Pneumatic postcranial bones in dinosaurs and other archosaurs
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AbstractAmong amniotes, pneumatic postcranial bones are shared by Aves, Theropoda, Sauropoda, and Pterosauria. Comparison of pneumatic characters in the vertebrae of birds and non-avian dinosaurs demonstrates that the Theropoda and the Sauropoda had pneumatic vertebrae and ribs. Pneumatic bones of the extinct taxa were invaded by diverticula of an air sac system homologous with that of birds. The nature and extent of pneumatic features in the vertebrae were studied with the aid of cut sections, standard x-rays, and computed tomography. Two types of dinosaurian pneumatic vertebrae are recognized based on internal structure: 1) simple (camerate), with relatively large, open chambers or fossae, as in Syntarsus, Torvosaurus, Saurornitholestes, and Barapasaurus; and 2) complex (camellate), with supernumerary, interconnected chambers as in Tyrannosauridae, Ornithomimidae, Aves, and Pterosauria. Among the Theropoda complex vertebrae are restricted almost exclusively to Tetanurae, suggesting that this is an apomorphic character state for that taxon. Pterosaurs, sauropods, and theropods share with birds dorsoventrally widelyseparated articular facets on the ribs. Pneumatization significantly reduced the mass of dinosaur vertebrae. The location of diverticula tracks and foramina in some non-avian tetanuran theropods suggests that the air-sac system played a role in thermoregulation, as in extant birds. Pneumatic characters are of major taxonomic and phyletic significance because they can be used to test phyletic hypotheses based on other character suites, as well as to diagnose taxa. It is demonstrated that segnosaur vertebrae were pneumatic and of the complex type. These characters, along with non-pneumatic characters, indicate the enigmatic segnosaurs are tetanuran theropods. The distribution of pneumatic characters suggests that Pterosauria, Theropoda, and Sauropoda share a close common ancestry. Pneumatic postcranial bones indistinguishable from those of theropods are present in Archaeopteryx. Therefore, postcranial pneumaticity in birds is not an adaptation but an exaptation to flight. Pneumatic para-axial postcranial bones are found only in pterosaurs and birds, indicating that they are an adaptation to flight.
Bibliography: p. 290-317