Protoceratops

Protoceratops was a small dinosaur that lived in Asia during the Late Cretaceous, around 75 to 71 million years ago. This dinosaur belonged to a group called protoceratopsids and had two known species: P. andrewsi and P. hellenikorhinus. Fossils of P. andrewsi were first found in the Mongolian Djadokhta Formation in 1923, while fossils of P. hellenikorhinus were discovered later in the Chinese Bayan Mandahu Formation in 2001.

Protoceratops was not very big, measuring up to about 2–2.5 meters long and weighing between 62 and 104 kilograms. These dinosaurs had a large skull, a short stiff neck, and a frill extending from the back of their skull. The frill may have been used for showing off, protecting their neck, or helping them chew food. They also had a small horn-like structure over their nose. Unlike larger dinosaurs such as Triceratops, Protoceratops walked on all fours, though younger ones could stand on two legs when needed.

These dinosaurs were plant-eaters with strong jaws and teeth made for chopping leaves and other plants. They lived in groups and took care of their young. Protoceratops laid soft-shelled eggs, which was unusual for dinosaurs. Fossils show that they were sometimes attacked by Velociraptor, with one famous specimen showing a Protoceratops and a Velociraptor locked in battle. Scientists used to think Protoceratops were night-dwellers, but now believe they were active during dawn and dusk.

History of discovery

Explorer and zoologist Roy Chapman Andrews led several expeditions to China and Mongolia in the early 1900s, organized by the American Museum of Natural History. During these trips, scientists found many fossils, including the first remains of Protoceratops. These discoveries helped scientists learn more about dinosaurs that lived in Asia millions of years ago.

In 1923, the team found a skull that would become the main example, or holotype, of Protoceratops. Later expeditions found more Protoceratops fossils, including complete skeletons and even eggs. These finds made Protoceratops one of the most well-known dinosaurs from Asia. Scientists also discovered a famous pair of fossils showing a Protoceratops and a Velociraptor frozen in a battle, giving us a rare glimpse into dinosaur behavior.

Description

Protoceratops was a small dinosaur that lived in Asia about 75 to 71 million years ago. There were two types: P. andrewsi and the bigger P. hellenikorhinus. Both were about 2 to 2.5 meters long and weighed between 62 to 104 kilograms.

Protoceratops had a big skull with a neck frill at the back. The frill looked different in each dinosaur — some were short, and others were almost half the length of the skull. The eyes were large and helped it see well. The lower jaw had teeth for chewing, and the front of the mouth had a beak-like shape.

Classification

Protoceratops was first placed in a new family called Protoceratopsidae in 1923. This family was noted for being more basic compared to other horned dinosaurs like Ceratopsidae. Scientists initially thought Protoceratops might be related to ankylosaurians, but later studies showed it was an early type of ceratopsian dinosaur, not directly linked to ankylosaurs.

Over time, scientists have debated how Protoceratops fits into the family tree of dinosaurs. Some suggested it was an ancestor to larger ceratopsians like Triceratops, while others argued it evolved separately. Recent studies show that Protoceratops and another dinosaur called Bagaceratops are closely related, forming their own group within the ceratopsians.

Paleobiology

Feeding

In 1955, paleontologist Georg Haas examined the skull shape of Protoceratops and suggested that its large neck frill was likely an attachment site for muscles that helped anchor the lower jaws, useful for feeding. In 1988, Yannicke Dauphin and colleagues described the enamel microstructure of Protoceratops, observing a non-prismatic outer layer. They concluded that enamel shape does not relate to the diet or function of the teeth as most animals do not necessarily use teeth to process food. The maxillary teeth of ceratopsians were usually packed into a dental battery that formed vertical shearing blades which probably chopped the leaves. This feeding method was likely more efficient in protoceratopsids as the enamel surface of Protoceratops was coarsely-textured and the tips of the micro-serrations developed on the basis of the teeth, probably helping to crumble vegetation.

In 1991, paleontologist Gregory S. Paul stated that some groups may have been opportunistic meat-eaters, including the members of Ceratopsidae and Protoceratopsidae. He pointed out that their prominent parrot-like beaks and shearing teeth along with powerful muscles on the jaws suggest an omnivore diet instead, much like pigs, hogs, boars and entelodonts. In 2004, You Hailu and Peter Dodson suggested that the premaxillary teeth of Protoceratops may have been useful for selective cropping and feeding.

