Why Did Flying Reptiles Have Such Extravagant Heads? Signalling in the Air
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
Some pterosaurs carried head crests larger than the rest of the skull, in shapes that look impossible for an animal that had to fly. The explanations proposed have narrowed considerably as evidence accumulated.
What the structures are
Crests take several forms and are built differently from one another. Some are bony extensions of the skull, hollow in places and solid in others. Some are soft tissue supported on a bony base, preserved only in exceptional specimens and frequently far larger than the bone suggests, which means reconstructions based on bone alone underestimate them systematically. Some are entirely soft and are known only from rare preservation. Sizes range from a low ridge to a structure several times the area of the skull, and shapes include blades, sails, forward-projecting spikes and rounded plates. The variation between closely related species is substantial, which is itself one of the more informative facts about them.
The explanations proposed
Several functions have been suggested and the evidence supports them unevenly:
- •Display and species recognition, which is now the leading explanation
- •A rudder for steering in flight, which the aerodynamics do not support well
- •A counterbalance for a long beak, which fails because many crests sit in the wrong position
- •Heat regulation, which a well-supplied soft structure could achieve
- •Sexual signalling, supported where crest size differs between individuals of the same species
- •A structure for cutting through water while fishing, which has been tested and largely dismissed
The evidence for display
The case rests on several independent lines. Crest form varies enormously between closely related species while the rest of the skeleton is similar, which is the pattern expected of a recognition signal and not of a structure serving a mechanical function, since aerodynamic requirements would converge rather than diverge. Where enough specimens of one species exist, crests appear in two size classes, with some individuals carrying large ones and others small or none, which is consistent with a difference between sexes and which has been argued in detail for at least one well-sampled species. Crests grow disproportionately with age, being small or absent in juveniles and developing at the size associated with maturity, which is the standard signature of a signalling structure rather than a functional one.
The soft tissue nobody expected
Exceptional preservation has repeatedly shown that the bone gives a poor guide to what these animals looked like. Specimens preserving soft tissue reveal crests extending well beyond the bony base, in one case more than doubling the apparent height, which means every reconstruction made from bone alone is wrong in a predictable direction. Similar preservation has revealed throat pouches, webbing between the toes, and a body covering of branched filaments that several researchers argue are related to feathers, which if correct pushes the origin of such structures much further back than the dinosaur record alone implies. Ultraviolet photography and chemical imaging have made faint soft-tissue traces visible on specimens that appeared to be bone only, which is steadily expanding what can be read from existing collections.
What it cost to carry
The aerodynamic consequences have been examined experimentally and they constrain the interpretation. Models of crested skulls tested in wind tunnels and simulated computationally show that a large crest generates drag and turning moments that the animal had to counteract, with the cost depending on shape and on whether the head could be held aligned with the airflow. Several studies conclude the penalty was real and manageable, which is exactly the situation a costly signal requires, since a structure that imposed no cost would carry no information about the individual bearing it. That framing resolves the apparent puzzle, since the question shifts from how the animal tolerated the crest to what the crest bought that justified tolerating it.
The takeaway
Crests are bony, soft or both, with soft extensions frequently far larger than the bone suggests, so bone-only reconstructions underestimate them. Form varies enormously between closely related species while the rest of the skeleton does not, which fits a recognition signal rather than a mechanical function. Crests develop at maturity and impose a measurable aerodynamic cost, which is what a costly signal requires.