Editing Protoceratops (section)

==Paleobiology==
===Feeding===
In 1955, paleontologist [[Georg Haas (paleontologist)|Georg Haas]] examined the overall skull shape of ''Protoceratops'' and attempted to reconstruct its [[Muscles of mastication|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.<ref>{{cite journal |last1=Haas|first1=G.|date=1955|title=The Jaw Musculature in Protoceratops and in Other Ceratopsians|journal=American Museum Novitates|number=1729|pages=1–24|hdl=2246/2444|url=https://digitallibrary.amnh.org/bitstream/handle/2246/2444//v2/dspace/ingest/pdfSource/nov/N1729.pdf?sequence=1&isAllowed=y}}</ref> Yannicke Dauphin and colleagues in 1988 described the [[Tooth enamel|enamel]] microstructure of ''Protoceratops'', observing a non-prismatic outer layer. They concluded that enamel shape does not relate to the [[Diet (nutrition)|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.<ref name=Yannicke1988>{{cite journal|last1=Dauphin|first1=Y.|last2=Jaeger|first2=J.-J.|last3=Osmólska|first3=H.|date=1988|title=Enamel microstructure of ceratopsian teeth (Reptilia, Archosauria)|journal=Geobios|volume=21|issue=3|pages=319–327|doi=10.1016/S0016-6995(88)80056-1|bibcode=1988Geobi..21..319D }}</ref>

In 1991, the paleontologist [[Gregory S. Paul]] stated that contrary to the popular view of ornithischians as obligate [[herbivore]]s, some groups may have been opportunistic [[Carnivore|meat-eater]]s, 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, [[Warthog|hog]]s, [[boar]]s and [[entelodont]]s. Such scenario indicates a possible competition with the more predatory [[theropods]] over [[Carrion|carcasses]], however, as the animal tissue ingestion was occasional and not the bulk of their diet, the [[Energy flow (ecology)|energy flow]] in [[ecosystem]]s was relatively simple.<ref>{{cite journal|last1=Paul|first1=G. S.|date=1991|title=The many myths, some old, some new, of dinosaurology|journal=Modern Geology|volume=16|pages=69–99|url=http://gspauldino.com/Myths.pdf}}</ref> You Hailu and Peter Dodson in 2004 suggested that the premaxillary teeth of ''Protoceratops'' may have been useful for selective cropping and feeding.<ref name=Hailu2004>{{cite book|last1=Hailu|first1=Y.|last2=Dodson|first2=P.|year=2004|chapter=Basal Ceratopsia|chapter-url=https://content.ucpress.edu/pages/2601001/2601001.ch22.pdf|editor-last1=Weishampel|editor-first1=D. B.|editor-last2=Dodson|editor-first2=P.|editor-last3=Osmólska|editor-first3=H.|title=The Dinosauria|edition=2nd|page=493|publisher=University of California Press|isbn=9780520941434}}</ref>

In 2009, Kyo Tanque and team suggested that basal ceratopsians, such as protoceratopsids, were most likely low [[Browsing (herbivory)|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 [[seed]]s or plant material not available to smaller basal ceratopsians.<ref>{{cite journal|last1=Tanoue|first1=K.|last2=Grandstaff|first2=B. S.|last3=You|first3=H.-L.|last4=Dodson|first4=P.|date=2009|title=Jaw Mechanics in Basal Ceratopsia (Ornithischia, Dinosauria)|journal=The Anatomical Record|volume=292|issue=9|pages=1352–1369|doi=10.1002/ar.20979|doi-access=free|pmid=19711460}}</ref>

[[David J. Button]] and [[Lindsay E. Zanno]] in 2019 performed a large phylogenetic analysis based on skull [[Biomechanics|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 force quotient|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.<ref>{{cite journal|last1=Button|first1=D. J.|last2=Zanno|first2=L. E.|date=2019|title=Repeated Evolution of Divergent Modes of Herbivory in Non-avian Dinosaurs|journal=Current Biology|volume=30|issue=1|pages=158–168|doi=10.1016/j.cub.2019.10.050|doi-access=free|pmid=31813611|s2cid=208652510|url=https://www.cell.com/current-biology/pdf/S0960-9822(19)31390-9.pdf}}</ref>