In 2009, Kyo Tanque and team suggested that basal ceratopsians, such as protoceratopsids, were most likely low browsers due to their relatively small body size. This low-browsing method would have allowed to feed on foliage and fruits within range, and large basal ceratopsians may have consumed tougher seeds or plant material not available to smaller basal ceratopsians.

In 2019, David J. Button and Lindsay E. Zanno performed a large phylogenetic analysis based on skull biomechanical characters to analyze the multiple emergences of herbivory among non-avian dinosaurs. Their results found that herbivorous dinosaurs mainly followed two distinct modes of feeding, either processing food in the gut or the mouth. Ceratopsians (including protoceratopsids), along with Euoplocephalus, Hungarosaurus, parkosaurid, ornithpod and heterodontosaurine dinosaurs, were found to be in the former category, indicating that Protoceratops and relatives had strong bite forces and relied mostly on its jaws to process food.

Ontogeny

Protoceratops underwent significant changes as it grew. The orbits, frontals, and lacrimals shrank in relative size as the animal aged; the top border of the nostrils became more vertical; the nasal bones progressively became elongated and narrowed; and the neck frill as a whole also increases in size with age. The neck frill specifically, changed from a small, flat, and almost rounded structure in juveniles to a large, fan-like one in fully mature Protoceratops individuals.

In 2001, Lambert and colleagues considered the development of the two nasal "horns" of P. hellenikorhinus to be a trait that was delayed in relation to the appearance of sexual-discriminant traits. In 2007, Makovicky and team conducted a histological analysis on several specimens of Protoceratops to provide insights into the life history of Protoceratops. The examined fossil bones indicated that Protoceratops slowed its ontogeny around 9–10 years of life, and it ceased around 11–13 years. David Hone and colleagues in 2016 found that the frill of Protoceratops was disproportionally smaller in juveniles, grew at a rapid rate than the rest of the animal during its ontogeny, and reached a considerable size only in large adult individuals.

In 2017, Mototaka Saneyoshi with team analyzed several Protoceratops specimens, noting that from perinate/juvenile to subadult individuals, the parietal and squamosal bones increased their sides to posterior sides of the skull. From subadult to adult individuals, the squamosal bone increased in size more than the parietal bone, and the frill expanded to a top direction.

In 2018, paleontologists Łucja Fostowicz-Frelik and Justyna Słowiak studied the bone histology of several specimens of P. andrewsi through cross-sections. The sampled elements consisted of neck frill, femur, tibia, fibula, ribs, humerus and radius bones, and showed that the histology of Protoceratops remained rather uniform throughout ontogeny. It was characterized by simple fibrolamellar bone with prominent woven-fibered bone and low bone remodeling. Most bones of Protoceratops preserve a large abundance of bone fibers, which likely gave strength to the organ and enhanced its elasticity. The team also find that the growth rate of the femur increased at the subadult stage, suggesting changes in bone proportions, such as the elongation of the hindlimbs.

Movement

In 1996, Tereshchenko reconstructed the walking model of Protoceratops where he considered the most likely scenario to be Protoceratops as an obligate quadruped given the proportions of its limbs. The main gait of Protoceratops was probably trot-like mostly using its hindlimbs and it is unlikely to have used an asymmetric gait. If trapped in a specific situation (like danger or foraging), Protoceratops could have employed a rapid, facultative bipedalism. He also noted that the flat and wide pedal unguals of Protoceratops may have allowed efficient walking through loose terrain, such as sand which was common on its surroundings.

Phil Senter in 2007 suggested that the hands of Protoceratops could reach the ground when the hindlimbs were upright, and the overall forelimb morphology and range of motion may reflect that it was at least a facultative quadruped. The forelimbs of Protoceratops could sprawl laterally but not for quadrupedal locomotion, which was accomplished with the elbows tucked in. In 2010 Alexander Kuznetsov and Tereshchenko analyzed several vertebrae series of Protoceratops to estimate overall mobility, and concluded that Protoceratops had greater lateral mobility in the presacral (pre-hip) vertebrae series and reduced vertical mobility in the cervical (neck) region. The fossilized footprint associated with the specimen ZPAL Mg D-II/3 described by Niedźwiedzki in 2012 indicates that Protoceratops was digitigrade, meaning that it walked with its toes supporting the body weight.

