Editing Triceratops

{{Short description|Genus of ceratopsid dinosaur from the Late Cretaceous}}
{{About|the dinosaur|the band|Triceratops (band)}}
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{{merge from|Tatankaceratops|discuss=Talk:Triceratops#Merge Proposal|date=February 2025}}
{{Use American English|date=September 2020}}
{{Use mdy dates|date=April 2025}}
{{Automatic taxobox
| fossil_range = [[Late Cretaceous]] {{Geological range|68|66|earliest=70}}
| image = LA-Triceratops_mount-2.jpg
| image_caption = Skeletal mount of a ''T. prorsus'' specimen at the [[Natural History Museum of Los Angeles County|Natural History Museum of Los Angeles]]
| taxon = Triceratops
| authority = [[Othniel Charles Marsh|Marsh]], 1889b
| type_species = {{extinct}}'''''Ceratops horridus''''' <!-- Do not change to "Triceratops". The species was initially described as "Ceratops horridus", and ICZN recommendation 67B states that type species should be cited in their original form. -->
| type_species_authority = Marsh, 1889a
| subdivision_ranks = Species
| subdivision = *{{extinct}}'''''T. horridus''''' <small>(Marsh, 1889a) Marsh 1889b</small>
*{{extinct}}'''''T. prorsus''''' <small>Marsh, 1890</small>
| synonyms = {{collapsible list|bullets = true|title=<small>List</small>
|''[[Agathaumas]]''? <small>[[Edward Drinker Cope|Cope]], 1872</small>
|''[[Polyonax]]''? <small>Cope, 1874</small>
|''Bison alticornis'' <small>Marsh, 1887</small>
|''Sterrholophus'' <small>Marsh, 1891</small>
|''[[Claorhynchus]]''? <small>Cope, 1892</small>
|''Ugrosaurus'' <small>Cobabe & Fastovsky, 1987</small>
|''[[Eotriceratops]]''? <small> Wu ''et al.'', 2007</small>
|''[[Nedoceratops]]''? <small>Ukrainsky, 2007</small>
|''[[Ojoceratops]]''? <small> Sullivan & Lucas, 2010</small>
|''[[Tatankaceratops]]''? <small>Ott & Larson, 2010</small>}}
}}

'''''Triceratops''''' ({{IPAc-en|t|r|aɪ|ˈ|s|ɛr|ə|t|ɒ|p|s}} {{respell|try|SERR|ə|tops}};<ref>{{Cite web|url=https://www.dictionary.com/browse/triceratops|title=Definition of triceratops {{!}} Dictionary.com|website=www.dictionary.com|language=en|access-date=September 30, 2022|archive-date=October 4, 2022|archive-url=https://web.archive.org/web/20221004023145/https://www.dictionary.com/browse/triceratops|url-status=live}}</ref> {{lit|three-horned face}}) is a [[genus]] of [[Chasmosaurinae|chasmosaurine]] [[Ceratopsia|ceratopsian]] [[dinosaur]] that lived during the late [[Maastrichtian]] age of the Late [[Cretaceous]] [[Period (geology)|period]], about 68 to 66 million years ago on the island continent of [[Laramidia]], now forming western North America. It was one of the last-known non-avian dinosaurs and lived until the [[Cretaceous-Paleogene extinction event|Cretaceous–Paleogene extinction event]] 66 million years ago. The name ''Triceratops'', which means 'three-horned face', is derived from the [[Ancient Greek|Greek]] words {{lang|grc-Latn|trí-}} ({{wikt-lang|grc|τρί-}}) meaning 'three', {{lang|grc-Latn|kéras}} ({{wikt-lang|grc|κέρας}}) meaning 'horn', and {{lang|grc-Latn|ṓps}} ({{wikt-lang|grc|ὤψ}}) meaning 'face'.

Bearing a large bony [[neck frill|frill]], three horns on the skull, and a large, four-legged body, exhibiting [[convergent evolution]] with [[bovine]]s and [[rhinoceros]]es, ''Triceratops'' is one of the most recognizable of all dinosaurs and the best-known ceratopsian. It was also one of the largest, measuring around {{cvt|8|-|9|m}} long and weighing up to {{cvt|6|-|10|MT}}. It shared the landscape with and was most likely preyed upon by ''[[Tyrannosaurus]]'', though it is less certain that two adults would battle in the fanciful manner often depicted in museum displays and popular media. The functions of the frills and three distinctive facial horns on its head have inspired countless debates. Traditionally, these have been viewed as defensive weapons against predators. More recent interpretations find it probable that these features were primarily used in species identification, courtship, and dominance display, much like the antlers and horns of modern [[ungulate]]s.

''Triceratops'' was traditionally placed within the "short-frilled" ceratopsids, but modern [[cladistic]] studies show it to be a member of [[Chasmosaurinae]], which usually have long frills. Two [[species]], '''''T. horridus''''' and '''''T. prorsus''''', are considered valid today. Seventeen different species, however, have been named throughout history. Research published in 2010 concluded that the contemporaneous ''[[Torosaurus]]'', a ceratopsid long regarded as a separate genus, represents ''Triceratops'' in its mature form. This view is still highly disputed, and much more data is needed to settle this ongoing debate.

''Triceratops'' has been documented by numerous remains collected since the genus was first described in 1889 by American paleontologist [[Othniel Charles Marsh]]. Specimens representing life stages from hatchling to adult have been found. As the [[archetypal]] ceratopsian, ''Triceratops'' is one of the most beloved, popular dinosaurs and has been featured in numerous films, postage stamps, and many other media types.<ref>{{Cite web|date=December 2, 2020|title=Melbourne Museum acquires world's most complete triceratops skeleton in 'immense' dinosaur deal|url=http://www.theguardian.com/culture/2020/dec/02/melbourne-museum-acquires-worlds-most-complete-triceratops-skeleton-in-immense-dinosaur-deal|access-date=February 10, 2021|website=the Guardian|language=en|archive-date=February 18, 2021|archive-url=https://web.archive.org/web/20210218041616/https://www.theguardian.com/culture/2020/dec/02/melbourne-museum-acquires-worlds-most-complete-triceratops-skeleton-in-immense-dinosaur-deal|url-status=live}}</ref>

==Discovery and identification==
{{See also|Timeline of ceratopsian research}}
[[Image:Triceratops alticornis.jpg|thumb|Illustration of specimen YPM 1871E, the horn cores that were erroneously attributed to ''Bison alticornis'', the first named specimen of ''Triceratops'']]
The first named [[fossil]] specimen now attributed to ''Triceratops'' is a pair of brow horns attached to a skull roof that were found by George Lyman Cannon near [[Denver]], [[Colorado]], in the spring of 1887.<ref name="KC06">{{Cite book |title=Horns and Beaks: Ceratopsian and Ornithopod Dinosaurs |last=Carpenter |first=K. |date=2006 |publisher=Indiana University Press |isbn=978-0-253-34817-3 |editor-last=Carpenter |editor-first=K. |location=Bloomington and Indianapolis |pages=349–364 |chapter=''Bison''" ''alticornis'' and O.C. Marsh's early views on ceratopsians}}</ref> This specimen was sent to [[Othniel Charles Marsh]], who believed that the [[geologic formation|formation]] from which it came dated from the [[Pliocene]] and that the bones belonged to a particularly large and unusual [[bison]], which he named ''Bison alticornis''.<ref name="KC06"/><ref name="OCM87">{{cite journal | last1 = Marsh | first1 = O.C. | year = 1887 | title = Notice of new fossil mammals | url = http://ajs.library.cmu.edu/books/pages.cgi?call=AJS_1887_034_1887&layout=vol0/part0/copy0&file=00000332 | journal = American Journal of Science | volume = 34 | issue = 202 | pages = 323–331 | bibcode = 1887AmJS...34..323M | doi = 10.2475/ajs.s3-34.202.323 | s2cid = 129984410 | access-date = October 19, 2021 | archive-date = September 29, 2018 | archive-url = https://web.archive.org/web/20180929001329/http://ajs.library.cmu.edu/books/pages.cgi?call=AJS_1887_034_1887&layout=vol0%2Fpart0%2Fcopy0&file=00000332 | url-status = live | doi-access = free }}</ref> He realized that there were horned dinosaurs by the next year, which saw his publication of the genus ''[[Ceratops]]'' from fragmentary remains,<ref name="OCM88">{{cite journal | last1 = Marsh | first1 = O.C. | year = 1888 | title = A new family of horned Dinosauria, from the Cretaceous | url = http://ajs.library.cmu.edu/books/pages.cgi?call=AJS_1888_036_1888&layout=vol0/part0/copy0&file=00000493 | journal = American Journal of Science | volume = 36 | issue = 216 | pages = 477–478 | bibcode = 1888AmJS...36..477M | doi = 10.2475/ajs.s3-36.216.477 | s2cid = 130243398 | access-date = October 19, 2021 | archive-date = February 18, 2020 | archive-url = https://web.archive.org/web/20200218190455/http://ajs.library.cmu.edu/books/pages.cgi?call=AJS_1888_036_1888&layout=vol0%2Fpart0%2Fcopy0&file=00000493 | url-status = live }}</ref> but he still believed ''B.&nbsp;alticornis'' to be a Pliocene [[mammal]]. It took a third and much more complete skull to fully change his mind. 

Although not confidently assignable, fossils possibly belonging to ''Triceratops'' were described as two taxa, ''[[Agathaumas|Agathaumas sylvestris]]'' and ''[[Polyonax|Polyonax mortuarius]]'', in 1872 and 1874, respectively, by Marsh's archrival [[Edward Drinker Cope]].<ref>Cope, E.D. (1872). "On the existence of Dinosauria in the Transition Beds of Wyoming". [[Proceedings of the American Philosophical Society]]. '''12''': 481–483.</ref><ref name=":0">Cope, E.D. (1874). Report on the stratigraphy and Pliocene vertebrate paleontology of northern Colorado. Bulletin of the U.S. Geological and Geographical Survey of the Territories. '''9''':9-28.</ref> ''Agathaumas'' was named based on a pelvis, several vertebrae, and a few ribs collected by [[Fielding Bradford Meek]] and Henry Martyn Bannister near the [[Green River, Wyoming|Green River]] of southeastern [[Wyoming]] from layers coming from the Maastrichtian [[Lance Formation]].<ref>[[Richard Swann Lull|Lull, R. S.]], & Wright, N. E. (1942). Hadrosaurian dinosaurs of North America(Vol. 40). Geological Society of America.</ref> Due to the fragmentary nature of the remains, it can only confidently be assigned to Ceratopsidae.<ref name="Dodhorned" /><ref name="RSL33" /> ''Polyonax mortuarius'' was collected by Cope himself in 1873 from northeastern Colorado, possibly coming from the Maastrichtian [[Denver Formation]].<ref>{{Cite web |title=Division of Paleontology |url=http://research.amnh.org/paleontology/search.php?action=detail&specimen_id=46569 |access-date=April 12, 2022 |website=research.amnh.org |archive-date=September 27, 2012 |archive-url=https://web.archive.org/web/20120927015429/http://research.amnh.org/paleontology/search.php?action=detail&specimen_id=46569 |url-status=live }}</ref><ref name=":0" /> The fossils only consisted of fragmentary horn cores, 3 dorsal vertebrae, and fragmentary limb elements.<ref name=":0" /> ''Polyonax'' has the same issue as ''Agathaumas'', with the fragmentary remains non-assignable beyond Ceratopsidae.<ref>{{cite book | editor-last=Weishampel | editor-first=David B. | editor-last2=Dodson | editor-first2=Peter | editor-last3=Osmo´lska | editor-first3=Halszka | title=The dinosauria | publisher=University of California Press | publication-place=Berkeley, Calif. | date=2004 | isbn=978-0-520-94143-4 | oclc=801843269 | last1=Dodson | first1= P. | last2=Forster | first2= C.A. | last3 = Sampson | first3 = S.D.| chapter = Ceratopsidae | pages = 494–513}}.</ref><ref name="Dodhorned" />

The ''Triceratops'' [[holotype]], YPM 1820, was collected in 1888 from the [[Lance Formation]] of Wyoming by fossil hunter [[John Bell Hatcher]], but Marsh initially described this specimen as another species of ''Ceratops''.<ref name="OCM89a">{{cite journal | last1 = Marsh | first1 = O.C. | year = 1889a | title = Notice of new American Dinosauria | url = http://ajs.library.cmu.edu/books/pages.cgi?call=AJS_1889_037_1889&layout=vol0/part0/copy0&file=00000339 | journal = American Journal of Science | volume = 37 | issue = 220 | pages = 331–336 | bibcode = 1889AmJS...37..331M | doi = 10.2475/ajs.s3-37.220.331 | s2cid = 131729220 | access-date = October 19, 2021 | archive-date = September 29, 2018 | archive-url = https://web.archive.org/web/20180929000453/http://ajs.library.cmu.edu/books/pages.cgi?call=AJS_1889_037_1889&layout=vol0%2Fpart0%2Fcopy0&file=00000339 | url-status = live }}</ref> Cowboy Edmund B. Wilson had been startled by the sight of a monstrous skull poking out of the side of a ravine. He tried to recover it by throwing a lasso around one of the horns. When it broke off, the skull tumbling to the bottom of the cleft, Wilson brought the horn to his boss. His boss was rancher and avid fossil collector Charles Arthur Guernsey, who just so happened to show it to Hatcher. Marsh subsequently ordered Hatcher to locate and salvage the skull.<ref name="Dodhorned"/> The holotype was first named ''Ceratops horridus''. When further preparation uncovered the third nose horn, Marsh changed his mind and gave the piece the new generic name ''Triceratops'' ({{lit|three horn face}}), accepting his ''Bison alticornis'' as another species of ''Ceratops''.<ref name="OCM89b">{{cite journal | last1 = Marsh | first1 = O.C. | year = 1889b | title = Notice of gigantic horned Dinosauria from the Cretaceous | url = http://ajs.library.cmu.edu/books/pages.cgi?call=AJS_1889_038_1889&layout=vol0/part0/copy0&file=00000183 | journal = American Journal of Science | volume = 38 | issue = 224 | pages = 173–175 | bibcode = 1889AmJS...38..173M | doi = 10.2475/ajs.s3-38.224.173 | s2cid = 131187857 | access-date = October 19, 2021 | archive-date = September 28, 2018 | archive-url = https://web.archive.org/web/20180928235311/http://ajs.library.cmu.edu/books/pages.cgi?call=AJS_1889_038_1889&layout=vol0%2Fpart0%2Fcopy0&file=00000183 | url-status = live }}</ref> It would, however, later be added to ''Triceratops''.<ref name="HML07" /> The sturdy nature of the animal's skull has ensured that many examples have been preserved as fossils, allowing variations between species and individuals to be studied. ''Triceratops'' remains have subsequently been found in [[Montana]] and [[South Dakota]] (and more in Colorado and Wyoming), as well as the Canadian provinces of [[Saskatchewan]] and [[Alberta]].

