Editing Protoceratops (section)

==Description==
[[File:Protoceratops size.png|thumb|left|Size comparison of two ''Protoceratops'' species]]
''Protoceratops'' was a relatively small-sized [[ceratopsia]]n, with both ''P. andrewsi'' and ''P. hellenikorhinus'' estimated up to {{convert|2|-|2.5|m|ft|abbr=on}} in length,<ref>{{cite book|last1=Holtz|first1=T. R.|last2=Rey|first2=L. V.|year=2007|title=Dinosaurs: The Most Complete, Up-to-Date Encyclopedia for Dinosaur Lovers of All Ages|publisher=Random House|isbn=9780375824197}} [https://www.geol.umd.edu/~tholtz/dinoappendix/HoltzappendixWinter2011.pdf Genus List for Holtz 2012][https://www.geol.umd.edu/~tholtz/dinoappendix/appendix.html Weight Information]</ref><ref>{{cite book|last1=Paul|first1=G. S.|year=2016|title=The Princeton Field Guide to Dinosaurs|publisher=Princeton University Press|isbn=9780691167664|edition=2nd|location=Princeton, New Jersey|pages=282}}</ref> and around {{convert|62|-|104|kg|lb|abbr=on}} in body mass.<ref>{{cite journal|last1=Campione|first1=N. E.|last2=Evans|first2=D. C.|date=2020|title=The accuracy and precision of body mass estimation in non-avian dinosaurs|journal=Biological Reviews|volume=95|issue=6|pages=1759–1797|doi=10.1111/brv.12638|pmid=32869488|s2cid=221404013|doi-access=free}} [https://onlinelibrary.wiley.com/doi/abs/10.1111/brv.12638#SupportingInformation Supporting Information]</ref> Although similar in overall body size, the latter had a relatively greater skull length.<ref name=Helleniko2001/> Both species can be differentiated by the following characteristics:

* '''''P. andrewsi''''' – Two teeth were present at the premaxilla; the snout was low and long; the nasal horn was a single, pointed structure; the bottom edge of the dentary was slightly curved.<ref name=Brown1940/><ref name=Helleniko2001/>
* '''''P. hellenikorhinus''''' – Absence of premaxillary teeth; the snout was tall and broad; the nasal horn was divided into two pointed ridges; the bottom edge of the dentary was straight.<ref name=Helleniko2001/>

===Skull===
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|image1=Protoceratops MPC-D 100 551 skull.png
|caption1=Skull of ''P. andrewsi'' (MPC-D 100/551) in left lateral (A1-A2), dorsal (A3-A4), and right lateral (A5-A6) views

|image2=Protoceratops MPC-D 100 505 skull.png
|caption2=Skull of ''P. andrewsi'' (MPC-D 100/505) in right lateral (A1) and left lateral (A2) views

}}
The [[skull]] of ''Protoceratops'' was relatively large compared to its body and robustly built. The skull of the type species, ''P. andrewsi'', had an average total length of nearly {{convert|50|cm|mm|abbr=on}}. On the other hand ''P. hellenikorhinus'' had a total skull length of about {{convert|70|cm|mm|abbr=on}}. The rear of the skull gave form to a pronounced [[neck frill]] (also known as "parietal frill") mostly composed of the {{dinogloss|parietal}} and {{dinogloss|squamosal}} bones. The exact size and shape of the frill varied by individual; some had short, compact frills, while others had frills nearly half the length of the skull. The squamosal touched the {{dinogloss|jugal}} (cheekbone) and was very enlarged and high having a curved end that built the borders of the frill. The parietals were the posteriormost bones of the skull and major elements of the frill. In a top view they had a triangular shape and were joined by the {{dinogloss|frontal|frontals}} (bones of the [[skull roof]]). Both parietals were [[coossified]] (fused), creating a long ridge on the center of the frill. The jugal was deep and sharply developed and along with the {{dinogloss|quadratojugal}} they formed a horn-like extension that pointed to below at the lateral sides of the skull. The {{dinogloss|epijugal}} (tip region of the jugal) was separated from the jugal by a prominent [[Suture (anatomy)|suture]]; this suture was more noticeable in adults. The surfaces around the epijugal were coarse, indicating that it was covered by a [[Keratin|horny]] sheath. Unlike the much derived [[ceratopsids]], the frontal and [[postorbital]] bones of ''Protoceratops'' were flat and lacked horn cores or supraorbital horns. The {{dinogloss|palpebral}} (small spur-like bone) joined the prefrontal over the front of the [[Orbit (anatomy)|orbit]] (eye socket). In ''P. hellenikorhinus'' the palpebral protruded upwards from the {{dinogloss|prefrontal}}, just above the orbit and slightly meeting the frontal, creating a small horn-like structure. The {{dinogloss|lacrimal}} was a near-rectangular bone located in front of the orbit, contributing to the shape of the latter. The [[sclerotic ring]] (structure that supports the [[eyeball]]), found inside the orbit, was circular in shape and formed by consecutive bony plates.<ref name=Brown1940/><ref name=Helleniko2001/>

