Abstract
The evolutionary origins of the avian air sac pulmonary system are enigmatic due to the rarity of soft-tissue preservation in fossils. Here, we test whether fine anchoring fibers on the endosteal bone of bird and non-avian dinosaur vertebrae-termed "pneumosteum"-are absent in taxa lacking pneumatic openings. We studied thin sections from the caudalmost cervical and cranial dorsal vertebrae of 21 extant amniotes to infer the presence or absence of invading diverticula through vertebral foramina. We also provide a differential diagnosis of the structural features of pneumosteum. We found that the secondarily aquatic Western grebe (Aechmophorus occidentalis) and Magellanic penguin (Spheniscus magellanicus) lack external pneumaticity and pneumosteum. In addition, the small passerine bird examined (Estrildidae spp.) exhibits invading diverticula but no pneumosteum. This suggests that ventilatory air sacs and associated diverticula can be present despite the absence of osteological and histologic correlates and that these features are lost when transitioning to an aquatic lifestyle or in small-bodied birds. In volant pneumatized birds, diverticula and pneumosteum are associated with pneumatic foramina. This suggests that, in fossil birds, pneumatic foramina are good indicators of the presence of pulmonary diverticula. Furthermore, the loss of invading respiratory diverticula and pneumatic osteological characters in the postcranial skeleton of pursuit diving birds serves as a reminder that adaptation to specific ecologies, such as an aquatic environment, may obscure our ability to reconstruct soft tissue systems accurately in fossil taxa when relying on osteological correlates.