Given what was out there to eat you in the Cretaceous, having plenty of body armor was an important evolutionary development. Ankylosaurs took this to an extreme, being described by Ohio University doctoral student Jason Bourke as looking like “rocks with eyes”.
There is, however, a price to pay for such defenses. Not only are they heavy to lug around, but overheating is a danger. Dinosaurs famously did not possess big, heat producing, brains, but they were still vulnerable to temperature fluctuations.
Bourke noticed that the nasal passages of ankylosaurs are exceptionally convoluted, unlike those of modern animals, and the trait is consistent across known species. We can see this because the bony upper body of many ankylosaur species tended to fossilize well. Although thought to have had a good sense of smell, the species' complex cavities make little sense for smelling.
Instead, Bourke proposes that they were surrounded by veins allowing them to act as heat exchangers, with air drawn through the passages and over the blood vessels, providing a cooling mechanism.
Using CT scans of the skulls of two species of ankylosaurs, Panoplosaurus mirus and the abundant Euoplocephalus tutus, Bourke applied computational fluid dynamics and found that when 15° C air was drawn in through the nose, it would have warmed before it was finally breathed out. In the process however, it would have cooled the blood to 18° C. By concentrating the cooling effect on the blood that was on its way to the brain, the vital organ would have been kept at a reasonable temperature.
The larger Euoplocephalus' skull was more efficiently shaped for this purpose than Panoplosaurus, Bourke found, reflecting the fact that its size would have increased the risk of overheating.
Modern mammals and birds achieve a similar effect using respiratory turbinates, warming and humidifying air as it enters the body. "This is the first time we've been able to show that an animal that doesn't have these turbinates found another way around heating the air up or cooling it down, just by making the airway superlong and then curling it around," Bourke said.
Hadrosaurs also have complex nasal cavities, and these have previously been thought useful for creating loud calls for mating or deterrence. Bourke suggests many types of dinosaurs may have used these nasal architecture for dual purposes. "If they made sound, at least, it probably was going to be enhanced by having this crazier nose shape,"
Ohio University. Model of the airflow in a Stegoceras, a dinosaur species with a more simply shaped skull.