Data from: 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.