Axial and appendicular pneumaticity in Archaeopteryx.

From the time of its discovery in 1860 to this day Archaeopteryx has been essential to our understanding of avian evolution. Despite the great diversity of plesiomorphic avialan (sensu Gauthier 1986) taxa discovered within the last decade, Archaeopteryx remains the most basal avialan taxon. A very unusual feature of extant birds is their lung structure, in which air diverticulae penetrate the bones. This has previously been reported in Archaeopteryx as well, in the cervical vertebrae of the Berlin specimen and in an anterior thoracal vertebra of the Eichstätt specimen. This indicates the presence of a cervical air sac. We show that the London specimen also has pneumatized anterior thoracal vertebrae, and, thus, that this feature was present in the most archaic avialans, as the London and Eichstätt specimens are different species. Furthermore, the pelvis of the London specimen shows clear signs of the presence of an abdominal air sac, indicating that at least two of the five air sacs present in modern birds were also present in Archaeopteryx. Evidence of pubic pneumaticity was also found in the same position in some extant ratites.