===Species===
[[File:Triceratops holotype.jpg|thumb|left|[[Type specimen]] YPM 1820 of the [[type species]], ''T. horridus'']]
After ''Triceratops'' was described, between 1889 and 1891, Hatcher collected another thirty-one of its skulls with great effort. The first species had been named ''T.&nbsp;horridus'' by Marsh. Its [[specific name (zoology)|specific name]] was derived from the Latin word {{wikt-lang|la|horridus}} meaning "rough" or "rugose", perhaps referring to the type specimen's rough texture, later identified as an aged individual. The additional skulls varied to a lesser or greater degree from the original holotype. This variation is unsurprising, given that ''Triceratops'' skulls are large three-dimensional objects from individuals of different ages and both sexes that which were subjected to different amounts and directions of pressure during fossilization.<ref name="Dodhorned"/>

In the first attempt to understand the many species, [[Richard Swann Lull]] found two groups, although he did not say how he distinguished them. One group composed of ''T.&nbsp;horridus'', ''T.&nbsp;prorsus'', and ''T.&nbsp;brevicornus'' ('the short-horned'). The other composed of ''T.&nbsp;elatus'' and ''T.&nbsp;calicornis''. Two species (''T.&nbsp;serratus'' and ''T.&nbsp;flabellatus'') stood apart from these groups.<ref name="HML07"/> By 1933, alongside his revision of the landmark 1907 Hatcher–Marsh–Lull [[monograph]] of all known ceratopsians, he retained his two groups and two unaffiliated species, with a third lineage of ''T.&nbsp;obtusus'' and ''T.&nbsp;hatcheri'' ('Hatcher's') that was characterized by a very small nasal horn.<ref name="RSL33"/> ''T. horridus–T. prorsus–T. brevicornus'' was now thought to be the most conservative lineage, with an increase in skull size and a decrease in nasal horn size. ''T. elatus–T. calicornis'' was defined by having large brow horns and small nasal horns.<ref name="RSL33"/><ref>{{Cite journal |last=Goussard |first=Florent |year=2006 |title=The skull of Triceratops in the palaeontology gallery, Muséum national d'Histoire naturelle, Paris |url=https://www.researchgate.net/publication/242181069 |journal=Geodiversitas |volume=28 |issue=3 |pages=467–476 |via=ResearchGate |access-date=December 22, 2017 |archive-date=October 19, 2021 |archive-url=https://web.archive.org/web/20211019153736/https://www.researchgate.net/publication/242181069_The_skull_of_Triceratops_in_the_palaeontology_gallery_Museum_National_d%27Histoire_Naturelle_Paris |url-status=live }}</ref> [[Charles Mortram Sternberg|Charles&nbsp;Mortram Sternberg]] made one modification by adding ''T.&nbsp;eurycephalus'' ('the wide-headed') and suggesting that it linked the second and third lineages closer together than they were to the ''T.&nbsp;horridus'' lineage.<ref name="CMS49"/>
[[File:Triceratops prorsus old.jpg|thumb|1896 skeletal restoration of ''T.&nbsp;prorsus'' by [[O. C. Marsh]], based on the holotype skull YPM 1822 and referred elements]]
With time, the idea that the differing skulls might be representative of individual variation within one (or two) species gained popularity. In 1986, [[John Ostrom]] and [[Peter Wellnhofer]] published a paper in which they proposed that there was only one species, ''Triceratops horridus''.<ref>{{Cite journal |last1=Ostrom |first1=J. H. |last2=Wellnhofer |first2=P. |year=1986 |title=The Munich specimen of ''Triceratops'' with a revision of the genus |journal=Zitteliana |volume=14 |pages=111–158}}</ref> Part of their rationale was that there are generally only one or two species of any large animal in a region. To their findings, Thomas Lehman added the old Lull–Sternberg lineages combined with maturity and [[sexual dimorphism]], suggesting that the ''T. horridus–T. prorsus–T. brevicornus'' lineage was composed of females, the ''T. calicornis–T. elatus'' lineage was made up of males, and the ''T. obtusus–T. hatcheri'' lineage was of pathologic old males.<ref name="TML90"/>

These findings were contested a few years later by paleontologist [[Catherine Forster]], who reanalyzed ''Triceratops'' material more comprehensively and concluded that the remains fell into two species, ''T.&nbsp;horridus'' and ''T.&nbsp;prorsus'', although the distinctive skull of ''T.''&nbsp;("''Nedoceratops''") ''hatcheri'' differed enough to warrant a separate genus.<ref name="Forster1996">{{Cite journal |last=Forster |first=C.A. |year=1996 |title=Species resolution in ''Triceratops'': cladistic and morphometric approaches |journal=Journal of Vertebrate Paleontology |volume=16 |issue=2 |pages=259–270 |doi=10.1080/02724634.1996.10011313|bibcode=1996JVPal..16..259F }}</ref> She found that ''T.&nbsp;horridus'' and several other species belonged together and that ''T.&nbsp;prorsus'' and ''T.&nbsp;brevicornus'' stood alone. Since there were many more specimens in the first group, she suggested that this meant the two groups were two species. It is still possible to interpret the differences as representing a single species with sexual dimorphism.<ref name="Dodhorned"/><ref name="TML98">{{Cite journal |last=Lehman |first=T. M. |year=1998 |title=A gigantic skull and skeleton of the horned dinosaur ''Pentaceratops sternbergi'' from New Mexico |journal=Journal of Paleontology |volume=72 |issue=5 |pages=894–906 |jstor=1306666|doi=10.1017/S0022336000027220 |bibcode=1998JPal...72..894L |s2cid=132807103 }}</ref>

In 2009, John Scannella and Denver Fowler supported the separation of ''T.&nbsp;prorsus'' and ''T.&nbsp;horridus'', noting that the two species are also separated stratigraphically within the Hell Creek Formation, indicating that they did not live together at the same time.<ref name="scannella&fowler2009">{{cite book|last1=Scannella|first1= J. B. |last2= Fowler|first2= D. W. |date=2009|chapter=Anagenesis in ''Triceratops'': evidence from a newly resolved stratigraphic framework for the Hell Creek Formation|pages= 148–149|title=9th North American Paleontological Convention Abstracts|publisher= Cincinnati Museum Center Scientific Contributions 3}}</ref>

====Valid species====

[[File:Smithsonian Museum of Natural History Triceratops.jpg|thumb|First mounted ''T. horridus'' skeleton (the holotype of ''T. "obtusus"''), nicknamed "Hatcher", [[Smithsonian Museum]]]]
[[File:Triceratops prorsus - IMG 0697.jpg|thumb|''T. prorsus'', [[Carnegie Museum of Natural History]]]]
* ''T. horridus'' <small>(Marsh, 1889) Marsh, 1889 (originally ''[[Ceratops]]'')</small> ([[type species]])
* ''T. prorsus'' <small>Marsh, 1890</small>

====Synonyms and doubtful species====
Some of the following species are [[Synonym (taxonomy)|synonyms]], as indicated in parentheses ("=''T.&nbsp;horridus''" or "=''T.&nbsp;prorsus''"). All the others are each considered a {{lang|la|[[nomen dubium]]}} ({{lit|dubious name}}) because they are based on remains too poor or incomplete to be distinguished from pre-existing ''Triceratops'' species.

* ''T. albertensis'' <small>[[Charles Mortram Sternberg|C. M. Sternberg]], 1949</small>
* ''T. alticornis'' <small>(Marsh 1887) [[John Bell Hatcher|Hatcher]], [[Othniel Charles Marsh|Marsh]], and [[Richard Swann Lull|Lull]], 1907 [originally ''[[Bison]] alticornis'', Marsh 1887, and ''[[Ceratops]] alticornis'', Marsh 1888]</small>
* ''T. brevicornus'' <small>Hatcher, 1905</small> (=''T. prorsus'')
* ''T. calicornis'' <small>Marsh, 1898</small> (=''T. horridus'')
* ''T. elatus'' <small>Marsh, 1891</small> (=''T. horridus'')
* ''T. eurycephalus'' <small>[[Erich Maren Schlaikjer|Schlaikjer]], 1935</small>
* ''T. flabellatus'' <small>Marsh, 1889</small> (= ''Sterrholophus'' <small>Marsh, 1891</small>) (=''T. horridus'')
* ''T. galeus'' <small>Marsh, 1889</small>
* ''T. hatcheri'' <small>(Hatcher & Lull 1905) Lull, 1933</small> (contentious; see ''[[Nedoceratops]]'' below)
* ''T. ingens'' <small>Marsh vide [[R. S. Lull|Lull]], 1915</small>
* ''T. maximus'' <small>[[Barnum Brown|Brown]], 1933</small>
* ''T. mortuarius'' <small>([[Edward Drinker Cope|Cope]], 1874) Kuhn, 1936</small> (''nomen dubium''; originally ''[[Polyonax mortuarius]]'')
* ''T. obtusus'' <small>Marsh, 1898</small> (=''T. horridus'')
* ''T. serratus'' <small>Marsh, 1890</small> (=''T. horridus'')
* ''T. sulcatus'' <small>Marsh, 1890</small>
* ''T. sylvestris'' <small>(Cope, 1872) Kuhn, 1936</small> (''[[nomen dubium]]''; originally ''[[Agathaumas sylvestris]]'')

==Description==
===Size===
[[File:Triceratops Scale V1.svg|thumb|Size comparison with ''T. horridus'' in blue and ''T. prorsus'' in red]]
''Triceratops'' was a very large animal, measuring around {{cvt|8|-|9|m}} in length and weighing up to {{Convert|6|-|10|MT|ST}}.<ref name="paul2010" /><ref>{{cite book |last=Holtz |first=Thomas R. Jr. |year=2011 |title=Dinosaurs: The Most Complete, Up-to-Date Encyclopedia for Dinosaur Lovers of All Ages |url=http://www.geol.umd.edu/~tholtz/dinoappendix/HoltzappendixWinter2011.pdf |quote=Winter 2011 Appendix |archive-date=August 12, 2017 |access-date=August 2, 2022 |archive-url=https://web.archive.org/web/20170812011954/http://www.geol.umd.edu/~tholtz/dinoappendix/HoltzappendixWinter2011.pdf |url-status=live }}</ref><ref>{{cite journal |last1=Stein |first1=Walter W. |title=TAKING COUNT: A Census of Dinosaur Fossils Recovered From the Hell Creek and Lance Formations (Maastrichtian). |journal=The Journal of Paleontological Sciences |date=2019 |volume=8 |pages=1–42 |url=https://www.aaps-journal.org/pdf/JPS.C.2019.01a.pdf |archive-date=October 8, 2024 |access-date=April 21, 2024 |archive-url=https://web.archive.org/web/20241008104837/https://www.aaps-journal.org/pdf/JPS.C.2019.01a.pdf |url-status=live }}</ref> A specimen of ''T.&nbsp;horridus'' named Kelsey measured {{convert|22|-|24|ft|order=flip|sp=us}} long, has a {{convert|6.5|ft|order=flip|adj=on|sp=us|sigfig=1}} skull, stood about {{convert|7.5|ft|order=flip|sp=us}} tall, and was estimated by the [[Black Hills Institute]] to weigh approximately {{convert|5.4|MT|ST}}.<ref>{{Cite web |url=https://www.bhigr.com/pages/info/info_klsy.htm |title=A ''Triceratops'' Named 'Kelsey' |website=www.bhigr.com |access-date=December 22, 2017 |archive-date=December 23, 2017 |archive-url=https://web.archive.org/web/20171223102110/https://www.bhigr.com/pages/info/info_klsy.htm |url-status=live }}</ref><ref>{{Cite web |website=www.bhigr.com|title=Kesley the ''Triceratops''|url=https://bhigr.com/wp-content/uploads/2022/04/KelseySkeleton_03-copy-scaled.jpg}}</ref>

===Skull===
[[File:Triceratops skull frills.jpg|thumb|left|upright|Front view of skull with a prominent [[epoccipital]] fringe, [[Houston Museum of Natural Science]]]]
Like all [[Chasmosaurinae|chasmosaurines]], ''Triceratops'' had a large skull relative to its body size, among the largest of all land animals. The largest-known skull, specimen [[Museum of Western Colorado|MWC]] 7584 (formerly [[Brigham Young University|BYU]] 12183), is estimated to have been {{convert|2.5|m|ft|sp=us}} in length when complete<ref name="ScanHorn2010">{{Cite journal |last1=Scannella |first1=J. |last2=Horner |first2=J.R. |year=2010 |title=''Torosaurus'' Marsh, 1891, is ''Triceratops'' Marsh, 1889 (Ceratopsidae: Chasmosaurinae): synonymy through ontogeny |journal=Journal of Vertebrate Paleontology |volume=30 |issue=4 |pages=1157–1168 |doi=10.1080/02724634.2010.483632|bibcode=2010JVPal..30.1157S |s2cid=86767957 }}</ref> and could reach almost a third of the length of the entire animal.<ref name="Lambert93">{{Cite book |title=The Ultimate Dinosaur Book |last=Lambert |first=D. |publisher=Dorling Kindersley, New York |year=1993 |isbn=978-1-56458-304-8 |pages=[https://archive.org/details/ultimatedinosaur00lamb/page/152 152–167] |url=https://archive.org/details/ultimatedinosaur00lamb/page/152 }}</ref>

The front of the head was equipped with a large beak in front of its teeth. The core of the top beak was formed by a special rostral bone. Behind it, the [[premaxilla]]e bones were located, embayed from behind by very large, circular nostrils. In chasmosaurines, the premaxillae met on their midline in a complex bone plate, the rear edge of which was reinforced by the "narial strut". From the base of this strut, a triangular process jutted out into the nostril. ''Triceratops'' differs from most relatives in that this process was hollowed out on the outer side. Behind the toothless premaxilla, the [[maxilla]] bore thirty-six to forty tooth positions, in which three to five teeth per position were vertically stacked. The teeth were closely appressed, forming a "dental battery" curving to the inside. The skull bore a single horn on the snout above the nostrils. In ''Triceratops'', the nose horn is sometimes recognisable as a separate ossification, the epinasal.<ref name="Dino2"/> 

