Editing Protoceratops (section)

===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>