Editing Protoceratops (section)

==Classification==
[[File:The dinosaur book - the ruling reptiles and their relatives (1951) (20213832929).jpg|thumb|left|Early interpretation of the evolutionary relationships of ''Protoceratops'' with [[ceratopsids]] upon its discovery; a notion now obsolete]]
''Protoceratops'' was in 1923 placed within the newly named [[Family (biology)|family]] [[Protoceratopsidae]] as the representative species by Granger and Gregory. This family was characterized by their overall primitive morphology in comparison to the more derived [[Ceratopsidae]], such as lack of well-developed horn cores and relative smaller body size. ''Protoceratops'' itself was considered by the authors to be somehow related to [[ankylosauria]]ns based on skull traits, with a more intensified degree to ''[[Triceratops]]'' and relatives.<ref name=Granger1923/> Gregory and Charles C. Mook in 1925 upon a more deeper analysis of ''Protoceratops'' and its overall morphology, concluded that this [[taxon]] represents a ceratopsian more primitive than ceratopsids and not an ankylosaur-ceratopsian ancestor.<ref name=Greggory1925/> In 1951 Edwin H. Colbert considered ''Protoceratops'' to represent a key ancestor for the ceratopsid lineage, suggesting that it ultimately led to the evolution of large-bodied ceratopsians such as ''[[Styracosaurus]]'' and ''Triceratops''. Such lineage was suggested to have started from the primitive ceratopsian ''[[Psittacosaurus]]''. He also regarded ''Protoceratops'' as one of the first "frilled" ceratopsians to appear in the fossil record.<ref>{{cite book|last1=Colbert|first1=E. H.|date=1951|chapter=The Kinds of Dinosaurs|chapter-url=https://archive.org/details/bookruli00colb/page/78/mode/2up?view=theater|title=The Dinosaur Book: The Ruling Reptiles and Their Relatives|pages=79–83|publisher=McGraw-Hill Book Company Inc.}}</ref>

However, in 1975 Maryanska and Osmolska argued that it is very unlikely that protoceratopsids evolved from [[psittacosaurid]]s, and also unlikely that they gave rise to the highly derived (advanced) ceratopsids. The first point was supported by the numerous anatomical differences between protoceratopsids and psittacosaurids, most notably the extreme reduction of some hand digits in the latter group—a trait much less pronounced in protoceratopsids. The second point was explained on the basis of the already derived anatomy in protoceratopsids like ''Bagaceratops'' or ''Protoceratops'' (such as the jaw morphology). Maryanska and Osmolska also emphasized that some early members of the Ceratopsidae reflect a much older evolutionary history.<ref name=Mary1975/> In 1998, paleontologist [[Paul Sereno]] formally defined Protoceratopsidae as the [[Taxonomic rank|branch]]-based [[clade]] including all [[coronosaurs]] closer to ''Protoceratops'' than to ''Triceratops''.<ref>{{cite journal|last1=Sereno|first1=P. C.|date=1998|title=A rationale for phylogenetic definitions, with application to the higher level taxonomy of Dinosauria|journal=Neues Jahrbuch für Geologie und Paläontologie – Abhandlungen|volume=210|issue=1|pages=41–83|doi=10.1127/njgpa/210/1998/41|url=https://d3qi0qp55mx5f5.cloudfront.net/paulsereno/i/docs/98-NJrbPalaeAbh-PhyloDefs.pdf?mtime=1591820269}}</ref>