The [[snout]] was formed by the {{dinogloss|nasal}}, {{dinogloss|maxilla}}r, {{dinogloss|premaxilla}}r and {{dinogloss|rostral}} bones. The nasal was generally rounded but some individuals had a sharp nasal boss (a feature that has been called "nasal horn"). In ''P. hellenikorhinus'' this boss was divided in two sharp and long ridges. The maxilla was very deep and had up to 15 [[Dental alveolus|alveoli]] ([[tooth]] sockets) on its underside or teeth bearing surface. The premaxilla had two alveoli on its lower edge—a character that was present at least on ''P. andrewsi''. The rostral bone was devoid of teeth, high and triangular in shape. It had a sharp end and rough texture, which reflects that a [[rhamphotheca]] (horny [[beak]]) was present. As a whole, the skull had four pairs of [[Fenestra (anatomy)|fenestra]]e (skull openings). The foremost hole, the [[nares]] (nostril opening), was oval-shaped and considerably smaller than the nostrils seen in ceratopsids. ''Protoceratops'' had large orbits, which measured around {{convert|5|cm|mm|abbr=on}} in diameter and had irregular shapes depending on the individual. The forward facing and closely located orbits combined with a narrow snout, gave ''Protoceratops'' a well-developed [[binocular vision]]. Behind the eye was a slightly smaller fenestra known as the [[infratemporal fenestra]], formed by the curves of the jugal and squamosal. The last openings of the skull were two parietal fenestrae (holes in the frill).<ref name=Brown1940/><ref name=Helleniko2001/>
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|image1=Protoceratops andrewsi (1).jpg

|caption1=''P. andrewsi'' skull

|image2=Protoceratops hellenikorhinus 1.jpg

|caption2=''P. hellenikorhinus'' skull}}
The lower jaw of ''Protoceratops'' was a large element composed of the {{dinogloss|predentary}}, {{dinogloss|dentary}}, {{dinogloss|coronoid}}, {{dinogloss|angular}} and {{dinogloss|surangular}}. The predentary (frontmost bone) was very pointed and elongated, having a V-shaped [[symphyseal]] (bone union) region at the front. The dentary (teeth-bearing bone) was robust, deep, slightly recurved, and fused to the angular and surangular. A large and thick ridge ran along the lateral surface of the dentary that connected the coronoid [[Process (anatomy)|process]]—a bony projection that extends upwards from the upper surface of the lower jaw behind the tooth row—and surangular. It bore up to 12–14 alveoli on its top margin. Both predentary and dentary had a series of foramina (small pits), the latter mostly on its anterior end. The coronoid (highest point of the lower jaw) was blunt-shaped and touched by the coronoid process of the dentary, being obscured by the jugal. The surangular was near triangular in shape and in old individuals it was coossified together with the coronoid process. The angular was located below the two latter bones and behind the dentary. It was a large and somewhat rounded bone that complemented the curvature of the dentary. On its inner surface it was attached to the {{dinogloss|articular}}. The articular was a smaller bone and had a concavity on its inner surface for the articulation with the quadrate.<ref name=Brown1940/><ref name=Helleniko2001/>

''Protoceratops'' had leaf-shaped dentary and maxillary teeth that bore several [[Denticle (tooth feature)|denticles]] (serrations) on their respective edges. The [[Crown (tooth)|crowns]] (upper exposed part) had two faces or lobes that were divided by a central ridge-like structure (also called "primary ridge"). The teeth were packed into a single row that created a shearing surface. Both dentary and maxillary teeth presented marked [[homodont]]y—a dental condition where the teeth share a similar shape and size. ''P. andrewsi'' bore two small, peg to spike-like teeth that were located on the underside of each premaxilla. The second premaxillary tooth was larger than the first one. Unlike dentary and maxillary teeth, the premaxillary dentition was devoid of denticles, having a relatively smooth surface. All teeth had a single root (lower part inserted in the alveoli).<ref name=Brown1940/><ref name=Yannicke1988/><ref>{{cite journal|last1=Tanoue|first1=K.|last2=You|first2=H.-L.|last3=Dodson|first3=P.|date=2009|title=Comparative anatomy of selected basal ceratopsian dentitions|journal=Canadian Journal of Earth Sciences|volume=46|number=6|pages=425–439 |doi=10.1139/E09-030|bibcode=2009CaJES..46..425S |s2cid=58910055 }}</ref>