In 2019, Słowiak and team described the limb elements of ZPAL Mg D-II/3, which represents a sub-adult individual, and noted a mix of characters typical of bipedal ceratopsians such as a narrow glenoid with scapular blade and an arched femur. The absence of these traits in mature individuals indicates that young Protoceratops were capable of facultative bipedal locomotion and adults had an obligate quadrupedal stance. Even though adult Protoceratops were stocky and quadruped, their tibia-femur length ratio—the tibia being longer than femur, a trait present in bipedal ceratopsians—suggests the ability to occasionally stand on their hindlimbs. Słowiak and team also suggested that the flat and wide hand unguals (claw bone) of Protoceratops may have been useful for moving on loose terrain (such as sand) without sinking.

Digging behavior

Longrich in 2010 proposed that Protoceratops may have used its hindlimbs to dig burrows or take shelter under bushes and/or scrapes to escape the hottest temperatures of the day. A digging action with the hindlimbs was likely facilitated by the strong caudofemoralis muscle and its large feet equipped with flat, shovel-like unguals. As this behavior would have been common in Protoceratops, it predisposed individuals to become entombed alive during the sudden collapse of their burrows and high energy sand-bearing events—such as sandstorms—and thus explaining the standing in-situ posture of some specimens. Additionally, Longrich suggested that a backward burrowing could explain the preservation of some specimens pointing forward with curved tails.

In 2019, Victoria M. Arbour and David C. Evans cited the robusticity of the ulna of Ferrisaurus as a useful feature for digging, which may have been also true for Protoceratops.

Tail function

Gregory and Mook in 1925 suggested that Protoceratops was partially aquatic because of its large feet—being larger than the hands—and the very long neural spines found in the caudal (tail) vertebrae. Brown and Schlaikjer in 1940 indicated that the expansion of the distal (lower) ischial end may reflect a strong ischiocaudalis muscle, which together with the high tail neural spines were used for swimming. Barsbold in his brief 1974 description of the Fighting Dinosaurs specimen accepted this hypothesis and suggested that Protoceratops was amphibious (water-adapted) and had well-developed swimming capacities based on its side to side flattened tail with very high neural spines.

Jack Bowman Bailey in 1997 disagreed with previous aquatic hypotheses and indicated that the high caudal neural spines were instead more reminiscent of bulbous tails of some desert lizard species (such as Heloderma or Uromastyx), which are related to store fat with metabolic water in the tail. He considered a swimming adaptation unlikely given the arid settings of the Djadokhta Formation.

In 2008, based on the occurrence of some Protoceratops specimens in fluvial (river-deposited) sediments from the Djadokhta Formation and heterocoelous (vertebral centra that are saddle-shaped at both ends) caudal vertebrae of protoceratopsids, Tereshchenko concluded that the elevated caudal spines are a swimming adaptation. He proposed that protoceratopsids moved through water using their laterally flattened tails as a paddle to aid in swimming. According to Tereschenko, Bagaceratops was fully aquatic while Protoceratops was only partially aquatic. Longrich in 2010 argued that the high tail and frill of Protoceratops may have helped it to shed excess heat during the day—acting as large-surface structures—when the animal was active in order to survive in the relatively arid environments of the Djadokhta Formation without highly developed cooling mechanisms.

In 2011, during the description of Koreaceratops, Yuong-Nam Lee and colleagues found the above swimming hypotheses hard to prove based on the abundance of Protoceratops in eolian (wind-deposited) sediments that were deposited in prominent arid environments. They also pointed out that while taxa such as Leptoceratops and Montanoceratops are recovered from fluvial sediments, they are estimated to be some of the poorest swimmers. Lee and colleagues concluded that even though the tail morphology of Koreaceratops—and other basal ceratopsians—does not argues against swimming habits, the cited evidence for it is insufficient.

