in the other groups. We also know that oxygen reached its lowest levels of the past 500 million years in the late Triassic. Something about saurischians enhanced their survival in a low-oxygen world. In another conversation with John Ruben, I was told that this only shows that the abdominal pump allowed this enhanced survival. But what about the bone pneumaticity shown only by the group that prospered through a very bad time in Earth’s history (at least if you were an air breather). And why did the other groups supposed to have had abdominal pumps do so badly—such as the phytosaurs, which went completely extinct?

The ground truth suggests that a long and slow drop in oxygen culminated in the Triassic mass extinction but that this extinction was really a double event, separated by between 3 and 7 million years. There are few places on land where this time interval with abundant vertebrate fossils can be found. We really do not know the pattern of vertebrate extinction as well as we do for the extinction in the sea. We do not know how rapidly the prominent vertebrate victims of the mass extinction such as the phytosaurs, aetosaurs, primitive thecodonts, tritylodont therapsids, and other large animals disappeared. But by the time that the gaudy Jurassic ammonites appeared in the seas in abundance, leaving behind an exuberant record of renewal in early Jurassic rocks, the dinosaurs had won the world.

What kind of lungs did they have? Here it can be proposed that they had lungs and a respiratory system that could deal with the greatest oxygen crisis the world was to know in the time of animals on Earth. Let’s formalize this:

Hypothesis 8.4: Saurischian dinosaurs had a lower extinction rate than any other terrestrial vertebrate group because of a competitively superior respiration system—the first air sac system.

Support for this hypothesis comes from the new anatomical findings of an air sac system in saurischians and the newly presented data (above) that saurischians were actually expanding in number across this mass extinction boundary. They emerged—perhaps gasping, but still standing, over the corpses of the simpler lunged—in a world we will look at in the next chapter.

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