Based on the fossil record, at least five major and global mass extinction events have occurred during the past 542 million years. The Permian-Triassic extinction event, sometimes informally called the Great Dying, was an extinction event that occurred approximately 251 million years ago. It was the Earth’s most severe extinction event, with about 90 percent of all marine species and 70 percent of terrestrial vertebrate species going extinct.
About 20 million years after the Great Dying, there were three major designs of animals; the crocodilians, the mammals and the dinosaurs. Dr. Peter Ward, biologist and earth scientist, gave an astonishing theory why dinosaurs survived and dominated the Earth, while the mammals and crocodilians merely survived.
He says that the evolution of the air sac system or what we call today ‘bird lungs’, which first appears in the earliest dinosaurs, may have been a response to the oxygen-depleted environment. Birds have lungs with special air sacs to hold the incoming and outgoing air. The oxygen-rich incoming air is kept totally separate from the oxygen-depleted outgoing air. The amount of oxygen is more efficiently extracted from the incoming air as compared to the lungs of the crocodiles or the mammals. Dinosaurs emerged and evolved because of this unusual respiration system with their ability to adapt to Earth’s changing environment.
The oxygen level in the air back some 230 million years ago was around 12% to 16%, which is less than the 21% we enjoy today. The ‘regular’ lungs mix the incoming air that is rich in oxygen, with the oxygen-depleted air that leaves the mouth. These lungs would work fine in 20 percent oxygen, but not so well in the 15 percent or so oxygen that was available then. (Floro Mercene)