The skull also featured a pair of supraorbital "brow" horns approximately {{convert|1|m|ft|sp=us}} long, with one above each eye.<ref>{{cite news|url=https://news.google.com/newspapers?nid=894&dat=20031118&id=OCNTAAAAIBAJ&pg=6720,2967799|title=Denver museum unveils 7-foot-long, 1,000-pound Triceratops skull|publisher=The Daily Courier|date=November 18, 2003|access-date=December 26, 2013|archive-date=May 19, 2021|archive-url=https://web.archive.org/web/20210519173018/https://news.google.com/newspapers?nid=894&dat=20031118&id=OCNTAAAAIBAJ&pg=6720,2967799|url-status=live}}</ref><ref>{{Cite journal |last1=Scannella |first1=John B. |last2=Fowler |first2=Denver W. |last3=Goodwin |first3=Mark B. |last4=Horner |first4=John R. |date=July 15, 2014 |title=Evolutionary trends in Triceratops from the Hell Creek Formation, Montana |journal=Proceedings of the National Academy of Sciences |language=en |volume=111 |issue=28 |pages=10245–10250 |doi=10.1073/pnas.1313334111 |issn=0027-8424 |pmid=24982159 |pmc=4104892|bibcode=2014PNAS..11110245S |doi-access=free }}</ref> The [[jugal bone]]s pointed downward at the rear sides of the skull and were capped by separate epijugals. With ''Triceratops'', these were not particularly large and sometimes touched the quadratojugals. The bones of the skull roof were fused and by a folding of the [[frontal bone]]s, a "double" skull roof was created. In ''Triceratops'', some specimens show a [[fontanelle]], an opening in the upper roof layer. The cavity between the layers invaded the bone cores of the brow horns.<ref name="Dino2"/>

[[File:Back_and_below_of_Triceratops_skull_-_Fergus_County_Montana_-_Museum_of_the_Rockies_-_2013-07-08.jpg|thumb|Back of skull, showing rounded joint which connected the head and neck]]
At the rear of the skull, the outer [[squamosal bone]]s and the inner [[parietal bone]]s grew into a relatively short, bony frill, adorned with [[epoccipitals]] in young specimens. These were low triangular processes on the frill edge, representing separate skin ossifications or [[osteoderms]]. Typically, with ''Triceratops'' specimens, there are two epoccipitals present on each parietal bone, with an additional central process on their border. Each squamosal bone had five processes. Most other ceratopsids had large parietal [[Fenestra (anatomy)|fenestrae]], openings in their frills, but those of ''Triceratops'' were noticeably solid,<ref>{{cite news |url=https://news.google.com/newspapers?nid=2249&dat=19011024&id=U5I-AAAAIBAJ&pg=5758,3411357 |title=Making A Triceratops. Science Supplies Missing Part! Of Skeleton |publisher=Boston Evening Transcript |date=October 24, 1901 |access-date=December 26, 2013 |archive-date=May 19, 2021 |archive-url=https://web.archive.org/web/20210519173017/https://news.google.com/newspapers?nid=2249&dat=19011024&id=U5I-AAAAIBAJ&pg=5758,3411357 |url-status=live }}</ref> unless the genus ''[[Torosaurus]]'' represents mature ''Triceratops'' individuals, which it most likely does not. Under the frill, at the rear of the skull, a huge [[occipital condyle]], up to {{convert|106|mm|sp=us}} in diameter, connected the head to the neck.<ref name="Dino2"/>

The lower jaws were elongated and met at their tips in a shared epidentary bone, the core of the toothless lower beak. In the dentary bone, the tooth battery curved to the outside to meet the battery of the upper jaw. At the rear of the lower jaw, the [[articular bone]] was exceptionally wide, matching the general width of the jaw joint.<ref name="Dino2"/> ''T.&nbsp;horridus'' can be distinguished from ''T.&nbsp;prorsus'' by having a shallower snout.<ref name="paul2010">{{cite book | url=https://archive.org/details/princetonfieldgu0000paul/page/265 | title=The Princeton Field Guide to Dinosaurs | publisher=Princeton University Press | last=Paul | first=G. S. | author-link=Gregory S. Paul | year=2010 | pages=[https://archive.org/details/princetonfieldgu0000paul/page/265 265–267] | isbn=978-0-691-13720-9 }}</ref>

===Postcranial skeleton===
[[File:Triceratops Raymond National Museum of Nature and Science.jpg|thumb|left|Specimen nicknamed "Raymond" that preserves the natural, non-pronated pose of the forelimb]]
Chasmosaurines showed little variation in their postcranial skeleton.<ref name="Dino2"/> The skeleton of ''Triceratops'' is markedly robust. Both ''Triceratops'' species possessed a very sturdy build, with strong limbs, short hands with three hooves each, and short feet with four hooves each.<ref name="fujiwara2009">{{Cite journal |last=Fujiwara |first=Shin-Ichi |date=December 12, 2009 |title=A reevaluation of the manus structure in ''Triceratops'' (Ceratopsia: Ceratopsidae) |journal=Journal of Vertebrate Paleontology |volume=29 |issue=4 |pages=1136–1147 |doi=10.1671/039.029.0406 |bibcode=2009JVPal..29.1136F |s2cid=86519018 |issn=0272-4634}}</ref> The [[vertebral column]] consisted of ten neck, twelve back, ten sacral, and about forty-five tail [[vertebra]]e. The front neck vertebrae were fused into a syncervical. Traditionally, this was assumed to have incorporated the first three vertebrae, thus implying that the frontmost [[atlas (anatomy)|atlas]] was very large and sported a neural spine. Later interpretations revived an old hypothesis by [[John Bell Hatcher]] that, at the very front, a vestige of the real atlas can be observed, the syncervical then consisting of four vertebrae. The vertebral count mentioned is adjusted to this view. In ''Triceratops'', the neural spines of the neck are constant in height and don't gradually slope upwards. Another peculiarity is that the neck ribs only begin to lengthen with the ninth cervical vertebra.<ref name="Dino2"/>

The rather short and high vertebrae of the back were, in its middle region, reinforced by ossified tendons running along the tops of the [[neural arch]]es. The straight sacrum was long and adult individuals show a fusion of all sacral vertebrae. In ''Triceratops'' the first four and last two sacrals had transverse processes, connecting the vertebral column to the pelvis, that were fused at their distal ends. Sacrals seven and eight had longer processes, causing the sacrum to have an oval profile in top view. On top of the sacrum, a neural plate was present formed by a fusion of the neural spines of the second through fifth vertebrae. ''Triceratops'' had a large pelvis with a long [[Ilium (bone)|ilium]]. The [[ischium]] was curved downwards. The foot was short with four functional toes. The phalangeal formula of the foot is 2-3-4-5-0.<ref name="Dino2"/>

[[File:Triceratops horridus.png|thumb|right|[[Life restoration]] of ''T. horridus'']]
Although certainly [[quadruped]]al, the posture of horned dinosaurs has long been the subject of some debate. Originally, it was believed that the front legs of the animal had to be [[Terrestrial locomotion#Posture|sprawling]] at a considerable angle from the [[thorax]] in order to better bear the weight of the head.<ref name="Dodhorned">{{Cite book |title=The Horned Dinosaurs |last=Dodson |first=P. |publisher=Princeton University Press |year=1996 |isbn=978-0-691-02882-8 |location=Princeton, New Jersey |url=https://archive.org/details/horneddinosaursn00dods_0 }}</ref> This stance can be seen in paintings by [[Charles R. Knight|Charles Knight]] and [[Rudolph F. Zallinger|Rudolph Zallinger]]. [[Ichnological]] evidence in the form of [[fossil trackway|trackways]] from horned dinosaurs and recent reconstructions of skeletons (both physical and digital) seem to show that ''Triceratops'' and other ceratopsids maintained an upright stance during normal locomotion, with the elbows flexed to behind and slightly bowed out, in an intermediate state between fully upright and fully sprawling, comparable to the modern rhinoceros.<ref name="fujiwara2009"/><ref name="CP01">{{cite journal | last1 = Christiansen | first1 = P. | last2 = Paul | first2 = G.S. | year = 2001 | title = Limb bone scaling, limb proportions, and bone strength in neoceratopsian dinosaurs | url = http://gspauldino.com/GaiaNeoceratopsian.pdf | journal = Gaia | volume = 16 | pages = 13–29 | access-date = October 29, 2012 | archive-date = August 19, 2018 | archive-url = https://web.archive.org/web/20180819045306/http://gspauldino.com/GaiaNeoceratopsian.pdf | url-status = live }}</ref><ref name="thompsonholmes2007">{{cite journal |last1=Thompson |first1=S. |last2=Holmes |first2=R. |year=2007 |title=Forelimb stance and step cycle in ''Chasmosaurus irvinensis'' (Dinosauria: Neoceratopsia) |journal=Palaeontologia Electronica |volume=10 |issue=1 |pages=17 p |url=http://palaeo-electronica.org/2007_1/step/index.html |access-date=November 20, 2010 |archive-date=December 11, 2018 |archive-url=https://web.archive.org/web/20181211003615/https://palaeo-electronica.org/2007_1/step/index.html |url-status=live }}</ref><ref name="regaetal2010">{{Cite book |title=New Perspectives on Horned Dinosaurs: The Royal Tyrrell Museum Ceratopsian Symposium |last1=Rega |first1=E. |last2=Holmes |first2=R. |last3=Tirabasso |first3=A. |publisher=Indiana University Press |year=2010 |isbn=978-0-253-35358-0 |editor-last=Ryan |editor-first=Michael J. |location=Bloomington and Indianapolis |pages=340–354 |chapter=Habitual locomotor behavior inferred from manual pathology in two Late Cretaceous chasmosaurine ceratopsid dinosaurs, ''Chasmosaurus irvinensis'' (CMN 41357) and ''Chasmosaurus belli'' (ROM 843) |editor2-last=Chinnery-Allgeier |editor2-first=Brenda J. |editor3-last=Eberth |editor3-first=David A.}}</ref>

The hands and forearms of ''Triceratops'' retained a fairly primitive structure when compared to other quadrupedal dinosaurs, such as [[thyreophora]]ns and many [[sauropod]]s. In those two groups, the forelimbs of quadrupedal species were usually rotated so that the hands faced forward with palms backward ("pronated") as the animals walked. ''Triceratops'', like other ceratopsians and related quadrupedal [[ornithopod]]s (together forming the [[Cerapoda]]), walked with most of their fingers pointing out and away from the body, the original condition for dinosaurs. This was also retained by bipedal forms, like [[theropod]]s. In ''Triceratops'', the weight of the body was carried by only the first three fingers of the hand, while digits 4 and 5 were vestigial and lacked claws or hooves.<ref name="fujiwara2009"/> The phalangeal formula of the hand is 2-3-4-3-1, meaning that the first or innermost finger of the forelimb has two bones, the next has three, the next has four, etc.<ref>{{Cite book |title=Introduction to the study of dinosaurs |last=Martin |first=Anthony J. |date=2006 |publisher=Blackwell Pub |isbn=978-1405134132 |edition=2nd |location=Malden, MA |oclc=61130756}}</ref>

=== Skin ===

[[File:Ceratopsia skin integument.png|right|thumb|alt=|Skin impressions of various ceratopsians; k is from the flank of ''T. horridus'' specimen HMNS PV.1506]]
Preserved skin from ''Triceratops'' is known. This skin consist of large scales, some of which exceed {{Convert|100|mm|in}} across, which have conical projections rising from their center. A preserved piece of skin from the frill of a specimen is also known, which consists of small polygonal basement scales.<ref>{{Cite journal |last1=Bell |first1=Phil R. |last2=Hendrickx |first2=Christophe |last3=Pittman |first3=Michael |last4=Kaye |first4=Thomas G. |last5=Mayr |first5=Gerald |date=August 12, 2022 |title=The exquisitely preserved integument of Psittacosaurus and the scaly skin of ceratopsian dinosaurs |journal=Communications Biology |language=en |volume=5 |issue=1 |pages=809 |doi=10.1038/s42003-022-03749-3 |issn=2399-3642 |pmc=9374759 |pmid=35962036}}</ref>

==Classification==

''Triceratops'' is the best-known genus of [[Ceratopsidae]], a family of large, mostly North American [[Ceratopsia|ceratopsians]]. The exact relationship of ''Triceratops'' among the other ceratopsids has been debated over the years. Confusion stemmed mainly from the combination of a short, solid frill (similar to that of [[Centrosaurinae]]), with long brow horns (more akin to [[Chasmosaurinae]]).<ref>{{cite web|url=http://dinosaurios.org/triceratops/|title=What is special about the Triceratops?|publisher=Dinosaurios.org|access-date=December 26, 2013|date=July 24, 2013|archive-date=May 31, 2020|archive-url=https://web.archive.org/web/20200531214022/http://dinosaurios.org/triceratops/|url-status=live}}</ref> In the first overview of ceratopsians, [[R. S. Lull]] hypothesized the existence of two lineages, one of ''[[Monoclonius]]'' and ''[[Centrosaurus]]'' leading to ''Triceratops'', the other with ''[[Ceratops]]'' and ''[[Torosaurus]]'', making ''Triceratops'' a centrosaurine as the group is understood today.<ref name="HML07">{{Cite book |title=The Ceratopsia |last1=Hatcher |first1=J. B. |last2=Marsh |first2=O. C. |last3=Lull |first3=R. S. |date=1907 |publisher=Government Printing Office |isbn=978-0-405-12713-7 |location=Washington, D.C.}}</ref> Later revisions supported this view when [[Lawrence Lambe]], in 1915, formally describing the first, short-frilled group as Centrosaurinae (including ''Triceratops''), and the second, long-frilled group as Chasmosaurinae.<ref name="RSL33">{{cite journal | last1 = Lull | first1 = R. S. | year = 1933 | title = A revision of the Ceratopsia or horned dinosaurs | url =https://archive.org/details/revisionofcerato33lull | journal = Memoirs of the Peabody Museum of Natural History | volume = 3 | issue = 3| pages = 1–175 |access-date=November 20, 2010 | doi=10.5962/bhl.title.5716}}</ref><ref name="LL15">{{cite book | last=Lambe | first=Lawrence M. | title=On Eoceratops canadensis, gen. nov., with remarks on other genera of Cretaceous horned dinosaurs | publisher= Geological Survey of Canada, Government Printing Bureau | publication-place=Ottawa | date=1915 | isbn=0-665-82611-7 | oclc=920394016}}</ref>

