The origin and evolution of air sacs in pterosaurs and their forerunners.

Abstract

Although the existence of postcranial pneumaticity and the inferred presence of air sacs connected to the lungs are well established in Pterosauria, the origin of this system in pterosaurs remains unclear. We investigated skeletal pneumaticity in the Triassic pterosauromorph Venetoraptor using microcomputed tomography, seeking insights into the origin of postcranial pneumaticity. Our analysis reveals distinct patterns of postcranial pneumaticity, providing insights into the evolution of the respiratory adaptations of pterosauromorpha. Venetoraptor exhibits a mosaic of pneumatic foramina and internal chambers in its vertebrae, suggesting the early evolution of an elaborate system of air sacs connected to the lungs, which suggests the presence of an elaborate respiratory system. These findings support the hypothesis that invasive air sacs predated true pterosaurs, providing advantages such as enhanced ventilatory efficiency, reduced skeletal mass, and increased mechanical strength, all crucial for powered flight. Our study underscores the significance of early pneumatic structures in shaping vertebrate flight evolution, positioning pterosauromorphs as one of the key intermediary lineages in the development of avian-style respiratory systems.