Triceratops

Triceratops (lit. 'three-horned face') is a genus of chasmosaurine ceratopsian dinosaur. It lived during the late Maastrichtian age of the Late Cretaceous period, about 69 to 66 million years ago, on the island continent of Laramidia, which now forms western North America. It was one of the last-known non-avian dinosaurs and lived until the Cretaceous–Paleogene extinction event, 66 million years ago. For the band, see Triceratops (band).

The name Triceratops comes from Ancient Greek words meaning 'three', 'horn', and 'face'. It has a large bony frill, three horns on its skull, and a large, four-legged body. Its body shape is similar to that of rhinoceroses, even though the two animals are not closely related, which is an example of convergent evolution. Triceratops is one of the most recognizable dinosaurs and the best-known ceratopsian.

Triceratops was also one of the largest of its kind. It measured around 8–9 m (26–30 ft) long and weighed up to 6–10 t. It shared its landscape with Tyrannosaurus rex, which most likely preyed upon it.

Scientists have long debated what the frills and horns were used for. Traditionally, they were thought to be weapons for defending against predators. More recent ideas suggest that these features were mainly used to identify the species, attract mates, and show dominance.

Triceratops was once placed among the "short-frilled" ceratopsids. However, modern cladistic studies show that it belongs to Chasmosaurinae, a group that usually has long frills. Today, two species are considered valid, though seventeen different species have been named over time. The first scientific description of the genus was made in 1889 by American paleontologist Othniel Charles Marsh. Many remains have been found, including fossils from hatchlings to adults. As the archetypal ceratopsian, Triceratops has appeared in many films, postage stamps, and other media.

Discovery and identification

See also: Timeline of ceratopsian research

The first named fossil now linked to Triceratops was a pair of brow horns attached to a skull roof. George Lyman Cannon found them near Denver, Colorado, in the spring of 1887. The fossil was sent to Othniel Charles Marsh, who first thought it came from a large, unusual bison and named it Bison alticornis. The next year, he realized that these were horned dinosaurs. Marsh's rival, Edward Drinker Cope, also named some older fossils that might belong to Triceratops, but these remains were too incomplete to be sure. The main specimen used to describe Triceratops was collected in 1888 by fossil hunter John Bell Hatcher. At first, Marsh named it as another kind of Ceratops. When more of the skull was prepared, he gave it the new name Triceratops, which means "three-horn face."

After Triceratops was described, Hatcher carefully collected thirty-one more of its skulls. Scientists have debated how many kinds, or species, of Triceratops existed. Early researchers grouped the skulls in different ways, but the skulls vary a lot, partly because they came from animals of different ages and both sexes. In 1986, John Ostrom and Peter Wellnhofer proposed that there was only one species, Triceratops horridus. Later, paleontologist Catherine Forster studied the fossils more fully and concluded that there were two species, T. horridus and T. prorsus. In 2014, John Scannella and his colleagues supported this idea, noting that the two species lived at different times.

Description

Triceratops was a very large animal. It measured around 8–9 meters (26–30 ft) long and weighed up to 6–10 metric tons (6.6–11.0 short tons). A T. horridus specimen named Kelsey measured 6.7–7.3 meters (22–24 ft) long, had a 2-meter (6.5 ft) skull, and stood about 2.3 meters (7.5 ft) tall. The Black Hills Institute estimated that Kelsey weighed about 5.4 metric tons (6.0 short tons). A T. horridus specimen named Big John was estimated to be 8.0 meters (26.1 ft) long and 3.0 meters (10 ft) tall. Like all chasmosaurines, Triceratops had a large skull for its body size, among the largest of any land animal. The largest-known skull, specimen MWC 7584 (formerly BYU 12183), is estimated to have been 2.5 meters (8.2 ft) long and could reach almost a third of the animal's length. A large beak sat in front of the teeth, built around a special rostral bone, with premaxillae bones behind it. A single horn sat on the snout above the nostrils, and a pair of brow horns about 1 meter (3.3 ft) long sat above the eyes. The jugal bones pointed downward at the back sides of the skull. The frontal bones folded to form a "double" skull roof, and some specimens show a fontanelle, an opening in the upper roof layer. At the back of the skull, the squamosal bones and parietal bones formed a short frill. In young specimens, the frill was decorated with epoccipitals, which were small triangular bony bumps or osteoderms. Unlike most of its relatives, the frill was solid and did not have fenestrae, which are openings. A huge occipital condyle, up to 106 millimeters (4.2 in) across, connected the head to the neck. The articular bone at the back of the lower jaw was very wide.

