Editing Triceratops (section)

==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}}</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}}</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>