Birds have evolved an unusual breathing system, and clues to its development lie in the remains of a recently-discovered 10-meter-long predatory dinosaur from the banks of Argentina’s Rio Colorado. The findings of paleontologist Paul Sereno (National Geographic Explorer-in-Residence at University of Chicago) and colleagues are published on September 30 in the open-access journal PLoS ONE.

“Among land animals, birds have a unique way of breathing. The lungs actually don’t expand,” Sereno notes. A bird’s breathing system consists of lungs and air sacs, or bellows. Some 25% of inhaled fresh air goes directly into the lungs, while the remaining 75% bypasses the lungs and flows into air sacs and into bones. Upon exhaling, the air sacs help pump fresh air into the lungs. It is this efficient breathing process that allows birds to fly higher and faster than bats, which expand their lungs inefficiently like all mammals.

Evidence of a similar breathing system was found in a recently discovered dinosaur. In 1996, Sereno and colleagues discovered the new dinosaur and gave it the name Aerosteon riocoloradensis, which means “air bones from the Rio Colorado”. Found in rocks that dated about 85 million years old (the Cretaceous period), Aerosteon survived in isolation in South America. The North American Allosaurus is Aerosteon‘s closest cousin, but they went extinct millions of years earlier and were replaced by tyrannosaurs.

“This dinosaur, unlike any other, provides more direct evidence of the bellows involved in bird breathing,” said co-author Ricardo Martínez (Universidad Nacional de San Juan, Argentina). The dinosaur’s bones have air pockets and a sponge-like texture called “pneumatization,” where air sacs from the lung enter bone. It is these air-filled bones that make it possible for birds to use their bellows breathing system.

Co-author Oscar Alcober (Universidad Nacional de San Juan, Argentina) added that, “Despite its huge body size and lack of a breastbone or birdlike ribcage, this meat-eater had lungs that already functioned quite a bit like a bird’s.” Aerosteon‘s hollow bones in front and behind the ribcage, such as the wishbone (furcula) and the main hip bone (ilium), provided the evidence that highlighted the connection with birds.

Co-author Jeffrey Wilson (University of Michigan) remarks that, “The ancient history of features like air sacs is full of surprising turns, the explanations for which must account for their presence in a huge predator like Aerosteon as well as in a chicken.”

According to Sereno, Aerosteon’s air sacs are also located in a strange place: “They come around the edge of the body and go into belly ribs. It looks like the beast had a system of air tubes under its skin.”

Why would dinosaurs evolve air sacs? The authors suggest that air sacs result in a more efficient lung, reduce upper body weight in tipsy two-legged runners, and help release excess body heat. This last point is particularly intriguing to Sereno – Aerosteon was most likely a high-energy predator that had feathers but did not have sweat glands like birds. Since the dinosaur weighed as much as an elephant, it may have used an air sac system under the skin to release unwanted heat.

Evidence for Avian Intrathoracic Air Sacs in a New Predatory Dinosaur from Argentina
Sereno PC, Martinez RN, Wilson JA, Varricchio DJ, Alcober OA, et al
PLoS ONE (2008). 3(9):e3303.
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Written by: Peter M Crosta