriken Shireh.[8][9][10] Since its discovery, the Tugriken Shireh locality has yielded some of the most significant specimens of *Protoceratops*, such as the Fighting Dinosaurs,[8] *in situ* individuals—a preservation condition also known as "standing" individuals or specimens in some cases—,[11] authentic nests,[12] and small herd-like groups.[13] Specimens from this locality are usually found in articulation, suggesting possible mass mortality events.[11]

Stephan N. F. Spiekman and colleagues reported a partial *P. andrewsi* skull (RGM 818207) in the collections of the Naturalis Biodiversity Center, Netherlands in 2015. Since *Protoceratops* fossils are only found in the Gobi Desert of Mongolia and this specimen was likely discovered during the Central Asiatic Expeditions, the team concluded that this skull was probably acquired by Delft University between 1940 and 1972 as part of a collection transfer.[14]

### Species and synonyms[edit]

Protoceratopsid remains were recovered in the 1970s from the Khulsan locality of the Barun Goyot Formation, Mongolia, during the work of several Polish-Mongolian paleontological expeditions. In 1975, Polish paleontologists Teresa Maryańska and Halszka Osmólska described a second species of *Protoceratops* which they named *P. kozlowskii*. This new species was based on the Khulsan material, mostly consisting of juvenile skull specimens. The specific name, *kozlowskii*, is in tribute to the Polish paleontologist Roman Kozłowski. They also named the new genus and species of protoceratopsid *Bagaceratops rozhdestvenskyi*, known from specimens of the nearby Hermiin Tsav locality.[9] In 1990 the Russian paleontologist Sergei Mikhailovich Kurzanov referred additional material from Hermiin Tsav to *P. kozlowskii*. However, he noted that there were enough differences between *P. andrewsi* and *P. kozlowskii*, and erected the new genus and combination *Breviceratops kozlowskii*.[15] Though *Breviceratops* has been regarded as a synonym and juvenile stage of *Bagaceratops*,[16][17] Łukasz Czepiński in 2019 concluded that the former has enough anatomical differences to be considered as a separate taxon.[18]

In 2001 Oliver Lambert with colleagues named a new and distinct species of *Protoceratops*, *P. hellenikorhinus*. The first known remains of *P. hellenikorhinus* were collected from the Bayan Mandahu locality of the Bayan Mandahu Formation, Inner Mongolia, in 1995 and 1996 during Sino-Belgian paleontological expeditions. The holotype (IMM 95BM1/1) and paratype (IMM 96BM1/4) specimens consist of large skulls lacking body remains. The holotype skull was found facing upwards, a pose that has been reported in *Protoceratops* specimens from Tugriken Shireh. The specific name, *hellenikorhinus*, is derived from Greek hellenikos (meaning Greek) and rhis (meaning nose) in reference to its broad and angular snout, which is reminiscent of the straight profiles of Greek sculptures.[19] In 2017 abundant protoceratopsid material was reported from Alxa near Bayan Mandahu,[20] and it may be referable to *P. hellenikorhinus*.[18]

Viktor Tereshchenko and Vladimir R. Alifanov in 2003 named a new protoceratopsid dinosaur from the Bayn Dzak locality, *Bainoceratops efremovi* . This genus was based on a few dorsal (back) vertebrae that were stated to differ from those of *Protoceratops*.[21] In 2006 North American paleontologists Peter Makovicky and Mark A. Norell suggested that *Bainoceratops* may be synonymous with *Protoceratops* as most of the traits used to separate the former from the latter have been reported from other ceratopsians including *Protoceratops* itself, and they are more likely to fall within the wide intraspecific variation range of the concurring *P. andrewsi*.[22] The authors Brenda J. Chinnery and Jhon R. Horner in 2007 during their description of *Cerasinops* stated that *Bainoceratops*, along with other dubious genera, was determined to be either a variant or immature specimen of other genera. Based on this reasoning, they excluded *Bainoceratops* from their phylogenetic analysis.[23]