===Postcranial skeleton===
[[File:Protoceratops andrewsi skeletal.png|thumb|Skeletal reconstruction of ''P. andrewsi'']]

The [[vertebral column]] of ''Protoceratops'' had nine cervical (neck), 12 dorsal (back), eight sacral (pelvic) and over 40 caudal (tail) vertebrae. The [[centra]] (centrum; body of the vertebrae) of the first three cervicals were coossified together ({{dinogloss|atlas}}, {{dinogloss|axis}} and third cervical respectively) creating a rigid structure. The neck was rather short and had poor flexibility. The atlas was the smallest cervical and consisted mainly of the centrum because the {{dinogloss|neural arch}} (upper, and pointy vertebral region) was a thin, narrow bar of bone that extended upwards and backward to the base of the axis neural {{dinogloss|neural spine|spine}}. The capitular facet (attachment site for [[Haemal arch|chevron]]s; also known as cervical ribs) was formed by a low projection located near the base of the neural arch. The anterior facet of the atlas centrum was highly concave for the articulation of the {{dinogloss|occiput|occipital condyle}} of the skull. The neural arch and spine of the axis were notably larger than the atlas itself and any other cervical. The axial neural spine was broad and backward developed being slightly connected to that of the third cervical. From the fourth to the ninth all cervicals were relatively equal in size and proportions. Their neural spines were smaller than the first three vertebrae and the development of the capitular facet diminished from the fourth cervical onwards.<ref name=Brown1940/><ref name=Tereschenkko2007>{{cite journal|last1=Tereschenko|first1=V. S.|date=2007|title=Key to Protoceratopoid Vertebrae (Ceratopsia, Dinosauria) from Mongolia|journal=Paleontological Journal|volume=41|number=2|pages=175–188|doi=10.1134/S0031030107020086|bibcode=2007PalJ...41..175T |s2cid=84954199 |url=https://www.researchgate.net/publication/226106749}}</ref><ref name=Kuznetsov2010>{{cite journal|last1=Kuznetsov|first1=A. N.|last2=Tereschenko|first2=V. S.|date=2010|title=A Method for Estimation of Lateral and Vertical Mobility of Platycoelous Vertebrae of Tetrapods|journal=Paleontological Journal|volume=44|number=2|pages=209–225|doi=10.1134/S0031030110020139|bibcode=2010PalJ...44..209K |s2cid=84321442 |url=https://www.researchgate.net/publication/225123163}}</ref>
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|image1 = Protoceratops ZPAL MgD-II 3 left humerus.png

|image2 = Protoceratops ZPAL MgD-II 3 radius & ulna.png

|image3 = Protoceratops ZPAL MgD-II 3 right ilium.png

|image4 = Protoceratops ZPAL MgD-II 3 ischia.png

|image5 = Protoceratops ZPAL MgD-II 3 right femur.png

|image6 = Protoceratops ZPAL MgD-II 3 right fibula & tibia.png

|image7 = Protoceratops hands.png

|image8 = Protoceratops feet.png

|footer = Forelimb (top), pelvic (middle), and hindlimb fossil bones (bottom) of specimen ZPAL Mg D-II/3
}}
The {{dinogloss|dorsals|dorsal vertebrae}} were similar in shape and size. Their neural spines were elongated and sub-rectangular in shape with a tendency to become more elongated in posterior vertebrae. The centra were large and predominantly amphiplatian (flat on both facets) and circular when seen from the front. Sometimes in old individuals the last dorsal vertebra was somewhat coosified to the first sacral. The {{dinogloss|sacrals|sacral vertebrae}} were firmly coosified giving form to the sacrum, which was connected to the inner sides of both ilia. Their neural spines were broad, not coosified, and rather consistent in length. The centra were mainly opisthocoelous (concave on the posterior facet and convex on the anterior one) and their size became smaller towards the end. The {{dinogloss|caudals|caudal vertebrae}} decreased in size progressively towards the end and had very elongated neural spines in the mid-series, forming a [[Neural spine sail|sail]]-like structure. This elongation started from the first to the fourteenth caudal. The centra were {{dinogloss|centrum|heterocoelous}} (saddle-shaped at both facets). On the anterior caudals they were broad, however, from the twenty-fifth onwards the centra became elongated alongside the neural spines. On the underside of the caudal vertebrae a series of chevrons were attached, giving form to the lower part of the tail. The first chevron was located at the union of the third and fourth caudals. Chevrons three to nine were the largest and from the tenth onwards they became smaller.<ref name=Brown1940/><ref name=Tereschenkko2007/><ref name=Kuznetsov2010/><ref name=Tereschhenko20133>{{cite journal|last1=Tereschhenko|first1=V. S.|last2=Singer|first2=T.|date=2013|title=Structural Features of Neural Spines of the Caudal Vertebrae of Protoceratopoids (Ornithischia: Neoceratopsia)|journal=Paleontological Journal|volume=47|issue=6|pages=618–630|doi=10.1134/S0031030113060105|bibcode=2013PalJ...47..618T |s2cid=84639150 |url=https://www.researchgate.net/publication/263677535}}</ref>

