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In birds, diverticula of the lungs and air sacs pneumatize specific regionsof the postcranial skeleton. Relationships among pulmonary components andskeletal regions they pneumatize allow inferences about pulmonary anatomy innon-avian dinosaurs. Fossae, foramina and chambers in the postcranial skeletonsof pterosaurs and saurischian dinosaurs are diagnostic for pneumaticity. Inbasal saurischians only the cervical skeleton is pneumatized, by cervical airsacs. In more derived saurischians pneumatization of posterior dorsal, sacral,and caudal vertebrae indicates abdominal air sacs. Abdominal air sacs insauropods are also indicated by a pneumatic hiatus (a gap in vertebralpneumatization) in Haplocanthosaurus. Minimally, saurischians had dorsallyattached diverticular lungs plus anterior and posterior air sacs, and all thepulmonary prerequisites for flow-through lung ventilation like that of birds.Pneumaticity reduced skeletal mass in saurischians. I propose the Air SpaceProportion (ASP) as a measure of proportional volume of air in pneumatic bones.The mean ASP of a sample of sauropod and theropod vertebrae is 0.61, so onaverage, air occupied more than half the volume of these vertebrae. InDiplodocus, pneumatization lightened the living animal by 7-10 percent, andthat does not include extraskeletal diverticula, air sacs, lungs, or trachea.If all these air reservoirs included, the specific gravity of Diplodocus is0.80, higher than published values for birds but lower than those for squamatesand crocodilians. Pneumatization of cervical vertebrae facilitated evolution oflong necks in sauropods. Necks longer than nine meters evolved at least fourtimes, in mamenchisaurs, diplodocids, brachiosaurids, and titanosaurs.Increases in the number of cervical vertebrae, their proportional lengths, andtheir internal complexity occurred in parallel in most of these lineages.

Postcranial pneumaticity in dinosaurs and the origin of the avian lung