Editing Protoceratops (section)

===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>