Tereschhenko in 2013 examined the structure of the caudal vertebrae spines of Protoceratops, concluding that it had adaptations for terrestrial and aquatic habits. Observations made found that the high number of caudal vertebrae may have been useful for swimming and use the tail to counter-balance weight. He also indicated that the anterior caudals were devoid of high neural spines and had increased mobility—a mobility that stars to decrease towards the high neural spines—, which suggest that the tail could be largely raised from its base. It is likely that Protoceratops raised its tail as a signal (display) or females could use this method during egg laying to expand and relax the cloaca.

In 2016, Hone and team indicated that the tail of Protoceratops, particularly the mid region with elevated neural spines, could have been used in display to impress potential mates and/or for species recognition. The tail may have been related with structures like the frill for displaying behavior.

Kim with team in 2019 cited the elongated tail spines as well-suited for swimming. They indicated that both Bagaceratops and Protoceratops may have used their tails in a similar fashion during similar situations, such as swimming, given how similar their postcranial skeletons were. The team also suggested that a swimming adaptation could have been useful to avoid aquatic predators, such as crocodylomorphs.

Social behavior

Tomasz Jerzykiewiczz in 1993 reported several monospecific (containing only one dominant species) death assemblages of Protoceratops from the Bayan Mandahu and Djadokhta formations. A group of five medium-sized and adult Protoceratops was observed at the Bayan Mandahu locality. Individuals within this assemblage were lying on their bellies with their heads facing upwards, side by side parallel-aligned, and inclined about 21 degrees from the horizontal plane. Two other groups were found at the Tugriken Shireh locality; one group containing six individuals and another group of about 12 skeletons.

In 2014, David W. E. Hone and colleagues reported and described two blocks containing death assemblages of P. andrewsi from Tugriken Shireh. The first block (MPC-D 100/526) comprises four juvenile individuals in close proximity with their heads pointing upwards, and the second block (MPC-D 100/534) is composed of two sub-adults with a horizontal orientation. Based on previous assemblages and the two blocks, the team determined that Protoceratops was a social dinosaur that formed herds throughout its life and such herds would have varied in composition, with some including adults, sub-adults, siblings from a single nest or local members of a herd joining shortly after hatching. However, as the group could have loss members by predation or other factors, the remnants individuals would aggregate into larger groups to increase their survival. Hone and colleagues in particular suggested that juveniles would aggregate primarily as a defense against predators and an increased protection from the multiple adults within the group. The team also indicated that, while Protoceratops provides direct evidence for the formation of single cohort aggregations throughout its lifespan, it cannot be ruled out the possibility that some Protoceratops were solitary.

Sexual dimorphism and display

Brown and Schlaikjer in 1940 upon their large analysis of Protoceratops noted the potential presence of sexual dimorphism among specimens in P. andrewsi, concluding that this condition could be entirely subjective or represent actual differences between sexes. Individuals with a high nasal horn, massive prefrontals, and frontoparietal depression were tentatively determined as males. Females were mostly characterized by the lack of well-developed nasal horns. In 1972 Kurzanov made comparisons between P. andrewsi skulls from Bayn Dzak and Tugriken Shireh, noting differences on the nasal horn within populations.

Peter Dodson in 1996 used anatomical characters of the skull in P. andrewsi to quantify areas subject to ontogenic changes and sexual dimorphism. In total, 40 skull characters were measured and compared, including regions like the frill and nasal horn. Dodson found most of these characters to be highly variable across specimens, especially the frill which he interpreted to have had a bigger role in displaying behavior than simply serving as a site of masticatory muscles. He considered unlikely such interpretation based on the relative fragility of some frill bones and the large individual variation, which may have affected the development of those muscles. The length of the frill was found by Dodson to have a rather irregular growth in specimens, as juvenile AMNH 6419 was observed with a frill length smaller than other juveniles. He agreed with Brown and Schlaikjer in that a high, well-developed nasal horn represents a male trait and the opposite indicates females. In addition, Dodson suggested that traits like the nasal horn and frill in male Protoceratops may have been important visual displays for attracting females and repelling other males, or even predators. Lastly, he noted that both males and females had not significant disparity in body size, and that sexual maturity in Protoceratops could be recognised at the moment when males can be distinguished from females.

