Mendoza, Argentina. The remains of a new ten-meter-long predatory dinosaur discovered along the banks of Argentina’s Rio Colorado are helping to unravel how birds evolved their unusual breathing system.
In September 2008, palaeontologists, led by the University of Chicago’s Paul Sereno, a National Geographic Explorer-in-Residence, have published an article about their discovery in the online journal Public Library of Science ONE. Joining Sereno to announce the discovery at a news conference in Mendoza, Argentina, held on 29 September 2008, were Ricardo Martinez and Oscar Alcober, both of the Universidad Nacional de San Juan, Argentina.
The discovery of this dinosaur builds on decades of paleontological research indicating that birds evolved from dinosaurs.
“Among land animals, birds have a unique way of breathing. The lungs actually don’t expand,” Sereno said. Instead, birds have developed a system of bellows, or air sacs, which help pump air through the lungs. This is the reason birds can fly higher and faster than bats, which, like all mammals, expand their lungs in a less efficient breathing process.
Discovered by Sereno and his colleagues in 1996, the new dinosaur is named Aerosteon riocoloradensis (meaning “air bones from the Rio Colorado”).
Sereno explained that “Aerosteon, found in rocks dating to the Cretaceous period about 85 million years old, represents a lineage surviving in isolation in South America. Its closest cousin in North American, Allosaurus, had gone extinct millions of years earlier and was replaced by tyrannosaurs.”
Laboratory technicians spent years cleaning and CT-scanning the bones, which were embedded in hard rock, to finally reveal the evidence of air sacs within Aerosteon’s body cavity. Previously, palaeontologists had found only tantalizing evidence in the backbone, outside the cavity of the lungs. “This dinosaur, unlike any other, provides more direct evidence of the bellows involved in bird breathing,” Martinez explained. Its bones have telltale pockets and a sponge-like texture called ‘pneumatisation’, in which air sacs from the lung invade bone. And air-filled bones are the hallmark of the bellows system of breathing in birds. As Alcober noted, “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.” Therefore, Aerosteon provides the evidence needed to seal the connection with birds — hollow bones in front and behind the ribcage, such as the wishbone (furcula) and the main hip bone (ilium).
Co-author of the PLoS ONE article, Jeffrey Wilson of the University of Michigan, went on to explain 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.”
Sereno further noted that Aerosteon has air sacs in an unusual 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,” he said.
The team highlighted three explanations for the evolution of air sacs in dinosaurs in their paper:
- The development of a more efficient lung.
- The reduction of upper body mass in tipsy two-legged runners.
- The release of excess body heat.
Sereno is especially intrigued by heat loss, given that Aerosteon was probably a high-energy predator with feathers, but without sweat glands, as in birds. At approximately 30 feet (9m) in length and weighing as much as an elephant, Aerosteon might well have used an air system under the skin to rid itself of unwanted heat.
Also contributing to the PLoS ONE article were David Varricchio of Montana State University and Hans Larsson of McGill University. The expedition that led to the discovery was supported by the National Geographic Society and The David and Lucille Packard Foundation.
For a copy of the research article, see http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0003303