### Eggs and nests[edit]

As part of the Third Central Asiatic Expedition of 1923, Andrews and team discovered the holotype specimen of *Oviraptor* in association with some of the first known fossilized dinosaur eggs (nest AMNH 6508), in the Djadokhta Formation. Each egg was elongated and hard-shelled, and due to the proximity and high abundance of *Protoceratops* in the formation, these eggs were believed at the time to belong to this dinosaur. This resulted in the interpretation of the contemporary *Oviraptor* as an egg predatory animal, an interpretation also reflected in its generic name.[24][3] In 1975, the Chinese paleontologist Zhao Zikui named the new oogenera *Elongatoolithus* and *Macroolithus*, including them in a new oofamily: the Elongatoolithidae. As the name implies, they represent elongated dinosaur eggs, including some of referred ones to *Protoceratops*.[25]

In 1994 the Russian paleontologist Konstantin E. Mikhailov named the new oogenus *Protoceratopsidovum* from the Barun Goyot and Djadokhta formations, with the type species *P. sincerum* and additional *P. fluxuosum* and *P. minimum*. This ootaxon was firmly stated as belonging to protoceratopsid dinosaurs since they were the predominant dinosaurs where the eggs were found and some skeletons of *Protoceratops* were found in close proximity to *Protoceratopsidovum* eggs. More specifically, Mikhailov stated that *P. sincerum* and *P. minimum* were laid by *Protoceratops*, and *P. fluxuosum* by *Breviceratops*.[26]

However, also during 1994, Norell and colleagues reported and briefly described a fossilized theropod embryo inside an egg (MPC-D 100/971) from the Djadokhta Formation. They identified this embryo as an oviraptorid dinosaur and the eggshell, upon close examination, turned out be that of elongatoolithid eggs and thereby the oofamily Elongatoolithidae was concluded to represent the eggs of oviraptorids. This find proved that the nest AMNH 6508 belonged to *Oviraptor* and rather than an egg-thief, the holotype was actually a mature individual that perished brooding the eggs.[27] Moreover, phylogenetic analyses published in 2008 by Darla K. Zelenitsky and François Therrien have shown that *Protoceratopsidovum* represents the eggs of a maniraptoran more derived than oviraptorids and not *Protoceratops*.[28] The description of the eggshell of *Protoceratopsidovum* has further confirmed that they in fact belong to a maniraptoran, possibly deinonychosaur taxon.[29]

Nevertheless, in 2011 an authentic nest of *Protoceratops* was reported and described by David E. Fastovsky and colleagues. The nest (MPC-D 100/530) containing 15 articulated juveniles was collected from the Tugriken Shireh locality of the Djadokhta Formation during the work of Mongolian-Japanese paleontological expeditions.[12] Gregory M. Erickson and team in 2017 reported an embryo-bearing egg clutch (MPC-D 100/1021) of *Protoceratops* from the also fossiliferous Ukhaa Tolgod locality, discovered during paleontological expeditions of the American Museum of Natural History and Mongolian Academy of Sciences. This clutch comprises at least 12 eggs and embryos with only 6 embryos preserving nearly complete skeletons.[30] Norell with colleagues in 2020 examined fossilized remains around the eggs of this clutch which indicate a soft-shelled composition.[31]

### Fighting Dinosaurs[edit]

The Fighting Dinosaurs specimen preserves a *Protoceratops* (MPC-D 100/512) and *Velociraptor* (MPC-D 100/25) fossilized in combat and provides an important window regarding direct evidence of predator-prey behavior in non-avian dinosaurs.[8][32] In the 1960s and early 1970s, many Polish-Mongolian paleontological expeditions were conducted to the Gobi Desert with the objective of fossil findings. In 1971, the expedition explored several localities of the Djadokhta and Nemegt formations. On August 3 several fossils of *Protoceratops* and *Velociraptor* were found including a block containing two of them at the Tugriken Shire locality (Djadokhta Formation) during fieldworks of the expedition. The individuals of this block were identified as a *P. andrewsi* and *V. mongoliensis*. Although it was not fully understood the conditions surrounding their burial, it was clear that they died simultaneously in struggle.[8]