===Ontogeny===
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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.<ref name=Brown1940/> 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. This was based on the fact that one small specimen (IMM 96BM2/1) has a skull size slightly larger than a presumed sexually mature ''P. andrewsi'' skull (AMNH 6409), and yet it lacks double nasal horns present in fully mature ''P. hellenikorhinus''.<ref name=Helleniko2001/>

Makovicky and team in 2007 conducted a [[histological]] analysis on several specimens of ''Protoceratops'' from the [[American Museum of Natural History]] collections to provide insights into the life history of ''Protoceratops''. The examined fossil bones indicated that ''Protoceratops'' slowed its [[ontogeny]] (growth) around 9–10 years of life, and it ceased around 11–13 years. They also observed that the maximum or latest stage of development of the neck frill and nasal horn occurred in the oldest ''Protoceratops'' individuals, indicating that such traits were ontogenically variable (meaning that they varied with age). Makovicky and team also stated that as the maximum/radical changes on the neck frill and nasal horn were present in most adult individuals, trying to differentiate [[sexual dimorphism]] (anatomical differences between sexes) in adult ''Protoceratops'' may not be a good practice.<ref>{{cite journal|last1=Makovicky|first1=P. J.|last2=Sadler|first2=R.|last3=Dodson|first3=P.|last4=Erickson|first4=G. M.|last5=Norell|first5=M. A.|date=2007|title=Life history of Protoceratops andrewsi from Bayn Zag, Mongolia|journal=Journal of Vertebrate Paleontology|volume=27|issue=supp. 003|pages=109A|doi=10.1080/02724634.2007.10010458|s2cid=220411226 }}</ref>

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 [[adult]]hood.<ref name=Hone2016/>
[[File:Protoceratops ontogeny sizes.png|thumb|Four growth stages of ''Protoceratops'', from left to right: adult, sub-adult, juvenile and small juvenile (near [[perinate]]). Scale bar is {{convert|1|m|ft|abbr=on}}]]
In 2017, Mototaka Saneyoshi with team analyzed several ''Protoceratops'' specimens from the [[Djadokhta Formation]], 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. The team concluded that the frill of ''Protoceratops'' can be characterized by these ontogenetic changes.<ref>{{cite journal|last1=Saneyoshi|first1=M.|last2=Mishima|first2=S.|last3=Tsogtbaatar|first3=K.|last4=Mainbayar|first4=B.|date=2017|title=Morphological changes of Protoceratops andrewsi skull with ontogenetic processes|journal=Naturalistae|number=21|pages=1–6|language=ja|url=http://www1.ous.ac.jp/garden/kenkyuhoukoku/21/Naturalistae-2017feb-1-6.pdf}}</ref>

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, [[Fiber|fibrous]] texture and filled with [[blood vessel]]s—with prominent [[Bone#Composition|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 (biology)|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]]''.<ref>{{cite journal|last1=Fostowicz-Frelik|first1=Ł.|last2=Słowiak|first2=J.|date=2018|title=Bone histology of Protoceratops andrewsi from the Late Cretaceous of Mongolia and its biological implications|journal=Acta Palaeontologica Polonica|volume=63|issue=3|pages=503–517|doi=10.4202/app.00463.2018|doi-access=free|url=https://www.app.pan.pl/archive/published/app63/app004632018.pdf}}</ref>

===Movement===
[[File:Protoceratops juvenile and adult differences.jpg|thumb|left|Key differences between ''Protoceratops'' adults and juveniles]]
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]] [[equation]]s also estimated the average maximum walking speed of ''Protoceratops'' at about 3&nbsp;km/h ([[kilometres per hour]]).<ref>{{cite journal|last1=Tereschhenko|first1=V. S.|date=1996|title=A Reconstruction of the Locomotion of Protoceratops|journal=Paleontological Journal|volume=30|issue=2|pages=232–245|url=https://www.researchgate.net/publication/288362836}}</ref>

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.<ref>{{cite journal|last1=Senter|first1=P.|date=2007|title=Analysis of forelimb function in basal ceratopsians|journal=Journal of Zoology|volume=273|issue=3|pages=305–314|doi=10.1111/j.1469-7998.2007.00329.x}}</ref> 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.<ref name=Kuznetsov2010/> 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.<ref name=Nied2012/>

In 2019 however, 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.<ref name=Justyna2019/>

