A palaeontologist considers the evolution of birds' mechanism of breathing.

During both inhalation and exhalation, the air in birds' lungs moves in just one direction, through small tubes. This is unusual: most animals move air tidally, in and out of dead-end gas-exchange structures. The question of when and how the avian breathing mechanism evolved is interesting to palaeontologists like me who study these unusual features.

Traditionally, the avian pattern of one-way breathing has been thought to depend on special accessory air sacs that work similarly to bellows. Largely because they don't have these air sacs, alligators have always been presumed to be tidal breathers. However, this has now been questioned by Colleen Farmer and Kent Sanders at the University of Utah in Salt Lake City, who suggest that alligators actually breathe like birds (C. G. Farmer & K. Sanders Science 327, 338–340; 2010).

By measuring air and water flows in the lungs of anaesthetized and dead alligators, respectively, the authors demonstrate unidirectional flow. They draw the reasonable inference that this bird-like breathing is characteristic of the archosaurs, a broad group that includes both alligators and birds.

The finding is leading to changes in the direction of palaeontological research. Farmer and Sanders' results imply that air sacs are not essential for unidirectional breathing. The function of these sacs in extinct ancestors of birds — dinosaurs such as theropods — should thus be reconsidered.

Unidirectional breathing probably appeared among ancestral archosaurs during the Early Triassic period, some 250 million years ago, a time of low oxygen levels that might have encouraged evolutionary experimentation with improved ventilation. This raises the question of whether the drastic conditions led to other notable changes in Triassic animals.

Discuss this paper at http://blogs.nature.com/nature/journalclub