Dinosaurs' superior lungs may have allowed them to outcompete early mammals, according to a new study of modern-day alligators.
Scientists found that a method of high-efficiency breathing used by birds is also employed by today's alligators, which share a common ancestor with dinosaurs.
In mammals, each fresh breath carries oxygen-rich air to "cul-de-sacs" in the lungs called alveoli.
Air circulating through these sacs transfers oxygen into the bloodstream that picks up the blood's carbon dioxide waste.
But birds don't have alveoli. Instead, the air flows in one direction into the birds' air sacs.
This adaptation keeps birds' lungs filled with "fresh" air, allowing them to breathe at altitudes that would kill other animals.
The team found that, similar to birds, air bypasses certain tiers of bronchi, or airways--only to flow back through those bronchi before being exhaled.
Such a pattern likely arose in the common ancestor of birds, dinosaurs, and alligators--called archosaurs--during the Triassic period, 251 to 199 million years ago.
During the Early Triassic Period, the atmosphere was lower in oxygen than it is today.
"We know that in birds this lung structure is part of the reason birds are good at exercising in rarefied air," said study leader C.G. Farmer, an evolutionary biologist at the University of Utah.
"Our data suggest the archosaurs had a competitive edge in their low-oxygen world."
Dinosaurs' high level of fitness could also explain why mammals remained so small until the mass extinction of the dinosaurs 65 million years ago.
"It's as though these animals were being repressed, or kept small, by the archosaurs."
Research appears January 15 in the journal Science.