Dinosaur air heads. The role of complicated sinuses and convoluted airways. Air spaces such as the airway and sinuses comprise a neglected anatomical system. CT scanning and advanced 3D computer visualization provided the most complete view ever of the air spaces in two groups of dinosaurs and their modern relatives (birds and crocodilians), with a mammal of parochial interest (humans) thrown in for comparison. Modern alligators and ostriches both show great diversity in air sinuses, revealing that the complex air spaces of dinosaurs were part of their archosaurian heritage. All of the air spaces were modeled in the predatory theropod dinosaurs Majungasaurus and Tyrannosaurus , revealing in both a simple arcing airway, large olfactory regions, and lots of sinuses. But the major finding was just how much of the head was occupied by air. The brain cavity was tiny in comparison. An outcome of digitally reconstructing all the head tissues was that head mass could be calculated, allowing an assessment of potential weight savings due to the sinuses. The fully fleshed-out head of Majungasaurus weighed 32 kg (70 lbs), whereas that of T. rex weighed an enormous 515 kg (over 1100 lbs, as much as five or six adult men). Having all the sinuses saved about 8% of head mass but as much as 18% of skull mass, which is significant given how metabolically expensive bone is to maintain. The skulls of armored ankylosaurian dinosaurs also were studied, particularly those of Panoplosaurus and Euoplocephalus . Although it previously had been widely thought that their snouts were riddled with sinuses, the new, surprising results were that, instead of sinuses, their snouts housed a long convoluted nasal airway. This “crazy-straw” airway ran close to large blood vessels and so may have provided a physiological mechanism for dealing with heat. Likewise, the complicated airway would have acted as a vocal resonator, perhaps allowing for species- or even individual specific vocalization. Clearly, the spaces that make up these dinosaurian “air heads” represent a new, largely untapped, window into dinosaur function and behavior.