===Digging behavior===
[[File:Protoceratops andrewsi right leg.jpg|thumb|Fossil cast of ''P. andrewsi'' showing left hindlimb, equipped with large, flat, shovel-like unguals]]
Longrich in 2010 proposed that ''Protoceratops'' may have used its hindlimbs to [[Fossorial|dig burrows]] or take shelter under [[bushes]] and/or scrapes to escape the hottest temperatures of the [[Daytime|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 [[burrow]]s and high energy sand-bearing events—such as [[sandstorm]]s—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.<ref name=Longriich20110/>

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''.<ref>{{cite journal|last1=Arbour|first1=V. M.|last2=Evans|first2=D. C.|date=2019|title=A new leptoceratopsid dinosaur from Maastrichtian-aged deposits of the Sustut Basin, northern British Columbia, Canada|journal=PeerJ|volume=7|pages=e7926|doi=10.7717/peerj.7926|doi-access=free|pmc=6842559|pmid=31720103}}</ref>

===Tail function===
[[File:Protoceratops tail spines (2).jpg|thumb|left|Elevated neural spines of the caudal (tail) vertebrae of an assigned ''Protoceratops'' specimen]]
Gregory and Mook in 1925 suggested that ''Protoceratops'' was partially [[Aquatic animal|aquatic]] because of its large feet—being larger than the hands—and the very long neural spines found in the caudal (tail) vertebrae.<ref name=Greggory1925/> 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 [[Aquatic locomotion|swimming]].<ref name=Brown1940/> 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.<ref name=Barsbolld1974/>

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.<ref>{{cite journal|last1=Bailey|first1=J. B.|date=1997|title=Neural Spine Elongation in Dinosaurs: Sailbacks or Buffalo-Backs?|journal=Journal of Paleontology|volume=71|issue=6|pages=1124–1146|doi=10.1017/S0022336000036076|jstor=1306608|bibcode=1997JPal...71.1124B |s2cid=130861276 |url=https://www.researchgate.net/publication/280721656}}</ref>

In 2008, based on the occurrence of some ''Protoceratops'' specimens in [[fluvial]] (river-deposited) [[sediment]]s from the Djadokhta Formation and {{dinogloss|centrum|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 [[Webbed foot|paddle]] to aid in swimming. According to Tereschenko, ''[[Bagaceratops]]'' was fully aquatic while ''Protoceratops'' was only partially aquatic.<ref>{{cite journal|last1=Tereschhenko|first1=V. S.|date=2008|title=Adaptive Features of Protoceratopsids (Ornithischia: Neoceratopsia)|journal=Paleontological Journal|volume=42|issue=3|pages=50–64|doi=10.1134/S003103010803009X|bibcode=2008PalJ...42..273T |s2cid=84366476 |url=https://www.researchgate.net/publication/226432157}}</ref> 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 down|cooling mechanisms]].<ref name=Longriich20110>{{cite book|last1=Longrich|first1=N. R.|date=2010|chapter=The Function of Large Eyes in Protoceratops: A Nocturnal Ceratopsian?|chapter-url=https://books.google.com/books?id=OWpQW_WhPAsC&pg=PA308|editor1-last=Ryan |editor1-first=M. J.|editor2-last=Chinnery-Allgeier|editor2-first=B. J.|editor3-last=Eberth|editor3-first=D. A.|title=New Perspectives on Horned Dinosaurs: The Royal Tyrrell Museum Ceratopsian Symposium|pages=308–327|publisher=Indiana University Press|isbn=978-0-253-35358-0}}</ref>
[[File:Koreaceratops_NT.jpg|thumb|''Koreaceratops'' restored in a swimming behavior. This hypothesis has not yet reached a consensus]]
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 [[Aeolian processes|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.<ref>{{cite journal|last1=Lee|first1=Y.-N.|last2=Ryan|first2=M. J.|last3=Kobayashi|first3=Y.|date=2011|title=The first ceratopsian dinosaur from South Korea|journal=Naturwissenschaften|volume=98|issue=1|pages=39–49|bibcode=2011NW.....98...39L|doi=10.1007/s00114-010-0739-y|pmid=21085924|s2cid=23743082|url=http://doc.rero.ch/record/31549/files/PAL_E590.pdf}}</ref>

Tereschhenko in 2013 examined the structure of the caudal vertebrae spines of ''Protoceratops'', concluding that it had adaptations for [[Terrestrial animal|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 (zoology)|display]]) or females could use this method during [[Oviparity|egg laying]] to expand and relax the [[cloaca]].<ref name=Tereschhenko20133/>