The specimen shortly became notorious and was nicknamed the Fighting Dinosaurs. It has been examined and studied by numerous researchers and paleontologists, debating on how the animals got buried and preserved altogether. Though a drowning scenario has been proposed by Barsbold,[32] such hypothesis is considered unlikely given the arid paleoenvironments or settings of the Djadokhta Formation. It is generally accepted that they were buried alive by either a collapsed dune or sandstorm.[33][34][35]

### Skin impressions and footprints[edit]

https://upload.wikimedia.org/wikipedia/commons/thumb/e/e5/Protoceratops\_with\_possible\_skin\_impressions.jpg/173px-Protoceratops\_with\_possible\_skin\_impressions.jpg

https://upload.wikimedia.org/wikipedia/commons/thumb/7/7b/Protoceratops\_footprint.png/261px-Protoceratops\_footprint.png

AMNH 6418 specimen with possible skin impressions (left), and line diagram of footprint associated with specimen ZPAL Mg D-II/3 (right)

During the Third Central Asiatic Expedition in 1923, a nearly complete *Protoceratops* skeleton (specimen AMNH 6418) was collected at the Flaming Cliffs. Unlike other specimens, it was discovered in a rolled-up position with its skull preserving a thin, hard, and wrinkled layer of matrix (surrounding sediments). This specimen was later described in 1940 by Brown and Schlaikjer, who discussed the nature of the matrix portion. They stated that this layer had a very skin-like texture and covered mostly the left side of the skull from the snout to the neck frill. Brown and Schlaikjer discarded the idea of possible skin impressions as this skin-like layer was likely a product of the decay and burial of the individual, making the sediments become highly attached to the skull.[5]

The potential importance of these remains were not recognized and given attention, and by 2020 the specimen has already been completely prepared losing all traces of this skin-like layer. Some elements were damaged in the process such as the rostrum.[36] In 2022 Phil R. Bell and colleagues briefly described these potential soft tissues based on the photographs provided by Brown and Schlaikjer, as well as other ceratopsian soft tissues.[37] However, although the initial perception was that the entire skin-like layer had been removed, photographs shared by Czepiński during the same year have revealed that the right side of the skull remains intact, retaining much of this layer and pending further analysis.[36]

Also from the context of the Polish-Mongolian paleontological expeditions, in 1965 an articulated subadult *Protoceratops* skeleton (specimen ZPAL Mg D-II/3) was collected from the Bayn Dzak locality of the Djadokhta Formation. In the 2000s during the preparation of the specimen, a fossilized cast of a four-toed digitigrade footprint was found below the pelvic girdle. This footprint was described in 2012 by Grzegorz Niedźwiedzki and colleagues who considered it to represent one of the first reported finds of a dinosaur footprint in association with an articulated skeleton, and also the first one reported for *Protoceratops*.[38] The limb elements of the skeleton of ZPAL Mg D-II/3 were described in 2019 by paleontologists Justyna Słowiak, Victor S. Tereshchenko and Łucja Fostowicz-Frelik.[39] Tereshchenko in 2021 fully described the axial skeleton of this specimen.[40]

## Description[edit]

*Protoceratops* was a relatively small-sized ceratopsian, with both *P. andrewsi* and *P. hellenikorhinus* estimated up to 2–2.5 m (6.6–8.2 ft) in length,[41][42] and around 62–104 kg (137–229 lb) in body mass.[43] Although similar in overall body size, the latter had a relatively greater skull length.[19] Both species can be differentiated by the following characteristics:

* ***P. andrewsi*** – Two teeth were present at the premaxilla; the snout was low and long; the nasal horn was a single, pointed structure; the bottom edge of the dentary was slightly curved.[5][19]
* ***P. hellenikorhinus*** – Absence of premaxillary teeth; the snout was tall and broad; the nasal horn was divided into two pointed ridges; the bottom edge of the dentary was straight.[19]

### Skull[edit]

https://upload.wikimedia.org/wikipedia/commons/thumb/6/6a/Protoceratops\_MPC-D\_100\_551\_skull.png/218px-Protoceratops\_MPC-D\_100\_551\_skull.png

Skull of *P. andrewsi* (MPC-D 100/551) in left lateral (A1-A2), dorsal (A3-A4), and right lateral (A5-A6) views

https://upload.wikimedia.org/wikipedia/commons/thumb/d/d1/Protoceratops\_MPC-D\_100\_505\_skull.png/218px-Protoceratops\_MPC-D\_100\_505\_skull.png

Skull of *P. andrewsi* (MPC-D 100/505) in right lateral (A1) and left lateral (A2) views

The skull of *Protoceratops* was relatively large compared to its body and robustly built. The skull of the type species, *P. andrewsi*, had an average total length of nearly 50 cm (500 mm). On the other hand *P. hellenikorhinus* had a total skull length of about 70 cm (700 mm). The rear of the skull gave form to a pronounced neck frill (also known as "parietal frill") mostly composed of the parietal and squamosal bones. The exact size and shape of the frill varied by individual; some had short, compact frills, while others had frills nearly half the length of the skull. The squamosal touched the jugal (cheekbone) and was very enlarged and high having a curved end that built the borders of the frill. The parietals were the posteriormost bones of the skull and major elements of the frill. In a top view they had a triangular shape and were joined by the frontals (bones of the skull roof). Both parietals were coossified (fused), creating a long ridge on the center of the frill. The jugal was deep and sharply developed and along with the quadratojugal they formed a horn-like extension that pointed to below at the lateral sides of the skull. The epijugal (tip region of the jugal) was separated from the jugal by a prominent suture; this suture was more noticeable in adults. The surfaces around the epijugal were coarse, indicating that it was covered by a horny sheath. Unlike the much derived ceratopsids, the frontal and postorbital bones of *Protoceratops* were flat and lacked horn cores or supraorbital horns. The palpebral (small spur-like bone) joined the prefrontal over the front of the orbit (eye socket). In *P. hellenikorhinus* the palpebral protruded upwards from the prefrontal, just above the orbit and slightly meeting the frontal, creating a small horn-like structure. The lacrimal was a near-rectangular bone located in front of the orbit, contributing to the shape of the latter. The sclerotic ring (structure that supports the eyeball), found inside the orbit, was circular in shape and formed by consecutive bony plates.[5][19]

The snout was formed by the nasal, maxillar, premaxillar and rostral bones. The nasal was generally rounded but some individuals had a sharp nasal boss (a feature that has been called "nasal horn"). In *P. hellenikorhinus* this boss was divided in two sharp and long ridges. The maxilla was very deep and had up to 15 alveoli (tooth sockets) on its underside or teeth bearing surface. The premaxilla had two alveoli on its lower edge—a character that was present at least on *P. andrewsi*. The rostral bone was devoid of teeth, high and triangular in shape. It had a sharp end and rough texture, which reflects that a rhamphotheca (horny beak) was present. As a whole, the skull had four pairs of fenestrae (skull openings). The foremost hole, the nares (nostril opening), was oval-shaped and considerably smaller than the nostrils seen in ceratopsids. *Protoceratops* had large orbits, which measured around 5 cm (50 mm) in diameter and had irregular shapes depending on the individual. The forward facing and closely located orbits combined with a narrow snout, gave *Protoceratops* a well-developed binocular vision. Behind the eye was a slightly smaller fenestra known as the infratemporal fenestra, formed by the curves of the jugal and squamosal. The last openings of the skull were two parietal fenestrae (holes in the frill).[5][19]

https://upload.wikimedia.org/wikipedia/commons/thumb/4/46/Protoceratops\_andrewsi\_%281%29.jpg/157px-Protoceratops\_andrewsi\_%281%29.jpg

