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 ungulates.

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.

Research published in 2010 concluded that the contemporaneous Torosaurus, a ceratopsid long regarded as a separate genus, represents Triceratops in its mature form.

[2] 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.

Although not confidently assignable, fossils possibly belonging to Triceratops were described as two taxa, Agathaumas sylvestris and Polyonax mortuarius, in 1872 and 1874, respectively, by Marsh's archrival Edward Drinker Cope.

[6][7] 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 of southeastern Wyoming from layers coming from the Maastrichtian Lance Formation.

[12][9] 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.

[15] 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. obtusus and T. hatcheri ('Hatcher's') that was characterized by a very small nasal horn.

[19] 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. horridus and T. prorsus, although the distinctive skull of T. ("Nedoceratops") hatcheri differed enough to warrant a separate genus.

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.

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 ornithopods (together forming the Cerapoda), walked with most of their fingers pointing out and away from the body, the original condition for dinosaurs.

Confusion stemmed mainly from the combination of a short, solid frill (similar to that of Centrosaurinae), with long brow horns (more akin to Chasmosaurinae).

[46] Mercuriceratops Judiceratops Chasmosaurus Mojoceratops Agujaceratops Pentaceratops aquilonius Williams Fork chasmosaur Pentaceratops sternbergii Utahceratops Kosmoceratops Anchiceratops Almond Formation chasmosaur Bravoceratops Coahuilaceratops Arrhinoceratops Titanoceratops Torosaurus Triceratops For many years after its discovery, the deeper evolutionary origins of Triceratops and its close relatives remained largely obscure.

[49] 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.

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.

For example, Bruce Erickson, a paleontologist of the Science Museum of Minnesota, has reported having seen 200 specimens of T. prorsus in the Hell Creek Formation of Montana.

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.

[56] 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.

[68] 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 H. Sternberg in 1917 and 70 years later by Robert Bakker.

[80][81] One skull was found with a hole in the jugal bone, apparently a puncture wound sustained while the animal was alive, as indicated by signs of healing.

[19][68][87] 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.

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.

Paleontologists investigating dinosaur ontogeny in Montana's Hell Creek Formation have recently presented evidence that the two represent a single genus.

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.

Longrich observed that another newly described genus, Tatankaceratops, displayed a strange mix of characteristics already found in adult and juvenile Triceratops.

The multituberculates represented include Paracimexomys,[109] the cimolomyids Paressonodon,[110] Meniscoessus, Essonodon, Cimolomys, Cimolodon, and Cimexomys, and the neoplagiaulacids Mesodma and Neoplagiaulax.

The metatherians are represented by the alphadontids Alphadon, Protalphodon, and Turgidodon, the pediomyids Pediomys,[109] Protolambda, and Leptalestes,[111] the stagodontid Didelphodon,[109] the deltatheridiid Nanocuris, the herpetotheriid Nortedelphys,[110] and the glasbiid Glasbius.

[113] 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.