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.<ref name=Hone2016/>

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]].<ref name=Kim2019>{{cite journal|last1=Kim|first1=B.|last2=Yun|first2=H.|last3=Lee|first3=Y.-N.|date=2019|title=The postcranial skeleton of Bagaceratops (Ornithischia: Neoceratopsia) from the Baruungoyot Formation (Upper Cretaceous) in Hermiin Tsav of southwestern Gobi, Mongolia|journal=Journal of the Geological Society of Korea|volume=55|number=2|pages=179–190|doi=10.14770/jgsk.2019.55.2.179|s2cid=150321203 |doi-access=|url=http://www.jgsk.or.kr/_common/do.php?a=current&b=21&bidx=1526&aidx=19356#JGSK_2019_v55n2_179_B19|url-access=subscription}}</ref>

===Social behavior===
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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 [[Degree symbol|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.<ref name=Jerzykiewiczz1993/>

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 behavior|social dinosaur]] that formed [[herd]]s 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 [[Sociality|aggregate]] into larger groups to increase their survival. Hone and colleagues in particular suggested that juveniles would aggregate primarily as a [[Anti-predator adaptation|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.<ref name=Hone2014>{{cite journal|last1=Hone|first1=D. W. E.|last2=Farke|first2=A. A.|last3=Watabe|first3=M.|last4=Shigeru|first4=S.|last5=Tsogtbaatar|first5=K.|date=2014|title=A New Mass Mortality of Juvenile Protoceratops and Size-Segregated Aggregation Behaviour in Juvenile Non-Avian Dinosaurs|journal=PLOS ONE|volume=9|issue=11|pages=e113306|doi=10.1371/journal.pone.0113306|doi-access=free|pmc=4245121|pmid=25426957|bibcode=2014PLoSO...9k3306H }}</ref>

===Sexual dimorphism and display===
[[File:Protoceratops variation.png|thumb|Diagram featuring specimens of ''P. andrewsi'' and ''P. hellenikorhinus'', showcasing a wide range of variability]]
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.<ref name=Brown1940/> In 1972 Kurzanov made comparisons between ''P. andrewsi'' skulls from Bayn Dzak and Tugriken Shireh, noting differences on the nasal horn within populations.<ref>{{cite journal|last1=Kurzanov|first1=S. M.|date=1972|title=Sexual dimorphism in protoceratopsians|journal=Paleontological Journal|issue=1|pages=91–97|language=ru}}</ref>

[[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 [[Display (zoology)|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.<ref>{{cite journal|last1=Dodson|first1=P.|date=1976|title=Quantitative Aspects of Relative Growth and Sexual Dimorphism in Protoceratops|journal=Journal of Paleontology|volume=50|issue=5|pages=929–940|jstor=1303590}}</ref>

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.<ref name=Helleniko2001/> Also in 2001, Tereschhenko analized 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.<ref>{{cite journal|last1=Tereschhenko|first1=V. S.|date=2001|title=Sexual Dimorphism in the Postcranial Skeleton of Protoceratopsids (Neoceratopsia, Protoceratopsidae) from Mongolia|journal=Paleontological Journal|volume=35|issue=4|pages=415–425|hdl=123456789/25744|url=https://www.researchgate.net/publication/272152287}}</ref>

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 [[Genetic variability|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''.<ref name=Handa2012>{{cite journal|last1=Handa|first1=N.|last2=Watabe|first2=M.|last3=Tsogtbaatar|first3=K.|date=2012|title=New Specimens of Protoceratops (Dinosauria: Neoceratopsia) from the Upper Cretaceous in Udyn Sayr, Southern Gobi Area, Mongolia|journal=Paleontological Research|volume=16|issue=3|pages=179–198|doi=10.2517/1342-8144-16.3.179|bibcode=2012PalRe..16..179H |s2cid=130903035 }}</ref> 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.<ref name=Czepiński2020/>
[[File:Protoceratops andrewsi male & femake.png|thumb|left|Hypothetical male (left, AMNH 6438) and female (AMNH 6466) ''P. andrewsi'' compared]]
However, Leonardo Maiorino with team in 2015 performed a large [[Morphometrics#Landmark-based geometric morphometrics|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''.<ref>{{cite journal|last1=Maiorino|first1=L.|last2=Farke|first2=A. A.|last3=Kotsakis|first3=T.|last4=Piras|first4=P.|date=2015|title=Males Resemble Females: Re-Evaluating Sexual Dimorphism in Protoceratops andrewsi (Neoceratopsia, Protoceratopsidae)|journal=PLOS ONE|volume=10|issue=5|pages=e0126464|doi=10.1371/journal.pone.0126464|doi-access=free|pmc=4423778|pmid=25951329}}</ref>