*P. andrewsi* skull

https://upload.wikimedia.org/wikipedia/commons/thumb/4/43/Protoceratops\_hellenikorhinus\_1.jpg/137px-Protoceratops\_hellenikorhinus\_1.jpg

*P. hellenikorhinus* skull

The lower jaw of *Protoceratops* was a large element composed of the predentary, dentary, coronoid, angular and surangular. The predentary (frontmost bone) was very pointed and elongated, having a V-shaped symphyseal (bone union) region at the front. The dentary (teeth-bearing bone) was robust, deep, slightly recurved, and fused to the angular and surangular. A large and thick ridge ran along the lateral surface of the dentary that connected the coronoid process—a bony projection that extends upwards from the upper surface of the lower jaw behind the tooth row—and surangular. It bore up to 12-14 alveoli on its top margin. Both predentary and dentary had a series of foramina (small pits), the latter mostly on its anterior end. The coronoid (highest point of the lower jaw) was blunt-shaped and touched by the coronoid process of the dentary, being obscured by the jugal. The surangular was near triangular in shape and in old individuals it was coossified together with the coronoid process. The angular was located below the two latter bones and behind the dentary. It was a large and somewhat rounded bone that complemented the curvature of the dentary. On its inner surface it was attached to the articular. The articular was a smaller bone and had a concavity on its inner surface for the articulation with the quadrate.[5][19]

*Protoceratops* had leaf-shaped dentary and maxillary teeth that bore several denticles (serrations) on their respective edges. The crowns (upper exposed part) had two faces or lobes that were divided by a central ridge-like structure (also called "primary ridge"). The teeth were packed into a single row that created a shearing surface. Both dentary and maxillary teeth presented marked homodonty—a dental condition where the teeth share a similar shape and size. *P. andrewsi* bore two small, peg to spike-like teeth that were located on the underside of each premaxilla. The second premaxillary tooth was larger than the first one. Unlike dentary and maxillary teeth, the premaxillary dentition was devoid of denticles, having a relatively smooth surface. All teeth had a single root (lower part inserted in the alveoli).[5][44][45]

### Postcranial skeleton[edit]

The vertebral column of *Protoceratops* had nine cervical (neck), 12 dorsal (back), eight sacral (pelvic) and over 40 caudal (tail) vertebrae. The centra (centrum; body of the vertebrae) of the first three cervicals were coossified together (atlas, axis and third cervical respectively) creating a rigid structure. The neck was rather short and had poor flexibility. The atlas was the smallest cervical and consisted mainly of the centrum because the neural arch (upper, and pointy vertebral region) was a thin, narrow bar of bone that extended upwards and backwards to the base of the axis neural spine. The capitular facet (attachment site for chevrons; also known as cervical ribs) was formed by a low projection located near the base of the neural arch. The anterior facet of the atlas centrum was highly concave for the articulation of the occipital condyle of the skull. The neural arch and spine of the axis were notably larger than the atlas itself and any other cervical. The axial neural spine was broad and backwards developed being slightly connected to that of the third cervical. From the fourth to the ninth all cervicals were relatively equal in size and proportions. Their neural spines were smaller than the first three vertebrae and the development of the capitular facet diminished from the fourth cervical onwards.[5][46][47]

https://upload.wikimedia.org/wikipedia/commons/thumb/a/a4/Protoceratops\_ZPAL\_MgD-II\_3\_left\_humerus.png/197px-Protoceratops\_ZPAL\_MgD-II\_3\_left\_humerus.png

https://upload.wikimedia.org/wikipedia/commons/thumb/1/1b/Protoceratops\_ZPAL\_MgD-II\_3\_radius\_%26\_ulna.png/87px-Protoceratops\_ZPAL\_MgD-II\_3\_radius\_%26\_ulna.png

