The Triassic world ended abruptly 200 million years ago in another greenhouse mass-extinction event. Those dinosaur families that survived evolved highly efficient mechanisms for breathing in an oxygen-deprived world. These mechanisms may have encouraged bipedalism (think T. rex and modern birds), because in bipedal reptiles, the chest is more open and breathing is not checked by motion the way it is in the waddling walk of four-legged reptiles. During the Jurassic period (from around 200 to 150 million years ago), oxygen levels rose again, until they approached the levels of today’s world.
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