In 1949, [[Charles Mortram Sternberg]] was the first to question this position, proposing instead that ''Triceratops'' was more closely related to ''[[Arrhinoceratops]]'' and ''[[Chasmosaurus]]'' based on skull and horn features, making ''Triceratops'' a chasmosaurine ("ceratopsine" in his usage) genus.<ref name="CMS49">{{cite journal | last1 = Sternberg | first1 = C. M. | year = 1949 | title = The Edmonton fauna and description of a new ''Triceratops'' from the Upper Edmonton member; phylogeny of the Ceratopsidae | journal = National Museum of Canada Bulletin | volume = 113 | pages = 33–46 }}</ref> He was largely ignored, with [[John Ostrom]]<ref name="Ostrom66">{{cite journal|author=Ostrom, J. H.|year=1966| title=Functional morphology and evolution of the ceratopsian dinosaurs| journal=[[Evolution (journal)|Evolution]]| volume=20| issue=3 | pages = 290–308 | doi=10.2307/2406631|pmid=28562975|jstor=2406631}}</ref> and later David Norman placing ''Triceratops'' within the Centrosaurinae.<ref>{{cite book |last=Norman |first=David |title=The Illustrated Encyclopaedia of Dinosaurs |year=1985 |publisher=Salamander Books |location=London |isbn=978-0-517-46890-6}}</ref>

Subsequent discoveries and analyses, however, proved the correctness of Sternberg's view on the position of ''Triceratops'', with Thomas Lehman defining both subfamilies in 1990 and diagnosing ''Triceratops'' as "ceratopsine" on the basis of several morphological features. Apart from the one feature of a shortened frill, ''Triceratops'' shares no derived traits with centrosaurines.<ref name="TML90">{{Cite book |title=Dinosaur Systematics: Perspectives and Approaches |last=Lehman |first=T. M. |date=1990 |publisher=Cambridge University Press |isbn=978-0-521-36672-4 |editor-last=Carpenter |editor-first=K. |location=Cambridge |pages=211–229 |chapter=The ceratopsian subfamily Chasmosaurinae: sexual dimorphism and systematics |editor2-last=Currie |editor2-first=P. J.}}</ref> Further research by [[Peter Dodson]], including a 1990 [[cladistic]] analysis and a 1993 study using resistant-fit theta-rho analysis, or RFTRA (a [[morphometrics|morphometric technique]] which systematically measures similarities in skull shape), reinforces ''Triceratops''{{'}} placement as a chasmosaurine.<ref>{{Cite book |title=The Dinosauria |last1=Dodson |first1=P. |last2=Currie |first2=P. J. |date=1990 |publisher=University of California Press |isbn=978-0-520-06727-1 |editor-last=Weishampel |editor-first=D. B. |location=Berkeley |pages=593–618 |chapter=Neoceratopsia |author-link=Peter Dodson |editor2-last=Dodson |editor2-first=P. |editor3-last=Osmólska |editor3-first=H.}}</ref><ref>{{Cite journal |last=Dodson |first=P. |year=1993 |title=Comparative craniology of the Ceratopsia |url=http://earth.geology.yale.edu/~ajs/1993/11.1993.07Dodson.pdf |journal=American Journal of Science |volume=293 |pages=200–234 |doi=10.2475/ajs.293.A.200 |bibcode=1993AmJS..293..200D |access-date=January 21, 2007 |archive-date=August 19, 2018 |archive-url=https://web.archive.org/web/20180819045309/http://earth.geology.yale.edu/~ajs/1993/11.1993.07Dodson.pdf |url-status=live }}</ref>

[[File:2014 Triceratops horridus fossil.jpg|alt=An imposing Triceratops fossil on display, lit by blue and yellow light.|thumb|'Horridus', the most complete ''Triceratops'' fossil known, on display at the [[Melbourne Museum]]. Restored and prepared by the team at Pangea Fossils in Victoria British Columbia, Canada.]]
The cladogram below follows Longrich (2014), who named a new species of ''[[Pentaceratops]]'', and included nearly all species of chasmosaurine.<ref name="longrich15">{{Cite journal |last=Longrich |first=N. R. |year=2014 |title=The horned dinosaurs Pentaceratops and Kosmoceratops from the upper Campanian of Alberta and implications for dinosaur biogeography |journal=Cretaceous Research |volume=51 |pages=292–308 |doi=10.1016/j.cretres.2014.06.011|bibcode=2014CrRes..51..292L }}</ref>
{{clade| style=font-size:85%;line-height:85%
|label1=[[Chasmosaurinae]]
|1={{clade
 |1=''[[Mercuriceratops]]''
 |2={{clade
  |1=''[[Judiceratops]]''
  |2={{clade
   |1={{clade
    |1=''[[Chasmosaurus]]''
    |2=''[[Mojoceratops]]'' }}
   |2={{clade
    |1=''[[Agujaceratops]]''
    |2={{clade
     |1={{clade
      |1=''[[Pentaceratops aquilonius]]''
      |2=Williams Fork chasmosaur
      |3={{clade
       |1=''[[Pentaceratops sternbergii]]''
       |2=''[[Utahceratops]]'' }} }}
     |2={{clade
      |1=''[[Kosmoceratops]]''
      |2={{clade
       |1={{clade
        |1=''[[Anchiceratops]]''
        |2=Almond Formation chasmosaur }}
       |2={{clade
        |1={{clade
         |1=''[[Bravoceratops]]''
         |2=''[[Coahuilaceratops]]'' }}
        |2={{clade
         |1=''[[Arrhinoceratops]]''
         |label2=[[Triceratopsini]]
         |2={{clade
          |1=''[[Titanoceratops]]''
          |2={{clade
           |1=''[[Torosaurus]]''
           |2='''''Triceratops'''''}} }} }} }} }} }} }} }} }} }} }} }}
[[File:Laramie-skull.jpg|thumb|Skull of specimen DMNH 48617 from the [[Laramie Formation]] of eastern [[Colorado]]. Based on the age of the formation, it may be the oldest ''Triceratops'' known.]]
For many years after its discovery, the deeper evolutionary origins of ''Triceratops'' and its close relatives remained largely obscure. In 1922, the newly discovered ''[[Protoceratops]]'' was seen as its ancestor by [[Henry Fairfield Osborn]],<ref name="Dodhorned"/> but many decades passed before additional findings came to light. Recent years have been fruitful for the discovery of several antecedents of ''Triceratops''. ''[[Zuniceratops]]'', the earliest-known ceratopsian with brow horns, was described in the late 1990s, and ''[[Yinlong]]'', the first known [[Jurassic]] ceratopsian, was described in 2005.

These new finds have been vital in illustrating the origins of ceratopsians in general, suggesting an Asian origin in the Jurassic and the appearance of truly horned ceratopsians by the beginning of the Late Cretaceous in North America.<ref name="Dino2">{{Cite book |title=The Dinosauria |last1=Dodson |first1=P. |last2=Forster |first2=C. A. |last3=Sampson |first3=S. D. |date=2004 |publisher=University of California Press |isbn=978-0-520-24209-8 |editor-last=Weishampel |editor-first=D. B. |edition=second |location=Berkeley |pages=494–513 |chapter=Ceratopsidae |editor2-last=Dodson |editor2-first=P. |editor3-last=Osmólska |editor3-first=H.}}</ref>

In [[phylogenetics|phylogenetic taxonomy]], the genus ''Triceratops'' has been used as a reference point in the definition of Dinosauria. Dinosaurs have been designated as all descendants of the [[most recent common ancestor]] of ''Triceratops'' and [[Neornithes|modern bird]]s.<ref>{{cite journal | last1 = Gauthier | first1 = J. A. | year = 1986 | title = Saurischian monophyly and the origin of birds. The Origin of Birds and the Evolution of Flight, K. Padian (ed.) | journal = Memoirs of the California Academy of Sciences | volume = 8 | pages = 1–55 }}</ref> Furthermore, [[Ornithischia]] has been defined as those dinosaurs more closely related to ''Triceratops'' than to modern birds.<ref>{{cite journal | last1 = Sereno | first1 = P. C. | year = 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 }}</ref>

==Paleobiology==
[[File:Tyrannosaurus and Triceratops at Natural History Museum of Los Angeles County.jpg|thumb|left|A ''Triceratops'' mounted next to a ''Tyrannosaurus'' at the [[Los Angeles Natural History Museum]]]]
Although ''Triceratops'' is commonly portrayed as a [[herd]]ing animal, there is currently little evidence to suggest that they lived in herds. While several other ceratopsians are known from [[bone bed]]s preserving bones from two to hundreds or even thousands of individuals, there is currently only one documented bonebed dominated by ''Triceratops'' bones: a site in southeastern Montana with the remains of three juveniles. It may be significant that only juveniles were present.<ref name="JCMetal09">{{cite journal |doi=10.1080/02724634.2009.10010382 |last1=Mathews |first1=Joshua C. |last2=Brusatte |first2=Stephen L. |last3=Williams |first3=Scott A. |last4=Henderson |first4=Michael D. |year=2009 |title=The first ''Triceratops'' bonebed and its implications for gregarious behavior |journal=Journal of Vertebrate Paleontology |volume=29 |issue=1 |pages=286–290|bibcode=2009JVPal..29..286M |s2cid=196608646 }}</ref> In 2012, a group of three ''Triceratops'' in relatively complete condition, each of varying sizes from a full-grown adult to a small juvenile, were found near [[Newcastle, Wyoming]]. The remains are currently under excavation by paleontologist Peter Larson and a team from the [[Black Hills Institute]]. It is believed that the animals were traveling as a family unit, but it remains unknown if the group consists of a mated pair and their offspring, or two females and a juvenile they were caring for. The remains also show signs of predation or scavenging from ''[[Tyrannosaurus]]'', particularly on the largest specimen, with the bones of the front limbs showing breakage and puncture wounds from ''Tyrannosaurus'' teeth.<ref>{{Cite news |url=http://www.cnn.com/2013/06/03/us/triceratops-found/index.html |title=Triceratops trio unearthed in Wyoming – CNN |last=Smith |first=Matt |date=June 4, 2013 |work=CNN |access-date=December 22, 2017 |archive-date=August 1, 2017 |archive-url=https://web.archive.org/web/20170801233836/http://www.cnn.com/2013/06/03/us/triceratops-found/index.html |url-status=live }}</ref> In 2020, Illies and Fowler described the [[Ossification|co-ossified]] distal caudal vertebrae of ''Triceratops''. According to them, this pathology could have arisen after one ''Triceratops'' accidentally stepped on the tail of another member of the herd.<ref>{{Cite journal |last1=Illies |first1=M. M. Canoy |last2=Fowler |first2=D. W. |year=2020 |title=''Triceratops'' with a kink: Co-ossification of five distal caudal vertebrae from the Hell Creek Formation of North Dakota |journal=Cretaceous Research |volume=108 |pages=104355 |doi=10.1016/j.cretres.2019.104355|bibcode=2020CrRes.10804355C |s2cid=214215413 }}</ref><ref>{{cite news |last1=Barrera |first1=Nathanial A. |title=More than old bones: New study sheds light on Triceratops behavior and living habits |url=https://www.thedickinsonpress.com/incoming/4858206-More-than-old-bones-New-study-sheds-light-on-Triceratops-behavior-and-living-habits |access-date=March 31, 2020 |work=The Dickinson Press |date=January 9, 2020 |language=en |archive-date=May 27, 2020 |archive-url=https://web.archive.org/web/20200527200052/https://www.thedickinsonpress.com/incoming/4858206-More-than-old-bones-New-study-sheds-light-on-Triceratops-behavior-and-living-habits |url-status=live }}</ref>

[[File:Triceratops sp. (Hell Creek Formation, Upper Cretaceous; Montana, USA).jpg|thumb|''Triceratops'' rib with [[theropod]] tooth marks at the middle]]
For many years, ''Triceratops'' finds were known only from solitary individuals.<ref name="JCMetal09"/> These remains are very common. For example, [[Bruce Erickson (paleontologist)|Bruce Erickson]], a paleontologist of the [[Science Museum of Minnesota]], has reported having seen 200 specimens of ''T.&nbsp;prorsus'' in the [[Hell Creek Formation]] of [[Montana]].<ref>{{Cite journal |last=Erickson |first=B. R. |year=1966 |title=Mounted skeleton of ''Triceratops prorsus'' in the Science Museum |journal=Scientific Publications of the Science Museum |volume=1 |pages=1–16}}</ref> Similarly, [[Barnum Brown]] claimed to have seen over 500 skulls in the field.<ref name="Dodhorned"/>{{rp|79}} Because ''Triceratops'' teeth, horn fragments, frill fragments, and other skull fragments are such abundant fossils in the [[Lancian|Lancian faunal stage]] of the late [[Maastrichtian]] ([[Late Cretaceous]], 66&nbsp;mya) of western North America, it is regarded as one of the dominant herbivores of the time, if not the most dominant. In 1986, [[Robert Bakker]] estimated it as making up five sixths of the large dinosaur fauna at the end of the Cretaceous.<ref name="RTB86">{{cite book|last=Bakker|first= R. T.|date=1986|title=The Dinosaur Heresies: New Theories Unlocking The Mystery of the Dinosaurs and Their Extinction|url=https://archive.org/details/dinosaurheresies00robe|url-access=registration|publisher=William Morrow|location= New York|isbn=978-0-14-010055-6|page=[https://archive.org/details/dinosaurheresies00robe/page/438 438]}}</ref> Unlike most animals, skull fossils are far more common than [[postcrania]]l bones for ''Triceratops'', suggesting that the skull had an unusually high [[preservation potential]].<ref name="KD94">{{cite book |last=Derstler|first= K. |year=1994 |editor-last=Nelson|editor-first= G. E. |title=The Dinosaurs of Wyoming |series=Wyoming Geological Association Guidebook, 44th Annual Field Conference |chapter=Dinosaurs of the Lance Formation in eastern Wyoming |publisher=Wyoming Geological Association |pages=127–146}}</ref>