All vertebrae of ''Protoceratops'' had ribs attached on the lateral sides, except for the series of caudals. The first five cervical ribs (sometimes called chevrons) were some of the shortest ribs, and among them the first two were longer than the rest. The third to the sixth dorsal (thoracic) ribs were the longest ribs in the skeleton of ''Protoceratops'', the following ribs became smaller in size as they progressed toward the end of the vertebral column. The two last dorsal ribs were the smallest, and the last of them was in contact with the internal surfaces of the ilium. Most of the sacral ribs were fused into the sacrum, and had a rather curved shape.<ref name=Brown1940/>
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|image1=Protoceratops andrewsi Restoration.png

|image2=Protoceratops hellenikorhinus Restoration.png

|footer=[[Paleoart|Life restorations]] of ''P. andrewsi'' (top) and ''P. hellenikorhinus'' (bottom)
}}
The [[pectoral girdle]] of ''Protoceratops'' was formed by the {{dinogloss|scapulocoracoid}} (fusion of the coracoid and scapula) and clavicle. The {{dinogloss|scapula|scapulae}} (shoulder blades) were relatively large and rounded on their inner sides. At their upper region, the scapulae were wide. At their lower region, the scapulae meet the coracoids. The {{dinogloss|coracoid|coracoids}} were relatively elliptical, and sometimes coosified (fused) to the scapulae. The clavicle of ''Protoceratops'' was an U to slightly V-shaped element that joined to the upper border of the scapulocoracoid. In its general form, the forelimbs of ''Protoceratops'' were shorted than the hindlimbs, and composed by the humerus, radius, and ulna. The {{dinogloss|humerus}} (upper arm bone) was large and slender, and at the lower part it met with both radius and ulna. The {{dinogloss|ulna}} had a slightly recurved shape and was longer than the radius. A concavity was present on its upper part, serving as the connection with the humerus and forming the elbow. The {{dinogloss|radius}} was a rather short bone with a straight shape. The [[Manus (anatomy)|manus]] (hand) of ''Protoceratops'' had five [[Digit (anatomy)|digits]] (fingers). The first three fingers had [[ungual]]s (claw bones) and were the largest digits. The last two were devoid of unguals and had a small size, mostly [[vestigial]] (retained, but without important function). Both hand and feet unguals were flat, blunt and hoof-like.<ref name=Brown1940/><ref name=Justyna2019/>

The [[pelvic girdle]] was formed by the {{dinogloss|ilium}}, {{dinogloss|pubis}}, and {{dinogloss|ischium}}. The ilium was a large element, having a narrow preacetabular process (anterior end) and a wide postacetabular process (posterior end). The pubis was the smallest element of the pelvic girdle and it had an irregular shape, although its lower end was developed into a pointed bony projection downward. The ischium was the longest bone of the pelvic girdle. It had an elongated shaft with a somewhat wide lower end. The hindlimbs of ''Protoceratops'' were rather long, with a slighter longer tibia (lower leg bone) than femur (thigh bone). The {{dinogloss|femur}} (thighbone) was robust and had a rather rounded and pronounced [[greater trochanter]], which was slightly recurved into the inner sides. The {{dinogloss|tibia}} (shinbone) was long and slender with a wide lower end. On its upper region a concavity was developed for the joint with the smaller {{dinogloss|fibula}}. The [[Pes (anatomy)|pes]] (foot) were composed of four {{dinogloss|metatarsal}} and four toes which bore shovel-like pedal unguals. The first metatarsal and toe were the smallest, while the other elements were of similar shape and length.<ref name=Brown1940/><ref name=Justyna2019/>