Figure 1. Nasal passages of endotherms. Respiratory turbinates (rt) are complex bony or cartilaginous structures in the nasal cavities of mammals (top left) and birds (bottom left), which help limit heat and water loss during breathing. These structures are missing in all modern reptiles. Also shown are cross-sections of representative birds (bottom right) and mammals (top right).

Figure 2. Relationship between nasal passage cross-sectional area and body mass in modern endotherms and ectotherms. The larger dimensions in endotherms compensate for the increase in resistance to air flow presented by the respiratory turbinates. Also plotted are three genera of Late Cretaceous dinosaurs, the hadrosaurid Hypacrosaurus and the theropods Nanotyrannus and Ornithomimis. Data from Ruben et al. (1996).

Figure 3. Lung ventilation in modern birds. Specialized hinged ribs and joints between sternal ribs and the lateral edge of the sternum cause an up-and-down rocking motion that is responsible for the ventilation of the airsacs (top). The sternum of modern birds is therefore characterized by a robust lateral edge with transversely oriented joint facets that articulate with the sternal ribs (bottom).

Figure 4. Lung ventilation in dinosaurs. Dinosaurs lacked the specialized hinged ribs and sternocostal joints associated with lung ventilation in modern birds. But preserved soft tissues in the abdominal cavity of the compsognathid Sinosauropteryx (left) and the maniraptoran Scipionyx (right; bottom image under ultraviolet illumination) suggest that theropods may have piston-like liver movements to enhance lung ventilation. Bar, 1 cm.