Dinosaurs had always been built to fly. It started with their commitment to bipedalism. Their forelimbs, no longer required for running, were reduced, the hands left free for other activities, such as grasping prey, climbing, or flying. In sauropods, dinosaurs reverted to being quadrupeds and became the largest land animals ever to have lived, some measuring more than 50 meters long and weighing more than 70 tons. But dinosaurs also excelled at being small. The peculiar, bat-like Yi was no bigger than a starling.
How did dinosaurs get to be so very large—and so very small?
It began with the way they breathed. Dinosaurs and their immediate relatives evolved a one-way system for air handling, which made breathing very efficient. Air entered the lungs but did not immediately come out again. Instead, the air was shunted, guided by one-way valves, through an extensive system of air sacs throughout the body. Air spaces surrounded the internal organs and even penetrated the bones. Dinosaurs were literally full of air.
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