https://upload.wikimedia.org/wikipedia/commons/thumb/7/7f/Protoceratops\_ZPAL\_MgD-II\_3\_right\_ilium.png/110px-Protoceratops\_ZPAL\_MgD-II\_3\_right\_ilium.png

https://upload.wikimedia.org/wikipedia/commons/thumb/6/6d/Protoceratops\_ZPAL\_MgD-II\_3\_ischia.png/174px-Protoceratops\_ZPAL\_MgD-II\_3\_ischia.png

https://upload.wikimedia.org/wikipedia/commons/thumb/1/15/Protoceratops\_ZPAL\_MgD-II\_3\_right\_femur.png/158px-Protoceratops\_ZPAL\_MgD-II\_3\_right\_femur.png

https://upload.wikimedia.org/wikipedia/commons/thumb/8/85/Protoceratops\_ZPAL\_MgD-II\_3\_right\_fibula\_%26\_tibia.png/126px-Protoceratops\_ZPAL\_MgD-II\_3\_right\_fibula\_%26\_tibia.png

https://upload.wikimedia.org/wikipedia/commons/thumb/e/ea/Protoceratops\_hands.png/109px-Protoceratops\_hands.png

https://upload.wikimedia.org/wikipedia/commons/thumb/d/db/Protoceratops\_feet.png/175px-Protoceratops\_feet.png

Forelimb (top), pelvic (middle), and hindlimb fossil bones (bottom) of specimen ZPAL Mg D-II/3

The dorsal vertebrae were similar in shape and size. Their neural spines were elongated and sub-rectangular in shape with a tendency to become more elongated in posterior vertebrae. The centra were large and predominantly amphiplatian (flat on both facets) and circular when seen from the front. Sometimes in old individuals the last dorsal vertebra was somewhat coosified to the first sacral. The sacral vertebrae were firmly coosified giving form to the sacrum, which was connected to the inner sides of both ilia. Their neural spines were broad, not coosified, and rather consistent in length. The centra were mainly opisthocoelous (concave on the posterior facet and convex on the anterior one) and their size became smaller towards the end. The caudal vertebrae decreased in size progressively towards the end and had very elongated neural spines in the mid-series, forming a sail-like structure. This elongation started from the first to the fourteenth caudal. The centra were heterocoelous (saddle-shaped at both facets). On the anterior caudals they were broad, however, from the twenty-fifth onwards the centra became elongated alongside the neural spines. On the underside of the caudal vertebrae a series of chevrons were attached, giving form to the lower part of the tail. The first chevron was located at the union of the third and fourth caudals. Chevrons three to nine were the largest and from the tenth onwards they became smaller.[5][46][47][48]

All vertebrae of *Protoceratops* had ribs attached on the lateral sides, except for the series of caudals. The first five cervical ribs (sometimes called chevrons) were some of the shortest ribs, and among them the first two were longer than the rest. The third to the sixth dorsal (thoracic) ribs were the longest ribs in the skeleton of *Protoceratops*, the following ribs became smaller in size as they progressed toward the end of the vertebral column. The two last dorsal ribs were the smallest, and the last of them was in contact with the internal surfaes of the ilium. Most of the sacral ribs were fused into the sacrum, and had a rather curved shape.[5]

https://upload.wikimedia.org/wikipedia/commons/thumb/8/84/Protoceratops\_andrewsi\_Restoration.png/228px-Protoceratops\_andrewsi\_Restoration.png

https://upload.wikimedia.org/wikipedia/commons/thumb/a/ac/Protoceratops\_hellenikorhinus\_Restoration.png/228px-Protoceratops\_hellenikorhinus\_Restoration.png

Life restorations of *P. andrewsi* (top) and *P. hellenikorhinus* (bottom)