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 [[Protection|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 [[Abundance (ecology)|abundant]] and medium-sized dinosaur.<ref name=Hone2016>{{cite journal|last1=Hone|first1=D. W. E.|last2=Wood|first2=D.|last3=Knell|first3=R. J.|date=2016|title=Positive allometry for exaggerated structures in the ceratopsian dinosaur Protoceratops andrewsi supports socio-sexual signaling|journal=Palaeontologia Electronica|number=19.1.5A|pages=1–13|doi=10.26879/591|doi-access=free|url=https://palaeo-electronica.org/content/pdfs/591.pdf}}</ref>

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''.<ref name=Tereschenko2018/>

In 2020 nevertheless, Andrew C. Knapp and team conducted morphometric analyses of a large sample of ''P. andrewsi'' specimens, primarily confluding 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 [[Animal communication|signaling]] within the species, consistent with [[Mate choice|selection of potential mates]] with quality [[Biological ornament|ornamentation]] and hence [[reproductive success]], or [[dominance signal]]ing. 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.<ref>{{cite journal|last1=Knapp|first1=A. C.|last2=Knell|first2=R. J.|last3=Hone|first3=D. W. E.|date=2021|title=Three-dimensional geometric morphometric analysis of the skull of Protoceratops andrewsi supports a socio-sexual signalling role for the ceratopsian frill|journal=Proceedings of the Royal Society B: Biological Sciences|volume=288|number=1944|doi=10.1098/rspb.2020.2938|doi-access=free|pmid=33529562|pmc=7893235 }}</ref>

===Reproduction===
[[File:Protoceratops & juveniles.jpg|thumb|Skeletal mount of ''Protoceratops'' with juveniles]]
In 1989, Walter P. Coombs concluded that [[crocodilian]]s, [[ratite]] and [[megapode]] birds were suitable modern analogs for dinosaur [[Nesting instinct|nesting behavior]]. He largely considered [[elongatoolithid]] eggs to belong to ''Protoceratops'' because adult skeletons were found in close proximity to [[nest]]s, 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 [[Philopatry|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, [[Impact crater|crater]]-shaped center structure with the eggs arranged in semicircular fashion.<ref>{{cite book|last1=Coombs|first1=W. P.|year=1989|chapter=Modern analogs for dinosaur nesting and parental behavior|editor-last1=Farlow|editor-first1=J. O.|title=Paleobiology of the dinosaurs|series=Geological Society of America Special Papers |publisher=Boulder|location=Colorado|volume=Geological Society of America Special Paper 238|pages=21–54|doi=10.1130/SPE238-p21|isbn=0-8137-2238-1 }}</ref> 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.<ref>{{cite journal|last1=Thulborn|first1=R. A.|date=1992|title=Nest of the dinosaur Protoceratops|journal=Lethaia|volume=25|issue=2|pages=145–149|doi=10.1111/j.1502-3931.1992.tb01379.x|bibcode=1992Letha..25..145T |url=https://www.academia.edu/1049650}}</ref>
[[File:Protoceratops nest MPC-D 100 530 line.png|thumb|left|''Protoceratops'' nest MPC-D 100/530. Scale bar is {{convert|10|cm|mm|abbr=on}}]]
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 {{convert|70|cm|mm|abbr=on}}. 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 [[perinate]]s 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 (phylogenetics)|basal]] (primitive) ceratopsian, the finding may imply that other ceratopsians provided care for their young as well.<ref name=Fastovsky2011>{{cite journal|last1=Fastovsky|first1=D. E.|last2=Weishampel|first2=D. B.|last3=Watabe|first3=M.|last4=Barsbold|first4=R.|last5=Tsogtbaatar|first5=K.|last6=Narmandakh|first6=P.|date=2011|title=A nest of Protoceratops andrewsi (Dinosauria, Ornithischia)|journal=Journal of Paleontology|volume=85|issue=6|page=1035−1041|doi=10.1666/11-008.1|jstor=41409110|s2cid=129085129 |url=https://www.researchgate.net/publication/261971168}}</ref>

