What Was Triceratops? Horns, Frill and the Last Great Plant-Eater

Triceratops was a ceratopsian, the horned dinosaurs, a group that began as parrot-beaked bipeds the size of a dog in Asia and produced, in North America in the late Cretaceous, a radiation of quadrupeds with elaborate frills and horns; Triceratops was the largest and the last. It walked on four pillar-like legs with the front feet splayed, weighed six to twelve tonnes, and carried a head up to 2.5 metres long, one of the largest skulls of any land animal, in which the frill was a solid shelf of bone and the horns were cores of bone that in life were sheathed in keratin and longer than the fossils show. The beak was a parrot's, the jaws held batteries of hundreds of teeth arranged in vertical columns and replaced continuously, and the whole was built for shearing tough plants.

The obvious answer, defence against Tyrannosaurus, is supported by the fossils: Triceratops frills and skulls carry bite marks and, in some, healed wounds of the right size and spacing for tyrannosaur teeth, and a tyrannosaur's brow shows a healed puncture that a horn could have made. But horns and frills of the ceratopsians vary enormously between species in ways that defence does not explain, and the current view is that they were mainly for each other:

• •Display: the frill was a billboard, probably coloured, that identified the species and advertised the individual's maturity and condition, as antlers and horns do in living animals
• •Combat between rivals: the lesions on Triceratops skulls cluster where the brow horns of an opponent would strike in a locked-horns contest, and are rarer on the species with different horn arrangements
• •Defence: the horns worked against predators too, and an adult with its head lowered presented two spears and a shield
• •Not heat regulation: the frill's blood vessels once suggested a radiator, but the pattern of growth points to display

Triceratops skulls are known from hatchlings the size of a dog to adults, and they change so much on the way that the young were once thought different species. Juveniles had short straight horns that curved backward; as the animal grew the horns swung forward and lengthened, the frill grew and its edge lost its spiky ornaments, and the bone of the frill thinned. In 2010 the palaeontologists John Scannella and Jack Horner argued that Torosaurus, a rarer frilled dinosaur from the same rocks with two holes in its frill, was simply the fully mature Triceratops, its frill having grown and thinned into windows in old age; other workers have found the two forms overlapping in age and kept them separate, and the question is open. What is settled is that a Triceratops kept changing its head throughout its life, which is what a display organ does.

The animal ate low vegetation, palms, cycads, ferns and the flowering shrubs that had spread through the Cretaceous, shearing it with its beak and grinding it in the tooth batteries, and its bulk suggests a slow fermenting gut. It is found mostly as single individuals, unlike the herds of its relative Centrosaurus, though a bone bed in Montana with three juveniles together suggests the young grouped; it lived alongside the duck-billed Edmontosaurus, the armoured Ankylosaurus, the dome-headed Pachycephalosaurus and Tyrannosaurus in a warm, wet lowland of rivers and swamps that is now the Hell Creek badlands of Montana and the Dakotas. Its bones are the most abundant large dinosaur fossils in those beds, about 40 percent of the large animals found, which suggests it was as common on the ground as the fossils imply.

Triceratops is found up to the very top of the Cretaceous rocks, within metres of the layer of iridium that marks the asteroid's fall 66 million years ago, and a Triceratops horn found in 2011 in Montana lies 13 centimetres below the boundary, which made it, when found, the youngest known non-bird dinosaur fossil. It was there at the end. The animal that was first taken for a bison has become, with its enemy, the pairing that stands for the dinosaurs' last day, and the two of them are found in the same rocks with the marks of each other on their bones.

When the idea that Torosaurus was simply an old Triceratops was published in 2010, headlines announced that Triceratops had never existed. That had it backwards. Scientific names for animals follow an international code, and one of its main rules is priority: when two names turn out to belong to the same animal, the one published first normally stands. Othniel Marsh named Triceratops in 1889 and Torosaurus in 1891, so if the two were ever shown to be one animal, Torosaurus would be the name that disappeared.

The best known example of this rule at work is Brontosaurus. Marsh named Apatosaurus in 1877 and Brontosaurus two years later, and when the two were judged to be the same kind of animal early in the twentieth century, Brontosaurus lost out, even though the public kept using it. In 2015 a detailed study of sauropod skeletons argued that the differences were large enough to separate them again, and Brontosaurus returned as a valid genus, though not everyone agrees. These cases show that dinosaur names reflect the current best judgement about how fossils are related, and that judgement can change as more specimens are studied.

For a century palaeontologists argued over how Triceratops held its front legs. The joints of the shoulder and elbow seemed to suggest a lizard-like sprawl, with the elbows sticking out sideways, and early museum skeletons were mounted that way, although an animal of several tonnes would have had to work hard to hold itself up in that pose. Others drew it with straight legs beneath the body like a rhinoceros, which the bones did not easily allow. Fossil trackways of horned dinosaurs helped settle the matter, because they show the front footprints placed fairly close to the line of the body rather than far out to the sides.

Digital modelling made the test more exact. When the Smithsonian's famous Triceratops skeleton, mounted in the early twentieth century, was taken down around 2000, researchers scanned its bones and found that the mount combined bones from several individuals of different sizes and that its posture was wrong. The view that emerged from scans, joint modelling and trackways together is a compromise: the front limbs were held more upright than a lizard's, but with the elbows bent somewhat outward and the hands planted beneath the shoulders. The answer came from bringing several independent kinds of evidence to bear on the same question.

In 2021 an unusually large and complete Triceratops skeleton nicknamed Big John, found in South Dakota, was sold at auction in Paris for around 6.6 million euros, at the time a record for a dinosaur sold at auction in Europe. The buyer was a private collector, and the skeleton went on display rather than into a public research collection.

Sales like this worry many palaeontologists. When important fossils go to private owners, scientists may lose access to them, and research based on specimens that can be sold again is hard to check. High prices also encourage commercial digging that can damage sites. Others argue that commercial collectors find fossils that would otherwise erode away. The debate has grown as dinosaur prices have soared.

Triceratops was an elephant-sized horned dinosaur with a bony frill, two long brow horns and a nose horn, a parrot beak and shearing tooth batteries, that grazed the floodplains of western North America in the last two million years of the Cretaceous and was the commonest large animal there. Its horns and frill were mainly for display and contests with its own kind, worked against Tyrannosaurus too, and changed shape throughout its life; it survived to within centimetres of the asteroid layer.