What Is a Ceratopsian? Frills and Horns That Changed With Age

Ceratopsians are a branch of the plant-eating ornithischian dinosaurs defined by a bone at the tip of the upper jaw forming a parrot-like beak, which is present even in the earliest and smallest members. From that beginning the group ranged from bipedal animals the size of a dog in the early Cretaceous to quadrupedal giants weighing several tonnes at the end of it. The characteristic features of the large forms are the frill, an extension of the bones at the back of the skull projecting backwards over the neck, and the horns, which vary between species in number, position, size and orientation. Their teeth formed shearing batteries rather than grinding ones, cutting vegetation rather than crushing it, which is a different solution from the hadrosaur one and suited different plants. The skulls are among the largest of any land animal, reaching over two and a half metres in some species, which required substantial neck musculature and a reinforced skeleton.

Several explanations have been proposed and the evidence supports some more than others:

• •Protection of the neck, which is undermined by the large openings in most frills, since a shield with holes in it is a poor shield
• •Muscle attachment for the jaw, which is anatomically plausible for the front portion and does not explain the size or the ornamentation
• •Thermoregulation, with blood vessels in the frill shedding heat, which is possible as a secondary function and does not explain the variation between species
• •Display to rivals and mates, supported by the growth pattern, since the structures develop most rapidly around the age of sexual maturity
• •Species recognition, supported by the fact that closely related species living alongside each other differ sharply in exactly these structures and in almost nothing else
• •Combat, supported by healed injuries on some skulls consistent with horn-to-horn pushing, and by computer modelling showing that some horn arrangements could interlock

A substantial revision of the group came from recognising that juveniles do not look like small adults. Growth series assembled from bone beds show frills changing shape, horns curving in different directions at different ages, and spikes along the frill margin growing, fusing and in some cases resorbing entirely. That has two consequences. It supports the display interpretation directly, since structures that develop at maturity and continue changing are behaving like signals rather than like armour. And it caused a reassessment of the species list, with the proposal that several named species are growth stages of others, which has been argued vigorously and accepted in some cases and rejected in others. The most publicised case concerns whether a small flat-skulled form is the juvenile of a larger spiky-frilled one, which has been argued in print for over a decade without settling, and the underlying difficulty is that testing it requires specimens of intermediate age from the same rock unit, which are scarce.

Many ceratopsian specimens come from deposits containing dozens to thousands of individuals of a single species and almost nothing else, which is the primary evidence that these animals lived in groups. The explanation for how such accumulations formed is generally drowning during river crossings or floods, on the pattern documented in modern mass mortality events among migrating herds, though other mechanisms including drought around waterholes have been proposed for some sites. Those deposits are what make growth series possible, since a single event sampling a population captures animals of every age simultaneously. They also permit measurement of variation within a species, which has repeatedly shown that features used to separate species fall within the range of variation of one, and several names have been sunk as a result. The distribution of species across western North America shows different forms in different regions at the same time, which has been interpreted as evidence for provinciality and rapid speciation, and the pattern is one of the better-sampled cases of dinosaur biogeography.

A parrot-like beak bone defines the group from its smallest early members to multi-tonne giants with skulls over two metres long. Frills full of large openings make poor armour, and the structures develop fastest around maturity and differ sharply between species that are otherwise alike, which is what display and species recognition require. Juveniles look nothing like adults, which has collapsed several named species into growth stages.