In 2001, Lambert and team upon the description of P. hellenikorhinus also noted variation within individuals. For instance, some specimens (e.g., holotype IMM 95BM1/1) preserve high nasal bones with a pair of horns; relatively short antorbital length; and vertically oriented nostrils. Such traits were regarded as representing male P. hellenikorhinus. The other group of skulls is characterized by low nasals that have undeveloped horns; a relatively longer antorbital length; and more oblique nostrils. These individuals were considered as females. The team however, was not able to produce deeper analysis regarding sexual dimorphism in P. hellenikorhinus due to the lack of complete specimens. Also in 2001, Tereschhenko analyzed several specimens of P. andrewsi to evaluate sexual dimorphism. He found 19 anatomical differences in the vertebral column and pelvic region of regarded male and female Protoceratops individuals, which he considered to represent actual sexual characters.

In 2012, Naoto Handa and colleagues described four specimens of P. andrewsi from the Udyn Sayr locality of the Djadokhta Formation. They indicated that sexual dimorphism in this population was marked by a prominent nasal horn in males—trait also noted by other authors—relative wider nostrils in females, and a wider neck frill in males. Despite maintaining the skull morphology of most Protoceratops specimens (such as premaxillary teeth), the neck frill in this population was straighter with a near triangular shape. Handa and team in addition found variation across this Udyn Sayr sample and classified them in three groups. First group includes individuals with a well-developed bony ridge on the lateral surface of the squamosal bone, and the posterior border of the squamosal is backwards oriented. Second group had a fairly rounded posterior border of the squamosal, and a long and well-developed bony ridge on the posterior border of the parietal bone. Lastly, the third group was characterized by a curved posterior border of the squamosal and a notorious rugose texture on the top surface of the parietal. Such skull traits were regarded as marked intraspecific variation within Protoceratops, and they differ from other populations across the Djadokhta Formation (like Tugriken Shireh), being unique to the Udyn Sayr region. These neck frill morphologies differ from those of Protoceratops from the Djadokhta Formation in the adjacent dinosaur locality Tugrikin Shire. The morphological differences among the Udyn Sayr specimens may indicate intraspecific variation of Protoceratops. A large and well-developed bony ridge on the parietal has been observed on another P. andrewsi specimen, MPC-D 100/551, also from Udyn Sayr.

However, Leonardo Maiorino with team in 2015 performed a large geometric morphometric analysis using 29 skulls of P. andrewsi to evaluate actual sexual dimorphism. Obtained results indicated that other than the nasal horn—which remained as the only skull trait with potential sexual dimorphism—all previously suggested characters to differentiate hyphotetical males from females were more linked to ontogenic changes and intraspecific variation independent of sex, most notably the neck frill. The geometrics showed no consistent morphological differences between specimens that were regarded as males and females by previous authors, but also a slight support for differences in the rostrum across the sample. Maiorino and team nevertheless, cited that the typical regarded Protoceratops male, AMNH 6438, pretty much resembles the rostrum morphology of AMNH 6466, a typical regarded female. However, they suggested that authentic differences between sexes could be still present in the postcranial skeleton. Although previously suggested for P. hellenikorhinus, the team argued that the sample used for this species was not sufficient, and given that sexual dimorphism was not recovered in P. andrewsi, it is unlikely that it occurred in P. hellenikorhinus.

In 2016, Hone and colleagues analyzed 37 skulls of P. andrewsi, finding that the neck frill of Protoceratops (in both length and width) underwent positive allometry during ontongeny, that is, a faster growth/development of this region than the rest of the animal. The jugal bones also showed a trend towards an increase in relative size. These results suggest that they functioned as socio-sexual dominance signals, or, they were mostly used in display. The use of the frill as a displaying structure may be related to other anatomical features of Protoceratops such as the premaxillary teeth (at least for P. andrewsi) which could have been used in display or intraspecific combat, or the high neural spines of tail. On the other hand, Hone and team argued that if neck frills were instead used for protective purposes, a large frill may have acted as an aposematic (warning) signal to predators. However, such strategies are most effective when the taxon is rare in the overall environment, opposed to Protoceratops which appears to be an extremely abundant and medium-sized dinosaur.

