Avian dinosaurs—aka birds—have a streamlined way of breathing. Instead of sending air in and out of tiny sacs in the lungs like some other animals do, their breath flows in a single direction through a series of tubes. A new study reveals that birds are not alone in this adaptation: alligators also rely on this one-way inhale/exhale, suggesting that this form of respiration emerged a lot earlier in evolutionary time than had been previously thought.
These findings, published online January 14 in Science, indicate that this method of breathing likely emerged more than 246 million years ago, during the Triassic period, before the lineage that gave rise to alligators and birds split—rather than in later bird relatives.
"Our data provide evidence that unidirectional flow predates the origins of pterosaurs, dinosaurs and birds and evolved in the common ancestor of the crocodilian and bird lineages," Collen Farmer, an assistant professor of biology at the University of Utah in Salt Lake City and principal researcher, said in a prepared statement. (The precise common ancestor of birds and crocodilians, an archosaur, remains unknown, but Farmer speculates that it might have been "a small, relatively agile, insect-eating animal.")
Today, having this unidirectional airflow helps birds soar to heights that Farmer said would "render mammals comatose." But could this little breathing trick have helped both the bird's flightless ancestors and the ancient crocodilians outlast others?
"The real importance of this air-flow discovery in gators is it may explain the turnover in fauna between the Permian and the Triassic," said Farmer.
"Many archosaurs, such as pterosaurs, apparently were capable of sustaining vigorous exercise" despite a relatively oxygen-poor atmosphere, Farmer said. At that point in time, the planet was hot and dry, containing about 12 percent oxygen (compared to current levels of 21 percent) in the atmosphere, and a unidirectional flow might have meant better oxygen-intake efficiency in this harsher environment. "Lung design may have played a key role in this capacity because the lung is the first step in the cascade of oxygen from the atmosphere to the animal's tissues."
The researchers were tipped off to this deep link by some anatomical similarities among bird and alligator lungs. Living members of the Crocodilia order, which includes today's crocs and gators, have long been a useful reference for evolutionary study because they have changed little in the millions of years they've been around. To confirm the respiration suspicions, Farmer and her colleague Kent Sanders, of the University of Utah Health Sciences Center, examined air flow through the lungs of live (though sedated) and dead (donated) alligators. They also removed some lungs and filled them with saline that contained small fluorescent beads to better understand how the fluid would move inside the lungs. Examining the fluid flow through all of these lungs, Farmer and Sanders concluded that substances were moving "in a strikingly bird-like pattern."
Previous research has suggested that dinosaurs breathed like birds, but these new findings seem to indicate that even before the dinos came along, the lungs of early archosaurs weren't waiting to exhale.
Image of CT scan of American alligator courtesy of C. G. Farmer