Pulmonary anatomy in the Nile crocodile and the evolution of unidirectional airflow in Archosauria

Abstract

The lungs of birds have long been known to move air in only one direction during both inspiration and expiration through most of the tubular gas-exchanging bronchi (parabronchi). Recently a similar pattern of airflow has been observed in American alligators, a sister taxon to birds. The pattern of flow appears to be due to the arrangement of the primary and secondary bronchi, which, via their branching angles, generate inspiratory and expiratory aerodynamic valves. Both the anatomical similarity of the avian and alligator lung and the similarity in the patterns of airflow raise the possibility that these features are plesiomorphic for Archosauria and therefore did not evolve in response to selection for flapping flight or an endothermic metabolism, as has been generally assumed. To further test the hypothesis that unidirectional airflow is ancestral for Archosauria, we measured airflow in the lungs of the Nile crocodile (Crocodylus niloticus). As in birds and alligators, air flows cranially to caudally in the cervical ventral bronchus, and caudally to cranially in the dorsobronchi in the lungs of Nile crocodiles. We also visualized the gross anatomy of the primary, secondary and tertiary pulmonary bronchi of C. niloticus using computed tomography (CT) and microCT. The cervical ventral bronchus, cranial dorsobronchi and cranial medial bronchi display similar characteristics to their proposed homologues in the alligator, while there is considerable variation in the tertiary and caudal group bronchi. Our data indicate that the aspects of the crocodilian bronchial tree that maintain the aerodynamic valves and thus generate unidirectional airflow, are ancestral for Archosauria.

A phenology of the evolution of endothermy in birds and mammals

“…All archosaurs (crocodiles, pterosaurs, dinosaurs and birds) have a unidirectional flow of air through the lungs during both inhalation and exhalation which is more efficient than costal breathing in mammals (Farmer & Sanders, 2010;Schachner et al, 2013). The lungs are comprised of open-ended tubes rather than the blind-ending alveoli sacs where tidal gas exchange occurs in mammalian lungs (Schachner et al, 2013).…”

“…The lungs are comprised of open-ended tubes rather than the blind-ending alveoli sacs where tidal gas exchange occurs in mammalian lungs (Schachner et al, 2013). In the derived sauropsid state in Dinosauromorpha (Pterosauria + Ornithischia + Sauropodomorpha + Theropoda) the non-gas-exchange pulmonary space of the abdominal, thoracic, cervical, and clavicular air sacs permeate the postcranial skeleton via pneumatopores to form pneumatized, light-weight bone with unknown respiratory function(s) (O'Connor & Claessens, 2005;Sereno et al, 2008;Claessens, O'Connor & Unwin, 2009;Benson et al, 2012;Butler, Barrett & Gower, 2012;Schachner et al, 2013). Lung ventilation in birds differs markedly from mammals because birds lack a diaphragm.…”

Effect of pre-cardiac and adult stages of Dirofilaria immitis in pulmonary disease of cats: CBC, bronchial lavage cytology, serology, radiographs, CT images, bronchial reactivity, and histopathology

“…For color illustration of lung lesions, each structure of the anatomy was mapped with manual identification of discrete of areas of lung lesions on individual CT slices and Avizo 7.0 software. Three-dimensional reconstructions were rendered as has been described (Schachner et al, 2013(Schachner et al, , 2014.…”

Unidirectional pulmonary airflow patterns in the savannah monitor lizard

“…1c; Cb). This bronchus is anatomically reminiscent of the cervical ventral bronchus in crocodilians 6,9 . Arising sequentially from the lateral surface of the primary bronchus is a series of 9-11 variably shaped large sac-like bronchi, termed lateral bronchi ( Fig.…”