Furthermore, with the re-examinations of ''[[Turanoceratops]]'' in 2009 and ''[[Zuniceratops]]''—two critical ceratopsian taxa regarding the evolutionary history of ceratopsids—in 2010 it was concluded that the origin of ceratopsids is unrelated to, and older than the fossil record of ''Protoceratops'' and relatives.<ref>{{cite journal|last1=Sues|first1=H.-C.|last2=Averianov|first2=A.|date=2009|title=Turanoceratops tardabilis—the first ceratopsid dinosaur from Asia|journal=Naturwissenschaften|volume=96|issue=5 |pages=645–652|bibcode=2009NW.....96..645S|doi=10.1007/s00114-009-0518-9|pmid=19277598|s2cid=21951969 |url=https://www.academia.edu/5744203}}</ref><ref>{{cite book|last1=Wolfe|first1=D. G.|last2=Kirkland|first2=J. I.|last3=Smith|first3=D.|last4=Poole|first4=K.|last5=Chinnery-Allgeier|first5=B.|last6=McDonald|first6=A.|date=2010|chapter=Zuniceratops christopheri: The North American Ceratopsid Sister Taxon Reconstructed on the Basis of New Data|chapter-url=https://books.google.com/books?id=cDnYPBjTkaIC&q=Zuniceratops|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=91–98|publisher=Indiana University Press|isbn=978-0-253-35358-0}}</ref> In most recent/modern phylogenetic analyses ''Protoceratops'' and ''Bagaceratops'' are commonly recovered as [[sister taxa]], leaving the interpretations proposing direct relationships with more derived ceratopsians unsupported.<ref>{{cite journal |author1=Yiming He |author2=Peter J. Makovicky |author3=Kebai Wang |author4=Shuqing Chen |author5=Corwin Sullivan |author6=Fenglu Han |author7=Xing Xu |author8=Michael J. Ryan |author9=David C. Evans |author10=Philip J. Currie |author11=Caleb M. Brown |author12=Don Brinkman |year=2015 |title=A New Leptoceratopsid (Ornithischia, Ceratopsia) with a Unique Ischium from the Upper Cretaceous of Shandong Province, China |journal=PLOS ONE |volume=10 |issue=12 |pages=e0144148 |doi=10.1371/journal.pone.0144148 |pmid=26701114 |pmc=4689537 |bibcode=2015PLoSO..1044148H |doi-access=free }}</ref>
[[File:Ceratops.gif|thumb|''Protoceratops'' (A, D, E) compared to other ceratopsians]]
In 2019 Czepiński analyzed a vast majority of referred specimens to the ceratopsians ''[[Bagaceratops]]'' and ''[[Breviceratops]]'', and concluded that most were in fact specimens of the former. Although the genera ''Gobiceratops'', ''Lamaceratops'', ''Magnirostris'', and ''Platyceratops'', were long considered valid and distinct taxa, and sometimes placed within Protoceratopsidae, Czepiński found the diagnostic (identifier) features used to distinguish these taxa to be largely present in ''Bagaceratops'' and thus becoming synonyms of this genus. Under this reasoning, Protoceratopsidae consists of ''Bagaceratops'', ''Breviceratops'', and ''Protoceratops''. Below are the proposed relationships among Protoceratopsidae by Czepiński:<ref name=Czepiński19/>
{{clade| style=font-size:90%;line-height:90%
|label1=[[Protoceratopsidae]]
|1={{clade
|1={{clade
|1='''''Protoceratops andrewsi'''''
|2={{clade
|1='''''Protoceratops hellenikorhinus'''''
|2={{clade
|1=''[[Breviceratops]]''
|2={{clade
|1=''[[Bagaceratops]]'' }} }} }} }} }} }}

In 2019 Bitnara Kim and colleagues described a relatively well-preserved ''Bagaceratops'' skeleton from the [[Barun Goyot Formation]], noting numerous similarities with ''Protoceratops''. Even though their respective skull anatomy had substantial differences, their postcranial skeleton was virtually the same. The [[phylogenetic analysis]] performed by the team recovered both protoceratopsids as sister taxa, indicating that ''Bagaceratops'' and ''Protoceratops'' were anatomically and [[Systematics|systematically]] related. Below is the obtained [[cladogram]], showing the position of ''Protoceratops'' and ''Bagaceratops'':<ref name=Kim2019/>
[[File:Protoceratopsidae size comparison.png|thumb|Size of ''Protoceratops'' (1, 3) compared with other protoceratopsids]]
{{clade| style=font-size:90%;line-height:90%
|label1=[[Coronosauria]]
|1={{clade
|1={{clade
|1=''[[Graciliceratops]]''
|2={{clade
|label1=[[Protoceratopsidae]]
|1={{clade
|1=''[[Bagaceratops]]''
|2=''Protoceratops'' }}
|2={{clade
|1=''[[Zuniceratops]]''
|2={{clade
|1=''[[Turanoceratops]]''
|2={{clade
|1=[[Ceratopsidae]] }} }} }} }} }}
|2={{clade
|1=[[Leptoceratopsidae]] }} }} }}