The pectoral girdle of *Protoceratops* was formed by the scapulocoracoid (fusion of the coracoid and scapula) and clavicle. The scapulae (shoulder blades) were relatively large and rounded on their inner sides. At their upper region, the scapulae were wide. At their lower region, the scapulae meet the coracoids. The coracoids were relatively elliptical, and sometimes coosified (fused) to the scapulae. The clavicle of *Protoceratops* was an U to slightly V-shaped element that joined to the upper border of the scapulocoracoid. In its general form, the forelimbs of *Protoceratops* were shorted than the hindlimbs, and composed by the humerus, radius, and ulna. The humerus (upper arm bone) was large and slender, and at the lower part it meet with both radius and ulna. The radius had a slightly recurved shape and was longer than the ulna. A concavity was present on its upper part, serving as the connection with the humerus and forming the elbow. The ulna was a rather short bone with a straight shape. The manus (hand) of *Protoceratops* had five digits (fingers). The first three fingers had unguals (claw bones) and were the largest digits. The last two were devoid of unguals and had a small size, mostly vestigial (retained, but without important function). Both hand and feet unguals were flat, blunt and hoof-like.[5][39]

The pelvic girdle was formed by the ilium, pubis, and ischium. The ilium was a large element, having a narrow preacetabular process (anterior end) and a wide postacetabular process (posterior end). The pubis was the smallest element of the pelvic girdle and it had an irregular shape, although its lower end was developed into a pointed bony projection downwards. The ischium was the longest bone of the pelvic girdle. It had an elongated shaft with a somewhat wide lower end. The hindlimbs of *Protoceratops* were rather long, with a slighter longer tibia (lower leg bone) than femur (thigh bone). The femur (thighbone) was robust and had a rather rounded and pronounced greater trochanter, which was slightly recurved into the inner sides. The tibia (shinbone) was long and slender with a wide lower end. On its upper region a concavity was developed for the joint with the smaller fibula. The pes (foot) were composed of four metatarsal and four toes which bore shovel-like pedal unguals. The first metatarsal and toe were the smallest, while the other elements were of similar shape and length.[5][39]

## Classification[edit]

*Protoceratops* was in 1923 placed within the newly named family Protoceratopsidae as the representative species by Granger and Gregory. This family was characterized by their overall primitive morphology in comparison to the more derived Ceratopsidae, such as lack of well-developed horn cores and relative smaller body size. *Protoceratops* itself was considered by the authors to be somehow related to ankylosaurians based on skull traits, with a more intensified degree to *Triceratops* and relatives.[1] Gregory and Charles C. Mook in 1925 upon a more deeper analysis of *Protoceratops* and its overall morphology, concluded that this taxon represents a ceratopsian more primitive than ceratopsids and not an ankylosaur-ceratopsian ancestor.[6] In 1951 Edwin H. Colbert considered *Protoceratops* to represent a key ancestor for the ceratopsid lineage, suggesting that it ultimately led to the evolution of large-bodied ceratopsians such as *Styracosaurus* and *Triceratops*. Such lineage was suggested to have started from the primitive ceratopsian *Psittacosaurus*. He also regarded *Protoceratops* as one of the first "frilled" ceratopsians to appear in the fossil record.[49]

However, in 1975 Maryanska and Osmolska argued that it is very unlikely that protoceratopsids evolved from psittacosaurids, and also unlikely that they gave rise to the highly derived (advanced) ceratopsids. The first point was supported by the numerous anatomical differences between protoceratopsids and psittacosaurids, most notably the extreme reduction of some hand digits in the latter group—a trait much less pronounced in protoceratopsids. The second point was explained on the basis of the already derived anatomy in protoceratopsids like *Bagaceratops* or *Protoceratops* (such as the jaw morphology). Maryanska and Osmolska also emphasized that some early members of the Ceratopsidae reflect a much older evolutionary history.[9] In 1998, paleontologist Paul Sereno formally defined Protoceratopsidae as the branch-based clade including all coronosaurs closer to *Protoceratops* than to *Triceratops*.[50]

