About 75 million years ago, in what is now Alberta, Canada, a dinosaur called Euoplocephalus took its final breath. That exhalation, like every other, was fleeting and insubstantial, but eons later, scientists can still reconstruct the path it took out of the dinosaur’s head. And that path, it turns out, was extraordinarily convoluted.
Euoplocephalus was one of the ankylosaurs—a group of tank-like species covered in bony plates. Their skulls and backs were armored. Their eyelids were occasionally armored. Even the nasal passages inside their skulls were lined with bone, preserving these delicate structures, usually lost to time.
More than a century ago, paleontologists first noticed that those passages included a weirdly complicated series of chambers and tubes. They interpreted these as a set of sinuses that branched from a simple central channel—a slightly more elaborate version of the setup that exists inside your nose. But in 2008, Lawrence Witmer and Ryan Ridgely from Ohio University worked out what was really going on when they put the skulls of several ankylosaurs in a medical CT scanner.
The scans revealed the unusual structure of the creatures’ nasal passages—not sinuses forking off a central channel, but a single airway that repeatedly twists and turns, like roller-coaster tracks or a Krazy Straw. These passages are more complex than the airways of other backboned animals, and they’re remarkably long. The skull of Euoplocephalus “is the length of your arm from the wrist and elbow, but its nasal passage, if stretched out, would run from your shoulder to your fingertip,” says Witmer. “I remember standing up at a paleontology meeting, holding up my hands, and saying, ‘I don’t believe it, but this is what we got.’”
Witmer and Ridgely thought that these convoluted airways acted as an elaborate air-conditioning system for the ankylosaurs’ brains. These were big, car-size animals whose bodies would have retained a lot of heat in the Mesozoic sun. “Hot blood would have come up from the core of their bodies to their brains,” says Witmer. “And while these dinosaurs’ brains were famously small, they were still brains.” Brains are especially sensitive to rises in temperature, which is why confusion and fainting are among the first signs of heatstroke. So how did ankylosaurs and other giant dinosaurs keep their noggins from cooking?
It was all in the nose, Witmer guessed. The vessels carrying blood from an ankylosaur’s body to its head ran alongside its long nasal canal. Every time the dinosaur inhaled, cool air would have meandered through that twisty airway, absorbing the heat from the adjacent blood and cooling it before it hit the brain.
Witmer’s colleagues Jason Bourke and Ruger Porter have now tested this idea. They used medical scanners to create digital replicas of the skulls of two ankylosaurs—Euoplocephalus and Panoplosaurus. Then they simulated the flow of air through these virtual noses, using techniques that are more commonly used by aerospace engineers.
These simulations revealed that, on an inhale, the dinosaurs’ long nasal passages gradually heated air by up to 36 degrees Fahrenheit, taking it from room temperature to body temperature and substantially cooling the adjacent blood. When the dinosaur exhaled, along the same twisty tubes, the air would return most of that heat back to the body. (Our own simple noses work on a similar principle, which is why your breath feels hotter coming from your mouth than from your nose.)
The team members also played around with their virtual skulls. In one experiment, they gave their ankylosaurs short and simple airways, much like ours. In another, they straightened the animals’ airways so they kept their normal length but lacked any twists. In both cases, the heating effect became far less efficient. Inhaled air picked up less heat, and it did so at the very end of the passages—too late to cool the adjacent blood vessels. It’s the passages’ length and their curviness that make them efficient air conditioners.
“This is a fascinating deep dive into an aspect of dinosaur biology that’s been difficult to study—how a dinosaur’s breath traveled through its skull,” says Victoria Arbour, an ankylosaur expert at the Royal Ontario Museum in Toronto. “It makes a lot of sense [especially since] many ankylosaurs lived in arid or tropical environments. It’s easy to see how this adaptation arose.”
Matthew Vickaryous from the University of Guelph notes that of the two species that the team studied, Euoplocephalus was bigger and had more complex nasal passages. Are those two things related? Do bigger species, which are more likely to suffer from overheating, need twistier noses? What kind of zany structures lurked inside the snout of the largest ankylosaur—the eponymous, eight-meter-long Ankylosaurus? Today it’s possible to answer these questions, because “CT data are now available for an increasing number of ankylosaurs,” Vickaryous says.
Tetsuto Miyashita from the University of Chicago agrees. He credits Witmer’s team for “pioneering a new genre in paleontology” in which they fuse the hard physics with messy biology. “What’s next?” Miyashita asks. “Well, no one has reconstructed resonation in those virtual nasal airway models to see whether [the passages] work like a trumpet. I hope they give that a try.”