The concentration of oxygen in the atmosphere is currently 21 percent, but it has been both higher and lower during the Phanerozoic

Question

At the start of the Permian it reached a maximum of 30 percent to 32 percent and then declined throughout the Permian and Triassic, reaching its minimum of 13 percent to 15 percent at the end of the Triassic. Dinosaurs and mammals both appeared in the Triassic, but dinosaurs flourished and radiated into a variety of large-bodied lineages, whereas mammals remained small and did not radiate into multiple lineages. Recall the characteristics of the respiratory systems of sauropsids and synapsids described in Chapter 11 to explain how the low concentration of atmospheric oxygen in the Permian may have favored the development of dinosaurs over mammals.

Answer 1

The evidence of postcranial skeletal pneumatization in all the major lineages of Mesozoic archosaurs suggests that these animals had sauropsid faveolar lungs and a through-flow pattern of ventilation like that of extant birds. The avian respiratory system extracts oxygen from air more effectively than the alveolar lungs and tidal ventilation system of mammals, and that difference in respiration may have contributed to the success of dinosaurs and pterosaurs in the Mesozoic.
Source
Ward, P., and R. Berner. 2011 . Why were there dinosaurs? Why are there birds? In G. Dyke and G. Kaiser (eds.), Living Dinosaurs, the Evolutionary History of Modern Birds. Chichester, West Sussex, UK: Wiley, 30-38