Postcranial pneumatization in Archaeopteryx

Pneumatization of the postcranial skeleton by the lungs is thought to be a hallmark of the avian skeleton, and to be an adaptation for flight by reducing weight. Pneumatic features have, however, remained elusive in the primitive avialan Archaeopteryx lithographica. The hollow long bones of Archaeopteryx were first interpreted to be pneumatized, but this interpretation was later rejected because of an absence of pneumatic foramina in these bones that connect their interiors with the respiratory system. Pneumatic features have also been recognized in the axial skeleton of many non-avialan theropod dinosaurs (and some other archosaurs of the bird clade). The purported lack of postcranial pneumatic features in Archaeopteryx has been interpreted as a primitive condition of avialans; this raises doubts about the homology between postcranial pneumatic features of birds and non- avialan theropods. Here we re-examine two specimens of Arhaeopteryx. These specimens show evidence of vertebral pneumaticity in the cervical and anterior thoracic vertebrae, thus confirming the phylogenetic continuity between the pneumatic systems of non-avialan theropods and living birds.