Pterosaur respiratory system. Artwork of a pair of flying Anhanguera sp. pterosaurs. The main pterosaur has been cut-away to reveal bird-like pulmonary air sac respiratory system (green and blue) and lungs (red). Some scientists have speculated that pterosaurs evolved this respiratory system before birds had evolved. Pulmonary air sacs store air and act like bellows, allowing the lungs to maintain a fixed volume with fresh air flowing through them. Powered flight requires a high oxygen demand. Anhanguera lived from the Early to Late Cretaceous (112-94 million years ago).