Feeding In 1955, paleontologist
Georg Haas examined the overall skull shape of
Protoceratops and attempted to reconstruct its
jaw musculature. He suggested that the large
neck frill was likely an attachment site for masticatory muscles. Such placement of the muscles may have helped to anchor the lower jaws, useful for feeding. Yannicke Dauphin and colleagues in 1988 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. Based on their respective peg-like shape and reduced microornamentation, Dauphin and colleagues suggested that the premaxillary teeth of
Protoceratops had no specific function. In 1991, the paleontologist
Gregory S. Paul stated that contrary to the popular view of ornithischians as obligate
herbivores, 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. Such scenario indicates a possible competition with the more predatory
theropods over
carcasses, however, as the animal tissue ingestion was occasional and not the bulk of their diet, the
energy flow in
ecosystems was relatively simple. You Hailu and Peter Dodson in 2004 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.
David J. Button and
Lindsay E. Zanno in 2019 performed a large phylogenetic analysis based on skull
biomechanical characters—provided by 160
Mesozoic dinosaur species—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—characterized by relatively gracile skulls and low
bite forces—or the mouth, which was characterized by features associated with extensive processing such as high bite forces and robust jaw musculature. Ceratopsians (including protoceratopsids), along with
Euoplocephalus,
Hungarosaurus,
parkosaurid,
ornithopod 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 Brown and Schlaikjer in 1940 upon their large description and revision of
Protoceratops remarked that the orbits, frontals, and lacrimals suffered a shrinkage 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, underwent a dramatic change from a small, flat, and almost rounded structure in juveniles to a large, fan-like one in fully mature
Protoceratops individuals. David Hone and colleagues in 2016 upon their analysis of
P. andrewsi neck frills, 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. Other changes during ontogeny include the elongation of the premaxillary teeth that are smaller in juveniles and enlarged in adults, and the enlargement of middle neural spines in the tail or caudal vertebrae, which appear to grow much taller when approaching
adulthood. In 2018, paleontologists Łucja Fostowicz-Frelik and Justyna Słowiak studied the bone histology of several specimens of
P. andrewsi through cross-sections, in order to analyze the growth changes in this dinosaur. 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—bony tissue with an irregular,
fibrous texture and filled with
blood vessels—with prominent
woven-fibered bone and low
bone remodeling. Most bones of
Protoceratops preserve a large abundance of bone fibers (including
Sharpey's fibres), 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. This growth rate is mostly similar to that of other small herbivorous dinosaurs such as primitive
Psittacosaurus or
Scutellosaurus.
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. Tereshchenko using
speed equations also estimated the average maximum walking speed of
Protoceratops at about 3 km/h (
kilometres per hour). Upon the analysis of the forelimbs of several ceratopsians, 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 (optional) 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.
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. In 2008, based on the occurrence of some
Protoceratops specimens in
fluvial (river-deposited)
sediments from the Djadokhta Formation and (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.
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.
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.
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. 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. 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.
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 . 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 long and 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, a subsequent study in 2021 found that
Protoceratops had a greater capability of nocturnal vision than did
Velociraptor. ==Paleoenvironment==