Abstract
The question of whether the iconic avialan Archaeopteryx was capable of active flapping flight or only passive gliding is still unresolved. This study contributes to this debate by reporting on two key aspects of this fossil that are visible under ultraviolet (UV) light. In contrast to previous studies, we show that most of the vertebral column of the Berlin Archaeopteryx possesses intraosseous pneumaticity, and that pneumatic structures also extend beyond the anterior thoracic vertebrae in other specimens of Archaeopteryx. With a minimum Pneumaticity Index (PI) of 0.39, Archaeopteryx had a much more lightweight skeleton than has been previously reported, comprising an air sac-driven respiratory system with the potential for a bird-like, high-performance metabolism. The neural spines of the 16th to 22nd presacral vertebrae in the Berlin Archaeopteryx are bridged by interspinal ossifications, and form a rigid notarium-like structure similar to the condition seen in modern birds. This reinforced vertebral column, combined with the extensive development of air sacs, suggests that Archaeopteryx was capable of flapping its wings for cursorial and/or aerial locomotion.
Highlights
Living birds have extensively pneumatized postcrania, due to their unique and extremely efficient respiratory system[1,2]
In the Berlin Archaeopteryx, the vertebral cortex has been abraded in most presacral vertebrae, possibly due to repeated preparation[15,18], exposing the internal bone structure of the vertebrae and ribs
We demonstrate that Archaeopteryx possessed a derived, bird-like postcranial pneumatization pattern that comprises intraosseous pneumatic camellae and camerae within the presacral vertebral column and caudal vertebrae
Summary
Living birds have extensively pneumatized postcrania, due to their unique and extremely efficient respiratory system[1,2]. The large cavities within pneumatic vertebrae of dinosaurs have been descriptively separated according to their architecture into larger and rounded camerae, and smaller and more angular-walled camellae[8,9,10]. Both types of internal pneumatic structures can occur in the same skeletal element, and multiple camellae can create a specific honeycomb-like pattern as in extant birds, described as somphospondylous[10]. The new UV findings we present here extend these results, enable a detailed account of all unambiguous pneumatic structures in the postcranial www.nature.com/scientificreports skeleton of MB.Av.[101], and confirm that numerous postcranial bones of Archaeopteryx were reduced in mass via hollow interiors (Table S1)
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