Tereschenko in 2018 examined the cervical vertebrae series of six P. andrewsi specimens. Most of them had differences in the same exact vertebra, such as the shape and proportions of the vertebral centra and orientation of neural arches. According these differences, four groups were identified, concluding that individual variation was extended to the vertebral column of Protoceratops.

In 2020 nevertheless, Andrew C. Knapp and team conducted morphometric analyses of a large sample of P. andrewsi specimens, primarily confluing that the neck frill of Protoceratops has no indicators or evidence for being sexually dimorphic. Obtained results showed instead that several regions of the skull of Protoceratops independently varied in their rate of growth, ontogenetic shape and morphology; a high growth of the frill during ontogeny in relation to other body regions; and a large variability of the neck frill independent of size. Knapp and team noted that results of the frill indicate that this structure had a major role in signaling within the species, consistent with selection of potential mates with quality ornamentation and hence reproductive success, or dominance signaling. Such use of the frill may suggest that intraspecific social behavior was highly important for Protoceratops. Results also support the general hypothesis that the neck frill of ceratopsians functioned as a socio-sexual signal structure.

Reproduction

In 1989, Walter P. Coombs concluded that crocodilians, ratite and megapode birds were suitable modern analogs for dinosaur nesting behavior. He largely considered elongatoolithid eggs to belong to Protoceratops because adult skeletons were found in close proximity to nests, interpreting this as an evidence for parental care. Furthermore, Coombs considered the large concentration of Protoceratops eggs at small regions as an indicator of marked philopatric nesting (nesting in the same area). The nest of Protoceratops would have been excavated with the hindlimbs and was built in a mound-like, crater-shaped center structure with the eggs arranged in semicircular fashion. Richard A. Thulborn in 1992 analyzed the different types of eggs and nests—the majority of them, in fact, elongatoolithid—referred to Protoceratops and their structure. He identified types A and B, both of them sharing the elongated shape. Type A eggs differed from type B eggs in having a pinched end. Based on comparisons with other ornithischian dinosaurs such as Maiasaura and Orodromeus—known from more complete nests—Thulborn concluded that most depictions of Protoceratops nests were based on incompletely preserved clutches and mostly on type A eggs, which were more likely to have been laid by an ornithopod. He concluded that nests were built in a shallow mound with the eggs laid radially, contrary to popular restorations of crater-like Protoceratops nests.

In 2011, the first authentic nest of Protoceratops (MPC-D 100/530) from the Tugriken Shireh locality was described by David E. Fastovsky and team. As some individuals are closely appressed along the well-defined margin of the nest, it may have had a circular or semi-circular shape—as previously hypothetized—with a diameter of 70 cm (700 mm). Most of the individuals within the nest had nearly the same age, size and growth, suggesting that they belonged to a single nest, rather than an aggregate of individuals. Fastovsky and team also suggested that even though the individuals were young, they were not perinates based on the absence of eggshell fragments and their large size compared to even more smaller juveniles from this locality. The fact that the individuals likely spend some time in the nest after hatching for growth suggests that Protoceratops parents might have cared for their young at nests during at least the early stages of life. As Protoceratops was a relatively basal (primitive) ceratopsian, the finding may imply that other ceratopsians provided care for their young as well.

In 2017, Gregory M. Erickson and colleagues determined the incubation periods of P. andrewsi and Hypacrosaurus by using lines of arrested growth (LAGS; lines of growth) of the teeth in embryonic specimens (Protoceratops egg clutch MPC-D 100/1021). The results suggests a mean embryonic tooth replacement period of 30.68 days and relatively plesiomorphically (ancestral-shared) long incubation times for P. andrewsi, with a minimum incubation time of 83.16 days. Norell and team in 2020 analyzed again this clutch and concluded that Protoceratops laid soft-shelled eggs. Most embryos within this clutch have a flexed position and the outlines of eggs are also present, suggesting that they were buried in ovo (in the egg). The outlines of eggs and embryos indicates ellipsoid-shaped eggs in life with dimensions about 12 cm (120 mm) long and 6 cm (60 mm) wide. Several of the embryos were associated with a black to white halo (circumference). Norell and team performed histological examinations to its chemical composition, finding traces of proteinaceous eggshells, and when compared to other sauropsids the team concluded that they were not biomineralized in life and thus soft-shelled. Given that soft-shelled eggs are more vulnerable to deshydratation and crushing, Protoceratops may have buried its eggs in moisturized sand or soil. The growing embryos therefore relied on external heat and parental care.

