Nothing looks as lazy as an alligator on a hot summer day, but new analyses of the reptile's lungs show that they're structured surprisingly like those of birds, which researchers had thought were unique. The findings could help to explain why the archosaurs--the ancestors of crocodilians, dinosaurs, and modern birds--emerged to dominate the world some 250 million years ago.
As well-studied as dinosaurs have been over the past couple of centuries, paleontologists have learned little about their breathing, because lungs do not fossilize. One way to fill the gap and to deduce dinosaurs' respiratory physiology is to examine the lungs of their closest living descendants--birds. In the lungs of mammals, air flows in and out of small sacs, where oxygen is extracted and carbon dioxide released. But birds breathe in one direction, like a jet engine, with the cells of the lungs absorbing oxygen from the air as it passes by. The arrangement ensures that the lungs continually contain fresh air and is ideal for a creature that spends much of its waking life in flight, which requires lots of oxygen.
But what about other dinosaur relatives that are alive today? Evolutionary biologists C. G. Farmer and Kent Sanders of the University of Utah in Salt Lake City attempted to determine how the lungs of crocodilians--the group that includes crocodiles and alligators--work. "Birds and crocs are sister groups," Farmer says. "If we find features that are similar in birds and crocs, it is probably because they were inherited from their common ancestor."
Over the past 5 years, the two researchers tracked airflow patterns in the animals' lungs by using MRI scans on living gators and pushing fluids containing tracking agents through lungs removed from dissected specimens. In today's issue of Science, the pair reports that, like those of birds, alligator lungs also use one-directional airflow. Instead of moving into and out of air sacs as in mammals, the air flows continuously through the lungs. It enters the windpipe and moves through the lungs toward the tail, then back out the nostrils through the windpipe. The structure allows alligators, when they need it, to move a lot more air through their lungs and absorb a lot more oxygen than mammals can. The trait "is not unique to birds," Farmer says.
"It's an absolutely spectacular finding," says morphologist Adam Summers of the University of Washington, Seattle, who was not involved in the research. "I'm shocked that we didn't know how alligators breathed." Summers says that one-way flow could explain why archosaurs "went from bit players to dinos" when Earth's oxygen levels dipped suddenly about 250 million years ago following a major extinction event. The ability to force a lot of oxygen into the lungs, he explains, meant that the animals suddenly were better adapted to the thinner air than were the mammal ancestors that were alive at the same time.
There's another possibility, Summers says. "We don't know how other critters breathe--lizards, for example." It could be that unidirectional airflow is the pattern for all nonmammalian land vertebrates, he says. That would make mammals, rather than birds, unique in the way they breathe. "It reopens a lot of questions that we thought were closed," he says.