Analysis of the endocranial anatomy of ''Triceratops'' suggest its sense of smell was poor compared to that of other dinosaurs. Its ears were attuned to low frequency sounds, given the short cochlear lengths recorded in an analysis by Sakagami ''et al,''. This same study also suggests that ''Triceratops'' held its head about 45 degrees to the ground, an angle which would showcase the horns and frill most effectively that simultaneously allowed the animal to take advantage of food through grazing.<ref>{{Cite journal|doi = 10.7717/peerj.9888|title = Endocranial anatomy of the ceratopsid dinosaur Triceratops and interpretations of sensory and motor function|year = 2020|last1 = Sakagami|first1 = Rina|last2 = Kawabe|first2 = Soichiro|journal = PeerJ|volume = 8|pages = e9888|pmid = 32999761|pmc = 7505063| doi-access=free }}</ref>  
 
A 2022 study by Wiemann and colleagues of various dinosaur genera, including ''Triceratops'', suggests that it had an [[ectothermic]] (cold blooded) or [[gigantothermic]] metabolism, on par with that of modern reptiles. This was uncovered using the [[spectroscopy]] of lipoxidation signals, which are byproducts of [[oxidative phosphorylation]] and correlate with metabolic rates. They suggested that such metabolisms may have been common for ornithischian dinosaurs in general, with the group evolving towards ectothermy from an ancestor with an [[endothermic]] (warm blooded) metabolism.<ref name="wiemann2022">{{cite journal |last1=Wiemann |first1=J. |last2=Menéndez |first2=I. |last3=Crawford |first3=J.M. |first4=M. |last4=Fabbri |first5=J.A. |last5=Gauthier |first6=P.M. |last6=Hull |first7=M.A. |last7=Norell |first8=D.E.G. |last8=Briggs |title=Fossil biomolecules reveal an avian metabolism in the ancestral dinosaur |journal=Nature |year=2022 |volume=606 |issue=7914 |pages=522–526 |doi=10.1038/s41586-022-04770-6|pmid=35614213 |bibcode=2022Natur.606..522W |s2cid=249064466 |url=https://resolver.caltech.edu/CaltechAUTHORS:20220531-924660000 }}</ref> An isotopic analysis study by Rooij and colleagues suggested that ''Triceratops'' was gigantothermic, if not endothermic due to its large body volume.<ref name=":1" /> The very same study conducted by Wiemann et al., revealed that ''Triceratops'' lived in environments that consisted of floodplains and inland forests.<ref>{{Cite journal |last1=de Rooij |first1=Jimmy |last2=van der Lubbe |first2=Jeroen H. J. L. |last3=Verdegaal |first3=Suzan |last4=Hulscher |first4=Megan |last5=Tooms |first5=Daphne |last6=Kaskes |first6=Pim |last7=Verhage |first7=Oeki |last8=Portanger |first8=Leonie |last9=Schulp |first9=Anne S. |date=2022-12-01 |title=Stable isotope record of Triceratops from a mass accumulation (Lance Formation, Wyoming, USA) provides insights into Triceratops behaviour and ecology |url=https://www.sciencedirect.com/science/article/pii/S003101822200445X |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |volume=607 |pages=111274 |doi=10.1016/j.palaeo.2022.111274 |bibcode=2022PPP...60711274D |issn=0031-0182}}</ref>

===Dentition and diet===
[[File:Tric1.JPG|thumb|Close up of the jaws and teeth]]
''Triceratops'' were [[herbivorous]] and, because of their low slung head, their primary food was probably low growing vegetation, although they may have been able to knock down taller plants with their horns, [[beak]], and sheer bulk.<ref name="Dino2"/><ref>{{cite journal | last1 = Tait | first1 = J. | last2 = Brown | first2 = B. | year = 1928 | title = How the Ceratopsia carried and used their head | journal = Transactions of the Royal Society of Canada | volume = 22 | pages = 13–23 }}</ref> The jaws were tipped with a deep, narrow beak, believed to have been better at grasping and plucking than biting.<ref name="Ostrom66"/><ref>{{Cite journal |last1=Erickson |first1=Gregory M. |last2=Sidebottom |first2=Mark A. |last3=Kay |first3=David I. |last4=Turner |first4=Kevin T. |last5=Ip |first5=Nathan |last6=Norell |first6=Mark A. |last7=Sawyer |first7=W. Gregory |last8=Krick |first8=Brandon A. |date=2015-06-05 |title=Wear biomechanics in the slicing dentition of the giant horned dinosaur Triceratops |journal=Science Advances |volume=1 |issue=5 |pages=e1500055 |doi=10.1126/sciadv.1500055 |pmc=4640618 |pmid=26601198|bibcode=2015SciA....1E0055E }}</ref>

''Triceratops'' teeth were arranged in groups called batteries, which contained 36 to 40 tooth columns in each side of each jaw and 3 to 5 stacked teeth per column, depending on the size of the animal.<ref name="Dino2"/> This gives a range of 432 to 800 teeth, of which only a fraction were in use at any given time (as tooth replacement was continuous throughout the life of the animal).<ref name="Dino2"/> They functioned by shearing in a vertical to near-vertical orientation.<ref name="Dino2"/> Additionally, their teeth wore as they fed, creating fullers that minimised friction as they masticated.<ref>{{Cite journal |last1=Erickson |first1=Gregory M. |last2=Sidebottom |first2=Mark A. |last3=Kay |first3=David I. |last4=Turner |first4=Kevin T. |last5=Ip |first5=Nathan |last6=Norell |first6=Mark A. |last7=Sawyer |first7=W. Gregory |last8=Krick |first8=Brandon A. |date=June 5, 2015 |title=Wear biomechanics in the slicing dentition of the giant horned dinosaur Triceratops |journal=[[Science Advances]] |language=en |volume=1 |issue=5 |pages=e1500055 |doi=10.1126/sciadv.1500055 |issn=2375-2548 |pmc=4640618 |pmid=26601198 |bibcode=2015SciA....1E0055E }}</ref> The great size and numerous teeth of ''Triceratops'' suggests that they ate large volumes of [[Flowering plant|fibrous plant]] material.<ref>{{Cite journal |last1=Erickson |first1=Gregory M. |last2=Sidebottom |first2=Mark A. |last3=Kay |first3=David I. |last4=Turner |first4=Kevin T. |last5=Ip |first5=Nathan |last6=Norell |first6=Mark A. |last7=Sawyer |first7=W. Gregory |last8=Krick |first8=Brandon A. |date=June 5, 2015 |title=Wear biomechanics in the slicing dentition of the giant horned dinosaur Triceratops |journal=Science Advances |language=en |volume=1 |issue=5 |pages=e1500055 |doi=10.1126/sciadv.1500055 |issn=2375-2548 |pmc=4640618 |pmid=26601198 |bibcode=2015SciA....1E0055E }}</ref><ref>{{Cite journal |last1=Maiorino |first1=Leonardo |last2=Farke |first2=Andrew A. |last3=Kotsakis |first3=Tassos |last4=Teresi |first4=Luciano |last5=Piras |first5=Paolo |date=September 11, 2015 |title=Variation in the shape and mechanical performance of the lower jaws in ceratopsid dinosaurs (Ornithischia, Ceratopsia) |url=http://dx.doi.org/10.1111/joa.12374 |journal=Journal of Anatomy |volume=227 |issue=5 |pages=631–646 |doi=10.1111/joa.12374 |pmid=26467240 |issn=0021-8782|pmc=4609198 }}</ref><ref>{{Cite web |last=Carter |first=Nicholas |date=January 29, 2019 |title=A Day in the Life of Triceratops |url=https://dinomuseum.ca/2019/01/a-day-in-the-life-of-triceratops |access-date=April 15, 2025 |website=Philip J. Currie Dinosaur Museum |language=en-CA}}</ref> Other plants that were a part of Triceratops's diet included [[Populus|''Populus'' plants]], [[Pine|Pine plants]], [[Platanus|''Platanus'' plants]], [[Hazel|''Hazel'' plants]], and [[Taxodium|''Taxodium'' plants]].<ref>{{Cite web |date=May 2, 2017 |title=A triceratopsian diet |url=https://www.mcgill.ca/redpath/article/triceratopsian-diet |access-date=April 15, 2025 |website=Redpath Museum |language=en}}</ref> Some researchers suggest it, along with its cousin ''Torosaurus'' ate [[Arecaceae|palms]] and [[cycad]]s<ref>{{Cite journal |last=Ostrom |first=J. H. |year=1964 |title=A functional analysis of jaw mechanics in the dinosaur ''Triceratops'' |url=http://www.peabody.yale.edu/scipubs/bulletins_postillas/ypmP088_1964.pdf |journal=Postilla |volume=88 |pages=1–35 |access-date=November 20, 2010 |archive-date=June 26, 2010 |archive-url=https://web.archive.org/web/20100626220725/http://www.peabody.yale.edu/scipubs/bulletins_postillas/ypmP088_1964.pdf |url-status=dead }}</ref><ref>{{cite book | last1 = Weishampel | first1 = D. B. | journal = Advances in Anatomy, Embryology, and Cell Biology | title = Evolution of Jaw Mechanisms in Ornithopod Dinosaurs | year = 1984 | volume = 87 | pages = 1–110 | pmid = 6464809 | doi=10.1007/978-3-642-69533-9| series = Advances in Anatomy Embryology and Cell Biology | isbn = 978-3-540-13114-4 | s2cid = 12547312 }}</ref><ref>{{Cite journal |last1=Fricke |first1=Henry C. |last2=Pearson |first2=Dean A. |date=2008 |title=Stable isotope evidence for changes in dietary niche partitioning among hadrosaurian and ceratopsian dinosaurs of the Hell Creek Formation, North Dakota |url=http://dx.doi.org/10.1666/08020.1 |journal=Paleobiology |volume=34 |issue=4 |pages=534–552 |doi=10.1666/08020.1 |bibcode=2008Pbio...34..534F |issn=0094-8373|url-access=subscription }}</ref> and others suggest it ate [[fern]]s, which then grew in prairies.<ref>{{Cite book |title=The Origins of Angiosperms and their Biological Consequences |last1=Coe |first1=M. J. |last2=Dilcher |first2=D. L. |last3=Farlow |first3=J. O. |last4=Jarzen |first4=D. M. |last5=Russell |first5=D. A. |date=1987 |publisher=Cambridge University Press |isbn=978-0-521-32357-4 |editor-last=Friis |editor-first=E. M. |pages=225–258 |chapter=Dinosaurs and land plants |editor2-last=Chaloner |editor2-first=W. G. |editor3-last=Crane |editor3-first=P. R.}}</ref><ref name=":1">{{Cite journal |last1=de Rooij |first1=Jimmy |last2=van der Lubbe |first2=Jeroen H. J. L. |last3=Verdegaal |first3=Suzan |last4=Hulscher |first4=Megan |last5=Tooms |first5=Daphne |last6=Kaskes |first6=Pim |last7=Verhage |first7=Oeki |last8=Portanger |first8=Leonie |last9=Schulp |first9=Anne S. |date=December 1, 2022 |title=Stable isotope record of Triceratops from a mass accumulation (Lance Formation, Wyoming, USA) provides insights into Triceratops behaviour and ecology |url=https://www.sciencedirect.com/science/article/pii/S003101822200445X |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |volume=607 |pages=111274 |doi=10.1016/j.palaeo.2022.111274 |bibcode=2022PPP...60711274D |issn=0031-0182 |archive-date=February 26, 2024 |access-date=November 22, 2024 |archive-url=https://web.archive.org/web/20240226074157/https://www.sciencedirect.com/science/article/pii/S003101822200445X |url-status=live }}</ref> Studies of the isotopes of ceratopsian and hadrosaur teeth revealed that ''Triceratops'' and ''Edmontosaurus'' respectively engaged in [[Ecological niche|niche partitioning]].<ref>{{Cite journal |last1=Fricke |first1=Henry C. |last2=Pearson |first2=Dean A. |date=October 2008 |title=Stable isotope evidence for changes in dietary niche partitioning among hadrosaurian and ceratopsian dinosaurs of the Hell Creek Formation, North Dakota |url=https://www.cambridge.org/core/journals/paleobiology/article/abs/stable-isotope-evidence-for-changes-in-dietary-niche-partitioning-among-hadrosaurian-and-ceratopsian-dinosaurs-of-the-hell-creek-formation-north-dakota/C96E90222D1D728292F0343828FC213B |journal=Paleobiology |language=en |volume=34 |issue=4 |pages=534–552 |doi=10.1666/08020.1 |bibcode=2008Pbio...34..534F |issn=0094-8373|url-access=subscription }}</ref><ref>{{Cite journal |last1=Sander |first1=P. M. |last2=Gee |first2=C. T. |last3=Hummel |first3=J. |last4=Clauss |first4=Marcus |date=2010-06-23 |editor-last=Gee |editor-first=C. T. |title=Mesozoic plants and dinosaur herbivory |url=https://www.zora.uzh.ch/id/eprint/35283/ |journal=Life of the Past |pages=331–359 |doi=10.5167/uzh-35283}}</ref><ref>{{Cite journal |last1=Barrett |first1=Paul M. |last2=Rayfield |first2=Emily J. |date=2006-04-01 |title=Ecological and evolutionary implications of dinosaur feeding behaviour |url=https://www.cell.com/trends/ecology-evolution/abstract/S0169-5347(06)00023-1 |journal=Trends in Ecology & Evolution |language=English |volume=21 |issue=4 |pages=217–224 |doi=10.1016/j.tree.2006.01.002 |issn=0169-5347 |pmid=16701088}}</ref>

===Functions of the horns and frill===

[[File:UCMP Triceratops right.JPG|left|thumb|Juvenile and adult skulls—the juvenile skull is about the size of an adult human head]]
There has been much speculation over the functions of ''Triceratops''{{'}} head adornments. The two main theories have revolved around use in combat and in courtship display, with the latter now thought to be the most likely primary function.<ref name="Dino2"/>

Early on, Lull postulated that the frills may have served as anchor points for the jaw muscles to aid chewing by allowing increased size and power for the muscles.<ref>{{cite journal | last1 = Lull | first1 = R. S. | year = 1908 | title = The cranial musculature and the origin of the frill in the ceratopsian dinosaurs | url = https://zenodo.org/record/1450156 | journal = American Journal of Science | volume = 4 | issue = 25 | pages = 387–399 | doi = 10.2475/ajs.s4-25.149.387 | bibcode = 1908AmJS...25..387L | access-date = June 12, 2019 | archive-date = November 15, 2019 | archive-url = https://web.archive.org/web/20191115184638/https://zenodo.org/record/1450156 | url-status = live }}</ref> This has been put forward by other authors over the years, but later studies do not find evidence of large muscle attachments on the frill bones.<ref name="Forster90">{{cite thesis|last=Forster|first= C. A. |date=1990|title= The cranial morphology and systematics of ''Triceratops'', with a preliminary analysis of ceratopsian phylogeny|degree= Ph.D. Dissertation|publisher= University of Pennsylvania|location= Philadelphia|pages= 227 }}</ref>

