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The newfound Aerosteon riocoloradensis had both lungs (in red) and air sacs (in other colors), much like modern-day birds.

The 33-foot (10-meter) long dinosaur—described in a September 2008 study—has given scientists new insight into the evolution of bird lungs.
(courtesy Todd Marshall, National Geographic Society)


A new predatory dinosaur with a birdlike breathing system found in Argentina may help scientists better understand the evolution of birds' lung systems.
The elephant-size dinosaur Aerosteon riocoloradensis lived 85 million years ago during the Cretaceous period.

The fossil provides the first evidence of dinosaur air sacs, which pump air into the lungs and are used by modern-day birds, said Paul Sereno, the project's lead researcher and a National Geographic explorer-in-residence. (The National Geographic Society owns National Geographic News.)

Scientists have known dinosaurs used the pumplike apparatus to breathe, but the new find cements the connection between dinosaur and avian evolution, said Sereno, a paleontologist at the University of Chicago.

"This leaves little discussion that air sacs existed and that meat-eaters really do have lung structures that resemble birds," Sereno said.

Building a Case

Verifying that dinosaurs had bird-like breathing systems has been difficult because lungs do not fossilize, according to Sereno.

In Argentina, Sereno's team found the wishbone, hipbone, and stomach ribs of the newly found dinosaur species hollowed out—a telltale sign of air sacs.

Aerosteon had a sophisticated and extensive breathing system, said Brooks Britt, a paleontologist at Brigham Young University who studies birdlike dinosaurs and is familiar with Sereno's research.

Britt studies Allosaurus, the new dinosaur's closest relative, which lived 150 million years ago during the Jurassic period.

The two giants had something in common when it comes to birdlike features.

Allosaurus had air-filled bones in its vertebrae and possibly other bones. These spongelike textured bones are "the hallmark of the bellows system of breathing in birds," Ricardo Martínez of the Universidad Nacional de San Juan in Argentina said in a press release. Martínez, Sereno, and colleagues published their work today in the journal PLoS One.

The Evolution of Flight

Birds have a unique way of breathing in which the lungs don't expand, Sereno explained. Instead, the air sacs do the pumping. "It's the reason birds can fly higher and faster than bats, which, like all mammals, expand their lungs in a less efficient breathing process, he said.

Despite their birdlike breathing systems, Aerosteon and similar dinosaurs lacked the large sternum of birds, a specialized rib arrangement, and birdlike hips necessary for flight, Brigham Young's Britt said.

"It shows that evolution is not a chalk line—there are many dead ends," he added.

The connection to the much older Allosaurus also suggests that these types of breathing systems were around in the Jurassic, and that the Allosaurus may have had a more sophisticated breathing system than is currently known, Britt said.

The new dinosaur probably had feathers, but did not actually fly, Sereno said, suggesting that even though this species was birdlike, feathers and air sacs didn't necessarily evolve for flight.

"We suggest that air sacs played a role in cooling off the animal," Sereno said. "Our conclusion is that feathers were for insulation, keeping the animal warm."

Aerosteon probably didn't have sweat glands, which help keep mammals cool, Sereno said. A warm-blooded, active animal may then develop air sacs to cool off, he said.

The research team also suggests that air sacs could have been used for the development of a more efficient lung or to help balance the weight of the dinosaur's top-heavy, two-legged frame.

Isolated

Finding a dinosaur living in isolation is "remarkable," Sereno said. It's like finding a piece of the Jurassic period in the Cretaceous, he added.

Sereno and his team found the Aerosteon fossils in 1996, along the banks of the Colorado River in Mendoza Province, but were able to study them closely only after an arduous and meticulous process of detaching the bones from solid rock.

While researchers were preparing the hipbone, they noticed a crack, Sereno said. This led them to look at a cross section of the bone, which revealed holes related to the air sacs.

Aerosteon may have been able to reach the size of a Tyrannosaurus Rex, though the specimen found by Sereno's team measures about 33 feet (10 meters).

The research team found crocodiles, among other animals, at the site, where the climate 85 million years ago resembled a "lush, forested area," Sereno said.