In 2017, Gregory M. Erickson and colleagues determined the [[Egg incubation|incubation]] periods of ''P. andrewsi'' and ''[[Hypacrosaurus]]'' by using [[lines of arrested growth]] (LAGS; lines of growth) of the teeth in [[embryo]]nic specimens (''Protoceratops'' egg clutch MPC-D 100/1021). The results suggests a mean embryonic tooth replacement period of 30.68 days and relatively [[Plesiomorphy and symplesiomorphy|plesiomorphically]] (ancestral-shared) long incubation times for ''P. andrewsi'', with a minimum incubation time of 83.16 days.<ref name=Erickson2017>{{cite journal|last1=Erickson|first1=G. M.|last2=Zelenitsky|first2=D. K.|last3=Kay|first3=D. I.|last4=Norrell|first4=M. A.|date=2017|title=Dinosaur incubation periods directly determined from growth-line counts in embryonic teeth show reptilian-grade development|journal=Proceedings of the National Academy of Sciences|volume=114|issue=3|pages=540–545|doi=10.1073/pnas.1613716114|doi-access=free|pmid=28049837|pmc=5255600|bibcode=2017PNAS..114..540E }}</ref> 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 {{convert|12|cm|mm|abbr=on}} long and {{convert|6|cm|mm|abbr=on}} 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 [[protein]]aceous eggshells, and when compared to other [[sauropsid]]s the team concluded that they were not [[Biomineralization|biomineralized]] in life and thus soft-shelled. Given that soft-shelled eggs are more vulnerable to [[Desiccation|deshydratation]] and crushing, ''Protoceratops'' may have buried its eggs in [[Moisture|moisturized]] sand or [[soil]]. The growing embryos therefore relied on external heat and parental care.<ref name=Norell2020S>{{cite journal|last1=Norell|first1=M. A.|last2=Wiemann|first2=J.|last3=Fabbri|first3=M.|last4=Yu|first4=C.|last5=Marsicano|first5=C. A.|last6=Moore-Nall|first6=A.|last7=Varricchio|first7=D. J.|last8=Pol|first8=D.|last9=Zelenitsky|first9=D. K.|date=2020|title=The first dinosaur egg was soft|journal=Nature|volume=583|issue=7816|pages=406–410|bibcode=2020Natur.583..406N|doi=10.1038/s41586-020-2412-8|pmid=32555457|s2cid=219730449 |url=http://staff.mef.org.ar/images/investigadores/diego_pol/papers/108.pdf}}</ref>

===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 [[Injury|external damage]].<ref name=Tereschenko2018>{{cite journal|last1=Tereschenko|first1=V. S.|date=2018|title=On Polymorphism of Protoceratops andrewsi Granger et Gregory, 1923 (Protoceratopidae, Neoceratopsia)|journal=Paleontological Journal|volume=52|number=4|pages=429–444|doi=10.1134/S0031030118040135|bibcode=2018PalJ...52..429T |s2cid=92796229 |url=https://www.researchgate.net/publication/327422182}}</ref>

===Predator–prey interactions===
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|image1 = Fighting dinosaurs (1).jpg
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|footer = Fossil cast of the Fighting Dinosaurs specimen (left) and life restoration of same depicting the fight (right)
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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.<ref name=Barsbolld1974>{{cite journal|last1=Barsbold|first1=R.|date=1974|title=Поединок динозавров|trans-title=Dueling dinosaurs|journal=Priroda|volume=2|pages=81–83|language=ru}}</ref>

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.<ref name=Osmolska1993>{{cite journal|last1=Osmólska|first1=H.|date=1993|title=Were the Mongolian Fighting Dinosaurs really fighting?|journal=Rev. Paleobiol.|volume=7|pages=161–162}}</ref>

