Let's begin by looking back to the middle part of the Triassic period. In this middle-late Triassic world, 215 million years ago, on land at least we seem to arriaved among a veritable smorgasbord of animal body plans. Many quite different kinds of vertebrats inhabit this world. Dog-like creatures walk beneath the conifer- and tree-fern dominated vegetation. They are cyonodonts, carnivorous vrieties, but there are massive herbivores belonging to the same group as well. They are all very mammalian in appearance and behavior, except in one aspect. They move little and seem to tire easily. The carnivores mostly lay in wait, and the herbivores browse stolidly. The cynodonts are not the only mammal-like reptiles here, for rhino-sized dicynodonts also browse the low brushy vegetation. Their odd, name-giving tusks extending from a parrot-like beaked mouth make them look like nothing if our world, and seeing them harkens memories of the late Permian world prior to the great Permian mass extinction. All are panting heavily and give the impression of animals having just engaged in strenuous exercise. But most have been motionless; yet they pant for a good reason. The level of atmospheric oxygen at this time is equivalent to being higher than 10,000 feet in our world. Except that here we are not atop any mountain: the low swamps and nearby arm of the sea attest to our being at sea level.
Out of Thin Air, Ward, P. pgs 161-162.
Okay, so, first off, a retraction. I stated that it was 5k feet and panting. I misremembered.
Secondly, I don't have the time to transcribe the whole scene. I am a reasonablly quick typist, but I have accuracy issues and I do have a job too.
Damnit. I don't have time to write more. Alas.