What Is a Frill? The Bony Shield That Was Probably Not Armour
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Horned dinosaurs carried a sheet of bone extending backwards from the skull over the neck, sometimes more than a metre across. The obvious explanation is protection, and the evidence points somewhere else entirely.
What it is made of
The structure is formed from two bones at the back of the skull, extended rearwards and upwards into a sheet whose margin frequently carries spikes, hooks or scalloped projections. In many species the sheet contains large openings, so the bony part is a frame rather than a shield, with the gaps closed in life by soft tissue. The surface bears channels for blood vessels, indicating a substantial supply of blood to whatever covered it, and the covering was probably keratin or skin rather than muscle. The margin and the ornamentation vary enormously between species and are the primary means of telling them apart, while the underlying arrangement stays much the same. Growth changes the structure substantially, with juveniles having short simple frills and adults developing the elaborate forms, which is a pattern with implications for how the structure was used.
The case against armour
Several observations undermine the protective explanation, which was the original one:
- •The large openings in many species, which would be useless against a bite and are the opposite of what armour requires
- •The thinness of the bone in several forms, measured at a few millimetres in places
- •The blood supply indicated by surface channels, which is wasteful in a purely protective structure and useful in a display one
- •The enormous variation between species, which is characteristic of signalling structures and not of defensive ones
- •The late development of the elaborate form, appearing around the age at which the animal would have been breeding
- •Bite marks and healed injuries on frills, which show damage occurred without showing the structure was for absorbing it
What it was probably for
The favoured explanation is species recognition and display, and the reasoning rests on the pattern of variation. Structures used for signalling diverge rapidly and arbitrarily between related species, since the requirement is to be distinguishable rather than to be good at anything, and the horned dinosaurs are a textbook case, with closely related animals differing almost exclusively in the arrangement of horns and frill ornament while being nearly identical elsewhere. Statistical work comparing the rate of change in these structures with the rate in the rest of the skeleton supports that reading. Sexual selection is a plausible component, though establishing it requires showing a difference between sexes and the evidence for that is weak in this group. Combat between rivals is also supported in some species by injury patterns consistent with horn locking, and a frill could shield the neck during such a contest without being armour against predators.
The growth problem
Because the structure changes so much as an animal grows, distinguishing species from growth stages in this group has been genuinely difficult and has produced long-running disputes. Several named species have been argued to be juveniles of others, on the reasoning that the differences between them match the changes seen within a known growth series, and the argument has been accepted in some cases and rejected in others. The most contested examples involve animals whose adult and juvenile forms differ so much that nobody would place them together without intermediate specimens. Resolving such questions requires large samples covering the full range of sizes, bone sectioning to establish maturity independently of size, and careful attention to which features change with growth and which do not. The dispute matters beyond naming, since the number of species present affects every conclusion about diversity, geographic range and how quickly the group was evolving.
The heat and muscle ideas
Two further explanations have been proposed and both have problems. Temperature regulation is suggested by the blood supply, on the reasoning that a large thin well-supplied surface sheds heat efficiently, and the idea has been applied to several extinct structures including the plates of stegosaurs. It faces the objection that species with the largest frills are not those with the greatest need, and that the blood supply may simply have served the covering tissue. Muscle attachment is the other, with the frill proposed as an anchor for jaw muscles, which is anatomically plausible for the front portion and does not explain the extended rear part or the ornament. The current position accommodates several functions, with the structure probably serving display primarily while contributing to whatever else the anatomy permitted, which is the usual outcome for large conspicuous features.
The takeaway
The sheet is built from two skull bones and frequently contains large openings, which makes it a frame rather than a shield, with thin bone and a substantial blood supply. Variation between closely related species is concentrated almost entirely in the frill and horns, which is the signature of a signalling structure. The elaborate form develops around breeding age rather than being present in juveniles.