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AVIAN AIRWAYSThe new dinosaur species Aerosteon riocoloradensis had lungs (red) and air sacs (other colors) similar to those of modern birds. Full StoryTodd Marshall, Project Exploration

Paleontologists have discovered a new species of carnivorous dinosaur that possessed an interesting feature: It breathed like a bird.

Unlike mammals, birds have evolved a highly efficient system of breathing in which multiple bellows, or air sacs, in the rib cage push oxygen-rich air through a fixed lung.

The fossils from this new dinosaur — called Aerosteon riocoloradensis, which means air bones from the Río Colorado, the Argentine river near where they were found — show characteristically avian features, including air-filled bones in the rib cage that show the hallmark imprints of air sacs, the team reports online September 30 in PLoS ONE.

Scientists are left to wonder why a birdlike system of breathing would be used both by a 10-meter long predatory dinosaur that weighed as much as an Indian elephant, and by a chicken.

In birds, air goes in one end of a lung and comes out the other, a system called flow-through lungs. This allows more oxygen to be absorbed than the mammalian way of breathing, in which air comes in and goes out the same way, leaving some stale air behind after each breath. Birds’ small, lightweight lungs are also a big benefit for flying.

"The real trick is understanding how the air sac is useful to both birds and dinosaurs," says study coauthor Jeffrey Wilson of the University of Michigan in Ann Arbor. "We know now that the machinery [of the bird lung] was in place 86 million years ago in animals that look nothing like a bird."

Soft tissues, like lungs, disintegrate long before scientists can find them, but can leave imprints, or "stamps," on hard bones, explains Wilson.

"Since generally all we have is bones in the fossil record, we hit the jackpot with Aerosteon, which shows the presence of these air sacs,” explains study lead author Paul Sereno of the University of Chicago.

The idea of birds and dinosaurs sharing breathing styles has been around for a while. In 2005, a team led by Patrick O’Connor of Ohio University in Athens found a different type of predatory dinosaur with air-filled bones, indicating birdlike breathing. The new fossils from Aerosteon riocoloradensis provide much-needed clarity. "The specimen represents an additional data point to incorporate into existing models," comments O’Connor.

In addition to shedding light on the mysterious beginnings of modern bird features, this newly discovered dinosaur unravels some deeper questions about dinosaur lineages.

"Aerosteon represents a surviving lineage never before found and possibly existing in the Late Cretaceous only on the isolated continent of South America," says Sereno.

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