What Is a Dinosaur Crest? Headgear Built for Being Seen and Heard

The lambeosaurine hadrosaurs, duck-billed plant eaters of the late Cretaceous, carried hollow crests of varied and extravagant shapes, from the long backward-swept tube of Parasaurolophus to the hatchet shape of Lambeosaurus and the round helmet of Corythosaurus. The crest was not solid bone but an enlargement of the nasal passages, so air drawn in through the nostrils travelled up into the crest, looped around and came back down to the throat. That immediately suggests a function connected with breathing or sound, and researchers tested it directly by building computer models of the internal passages from scans and calculating the resonant frequencies, then producing the sound. The result is a deep, low, carrying tone, comparable to a brass instrument, and different crest shapes and sizes produce different pitches. Juveniles had small crests that grew through life, so a young animal would have sounded different from an adult, and males and females may have differed as well, which gives the structure exactly the properties a signalling system needs.

Head ornamentation evolved repeatedly and took many forms across unrelated groups:

• •Hollow nasal crests in lambeosaurine hadrosaurs, connected to the airway and acoustically resonant
• •Solid crests and bumps in other hadrosaurs, which could not have made sound and were presumably visual
• •Frills and horns in ceratopsians, enormously varied between species and growing dramatically with age
• •Domed skulls in pachycephalosaurs, long interpreted as battering rams and now generally read as display with some evidence of flank butting and of healed head injuries
• •Head crests in some theropods including Dilophosaurus and Cryolophosaurus, thin and clearly not weapons
• •Crests in pterosaurs, which are not dinosaurs and evolved comparable structures, some of them very large relative to the animal

Several explanations were offered historically and most have been abandoned for specific reasons. The snorkel hypothesis, that hollow crests let animals breathe while submerged, fails because the crests have no opening at the top. The air reservoir hypothesis, that they stored air for diving, fails on volume, since the capacity is far too small to matter. A cooling function is plausible as a secondary effect and does not explain the diversity of shapes. An improved sense of smell has some support from the enlarged nasal area and again does not explain why the shapes differ so much between closely related species. Display explains the pattern best, because the diagnostic feature of display structures is exactly that they differ sharply between closely related species while being similar in everything else, which is what allows animals to recognise their own kind. That species recognition function, combined with signalling age, fitness and possibly sex, accounts for the diversity, the growth trajectory and the cost.

Two developments shifted the field. Growth series recovered from bone beds showed that crests, frills and horns changed dramatically through an animal's life, which had the consequence that several named species turned out to be juveniles of others, and the resulting reduction in named species tightened the diversity picture considerably. Computed tomography allowed internal structures to be reconstructed without destroying specimens, which is what made the acoustic modelling possible and which also revealed the extent of air spaces inside skulls generally. Alongside these, comparison with living animals became more disciplined, since birds and crocodilians bracket dinosaurs phylogenetically, so a feature present in both is reasonably inferred in dinosaurs and one present in neither requires stronger evidence. That framework has supported reconstructions of soft tissue including possible inflatable structures, wattles and coloured coverings, none of which fossilise, and it is a reminder that the bone preserved is frequently the core of a larger structure that has left no trace.

Hollow hadrosaur crests are enlarged nasal passages looping up and back down, and modelling the internal geometry produces deep carrying tones that differ by species, age and possibly sex. Snorkel and air reservoir explanations fail on anatomy and volume. Display explains the pattern because closely related species differ sharply in exactly this feature and in little else, which is what species recognition requires.