Take a deep breath. You have pulled air into your lungs: into a network of ever-branching and thinning airways called bronchi, which thin further still into bronchioles and finally terminate in alveolar air sacks where oxygen is exchanged for carbon dioxide and other waste gasses. The used air can only exit your lungs through the same circuitous pathway it came in. Now exhale.

Dinosaurs didn’t breathe like we do. Scientists have long known that birds breathe differently than humans or other mammals. Instead of the two-way in-out model used by mammal lungs, birds use a more efficient and complex system in which air enters and, instead of flowing back out the same way, flows in a “loop.” When birds inhale, the new air is stored in air sacks before passing into the lung. Inside the lung, the air passes from chamber to chamber before being exhaled through the windpipe. Each chamber is able to extract more precious oxygen and the air sacks insure old air and new air aren’t mixed together. Such an efficient way of breathing makes sense for creatures who burn large amounts of oxygen in order to fly, and sometimes even have to fly at oxygen depleted altitudes. But did dinosaurs, who are undeniably closely related to birds, breathe the same way?

In 2010, a study published in the journal Science helped answer the question. Authors Colleen Farmer and Kent Sanders of the University of Utah discovered that alligators breathe in much the same way. Unlike birds, alligators lack the obvious air sacks thought to make one-way breathing possible. They do, however, have multi-chambered lungs which pass air in a loop to extract the maximum amount of oxygen. Such a similar breathing pattern likely stemmed from a common ancestor. Birds and alligators are related and both are related to dinosaurs. All three types of animals are members of the clade Archosauria. Their ancestors diverged from one another sometime during the Triassic.

The biggest question left, at this point in the investigation, was why did dinosaurs breathe in a loop? Oxygen levels in the atmosphere were extremely low during much of the Triassic Period. It is possible that animals living at that time needed to be extremely efficient with the air they breathed, especially animals as large as some dinosaurs.

The mystery deepened, however, with Farmer’s next study, published last December in the journal Nature. Another animal shares the one-way pattern of breathing: the monitor lizard. Monitor lizards are not members of Archosauria. In fact, if monitor lizards, alligators, dinosaurs, and birds all inherited their efficient lungs from a common ancestor, one-way breathing must be at least 20 million years older than previously thought. That could be old enough to place the evolution of the super-lung at a time when atmospheric oxygen levels were actually extremely high, during the Permian.

Even Farmer is at a loss to say why relatively slow moving creatures like lizards and alligators would need to develop multi-chambered lungs during an oxygen-rich time period. One possibility is that they didn’t. The ancestors of the monitor lizard may have developed one-way breathing on their own, without any relation to Archosauria. The lungs of the monitor lizard lack valves that push the air from one chamber to the next, a difference that could indicate that other similarities are deceptive. Further studies should reveal which other reptiles share breathing patterns with the monitor and the alligator. Measuring airflow in the tiny lungs of a gecko could be quite the challenge, but it may tell us if breathing in loops was what gave dinosaurs the edge they needed to become so large and so diverse.

Source:

Emma R. Schachner, Robert L. Cieri, James P. Butler & C. G. Farmer. Unidirectional pulmonary airflow patterns in the savannah monitor lizard. Nature, 2013 DOI: 10.1038/nature12871

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