Unidirectional pulmonary airflow patterns in the savannah monitor lizard

The unidirectional airflow patterns in the lungs of birds have long been considered a unique and specialized trait associated with the oxygen demands of their volant lifestyle, endothermic metabolism and unusual pulmonary architecture; however, the discovery of similar flow patterns in the lungs of crocodilians indicates that this character is likely ancestral for all archosaurs, the group that includes extant birds and crocodilians as well as their extinct relatives, such as pterosaurs and dinosaurs. Unidirectional flow in birds results from aerodynamic valves, rather than from sphincters or other physical mechanisms, and similar aerodynamic valves appear to be at work in crocodilians. Due to anatomical and developmental similarities in the primary and secondary bronchi of birds and crocodilians, these structures and airflow patterns may be homologous. Thus the origin of this pattern is at least as old as the split between crocodilians and birds, which occurred in the Triassic; however, it is possible that this pattern of flow is even older. This constitutes an alternative hypothesis, which can be tested by investigating patterns of airflow in members of the outgroup to birds and crocodilians, the Lepidosauromorpha (tuatara, lizards, and snakes). Here we show region-specific unidirectional airflow in the lungs of the savannah monitor lizard (Varanus exanthematicus). The presence of unidirectional flow in the lungs of V. exanthematicus thus gives rise to two possible evolutionary scenarios: (1) unidirectional airflow evolved independently in archosaurs and monitor lizards, or (2) these flow patterns are homologous in archosaurs and V. exanthematicus, having evolved only once in ancestral diapsids. If unidirectional airflow is plesiomorphic for Diapsida, this respiratory character can be reconstructed for extinct diapsids, and evolved in a small ectothermic tetrapod during the Paleozoic Era at least 100 million years before the origin of birds.