''Triceratops'' were long thought to have used their horns and frills in combat with large predators, such as ''[[Tyrannosaurus]]'', the idea being discussed first by [[Charles Hazelius Sternberg|Charles H. Sternberg]] in 1917 and 70 years later by Robert Bakker.<ref name="RTB86"/><ref>{{cite book|last=Sternberg|first= C. H. |publisher=C. H. Sternberg{{--}}The World Company Press |date=1917|title=Hunting Dinosaurs in the Badlands of the Red Deer River, Alberta, Canada|location= Lawrence, Kansas|pages= 261 |doi=10.5962/bhl.title.159811 }}</ref> There is evidence that ''Tyrannosaurus'' did have aggressive head-on encounters with ''Triceratops'', based on partially healed tyrannosaur tooth marks on a ''Triceratops'' brow horn and [[squamosal]]. The bitten horn is also broken, with new bone growth after the break. Which animal was the aggressor, however, is unknown.<ref name="JH08">{{Cite book |title=Tyrannosaurus rex, the Tyrant King (Life of the Past) |last=Happ |first=J. |publisher=Indiana University Press |year=2008 |isbn=978-0-253-35087-9 |editor-last=Larson |editor-first=P. |location=Bloomington |pages=355–368 |chapter=An analysis of predator-prey behavior in a head-to-head encounter between ''Tyrannosaurus rex'' and ''Triceratops'' |editor2-last=Carpenter |editor2-first=K.}}</ref>  Paleontologist Peter Dodson estimates that, in a battle against a bull ''Tyrannosaurus'', the ''Triceratops'' had the upper hand and would successfully defend itself by inflicting fatal wounds to the ''Tyrannosaurus'' using its sharp horns. ''Tyrannosaurus'' is also known to have fed on ''Triceratops'', as shown by a heavily tooth-scored ''Triceratops'' [[ilium (bone)|ilium]] and [[sacrum]].<ref name="erickson1996">{{Cite journal |last1=Erickson |first1=Gregory M. |last2=Olson |first2=Kenneth H. |date=March 19, 1996 |title=Bite marks attributable to Tyrannosaurus rex: Preliminary description and implications |journal=Journal of Vertebrate Paleontology |volume=16 |issue=1 |pages=175–178 |doi=10.1080/02724634.1996.10011297 |bibcode=1996JVPal..16..175E |issn=0272-4634 |url=https://zenodo.org/record/3732430 |access-date=June 3, 2020 |archive-date=October 19, 2021 |archive-url=https://web.archive.org/web/20211019153802/https://zenodo.org/record/3732430 |url-status=live }}</ref> 

In addition to combat with predators using its horns, ''Triceratops'' are popularly shown engaging each other in combat with horns locked. While studies show that such activity would be feasible, if unlike that of present-day horned animals,<ref>{{cite journal | last=Farke | first=A. A. | year=2004 | title=Horn Use in ''Triceratops'' (Dinosauria: Ceratopsidae): Testing Behavioral Hypotheses Using Scale Models | url=http://www.nhm.ac.uk/hosted_sites/pe/2004_1/horn/horn.pdf | access-date=November 20, 2010 | journal=Palaeo-electronica | volume=7 | issue=1 | pages=1–10 | archive-date=March 3, 2016 | archive-url=https://web.archive.org/web/20160303204748/http://www.nhm.ac.uk/hosted_sites/pe/2004_1/horn/horn.pdf | url-status=live }}</ref> there is disagreement about whether they did so. Although pitting, holes, lesions, and other damage on ''Triceratops'' skulls (and the skulls of other ceratopsids) are often attributed to horn damage in combat, a 2006 study finds no evidence for horn thrust injuries causing these forms of damage (with there being no evidence of infection or healing). Instead, non-pathological [[bone resorption]], or unknown bone diseases, are suggested as causes.<ref name="TF06">{{cite book|last1=Tanke|first1= D. H. |last2= Farke|first2= A. A. |date=2006|chapter= Bone resorption, bone lesions, and extracranial fenestrae in ceratopsid dinosaurs: a preliminary assessment|editor-last= Carpenter|editor-first= K. |title=Horns and Beaks: Ceratopsian and Ornithopod Dinosaurs|publisher= Indiana University Press|location= Bloomington|pages= 319–347|isbn=978-0-253-34817-3}}</ref> A 2009 study compared incidence rates of skull lesions and [[periosteal reaction]] in ''Triceratops'' and ''[[Centrosaurus]]'', showing that these were consistent with ''Triceratops'' using its horns in combat and the frill being adapted as a protective structure, while lower pathology rates in ''Centrosaurus'' may indicate visual use over physical use of cranial ornamentation or a form of combat focused on the body rather than the head.<ref>{{cite journal | last1 = Farke | first1 = A.A. | last2 = Wolff | first2 = E.D.S. | last3 = Tanke | first3 = D.H. | last4 = Sereno | first4 = Paul| year = 2009 | title = Evidence of Combat in ''Triceratops'' | journal = PLOS ONE | volume = 4 | issue = 1| page = e4252 | doi = 10.1371/journal.pone.0004252 | editor1-last = Sereno | editor1-first = Paul | bibcode = 2009PLoSO...4.4252F | pmid=19172995 | pmc=2617760| doi-access = free }} {{open access}}</ref><ref>{{Cite magazine |last=Wall |first=Michael |date=January 27, 2009 |title=Scars Reveal How Triceratops Fought |url=https://www.wired.com/2009/01/dinofight/ |archive-url=https://web.archive.org/web/20140820071319/https://www.wired.com/2009/01/dinofight/ |archive-date=August 20, 2014 |access-date=December 13, 2024 |magazine=Wired |language=en-US |issn=1059-1028}}</ref><ref>{{Cite web |last=Switek |first=Brian |date=February 5, 2009 |title=Triceratops v. Triceratops |url=https://www.smithsonianmag.com/science-nature/triceratops-v-triceratops-38373087/ |archive-url=https://web.archive.org/web/20140416192132/https://www.smithsonianmag.com/science-nature/triceratops-v-triceratops-38373087/ |archive-date=April 16, 2014 |access-date=December 13, 2024 |website=Smithsonian Magazine |language=en}}</ref> The frequency of injury was found to be 14% in ''Triceratops''.<ref>{{cite journal | last1 = Peterson | first1 = JE | last2 = Dischler | first2 = C | last3 = Longrich | first3 = NR | year = 2013 | title = Distributions of Cranial Pathologies Provide Evidence for Head-Butting in Dome-Headed Dinosaurs (Pachycephalosauridae) | journal = PLOS ONE | volume = 8 | issue = 7| page = e68620 | doi = 10.1371/journal.pone.0068620 | pmid=23874691 | pmc=3712952| bibcode = 2013PLoSO...868620P | doi-access = free }} {{open access}}</ref> The researchers also concluded that the damage found on the specimens in the study was often too localized to be caused by bone disease.<ref>{{cite magazine |url=https://www.wired.com/wiredscience/2009/01/dinofight/ |title=Scars Reveal How Triceratops Fought – |magazine=Wired |date=January 27, 2009 |access-date=August 3, 2010 |first=Michael |last=Wall |archive-date=January 12, 2014 |archive-url=https://web.archive.org/web/20140112143952/http://www.wired.com/wiredscience/2009/01/dinofight/ |url-status=live }}</ref> Histological examination reveals that the frill of ''Triceratops'' is composed of fibrolamellar bone.<ref>{{Cite book |title=Encyclopedia of Dinosaurs |last=Reid |first=R.E.H. |date=1997 |publisher=Academic Press |editor-last=Currie |editor-first=P. J. |location=San Diego, CA. |pages=329–339 |chapter=Histology of bones and teeth |editor2-last=Padian |editor2-first=K.}}</ref> This contains [[fibroblast]]s that play a critical role in wound healing and is capable of rapidly depositing bone during remodeling.<ref>{{cite journal | last1 = Horner | first1 = JR | last2 = Goodwin | first2 = MB | year = 2009 | title = Extreme Cranial Ontogeny in the Upper Cretaceous Dinosaur Pachycephalosaurus | journal = PLOS ONE | volume = 4 | issue = 10| page = e7626 | doi = 10.1371/journal.pone.0007626 | pmid = 19859556 | pmc=2762616| bibcode = 2009PLoSO...4.7626H | doi-access = free }}</ref><ref>{{cite journal | last1 = Horner | first1 = JR | last2 = Lamm | first2 = E | year = 2011 | title = Ontogeny of the parietal frill of Triceratops: a preliminary histological analysis | journal = Comptes Rendus Palevol | volume = 10 | issue = 5–6| pages = 439–452 | doi=10.1016/j.crpv.2011.04.006}}</ref>

[[File:Triceratops lesions.jpg|thumb|Examples of [[Periosteal reaction|periosteal reactive]] bone in selected specimens of ''Triceratops'']]
One skull was found with a hole in the [[Jugal bone#In dinosaurs|jugal bone]], apparently a puncture wound sustained while the animal was alive, as indicated by signs of healing. The hole has a diameter close to that of the distal end of a ''Triceratops'' horn. This and other apparent healed wounds in the skulls of ceratopsians have been cited as evidence of non-fatal intra-specific competition in these dinosaurs.<ref name="farlowanddodson1975">{{cite journal | last1 = Farlow | first1 = J. O. | last2 = Dodson | first2 = P. | year = 1975 | title = The behavioral significance of frill and horn morphology in ceratopsian dinosaurs | journal = Evolution | volume = 29 | issue = 2| pages = 353–361 | doi=10.2307/2407222| pmid = 28555861 | jstor = 2407222 }}</ref><ref>{{cite book|last=Martin|first= A. J.|date=2006|title= Introduction to the Study of Dinosaurs|edition= Second|location= Oxford|publisher= Blackwell Publishing|pages= 299–300|isbn=978-1-4051-3413-2}}</ref> Another specimen, referred to as "Big John", has a similar fenestra to the squamosal caused by what appears to be another ''Triceratops'' horn and the squamosal bone shows signs of significant healing, further vindicating the hypothesis that this ceratopsian used its horns for intra-specific combat.<ref>{{cite journal |last1=D’Anastasio |first1=Ruggero |last2=Cilli |first2=Jacopo |last3=Bacchia |first3=Flavio |last4=Fanti |first4=Federico |last5=Gobbo |first5=Giacomo |last6=Capasso |first6=Luigi |title=Histological and chemical diagnosis of a combat lesion in Triceratops |journal=Scientific Reports |date=April 7, 2022 |volume=12 |issue=1 |pages=3941 |doi=10.1038/s41598-022-08033-2 |pmid=35393445 |pmc=8990019 |bibcode=2022NatSR..12.3941D |language=en |issn=2045-2322}}</ref>

The large frill also may have helped to increase body area to [[thermoregulation|regulate body temperature]].<ref>{{Cite journal |last=Wheeler |first=P.E. |year=1978 |title=Elaborate CNS cooling structures in large dinosaurs |journal=[[Nature (journal)|Nature]] |volume=275 |issue=5679 |pages=441–443 |bibcode=1978Natur.275..441W |doi=10.1038/275441a0 |pmid=692723|s2cid=4160470 }}</ref> A similar theory has been proposed regarding the plates of ''[[Stegosaurus]]'',<ref>{{Cite journal |last1=Farlow |first1=J. O. |last2=Thompson |first2=C. V. |last3=Rosner |first3=D. E. |year=1976 |title=Plates of the dinosaur ''Stegosaurus'': Forced convection heat loss fins? |journal=[[Science (magazine)|Science]] |volume=192 |issue=4244 |pages=1123–5 |bibcode=1976Sci...192.1123F |doi=10.1126/science.192.4244.1123 |pmid=17748675|s2cid=44506996 }}</ref> although this use alone would not account for the bizarre and extravagant variation seen in different members of [[Ceratopsidae]], which would rather support the sexual display theory.<ref name="Dino2"/>

The theory that frills functioned as a sexual display was first proposed by Davitashvili in 1961 and has gained increasing acceptance since.<ref name="TML90"/><ref name="Forster90"/><ref name="Davitashvili61">{{cite book|title=Teoriya Polovogo Otbora (Theory of Sexual Selection)|year=1961|author=Davitashvili, L. Sh.|page=538|publisher=Izdatel'stvo Akademii nauk SSSR}}</ref> Evidence that visual display was important, either in courtship or other social behavior, can be seen in the ceratopsians differing markedly in their adornments, making each species highly distinctive. Also, modern living creatures with such displays of horns and adornments use them similarly.<ref name="farlowanddodson1975" /> A 2006 study of the smallest ''Triceratops'' skull, ascertained to be that of a juvenile, shows the frill and horns developed at a very early age, predating sexual development. That would suggest they were probably important for visual communication and species recognition in general.<ref>{{cite journal| author1=Goodwin, M.B.| author2=Clemens, W.A.| author3=Horner, J.R.| author4=Padian, K.| name-list-style=amp| url=http://www.ucmp.berkeley.edu/people/mbg/Goodwin_et_al_2006.pdf| title=The smallest known ''Triceratops'' skull: new observations on ceratopsid cranial anatomy and ontogeny| journal=Journal of Vertebrate Paleontology| volume=26| issue=1| pages=103–112| doi=10.1671/0272-4634(2006)26[103:TSKTSN]2.0.CO;2| year=2006| s2cid=31117040| issn=0272-4634| access-date=January 21, 2007| archive-url=https://web.archive.org/web/20151105200947/http://www.ucmp.berkeley.edu/people/mbg/Goodwin_et_al_2006.pdf| archive-date=November 5, 2015| url-status=dead}}</ref> However, the use of the exaggerated structures to enable dinosaurs to recognize their own species has been questioned, as no such function exists for such structures in modern species.<ref>{{Cite journal |last1=Hone |first1=D. W. E. |last2=Naish |first2=D. |date=2013 |title=The 'species recognition hypothesis' does not explain the presence and evolution of exaggerated structures in non-avialan dinosaurs |journal=Journal of Zoology |volume=290 |issue=3 |pages=172–180 |doi=10.1111/jzo.12035|doi-access=free }}</ref>

