 The palaeontologist Peter Ward has argued that the evolution of the air sac system, which first appears in the very earliest dinosaurs, may have been in response to the very low (11%) atmospheric oxygen of the Carnian and Norian ages of the Triassic Period.Birds have spurs called "uncinate processes" on the rear edges of their ribs, and these give the chest muscles more leverage when pumping the chest to improve oxygen supply.The size of the uncinate processes is related to the bird's lifestyle and oxygen requirements