Paleopathology

In 2018, Tereshchenko examined and described several articulated cervical vertebrae of P. andrewsi and reported the presence of two abnormally fused vertebrae (specimen PIN 3143/9). The fusion of the vertebrae was likely a product of disease or external damage.

Predator–prey interactions

Barsbold in 1974 shortly described the Fighting Dinosaurs specimen and discussed possible scenarios. The Velociraptor has its right leg pinned under the Protoceratops body with its left sickle claw oriented into the throat region. The Protoceratops bit the right hand of the predator, implying that it was unable to escape. Barsbold suggested that both animals drowned as they fell into a swamp-like body of water or, the relatively quicksand-like bottom of a lake could have kept them together during the last moments of their fight.

Osmólska in 1993 proposed another two hypotheses to explain their preservation. During the death struggle, a large dune may have collapsed simultaneously burying both Protoceratops and Velociraptor. Another proposal is that the Velociraptor was scavenging an already dead Protoceratops when it got buried and eventually killed by indeterminate circumstances.

In 1995, David M. Unwin and colleagues cast doubt on previous explanations especially a scavenging hypothesis as there were numerous indications of a concurrent death event. For instance, the Protoceratops has a semi-erect stance and its skull is nearly horizontal, which could have not been possible if the animal was already dead. The Velociraptor has its right hand trapped within the jaws of the Protoceratops and the left one grasping the Protoceratops skull. Moreover, it lies on the floor with its feet directed to the prey's belly and throat areas, indicating that this Velociraptor was not scavenging. Unwin and colleagues examined the sediments surrounding the specimen and suggested that the two were buried alive by a powerful sandstorm. They interpreted the interaction as the Protoceratops being grasped and dispatched with kicks delivered by the low-lying Velociraptor. They also considered possible that populations of Velociraptor were aware of crouching behaviors in Protoceratops during high-energy sandstorms and used it for successful hunts.

Kenneth Carpenter in 1998 considered the Fighting Dinosaurs specimen to be conclusive evidence for theropods as active predators and not scavengers. He suggested another scenario where the multiple wounds delivered by the Velociraptor on the Protoceratops throat had the latter animal bleeding to death. As a last effort, the Protoceratops bit the right hand of the predator and trapped it beneath its own weight, causing the eventual death and desiccation of the Velociraptor. The missing limbs of the Protoceratops were afterwards taken by scavengers. Lastly, both animals were buried by sand. Given that the Velociraptor is relatively complete, Carpenter suggested that it may have been completely or partially buried by sand.

In 2010, David Hone with team reported a new interaction between Velociraptor and Protoceratops based on tooth marks. Several fossils were collected at the Gate locality of the Bayan Mandahu Formation in 2008, including teeth and body remains of protoceratopsid and velociraptorine dinosaurs. The team referred these elements to Protoceratops and Velociraptor mainly based on their abundance across the unit, although they admitted that reported remains could represent different, yet related taxa (in this case, Linheraptor instead of Velociraptor). At least eight body fossils of Protoceratops present active teeth marks, which were interpreted as feeding traces. Much in contrast to the Fighting Dinosaurs specimen, the tooth marks are inferred to have been produced by the dromaeosaurid during late-stage carcass consumption either during scavenging or following a group kill. The team stated that feeding by Velociraptor upon Protoceratops was probably a relatively common occurrence in these environments, and that this ceratopsian actively formed part of the diet of Velociraptor.