Triceratops had a sturdy build, with strong limbs, short hands, and short feet, and chasmosaurines showed little variation in their skeletons. Its vertebral column had ten neck, twelve back, ten sacral, and about forty-five tail vertebrae. Its front neck bones were fused into a syncervical. Earlier scientists thought this included the first three vertebrae, but later work revived an old idea by John Bell Hatcher that a vestige of the real atlas, with its spinous process, could be seen at the very front. The back vertebrae had bone bands around the spinal canal. The sacrum, which connects to the pelvis, was fused in adults, and bone plates on top formed a carapace-like shape. The pelvis had a long ilium, and the ischium curved downward. Although certainly quadrupedal, its posture has long been debated. People once believed its front legs sprawled out from the thorax, as shown in paintings by Charles Knight and Rudolph Zallinger. Newer ichnological evidence from trackways and skeleton reconstructions suggests it kept an upright stance, similar to a modern rhinoceros. Tracks named Ceratopsipes goldenensis may have been made by Triceratops or the closely related Torosaurus. Its hands kept a fairly primitive structure compared with thyreophorans and many sauropods. Like other ceratopsians, ornithopods (together forming the Cerapoda), and theropods, most of its fingers pointed out and away from the body. Only the first three fingers carried its weight, and its phalangeal formula shows this pattern. Fossil skin from Triceratops has also been found. A specimen nicknamed "Lane" (HMNS PV.1506), found in Wyoming in 2002, keeps large areas of skin and is on display at the Houston Museum of Natural Science. This skin is preserved as a thin layer of clay, not as real skin, a method scientists learned about from studying two Edmontosaurus "mummy" specimens. Its large scales could be up to 90 millimetres (3.5 in) across, and larger scattered scales had small cone-like bumps. It has been suggested that these bumps probably did not have bristles, unlike those of Psittacosaurus.

Classification

Triceratops is the best-known kind of Ceratopsidae, a family of large, mostly North American ceratopsians. Scientists debated for many years where it fits among its relatives. It had a short, solid frill like Centrosaurinae, but long brow horns like Chasmosaurinae. In the first overview of ceratopsians, R. S. Lull suggested two lineages, and he placed Triceratops among the centrosaurines. In 1949, Charles Mortram Sternberg was the first to question this. He proposed that Triceratops was closer to Arrhinoceratops and Chasmosaurus, so it belonged with the chasmosaurines. Later discoveries and studies, including work by Peter Dodson, supported Sternberg's view.

The cladogram below follows Longrich (2014), who named a new species of Pentaceratops, and included nearly all species of chasmosaurine.

Scientists long did not know much about the early family history of Triceratops. In 1922, Henry Fairfield Osborn saw the newly found Protoceratops as its ancestor. Newer discoveries, such as Zuniceratops, the earliest-known ceratopsian with brow horns, and Yinlong, the first known Jurassic ceratopsian, have helped show how ceratopsians developed. In phylogenetic taxonomy, Triceratops is also used as a reference point for defining Dinosauria. Dinosaurs are described as all descendants of the most recent common ancestor of Triceratops and modern birds, and Ornithischia includes the dinosaurs more closely related to Triceratops than to modern birds.

Paleobiology

Many people picture Triceratops as a herding animal, but there is little evidence that it lived in herds. Only one bone beds site is dominated by Triceratops bones, and it holds the remains of three young animals. In 2012, three Triceratops of different sizes were found near Newcastle, Wyoming, and they may have traveled together as a family. Their bones show signs of attack or scavenging by Tyrannosaurus. Paleontologist Peter Larson and the Black Hills Institute are studying these remains.