===Evolution===
[[File:Bagaceratops & Protoceratops evolution.png|thumb|left|Hypothesized transition from ''P. andrewsi'' to ''B. rozhdestvenskyi'']]
Longrich and team in 2010 indicated that highly derived morphology of ''P. hellenikorhinus''—when compared to ''P. andrewsi''—indicates that this species may represent a lineage of ''Protoceratops'' that had a longer evolutionary history compared to ''P. andrewsi'', or simply a direct descendant of ''P. andrewsi''. The difference in morphologies between ''Protoceratops'' also suggests that the nearby [[Bayan Mandahu Formation]] is slightly younger than the Djadokhta Formation.<ref name=Longrich2010>{{cite journal|last1=Longrich|first1=N. R.|last2=Currie|first2=P. J.|last3=Dong|first3=Z.|date=2010|title=A new oviraptorid (Dinosauria: Theropoda) from the Upper Cretaceous of Bayan Mandahu, Inner Mongolia|journal=Palaeontology|volume=53|issue=5|pages=945–960|doi=10.1111/j.1475-4983.2010.00968.x|bibcode=2010Palgy..53..945L |doi-access=free}}</ref>

In 2020, Czepiński analyzed several long-undescribed protoceratopsid specimens from the Udyn Sayr and Zamyn Khondt localities of the Djadokhta Formation. One specimen (MPC-D 100/551B) was shown to present skull traits that are intermediate between ''Bagaceratops rozhdestvenskyi'' (which is native to adjacent Bayan Mandahu and [[Barun Goyot]]) and ''P. andrewsi''. The specimen hails from the Udyn Sayr locality, where ''Protoceratops'' remains are dominant, and given the lack of more conclusive anatomical traits, Czepiński assigned the specimen as ''Bagaceratops'' sp. He explained that the presence of this ''Bagaceratops'' specimen in such unusual locality could be solved by: (1) the coexistence and [[sympatric]] (altogether) evolution of both ''Bagaceratops'' and ''Protoceratops'' at this one locality; (2) the rise of ''B. rozhdestvenskyi'' in a different region and eventual migration to Udyn Sayr; (3) [[Hybrid (biology)|hybridization]] between the two protoceratopsids given the near placement of both Bayan Mandahu and Djadokhta; (4) [[anagenetic]] (progressive evolution) evolutionary transition from ''P. andrewsi'' to ''B. rozhdestvenskyi''. Among scenarios, an anagenetic transition was best supported by Czepiński given the fact that no definitive ''B. rozhdestvenskyi'' fossils are found in Udyn Sayr, as expected from a hybridization event; MPC-D 100/551B lacks a well-developed accessory antorbital fenestra (hole behind the nostril openings), a trait expected to be present if ''B. rozhdestvenskyi'' had migrated to the area; and many specimens of ''P. andrewsi'' recovered at Udyn Sayr already feature a decrease in the presence of primitive premaxillary teeth, hence supporting a growing change in the populations.<ref name=Czepiński2020>{{cite journal|last1=Czepiński|first1=Ł.|date=2020|title=New protoceratopsid specimens improve the age correlation of the Upper Cretaceous Gobi Desert strata|journal=Acta Palaeontologica Polonica|volume=65|issue=3|pages=481–497|doi=10.4202/app.00701.2019|doi-access=free|url=http://www.app.pan.pl/archive/published/app65/app007012019.pdf}}</ref>