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 [[sediment]]s 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.<ref name=Unwin1995>{{cite journal|last1=Unwin|first1=D. M.|last2=Perle|first2=A.|last3=Trueman|first3=C.|date=1995|title=Protoceratops and Velociraptor preserved in association: Evidence from predatory behavior in predatory dinosaurs?|journal=Journal of Vertebrate Paleontology|volume=15|issue=supp. 003|page=57A|doi=10.1080/02724634.1995.10011277}}</ref>
[[File:Fighting Dinosaurs size.png|thumb|left|Size of the Fighting Dinosaurs]]
[[Kenneth Carpenter]] in 1998 considered the Fighting Dinosaurs specimen to be conclusive evidence for theropods as active [[predator]]s and not scavengers. He suggested another scenario where the multiple [[wound]]s 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.<ref>{{cite journal|last1=Carpenter|first1=K.|date=1998|title=Evidence of predatory behavior by carnivorous dinosaurs|journal=Gaia|volume=15|pages=135–144|url=http://www.arca.museus.ul.pt/ArcaSite/obj/gaia/MNHNL-0000778-MG-DOC-web.PDF}}</ref>

In 2010, David Hone with team reported a new interaction between ''Velociraptor'' and ''Protoceratops'' based on [[Trace fossil|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 [[Carrion|carcass]] consumption either during scavenging or following a [[Pack hunter|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''.<ref>{{cite journal|last1=Hone|first1=D.|last2=Choiniere|first2=J.|last3=Sullivan|first3=C.|last4=Xu|first4=X.|last5=Pittman|first5=M.|last6=Tan|first6=Q.|date=2010|title=New evidence for a trophic relationship between the dinosaurs Velociraptor and Protoceratops|journal=Palaeogeography, Palaeoclimatology, Palaeoecology|volume=291|issue=3–4|pages=488–492|bibcode=2010PPP...291..488H|doi=10.1016/j.palaeo.2010.03.028}}</ref>

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 [[Bone fracture|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 [[herd]]ing 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.<ref name=Barsbold2016>{{cite journal|last1=Barsbold|first1=R.|date=2016|title=The Fighting Dinosaurs: The position of their bodies before and after death|journal=Paleontological Journal|volume=50|issue=12|pages=1412–1417|doi=10.1134/S0031030116120042|bibcode=2016PalJ...50.1412B |s2cid=90811750 }}</ref>

===Daily activity===
[[File:Protoceratops AMNH 6466 skull.jpg|thumb|Skull of ''P. andrewsi'' AMNH 6466, preserving sclerotic ring]]
In 2010, Nick Longrich examined the relatively large [[Orbit (anatomy)|orbital]] ratio and [[sclerotic ring]] of ''Protoceratops'', which he suggested as evidence for a [[nocturnal]] lifestyle. Based on the size of its sclerotic ring, ''Protoceratops'' had an unusually large [[eye]]ball among protoceratopsids. In birds, a medium-sized sclerotic ring indicates that the animal is a predator, a large sclerotic 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 (ecology)|abundance]] indicate it was not a predator, so if it was a [[Diurnality|diurnal]] animal, then it would have been expected to have a much smaller sclerotic ring size.<ref name=Longriich20110/>

However, in 2011, Lars Schmitz and Ryosuke Motani measured the dimensions of the sclerotic 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 sclerotic 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.<ref>{{cite journal|last1=Schmitz|first1=L.|last2=Motani|first2=R.|date=2011|title=Nocturnality in Dinosaurs Inferred from Scleral Ring and Orbit Morphology|journal=Science|volume=332|issue=6030|pages=705–708|bibcode=2011Sci...332..705S|doi=10.1126/science.1200043|pmid=21493820|s2cid=33253407}}</ref> However, a subsequent study in 2021 found that ''Protoceratops'' had a greater capability of nocturnal vision than did ''Velociraptor''.<ref>{{cite journal |last1=Choiniere |first1=Jonah N. |last2=Neenan |first2=James M. |last3=Schmitz |first3=Lars |last4=Ford |first4=David P. |last5=Chapelle |first5=Kimberley E. J. |last6=Balanoff |first6=Amy M. |last7=Sipla |first7=Justin S. |last8=Georgi |first8=Justin A. |last9=Walsh |first9=Stig A. |last10=Norell |first10=Mark A. |last11=Xu |first11=Xing |last12=Clark |first12=James M. |last13=Benson |first13=Roger B. J. |title=Evolution of vision and hearing modalities in theropod dinosaurs |journal=Science |date=7 May 2021 |volume=372 |issue=6542 |pages=610–613 |doi=10.1126/science.abe7941 |pmid=33958472 |bibcode=2021Sci...372..610C |s2cid=233872840 |url=https://www.science.org/doi/10.1126/science.abe7941 |language=en |issn=0036-8075}}</ref>