For many years, Triceratops fossils were known only from single animals, but they are very common. Bruce Erickson of the Science Museum of Minnesota reported seeing 200 specimens in the Hell Creek Formation of Montana, and Barnum Brown claimed to have seen over 500 skulls in the field. Its fossils are so common in western North America at the end of the Late Cretaceous that it is seen as one of the most common plant-eaters of its time. Skulls are far more common than postcranial bones, which suggests that the skull was unusually likely to be preserved. Robert Bakker estimated in 1986 that Triceratops made up five sixths of the large dinosaurs of the Lancian faunal stage of the late Maastrichtian.

Studies of its skull suggest that Triceratops had a poor sense of smell, but its ears were tuned to low sounds. It probably held its head about 45 degrees to the ground, which showed off its horns and frill while it grazed. A 2026 study suggested that its respiratory turbinates moistened the air it breathed, which cooled the blood going to the eyes and brain. A 2022 study suggested that it had a cold-blooded (ectothermic) or gigantothermic metabolism, similar to modern reptiles, while another study suggested it may have been warm-blooded (endothermic) because of its large body. Its environment included floodplains and inland forests.

Triceratops were herbivorous. Because its head was low, it probably ate low-growing plants, though it may have knocked down taller ones with its horns, beak, and bulk. Its teeth were arranged in groups called batteries, with many teeth in use and replaced throughout its life. Its diet may have included fibrous plant material, Populus, Pine, Platanus, Hazel, and Taxodium plants, as well as palms, cycads, and ferns.

There has been much speculation about the purpose of the horns and frill. The two main ideas were combat and courtship display, and display is now thought most likely to be the main purpose. Early on, Lull suggested the frill helped anchor jaw muscles, but later studies did not find evidence for this. For a long time, scientists thought Triceratops used its horns against predators such as Tyrannosaurus, an idea first discussed by Charles H. Sternberg in 1917. Fossil bite marks on a Triceratops horn and squamosal show that the two animals did have head-on encounters, but which animal attacked is unknown. Other Triceratops fossils show that Tyrannosaurus also fed on them.

Some damage on Triceratops skulls was once blamed on horn fights, but a 2006 study found no evidence for this. A 2009 study suggested that the frill protected the animal, and that its horns were used in combat. One skull has a hole in the jugal bone that was healed while the animal was alive, which suggests that Triceratops fought with other members of its own kind and survived. The frill may also have helped regulate body temperature, but this idea does not explain the wide variety of frill shapes. The theory that frills were used in courtship display was first proposed in 1961 and has gained support since. A 2006 study found that the frill and horns developed at a very early age, which suggests they were important for communication and recognizing members of the same species.

In 2006, John R. Horner and Mark Goodwin published the first large study of how Triceratops grew. They divided the animals into four general groups: babies, juveniles, subadults, and adults. Each group had its own features, and the horns changed shape as the animals grew.

Main article: Torosaurus

Torosaurus was first identified in 1891 from a pair of skulls. It was similar to Triceratops in age, location, and size, but it had a longer skull and two holes in its frill. In 2009, John Scannella suggested that Torosaurus was actually a mature form of Triceratops, and Horner and Scannella later argued this in print. Other scientists disagreed, pointing out that Triceratops and Torosaurus differed in ways that would be unusual for growth. Later studies by Farke and Maiorino concluded that they were separate kinds of dinosaurs, though the question remains debated. In 2022, researchers argued that some young specimens from Canada were a kind of Torosaurus.

Main article: Nedoceratops

Scientists have disagreed about whether Nedoceratops was a separate genus. Some considered it a growth stage of Triceratops, while Farke concluded that it was a valid kind of its own. Other researchers suggested that some newly described animals were also Triceratops or young individuals with unusual features.

Paleoecology

Triceratops lived in the Late Cretaceous of western North America. Its fossils have been found in the Evanston Formation, Scollard Formation, Laramie Formation, Lance Formation, Denver Formation, and Hell Creek Formation. These rock layers date to the time of the Cretaceous–Paleogene extinction event, dated to 66 ± 0.07 million years ago. Many animals and plants have been found in them, mostly from the Lance and Hell Creek Formations. Triceratops was one of the last ceratopsian genera to appear before the end of the Mesozoic. The related Torosaurus and the much more distantly related, smaller Leptoceratops also lived there, though their remains are rarely found. Chemical analysis suggests that Triceratops specimens from DTB (Darnell Triceratops Bonebed) may have lived on freshwater floodplains, although they likely were not limited to one environment.