In 2016, Barsbold re-examined the Fighting Dinosaurs specimen and found several anomalies within the Protoceratops individual: both coracoids have small bone fragments indicatives of a breaking of the pectoral girdle; the right forelimb and scapulocoracoid are torn off to the left and backward relative to its torso. He concluded that the prominent displacement of pectoral elements and right forelimb was caused by an external force that tried to tear them out. Since this event likely occurred after the death of both animals or during a point where movement was not possible, and the Protoceratops is missing other body elements, Barsbold suggested that scavengers were the most likely authors. Because Protoceratops is considered to have been a herding animal, another hypothesis is that members of a herd tried to pull out the already buried Protoceratops, causing the joint dislocation of limbs. However, Barsbold pointed out that there are no related traces within the overall specimen to support this latter interpretation. Lastly, he restored the course of the fight with the Protoceratops power-slamming the Velociraptor, which used its feet claws to damage the throat and belly regions and its hand claws to grasp the herbivore's head. Before their burial, the deathmatch ended up on the ground with the Velociraptor lying on its back right under the Protoceratops. After burial, either Protoceratops herd or scavengers tore off the buried Protoceratops to the left and backward, making both predator and prey to be slightly separated.

Daily activity

In 2010, Nick Longrich examined the relatively large orbital ratio and scleral ring of Protoceratops, which he suggested as evidence for a nocturnal lifestyle. Based on the size of its scleral ring, Protoceratops had an unusually large eyeball among protoceratopsids. In birds, a medium-sized scleral ring indicates that the animal is a predator, a large scleral ring indicates that it is nocturnal, and the largest ring size indicates it is an active nocturnal predator. Eye size is an important adaptation in predators and nocturnal animals because a larger eye ratio poses a higher sensitivity and resolution. Because of the energy necessary to maintain a larger eyeball and the weakness of the skull that corresponds with a larger orbit, Longrich argues that this structure may have been an adaptation for a nocturnal lifestyle. The jaw morphology of Protoceratops—more suitable for processing plant material—and its extreme abundance indicate it was not a predator, so if it was a diurnal animal, then it would have been expected to have a much smaller scleral ring size.

However, in 2011, Lars Schmitz and Ryosuke Motani measured the dimensions of the scleral ring and eye socket in fossil specimens of dinosaurs and pterosaurs, as well as some living species. They noted that whereas photopic (diurnal) animals have smaller scleral rings, scotopic (nocturnal) animals tend to have more enlarged rings. Mesopic (cathemeral) animals—which are irregularly active throughout the day and night—are between these two ranges. Schmitz and Motani separated ecological and phylogenetic factors and by examining 164 living species and noticed that eye measurements are quite accurate when inferring diurnality, cathemerality, or nocturnality in extinct tetrapods. The results indicated that Protoceratops was a cathemeral herbivore and Velociraptor primarily nocturnal, suggesting that the Fighting Dinosaurs deathmatch may have occurred at twilight or under low-light conditions. Lastly, Schmitz and Motani concluded that ecological niche was a potential main driver in the development of daily activity. However, a subsequent study in 2021 found that Protoceratops had a greater capability of nocturnal vision than did Velociraptor.

Paleoenvironment

The Bayan Mandahu Formation and the Djadokhta Formation are two rock layers where fossils of Protoceratops have been found. Both formations date back to the Late Cretaceous, about 75 to 71 million years ago. They were formed in dry, desert-like areas with sandy dunes and occasional rivers or lakes. These places had a hot, dry climate, much like the modern Gobi Desert.

Protoceratops lived alongside many other dinosaurs in these areas. In the Bayan Mandahu Formation, P. hellenikorhinus shared its home with predators like Velociraptor and Linheraptor, as well as other dinosaurs such as Machairasaurus and Wulatelong. In the Djadokhta Formation, P. andrewsi lived with dinosaurs like Velociraptor mongoliensis, Halszkaraptor, and Oviraptor. These formations give scientists clues about the kinds of animals that lived together and the tough, dry environments they survived in.

Main article: Bayan Mandahu Formation

Main articles: Djadokhta Formation , Protoceratops

Cultural significance

Some people think that ancient fossils of Protoceratops and other dinosaurs might have inspired stories about a mythical creature called the griffin. Griffins were described as large quadrupeds with bird-like beaks and claws, and they were said to guard gold deposits.

The idea comes from the fact that ancient people in Central Asia, near the Tian Shan and Altai Mountains, found many well-preserved dinosaur fossils. These areas were also rich in gold, which may have led to stories of griffins guarding treasure. However, some scientists argue that griffins more closely resemble real animals like big cats and were likely created from combinations of known species, rather than from fossil discoveries.