Anatomy, ontogeny, and evolution of the archosaurian respiratory system: a case study on Alligator mississippiensis and Struthio camelus

The avian lung is highly specialized and is both functionally and morphologically distinct from that of their closest extant relatives, the crocodilians. It is highly partitioned, with a unidirectionally ventilated and immobilized gas-exchanging lung, and fully decoupled, compliant, poorly vascularized ventilatory air-sacs. To understand the evolutionary history of the archosaurian (birds, crocodilians and their common ancestors) respiratory system, it is essential to determine which anatomical characteristics are shared between birds and crocodilians and the role these shared traits play in their respective respiratory biology. To begin to address this larger question, we examined the anatomy of the lung and bronchial tree of ten American alligators (Alligator mississippiensis) and eleven ostriches (Struthio camelus) across an ontogenetic series using traditional and micro-computed tomography (µCT), three-dimensional (3D) digital models, and morphometry. Intraspecific variation and lef...