Scientists have unearthed the remains of a large meat-eating dinosaur with a breathing apparatus much like a modern bird, fortifying the link between birds and dinosaurs and helping to explain the evolution of birds' unique system of breathing.
Pulled from 85-million-year-old rock along the banks of Rio Colorado in Argentina's Mendoza Province, this 33-foot-long, two-legged predator weighed as much as an elephant and likely had feathers, the scientists said.
But its method of breathing makes this dinosaur stand out, said Paul Sereno of the University of Chicago, who wrote about the find on Monday in the journal PLoS ONE.
Instead of lungs that expand and contract, Sereno thinks this beast had air sacs that worked like a bellows, blowing air into the beast's stiff lungs, much like modern birds.
"This dinosaur, unlike any other, provides more direct evidence of the bellows involved in bird breathing," Ricardo Martinez of the Universidad Nacional de San Juan, Argentina, who worked with Sereno on the research, said in a statement.
The team named the dinosaur Aerosteon riocoloradensis, meaning "air bones from the Rio Colorado," because its bones have pockets and a sponge-like texture called "pneumatization" in which air sacs from the lung invade the bone.
Most paleontologists believe birds evolved from small, feathered meat-eating dinosaurs, and the earliest known birds were strikingly similar to these dinosaurs.
The researchers think Aerosteon, a type of dinosaur called a theropod, may have evolved this breathing style in part to keep it from toppling over while chasing prey on its two massive legs. And it may have helped control body temperature.
"If dinosaurs and in particular theropods were 'warm-blooded' as many of us suspect and feathered for insulation, they would have had a major problem getting rid of heat at times. Perhaps this is why air sacs initially evolved, and then were co-opted for breathing," Sereno said.
Aerosteon was smaller than the very biggest meat-eaters, which included North America's Tyrannosaurus rex, Africa's Spinosaurus and Giganotosaurus, also found in Argentina.
Sereno thinks Aerosteon represents a separate line of predators that lived alongside and then outlasted Giganotosaurus. "This is one of the nice surprises of the find," he said in an e-mail.