Meat-eating dinosaurs from these formations include groups of dromaeosaurids, tyrannosaurids, ornithomimids, troodontids, avialans, and caenagnathids. From the Hell Creek Formation, dromaeosaurids include Acheroraptor and Dakotaraptor. Teeth once linked to Dromaeosaurus and Saurornitholestes are now thought to belong to Acheroraptor. Tyrannosauroids include Nanotyrannus and Tyrannosaurus. Ornithomimids include Struthiomimus and Ornithomimus, and an undescribed animal called "Orcomimus" may also be from the formation. Troodontids are represented by Pectinodon and Paronychodon in the Hell Creek Formation, with a possible species of Troodon from the Lance Formation. Teeth of an unknown coelurosaur, named Richardoestesia, are found in the Hell Creek and similar formations. Oviraptorosaurs from the Hell Creek Formation include Anzu. Avialans include Avisaurus, several species of Brodavis, other hesperornithoforms, and several true birds, including Cimolopteryx.

Ornithischians are common in the Scollard, Laramie, Lance, Denver, and Hell Creek Formations. Their main groups are ankylosaurians, ornithopods, ceratopsians, and pachycephalosaurians. Three ankylosaurians are known: Ankylosaurus, Denversaurus, and possibly a species of Edmontonia or an undescribed genus. Other ceratopsians from these formations include the leptoceratopsid Leptoceratops and the chasmosaurine ceratopsids Torosaurus, Nedoceratops, and Tatankaceratops. Ornithopods are common in the Hell Creek Formation, with several species of the thescelosaurine Thescelosaurus and the hadrosaurid Edmontosaurus. Several pachycephalosaurians have been found in the Hell Creek Formation and similar formations, including the derived pachycephalosaurids Stygimoloch, Dracorex, Pachycephalosaurus, Sphaerotholus, and an undescribed specimen from North Dakota. The first two might be junior synonyms of Pachycephalosaurus.

Mammals are plentiful in the Hell Creek Formation. Groups include multituberculates, metatherians, and eutherians. The multituberculates include Paracimexomys, the cimolomyids Paressonodon, Meniscoessus, Essonodon, Cimolomys, Cimolodon, and Cimexomys, and the neoplagiaulacids Mesodma and Neoplagiaulax. The metatherians include the alphadontids Alphadon, Protalphodon, and Turgidodon, the pediomyids Pediomys, Protolambda, and Leptalestes, the stagodontid Didelphodon, the deltatheridiid Nanocuris, the herpetotheriid Nortedelphys, and the glasbiid Glasbius. A few eutherians are known, including Alostera, Protungulatum, the cimolestids Cimolestes and Batodon, the gypsonictopsid Gypsonictops, and the possible nyctitheriid Paranyctoides.

Cultural significance

See also: Cultural depictions of dinosaurs

Since at least the early 1900s, Triceratops has been one of the most famous dinosaurs and a typical example of the ceratopsian dinosaurs. Museums often show it to the public because many of its fossils have been found. A mounted Triceratops skeleton was shown at the 1900 Paris Exposition, possibly the first time the dinosaur was displayed, and about 21,000 visitors saw it in the first two days. A model of Triceratops by American sculptor Louis Paul Jonas was also displayed at the 1964 New York World's Fair.

Triceratops appears in many dinosaur films, including The Lost World (1925) and Jurassic Park (1993). In books, Edgar Rice Burroughs called these animals "Gryfs" in his 1921 novel Tarzan the Terrible, and in the 1956 children's book The Enormous Egg by Oliver Butterworth, a Triceratops hatches from a chicken egg. After Tyrannosaurus and Stegosaurus, it is the dinosaur shown most often on stamps. Triceratops is the official state fossil of South Dakota and the official state dinosaur of Wyoming. In 1928, Charles R. Knight painted a mural showing a Tyrannosaurus fighting a Triceratops at the Field Museum of Natural History in Chicago, which helped make the two animals seem like enemies in people's minds. Paleontologist Robert Bakker said that no match between a predator and its prey has ever been more dramatic, and that the two giants fitting rivals in the last days of the last epoch of the Age of Dinosaurs.