( d in the last part of the Triassic peri...)Steep oxygen decline halted first land colonization by Earth's sea creatures

"Dinosaurs thrived and nothing else did. There's an explanation for that, and it is that the air sac breathing system in dinosaurs and their descendants, modern birds, is more efficient than systems used by other organisms," Ward said.

He and his colleagues tested that hypothesis by examining the breathing system used by birds. They found that at sea level birds breathe 30 percent more efficiently than mammals and at 5,000 feet in elevation birds are 200 percent more efficient.

Ward pictures a world in which dinosaurs were able to adapt to low atmospheric oxygen content relatively easily, and when oxygen levels rose the dinosaurs developed into giant creatures that dominated the Earth.

"I think of dinosaurs as the high-altitude Denver athletes of their day. They ran rings around their prey," he said.

Ward also began to wonder whether respiratory needs dictated how other organisms' bodies developed. He thought that perhaps, rather than being based on feeding and movement, body shape and design might largely be determined by respiratory efficiency. For instance, a mollusk shell is typically thought of as protection for the marine creature, he said, but it turns out the shell actually channels water across the gill to deliver oxygen.

"An unshelled mollusk has a far greater respiratory problem than a shelled mollusk," Ward said. "In many groups the shell is an active part of the respiratory system."