Editing Triceratops (section)

===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]]'')