 Some of "Aerosteon's" bones show pneumatisation (air-filled spaces), including pneumatic hollowing of the furcula and ilium, and pneumatisation of several gastralia, suggesting that it may have had a respiratory air-sac system similar to that of modern birds.These air sacs would have acted like bellows, moving air into and out of the animal's relatively inflexible lungs, instead of the lungs themselves being expanded and contracted as occurs with mammals.See avian respiratory system for more detailed information on this.