===Growth and ontogeny===
[[File:Triceratops ontogeny.jpg|thumb|Skull growth series]]
In 2006, the first extensive ontogenetic study of ''Triceratops'' was published in the journal ''[[Proceedings of the Royal Society]]''. The study, by [[John R. Horner]] and Mark Goodwin, found that individuals of ''Triceratops'' could be divided into four general ontogenetic groups: babies, juveniles, subadults, and adults. With a total number of 28 skulls studied, the youngest was only {{convert|38|cm|sp=us}} long. Ten of the 28 skulls could be placed in order in a growth series with one representing each age. Each of the four growth stages were found to have identifying features. Multiple ontogenetic trends were discovered, including the size reduction of the epoccipitals, development and reorientation of postorbital horns, and hollowing out of the horns.<ref name="horner2006">{{cite journal|last1=Horner|first1=J.R.|last2=Goodwin|first2=M.B.|year=2006|title=Major cranial changes during ''Triceratops'' ontogeny|journal=Proceedings of the Royal Society B: Biological Sciences|volume=273|issue=1602|pages=2757–2761|doi=10.1098/rspb.2006.3643|pmc=1635501|pmid=17015322}}</ref>

====''Torosaurus'' as growth stage of ''Triceratops''====
{{Main|Torosaurus}}
''[[Torosaurus]]'' is a ceratopsid genus first identified from a pair of skulls in 1891, two years after the identification of ''Triceratops'' by Othneil Charles Marsh. The genus ''Torosaurus'' resembles ''Triceratops'' in geological age, distribution, anatomy, and size, so it has been recognised as a close relative.<ref name="Farke-2006">{{Cite book |title=Horns and Beaks: Ceratopsian and Ornithopod Dinosaurs |last=Farke |first=A. A. |date=2006 |publisher=Indiana University Press |isbn=978-0-253-34817-3 |editor-last=Carpenter |editor-first=K. |location=Bloomington |pages=235–257 |chapter=Cranial osteology and phylogenetic relationships of the chasmosaurine ceratopsid ''Torosaurus latus''}}</ref> Its distinguishing features are an elongated skull and the presence of two ovular fenestrae in the frill. Paleontologists investigating dinosaur [[ontogeny]] in Montana's [[Hell Creek Formation]] have recently presented evidence that the two represent a single genus.

[[File:Torosaurus and Triceratops.tif|thumb|upright|A, ''Triceratops prorsus'' holotype YPM 1822 and B, ''Torosaurus latus'' ANSP 15192]]
John Scannella, in a paper presented in [[Bristol]] at the conference of the [[Society of Vertebrate Paleontology]] (September 25, 2009), reclassified ''Torosaurus'' as especially mature ''Triceratops'' individuals, perhaps representing a single sex. Horner, Scannella's mentor at Bozeman Campus, [[Montana State University]], noted that ceratopsian skulls consist of metaplastic bone. A characteristic of metaplastic bone is that it lengthens and shortens over time, extending and resorbing to form new shapes. Significant variety is seen even in those skulls already identified as ''Triceratops'', Horner said, "where the horn orientation is backwards in juveniles and forward in adults". Approximately 50% of all subadult ''Triceratops'' skulls have two thin areas in the frill that correspond with the placement of "holes" in ''Torosaurus'' skulls, suggesting that holes developed to offset the weight that would otherwise have been added as maturing ''Triceratops'' individuals grew longer frills.<ref name="growth09">{{cite web|url=https://www.sciencedaily.com/releases/2009/10/091031002314.htm|title=New Analyses Of Dinosaur Growth May Wipe Out One-third Of Species|date=October 31, 2009|work=Science News|publisher=ScienceDaily.com|access-date=November 3, 2009|archive-date=February 5, 2019|archive-url=https://web.archive.org/web/20190205024226/https://www.sciencedaily.com/releases/2009/10/091031002314.htm|url-status=live}}</ref> A paper describing these findings in detail was published in July 2010 by Scannella and Horner. It formally argues that ''Torosaurus'' and the similar contemporary ''[[Nedoceratops]]'' are synonymous with ''Triceratops''.<ref name="ScanHorn2010"/>

The assertion has since ignited much debate. Andrew Farke had, in 2006, stressed that no systematic differences could be found between ''Torosaurus'' and ''Triceratops'', apart from the frill.<ref name="Farke-2006"/> He nevertheless disputed Scannella's conclusion by arguing in 2011 that the proposed morphological changes required to "age" a ''Triceratops'' into a ''Torosaurus'' would be without precedent among ceratopsids. Such changes would include the growth of additional [[epoccipital]]s, reversion of bone texture from an adult to immature type and back to adult again, and growth of frill holes at a later stage than usual.<ref name="AF2011"/> A study by Nicholas Longrich and Daniel Field analyzed 35 specimens of both ''Triceratops'' and ''Torosaurus''. The authors concluded that ''Triceratops'' individuals too old to be considered immature forms are represented in the fossil record, as are ''Torosaurus'' individuals too young to be considered fully mature adults. The synonymy of ''Triceratops'' and ''Torosaurus'' cannot be supported, they said, without more convincing intermediate forms than Scannella and Horner initially produced. Scannella's ''Triceratops'' specimen with a hole on its frill, they argued, could represent a diseased or malformed individual rather than a transitional stage between an immature ''Triceratops'' and mature ''Torosaurus'' form.<ref name="longrichfieldstudy">{{Cite journal |last1=Longrich |first1=Nicholas R. |last2=Field |first2=Daniel J. |date=February 29, 2012 |title=Torosaurus Is Not Triceratops: Ontogeny in Chasmosaurine Ceratopsids as a Case Study in Dinosaur Taxonomy |journal=PLOS ONE |volume=7 |issue=2 |pages=e32623 |doi=10.1371/journal.pone.0032623 |pmid=22393425 |pmc=3290593 |issn=1932-6203|bibcode=2012PLoSO...732623L |doi-access=free }}</ref><ref name="bbcTriNotToro">{{Cite news |url=https://www.bbc.co.uk/news/science-environment-17192624 |title=Triceratops and Torosaurus dinosaurs 'two species, not one' |last=Bowdler |first=Neil |date=March 1, 2012 |work=BBC News |access-date=July 29, 2013 |language=en-GB |archive-date=March 15, 2013 |archive-url=https://web.archive.org/web/20130315215330/http://www.bbc.co.uk/news/science-environment-17192624 |url-status=live }}</ref>

====Other genera as growth stages of ''Triceratops''====
{{Main|Nedoceratops}}
[[File:Nedoceratops skull, PLoS ONE.png|thumb|Comparisons between the skulls of ''Triceratops'' and ''[[Nedoceratops]]'']]
Opinion has varied on the validity of a separate genus for ''Nedoceratops''. Scannella and Horner regarded it as an intermediate growth stage between ''Triceratops'' and ''Torosaurus''.<ref name="ScanHorn2010"/><ref>{{Cite journal | last1 = Scannella | first1 = J. B. | last2 = Horner | first2 = J. R. | editor1-last = Claessens | editor1-first = Leon | title = 'Nedoceratops': An Example of a Transitional Morphology | journal = PLOS ONE | volume = 6 | issue = 12 | pages = e28705 | year = 2011 | doi = 10.1371/journal.pone.0028705| pmid = 22194891 | bibcode = 2011PLoSO...628705S | pmc=3241274| doi-access = free }} {{open access}}</ref> Farke, in his 2011 redescription of the only known skull, concluded that it was an aged individual of its own valid [[taxon]], ''Nedoceratops hatcheri''.<ref name="AF2011">{{cite journal |last=Farke |first=Andrew A. |year=2011 |title= Anatomy and taxonomic status of the chasmosaurine ceratopsid ''Nedoceratops hatcheri'' from the Upper Cretaceous Lance Formation of Wyoming, U.S.A |journal=PLOS ONE |volume=6 |issue=1 |pages=e16196 |doi=10.1371/journal.pone.0016196 |editor1-last=Claessens |editor1-first=Leon |pmid=21283763 |pmc=3024410|bibcode=2011PLoSO...616196F |doi-access=free }} {{open access}}</ref> Longrich and Fields also did not consider it a transition between ''Torosaurus'' and ''Triceratops'', suggesting that the frill holes were pathological.<ref name="bbcTriNotToro"/>

As described above, Scannella had argued in 2010 that ''Nedoceratops'' should be considered a synonym of ''Triceratops''.<ref name="ScanHorn2010"/> Farke (2011) maintained that it represents a valid distinct genus.<ref name="AF2011"/> Longrich agreed with Scannella about ''Nedoceratops'' and made a further suggestion that the recently described ''[[Ojoceratops]]'' was likewise a synonym. The fossils, he argued, are indistinguishable from the ''Triceratops horridus'' specimens that were previously attributed to the defunct species ''Triceratops serratus''.

Longrich observed that another newly described genus, ''[[Tatankaceratops]]'', displayed a strange mix of characteristics already found in adult and juvenile ''Triceratops''. Rather than representing a distinct genus, ''Tatankaceratops'' could as easily represent a dwarf ''Triceratops'' or a ''Triceratops'' individual with a developmental disorder that caused it to stop growing prematurely.<ref name="Longrich">{{Cite journal|first=Nicholas R.|last= Longrich |year=2011 |title=''Titanoceratops ouranous'', a giant horned dinosaur from the Late Campanian of New Mexico |journal=Cretaceous Research |volume=32 |issue= 3|pages= 264–276|doi=10.1016/j.cretres.2010.12.007|bibcode= 2011CrRes..32..264L }}</ref>

==Paleoecology==
[[File:Upper Cretaceous Hell Creek dinosaur census.png|thumb|left|Pie chart of the time averaged census for large-bodied dinosaurs from the entire [[Hell Creek Formation]] in the study area]]
''Triceratops'' lived during the Late Cretaceous of western North America, its fossils coming from the [[Evanston Formation]], [[Scollard Formation]], [[Laramie Formation]], [[Lance Formation]], [[Denver Formation]], and [[Hell Creek Formation]].<ref name="weishampel"/> These fossil formations date back to the time of the [[Cretaceous–Paleogene extinction event]], which has been dated to 66&nbsp;±&nbsp;0.07 million years ago.<ref name="hcfage">{{Cite journal | doi = 10.1016/j.epsl.2011.03.008| title = Astronomical calibration of the Maastrichtian (Late Cretaceous)| journal = Earth and Planetary Science Letters| volume = 305| issue = 3–4| pages = 328–340| year = 2011| last1 = Husson | first1 = D. E. | last2 = Galbrun | first2 = B. | last3 = Laskar | first3 = J. | last4 = Hinnov | first4 = L. A. | last5 = Thibault | first5 = N. | last6 = Gardin | first6 = S. | last7 = Locklair | first7 = R. E. | bibcode = 2011E&PSL.305..328H}}</ref> Many animals and plants have been found in these formations, but mostly from the Lance Formation and Hell Creek Formation.<ref name="weishampel"/> ''Triceratops'' was one of the last ceratopsian genera to appear before the end of the Mesozoic. The related ''[[Torosaurus]]'' and more distantly related diminutive ''[[Leptoceratops]]'' were also present, though their remains have been rarely encountered.<ref name="Dodhorned"/> Isotopic analysis suggests ''Triceratops'' specimens from DTB (Darnell Triceratops Bonebed) could’ve inhabited freshwater floodplains, although the it likely wasn’t restricted to one environment.<ref name=":1" />

Theropods from these formations include genera of [[dromaeosaurids]], [[tyrannosaurids]], [[ornithomimids]], [[troodontids]],<ref name="weishampel">{{Cite book |title=The Dinosauria |last1=Weishampel |first1=D.B. |last2=Dodson |first2=Peter |last3=Osmólska |first3=H. |publisher=University of California Press |year=2004 |isbn=978-0-520-24209-8 |edition=Second |location=Berkeley |pages=861}}</ref> [[avialans]],<ref name="pnas">{{Cite journal | doi = 10.1073/pnas.1110395108| title = Mass extinction of birds at the Cretaceous-Paleogene (K-Pg) boundary| journal = Proceedings of the National Academy of Sciences| volume = 108| issue = 37| pages = 15253–15257| year = 2011| last1 = Longrich | first1 = N. R.| last2 = Tokaryk | first2 = T.| last3 = Field | first3 = D. J.| bibcode = 2011PNAS..10815253L | pmid=21914849 | pmc=3174646| doi-access = free}}</ref> and [[caenagnathids]].<ref name="Anzu">{{Cite journal | doi = 10.1371/journal.pone.0092022| pmid = 24647078| title = A New Large-Bodied Oviraptorosaurian Theropod Dinosaur from the Latest Cretaceous of Western North America| journal = PLOS ONE| volume = 9| issue = 3| pages = e92022| year = 2014| last1 = Lamanna | first1 = M. C. | last2 = Sues | first2 = H. D. | last3 = Schachner | first3 = E. R. | last4 = Lyson | first4 = T. R. | bibcode = 2014PLoSO...992022L | pmc=3960162| doi-access = free}} {{open access}}</ref> Dromaeosaurids from the Hell Creek Formation are ''[[Acheroraptor]]'' and ''[[Dakotaraptor]]''. Indeterminate dromaeosaurs are known from other fossil formations. Common teeth previously referred to ''[[Dromaeosaurus]]'' and ''[[Saurornitholestes]]'' were considered to be those of ''Acheroraptor''.<ref name="Acheroraptor">{{Cite journal | last1 = Evans | first1 = D. C. | last2 = Larson | first2 = D. W. | last3 = Currie | first3 = P. J. | doi = 10.1007/s00114-013-1107-5 | title = A new dromaeosaurid (Dinosauria: Theropoda) with Asian affinities from the latest Cretaceous of North America | journal = Naturwissenschaften | volume = 100 | issue = 11 | pages = 1041–1049 | year = 2013 | pmid =  24248432| bibcode = 2013NW....100.1041E | s2cid = 14978813 }}</ref> The tyrannosaurids from the formation are ''[[Nanotyrannus]]'' and ''[[Tyrannosaurus]]'', although the former is most likely a junior synonym of the latter. Among ornithomimids are the genera ''[[Struthiomimus]]'' and ''[[Ornithomimus]]''.<ref name="weishampel" /> An undescribed animal named "[[Orcomimus]]" could be from the formation.<ref name="tb1997">{{cite journal|last=Triebold|first=M.|year=1997|title=The Sandy site: Small dinosaurs from the Hell Creek Formation of South Dakota|editor-last=Wolberg|editor-first=D.|editor2-last=Stump|editor2-first=E.|editor3-last=Rosenberg|editor3-first=G.|journal=Dinofest International: Proceedings of a Symposium|pages=245–248}}</ref> Troodontids are only represented by ''[[Pectinodon]]'' and ''[[Paronychodon]]'' in the Hell Creek Formation with a possible species of ''[[Troodon]]'' from the Lance Formation. One species of unknown [[coelurosaur]] is known from teeth in the Hell Creek and similar formations by a single species, ''[[Richardoestesia]]''. Only three [[oviraptorosaur]]s are from the Hell Creek Formation: ''[[Anzu wyliei|Anzu]]'', ''[[Leptorhynchos (dinosaur)|Leptorhynchos]]''<ref name="Anzu" /> and a giant species of caenagnathid, very similar to ''[[Gigantoraptor]]'', from South Dakota. However, only fossilized foot prints were discovered.<ref>{{cite web|url=http://rmdrc.blogspot.com/2013/12/giant-oviraptor-tracks-from-hell-creek.html|last1=Maltese|first1=Anthony|title=Giant Oviraptor Tracks from the Hell Creek|publisher=RMDRC paleo lab|access-date=December 17, 2013|date=December 17, 2013|archive-date=November 12, 2020|archive-url=https://web.archive.org/web/20201112003952/http://rmdrc.blogspot.com/2013/12/giant-oviraptor-tracks-from-hell-creek.html|url-status=live}}</ref> The avialans known from the formation are ''[[Avisaurus]]'',<ref name="weishampel"/> multiple species of ''[[Brodavis]]'',<ref name="brodavis">{{Cite journal | doi = 10.1016/j.palwor.2012.02.005| title = A new evolutionary lineage of diving birds from the Late Cretaceous of North America and Asia| journal = [[Palaeoworld]]| volume = 21| pages = 59–63| year = 2012| last1 = Martin | first1 = L. D. | last2 = Kurochkin | first2 = E. N. | last3 = Tokaryk | first3 = T. T. }}</ref> and several other species of [[hesperornithoforms]], as well as several species of true [[bird]]s, including ''[[Cimolopteryx]]''.<ref name="pnas"/>

