Dinosaurs May Have Dominated Prehistoric Earth Due to Super-efficient Bird-like Lungs

Dinosaurs were able to thrive in the oxygen-thin atmosphere of prehistoric Earth because they had super-efficient bird-like lungs which may have given them an advantage over their competition, according to a study published in the journal Royal Open Society Science.

Birds—which are descended from a group of dinosaurs known as theropods—have a complex respiratory system which pumps sufficient oxygen around their bodies to sustain powered flight. However, whether this highly effective system first evolved in birds—which are technically living (avian) dinosaurs—or extinct (non-avian) dinosaurs has long remained unclear.

"Dinosaurs are a fascinating group of animals, and there are many open questions on dinosaur biology," Robert Brocklehurst, lead author of the study from the University of Manchester, told Newsweek. "How fast could they run? Were they warm-blooded? How did dinosaurs breathe? [These questions] don't get asked as often, but they are intimately linked with lung evolution, because oxygen availability sets a fundamental limit on what an animal is capable of."

"The respiratory system of non-avian dinosaurs has been the topic of considerable study over the years, both in an attempt to shed light on the biology of now extinct members of the dinosaur family, and in order to understand the origins and evolution of modern birds and reptiles," Brocklehurst said in a statement.

For the study, the researchers used a CT scanner to compare the lung cavity of 16 dinosaurs to those of 29 modern-day bird species, such as the ostrich, and four living crocodilians (like crocodiles and alligators)—a group that shares a common ancestor with both birds and dinosaurs.

"We went to museums in the U.K. and the U.S. to take 3D scans and photographs of bones from different dinosaurs, and compared these to bones from their closest living relatives, birds and crocodilians," Brocklehurst said. "We were particularly interested in the shape of the ribs and the vertebrae, as these act as structural supports around the lung and correlate with lung structure."

The scans revealed that all of the dinosaurs had vertebrae that were more similar in shape to those of birds than reptiles. Furthermore, dinosaur vertebrae intruded into the lung cavity, much like in living birds.

"In birds, the lung cavity is furrowed, with the ribs jutting out from the vertebrae and embedding themselves in the lung tissue," Brocklehurst said. "This helps to provide structural support for birds' lungs, which are immobile and ventilated by special air sacs."

Artist's illustration of dinosaurs. Chinese Academy of Sciences

"We thought some of the dinosaurs would have lungs more like birds, and others would be similar to reptiles, but this wasn't the case at all. Every dinosaur sample we scanned just looked like the birds we scanned," Brocklehurst said.

In addition, to the CT scans, the researchers examined the lungs removed from an alligator and an ostrich, finding that the bone support around the organs was very different in each animal.

While the alligator's lung cavity was smooth, allowing the lungs and other internal organs to glide as they pump air in and out, that of the ostrich was found to be furrowed—or folded—like in the dinosaurs.

This analysis suggests that all non-avian dinosaurs have costovertebral joints—the joints that connect the ribs to the vertebral column—that are more similar in structure to birds than to crocodilians.

"Our key findings were that dinosaurs had similar bony anatomy as living birds, with the ribs and vertebrae articulating in a similar way and providing similar structural support for the lungs," he said. "From this, we can infer dinosaurs had a bird-like lung. Unfortunately the only way to definitively prove this result would be to find fossilized lung tissue, which for dinosaurs no-one has."

The efficient bird-like lungs, would likely have enabled dinosaurs to adapt and thrive in the oxygen-thin atmosphere of the Mesozoic era (252 to 66 million years ago).

"Birds' lungs are particularly efficient at extracting oxygen from air even when oxygen concentration in the air is relatively low—like at altitude, for example," Brocklehurst said. "Back in the Mesozoic era, when dinosaurs ruled the world, the air had less oxygen in it than today. It's possible that bird-like lungs allowed dinosaurs to cope with this low oxygen better than other animals."

"A bird-like lung would also have given dinosaurs the oxygen they needed to sustain a fast metabolism and lots of physical activity," he said. "This has implications for how we generally think of dinosaurs—not sluggish, cold-blooded and 'reptilian' but active, warm-blooded and more bird-like.

Because birds are modern dinosaurs, it would not have been surprising necessarily if some of the more advanced, more bird-like dinosaurs had bird-like lungs (think Velociraptor for instance), according to Brocklehurst. What was surprising, the researchers say, is that they found evidence for bird-like lungs in all dinosaurs, even the really early more primitive ones.

"If even the very first dinosaurs to evolve had bird-like lungs, this goes some way to explaining why dinosaurs became the dominant animal species of their time," Bill Sellers, a co-author of the study from the University of Manchester, said in the statement.

"Other animal groups simply may not have had lungs as well suited to extracting oxygen from the air. That simple evolutionary difference may have let dinosaurs rule the world."

This article has been updated to include additional comments from Robert Brocklehurst.