What Was Triceratops? Horns, Frill and the Last Great Plant-Eater
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The first Triceratops horns to reach a scientist were sent to Yale in 1887 and identified as those of a giant extinct bison, and the animal they belonged to turned out to be among the last dinosaurs on Earth, an eight-metre, six-tonne plant-eater with a skull a third the length of its body, two metre-long brow horns, a nose horn and a bony frill, living on the floodplains of western North America in the final two million years before the asteroid. It is the commonest large dinosaur in those rocks, its bones are found by the hundred, and it shared its world with the predator that its horns were long assumed to be for.
The animal
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, the largest skull 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.
What the horns were for
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
Growing up
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.
How it lived
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.
The end
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.
The takeaway
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.