[[File:Hell Creek dinosaurs and pterosaurs by durbed.jpg|thumb|right|''Triceratops'' and other animals of the Hell Creek Formation]]
[[Ornithischia]]ns are abundant in the Scollard, Laramie, Lance, Denver, and Hell Creek Formation. The main groups of ornithischians are [[ankylosauria]]ns, [[ornithopod]]s, [[ceratopsia]]ns, and [[pachycephalosauria]]ns. Three ankylosaurians are known: ''[[Ankylosaurus]]'', ''[[Denversaurus]]'', and possibly a species of ''[[Edmontonia]]'' or an undescribed genus. Multiple genera of ceratopsians are known from the formation other than ''Triceratops''. These include the [[leptoceratopsid]] ''[[Leptoceratops]]'' and the [[chasmosaurine]] [[ceratopsids]] ''[[Torosaurus]]'',<ref name="weishampel" /> ''[[Nedoceratops]]'', and ''[[Tatankaceratops]]''.<ref name="tatanka">{{cite book|last1=Ott|first1=C.J.|last2=Larson|first2=P.L.|year=2010|chapter=A New, Small Ceratopsian Dinosaur from the Latest Cretaceous Hell Creek Formation, Northwest South Dakota, United States: A Preliminary Description|editor-last=Ryan|editor-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|location=Bloomington|publisher=Indiana University Press|pages=656}}</ref> Ornithopods are common in the Hell Creek Formation and are known from several species of the [[Thescelosaurinae|thescelosaurine]] ''[[Thescelosaurus]]'' and the [[hadrosaurid]]  ''[[Edmontosaurus]]''.<ref name="weishampel"/><ref name="hcfhadro">{{Cite journal |last1=Campione |first1=N. S. E. |last2=Evans |first2=D. C. |year=2011 |title=Cranial Growth and Variation in Edmontosaurs (Dinosauria: Hadrosauridae): Implications for Latest Cretaceous Megaherbivore Diversity in North America |journal=PLOS ONE |volume=6 |issue=9 |pages=e25186 |bibcode=2011PLoSO...625186C |doi=10.1371/journal.pone.0025186 |pmc=3182183 |pmid=21969872|doi-access=free }} {{open access}}</ref> Several [[pachycephalosauria]]ns have been found in the Hell Creek Formation and in similar formations. Among them are the derived [[pachycephalosaurids]] ''[[Stygimoloch]]'',<ref name="weishampel"/> ''[[Dracorex]]'',<ref name="bakker">{{cite journal|last1=Bakker|first1=R.T.|last2=Sullivan|first2=R.M.|last3=Porter|first3=V.|last4=Larson|first4=P.|last5=Saulsbury|first5=S.J.|year=2006|title=''Dracorex hogwartsia'', n. gen., n. sp., a spiked, flat-headed pachycephalosaurid dinosaur from the Upper Cretaceous Hell Creek Formation of South Dakota|editor-last=Lucas|editor-first=S.G.|editor2-last=Sullivan|editor2-first=R.M.|journal=Late Cretaceous Vertebrates from the Western Interior|series=New Mexico Museum of Natural History and Science Bulletin|volume=35|pages=331–345|url=http://econtent.unm.edu/cdm/ref/collection/bulletins/id/710|access-date=January 6, 2015|archive-date=November 19, 2018|archive-url=https://web.archive.org/web/20181119203737/http://econtent.unm.edu/cdm/ref/collection/bulletins/id/710|url-status=live}}</ref> ''[[Pachycephalosaurus]]'',<ref name="weishampel"/> ''[[Sphaerotholus]]'', and an undescribed specimen from North Dakota. The first two might be junior synonyms of ''Pachycephalosaurus''.

[[Mammal]]s are plentiful in the Hell Creek Formation. Groups represented include [[multituberculates]], [[metatheria]]ns, and [[eutheria]]ns. The multituberculates represented include ''[[Paracimexomys]],''<ref name="kj2004">{{Cite book |title=Mammals from the Age of Dinosaurs: Origins, Evolution, and Structure |last1=Kielan-Jaworowska |first1=Zofia |last2=Cifelli |first2=Richard L. |last3=Luo |first3=Zhe-Xi |date=2004 |publisher=Columbia University Press |isbn=978-0-231-11918-4 |location=New York |pages=98–99 |author-link=Zofia Kielan-Jaworowska}}</ref> the [[cimolomyidae|cimolomyids]] ''[[Paressonodon]]'',<ref name="mammal">{{Cite journal |last=Wilson |first=G. P. |year=2013 |title=Mammals across the K/Pg boundary in northeastern Montana, U.S.A.: Dental morphology and body-size patterns reveal extinction selectivity and immigrant-fueled ecospace filling |journal=Paleobiology |volume=39 |issue=3 |pages=429–469 |doi=10.1666/12041|bibcode=2013Pbio...39..429W |s2cid=36025237 |url=http://www.bioone.org/doi/pdf/10.4202/app.00117.2014 |url-access=subscription }}</ref> ''[[Meniscoessus]]'', ''[[Essonodon]]'', ''[[Cimolomys]]'', ''[[Cimolodon]]'', and ''[[Cimexomys]]'', and the [[neoplagiaulacidae|neoplagiaulacids]] ''[[Mesodma]]'' and ''[[Neoplagiaulax]]''. The metatherians are represented by the [[alphadontidae|alphadontids]] ''[[Alphadon]]'', ''[[Protalphodon]]'', and ''[[Turgidodon]]'', the [[pediomyidae|pediomyids]] ''[[Pediomys]]'',<ref name="kj2004"/> ''[[Protolambda]]'', and ''[[Leptalestes]]'',<ref name="mammalshcf">{{Cite journal |last1=Archibald |first1=J. D. |last2=Zhang |first2=Y. |last3=Harper |first3=T. |last4=Cifelli |first4=R. L. |year=2011 |title=Protungulatum, Confirmed Cretaceous Occurrence of an Otherwise Paleocene Eutherian (Placental?) Mammal |journal=Journal of Mammalian Evolution |volume=18 |issue=3 |pages=153–161 |doi=10.1007/s10914-011-9162-1|s2cid=16724836 }}</ref> the [[stagodontidae|stagodontid]] ''[[Didelphodon]]'',<ref name="kj2004"/> the [[deltatheridiidae|deltatheridiid]] ''[[Nanocuris]]'', the [[herpetotheriidae|herpetotheriid]] ''[[Nortedelphys]]'',<ref name="mammal"/> and the [[glasbiidae|glasbiid]] ''[[Glasbius]]''. A few eutherians are known, being represented by ''[[Alostera]]'',<ref name="kj2004"/> ''[[Protungulatum]]'',<ref name="mammalshcf"/> the [[cimolestidae|cimolestids]] ''[[Cimolestes]]'' and ''[[Batodon]]'', the [[gypsonictopsidae|gypsonictopsid]] ''[[Gypsonictops]]'' , and the possible [[nyctitheriidae|nyctitheriid]] ''[[Paranyctoides]]''.<ref name="kj2004"/>

[[File:Knight Triceratops.jpg|thumb|1901 illustration by [[Charles R. Knight]]]]
==Cultural significance==
{{See also|Cultural depictions of dinosaurs}}
''Triceratops'' is the official [[state fossil]] of [[South Dakota]].<ref>{{cite web|author=State of South Dakota |title=Signs and Symbols of South Dakota..... |url=http://www.state.sd.us/state/sdsym.htm |access-date=January 20, 2007 |url-status=dead |archive-url=https://web.archive.org/web/20080220004837/http://www.state.sd.us/state/sdsym.htm |archive-date=February 20, 2008 }}</ref> It is also the official state dinosaur of [[Wyoming]].<ref>{{cite web | author = State of Wyoming | title = State of Wyoming&nbsp;– General Information | url= http://wyoming.gov/general/general.asp | access-date = January 20, 2007 |archive-url = https://web.archive.org/web/20070210015835/http://wyoming.gov/general/general.asp |archive-date = February 10, 2007|url-status=dead}}</ref> In 1942, [[Charles R. Knight]] painted a mural incorporating a confrontation between a ''Tyrannosaurus'' and a ''Triceratops'' in the [[Field Museum of Natural History]] for the [[National Geographic Society]], establishing them as enemies in the popular imagination.<ref name="Bakker1986"/> Paleontologist [[Robert T. Bakker|Robert Bakker]] said of the imagined rivalry between ''[[Tyrannosauroidea|Tyrannosaurus]]'' and ''Triceratops'', "No matchup between predator and prey has ever been more dramatic. It's somehow fitting that those two massive antagonists lived out their co-evolutionary belligerence through the [[Cretaceous–Paleogene boundary|last days]] of the [[Maastrichtian|last epoch]] of the [[Mesozoic|Age of Dinosaurs]]."<ref name="Bakker1986">{{cite book|last=Bakker|first= R. T.|date= 1986|title=The Dinosaur Heresies|location=New York|publisher= Kensington Publishing| page= 240}} On that page, Bakker has his own ''T. rex''/''Triceratops'' fight.</ref>
{{clear}}

==References==
{{reflist}}

==External links==
{{Spoken Wikipedia|En-Triceratops-article.ogg|date=March 19, 2007}}
*{{Commons category-inline|Triceratops}}
*{{Wikibooks-inline|Wikijunior Dinosaurs/Triceratops}}
*{{Wikisource-inline|Notice of Gigantic Horned Dinosauria from the Cretaceous}}
*{{Wikispecies-inline}}
* [http://dinosaurpictures.org/Triceratops-pictures ''Triceratops''] at The Dinosaur Picture Database
* [http://www.livescience.com/24011-triceratops-facts.html LiveScience: Facts about ''Triceratops''] at LiveScience.com
* {{YouTube | id=jjwhEx4LwlE | title=Clash of the Dinosaurs: The Defenders – The Triceratops Threat}}
* [http://dml.cmnh.org/2002Jul/msg00898.html Dinosaur Mailing List post on ''Triceratops'' stance] {{Webarchive|url=https://web.archive.org/web/20071013213317/http://dml.cmnh.org/2002Jul/msg00898.html |date=October 13, 2007 }}
* [https://web.archive.org/web/20030202134913/http://www.mnh.si.edu/exhibits/triceratops/index.html Smithsonian Exhibit]
* [http://www.nhm.ac.uk/nature-online/life/dinosaurs-other-extinct-creatures/dino-directory/triceratops.html ''Triceratops'' in the Dino Directory]
* [https://web.archive.org/web/20090226220159/http://www.cbv.ns.ca/marigold/history/dinosaurs/datafiles/triceratops.html ''Triceratops''] (short summary and good color illustration)
* [http://www.enchantedlearning.com/subjects/dinosaurs/dinos/Triceratops.shtml ''Triceratops'' For Kids] (a fact sheet about the ''Triceratops'' with activities for kids)
* ''[https://www.bbc.co.uk/nature/life/Triceratops Triceratops]'', [[BBC]] Dinosaurs
* {{in lang|fr}} [http://www.dinosauria.ca/Dinos/triceratops.php Triceratops] {{Webarchive|url=https://web.archive.org/web/20161017201520/http://www.dinosauria.ca/Dinos/triceratops.php |date=October 17, 2016 }} – [https://web.archive.org/web/20160507062100/http://www.dinosauria.ca/liste.php Liste] de [http://www.dinosauria.ca/ Dinosauria] et [https://web.archive.org/web/20160427011327/http://www.dinosauria.ca/extinction.php Extinction]

{{Marginocephalia|C.}}
{{Taxonbar|from=Q14384|from2= Q3998678|from3= Q3998680}}
{{Portal bar|Dinosaurs|United States|Canada|Biology}}
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[[Category:Chasmosaurinae]]
[[Category:Dinosaur genera]]
[[Category:Maastrichtian dinosaurs]]
[[Category:Scollard Formation]]
[[Category:Lance Formation]]
[[Category:Laramie Formation]]
[[Category:Hell Creek Formation]]
[[Category:Taxa named by Othniel Charles Marsh]]
[[Category:Fossil taxa described in 1889]]
[[Category:Dinosaurs of Canada]]
[[Category:Dinosaurs of the United States]]