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
30 from that of their closest extant relatives, the crocodilians. It is highly partitioned, with a
31 unidirectionally ventilated and immobilized gas-exchanging lung, and functionally decoupled,
32 compliant, poorly vascularized ventilatory air-sacs. To understand the evolutionary history of
33 the archosaurian respiratory system, it is essential to determine which anatomical
34 characteristics are shared between birds and crocodilians and the role these shared traits play
35 in their respective respiratory biology. To begin to address this larger question, we examined
36 the anatomy of the lung and bronchial tree of ten American alligators (Alligator
37 mississippiensis) and eleven ostriches (Struthio camelus) across an ontogenetic series using
38 traditional and micro-computed tomography (µCT), three-dimensional (3D) digital models, and
39 morphometry. Intraspecific variation and left to right asymmetry were present in certain
40 aspects of the bronchial tree of both taxa but was particularly evident in the cardiac (medial)
41 region of the lungs of alligators and the caudal aspect of the bronchial tree in both species. The
42 cross-sectional area of the primary bronchus at the level of the major secondary airways and
43 cross-sectional area of ostia scaled either isometrically or negatively allometrically in alligators
44 and isometrically or positively allometrically in ostriches with respect to body mass. Of fifteen
45 lung metrics, five were significantly different between the alligator and ostrich, suggesting that
46 these aspects of the lung are more interspecifically plastic in archosaurs. One metric, the
47 distances between the carina and each of the major secondary airways, had minimal
48 intraspecific or ontogenetic variation in both alligators and ostriches, and thus may be a
49 conserved trait in both taxa. In contrast to previous descriptions, the 3D digital models and CT
50 scan data demonstrate that the pulmonary diverticula pneumatize the axial skeleton of the
51 ostrich directly from the gas-exchanging pulmonary tissues instead of the air sacs. Global and
52 specific comparisons between the bronchial topography of the alligator and ostrich reveal
53 multiple possible homologies, suggesting that certain structural aspects of the bronchial tree
54 are likely conserved across Archosauria, and may have been present in the ancestral
55 archosaurian lung.