Furthermore, with the re-examinations of *Turanoceratops* in 2009 and *Zuniceratops*—two critical ceratopsian taxa regarding the evolutionary history of ceratopsids—in 2010 it was concluded that the origin of ceratopsids is unrelated to, and older than the fossil record of *Protoceratops* and relatives.[51][52] In most recent/modern phylogenetic analyses *Protoceratops* and *Bagaceratops* are commonly recovered as sister taxa, leaving the interpretations proposing direct relationships with more derived ceratopsians unsupported.[53]

In 2019 Czepiński analyzed a vast majority of referred specimens to the ceratopsians *Bagaceratops* and *Breviceratops*, and concluded that most were in fact specimens of the former. Although the genera *Gobiceratops*, *Lamaceratops*, *Magnirostris*, and *Platyceratops*, were long considered valid and distinct taxa, and sometimes placed within Protoceratopsidae, Czepiński found the diagnostic (identifier) features used to distinguish these taxa to be largely present in *Bagaceratops* and thus becoming synonyms of this genus. Under this reasoning, Protoceratopsidae consists of *Bagaceratops*, *Breviceratops*, and *Protoceratops*. Below are the proposed relationships among Protoceratopsidae by Czepiński:[18]

In 2019 Bitnara Kim and colleagues described a relatively well-preserved *Bagaceratops* skeleton from the Barun Goyot Formation, noting numerous similarities with *Protoceratops*. Even though their respective skull anatomy had substantial differences, their postcranial skeleton was virtually the same. The phylogenetic analysis performed by the team recovered both protoceratopsids as sister taxa, indicating that *Bagaceratops* and *Protoceratops* were anatomically and systematically related. Below is the obtained cladogram, showing the position of *Protoceratops* and *Bagaceratops*:[54]

### Evolution[edit]

Longrich and team in 2010 indicated that highly derived morphology of *P. hellenikorhinus*—when compared to *P. andrewsi*—indicates that this species may represent a lineage of *Protoceratops* that had a longer evolutionary history compared to *P. andrewsi*, or simply a direct descendant of *P. andrewsi*. The difference in morphologies between *Protoceratops* also suggests that the nearby Bayan Mandahu Formation is slightly younger than the Djadokhta Formation.[55]

In 2020, Czepiński analyzed several long-undescribed protoceratopsid specimens from the Udyn Sayr and Zamyn Khondt localities of the Djadokhta Formation. One specimen (MPC-D 100/551B) was shown to present skull traits that are intermediate between *Bagaceratops rozhdestvenskyi* (which is native to adjacent Bayan Mandahu and Barun Goyot) and *P. andrewsi*. The specimen hails from the Udyn Sayr locality, where *Protoceratops* remains are dominant, and given the lack of more conclusive anatomical traits, Czepiński assigned the specimen as *Bagaceratops* sp. He explained that the presence of this *Bagaceratops* specimen in such unusual locality could be solved by: (1) the coexistence and sympatric (altogether) evolution of both *Bagaceratops* and *Protoceratops* at this one locality; (2) the rise of *B. rozhdestvenskyi* in a different region and eventual migration to Udyn Sayr; (3) hybridization between the two protoceratopsids given the near placement of both Bayan Mandahu and Djadokhta; (4) anagenetic (proggressive evolution) evolutionary transition from *P. andrewsi* to *B. rozhdestvenskyi*. Among scenarios, an anagenetic transition was best supported by Czepiński given the fact that no definitive *B. rozhdestvenskyi* fossils are found in Udyn Sayr, as expected from a hybridization event; MPC-D 100/551B lacks a well-developed accessory antorbital fenestra (hole behind the nostril openings), a trait expected to be present if *B. rozhdestvenskyi* had migrated to the area; and many specimens of *P. andrewsi* recovered at Udyn Sayr already feature a decrease in the presence of primitive premaxillary teeth, hence supporting a growing change in the populations.[56]