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 left to right asymmetry were present in certain aspects of
the bronchial tree of both taxa but was particularly evident in the
cardiac (medial) region of the lungs of alligators and the caudal aspect
of the bronchial tree in both species. The cross-sectional area of the
primary bronchus at the level of the major secondary airways and
cross-sectional area of ostia scaled either isometrically or negatively
allometrically in alligators and isometrically or positively
allometrically in ostriches with respect to body mass. Of fifteen lung
metrics, five were significantly different between the alligator and
ostrich, suggesting that these aspects of the lung are more
interspecifically plastic in archosaurs. One metric, the distances between
the carina and each of the major secondary airways, had minimal
intraspecific or ontogenetic variation in both alligators and ostriches,
and thus may be a conserved trait in both taxa. In contrast to previous
descriptions, the 3D digital models and CT scan data demonstrate that the
pulmonary diverticula pneumatize the axial skeleton of the ostrich
directly from the gas-exchanging pulmonary tissues instead of the air
sacs. Global and specific comparisons between the bronchial topography of
the alligator and ostrich reveal multiple possible homologies, suggesting
that certain structural aspects of the bronchial tree are likely conserved
across Archosauria, and may have been present in the ancestral
archosaurian lung.