Among extant tetrapods, pneumatic postcranial bones are only present in birds, and they are osteological correlates of the diverticular lungs and pulmonary air sacs. The presence of postcranial pneumaticity in sauropod and theropod dinosaurs suggests that some form of air sac system was also present in the dinosaurian ancestors of birds. In particular, anatomical and evolutionary patterns of pneumatization in nonavian saurischian dinosaurs are diagnostic for specific air sacs, including the cervical, clavicular, and abdominal air sacs. Pneumatic hiatuses are gaps in the pneumatization of the vertebral column and indicate pneumatization from multiple sources. A pneumatic hiatus in Haplocanthosaurus provides additional support for the presence of abdominal air sacs in sauropods. The origins of postcranial pneumaticity in archosaurs are enigmatic because the earliest putative traces of pneumaticity are difficult to distinguish from skeletal imprints of other soft tissues. Nevertheless, several lines of evidence suggest that air sac-driven lung ventilation was primitive for Saurischia.
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