reason: oxygen levels reached their lowest point in the Triassic, coincidentally at the time when a majority of Permian land animals were going extinct (leaving many empty niches and thus wholesale evolutionary and ecological opportunity), and many vertebrates responded to these two factors by rapidly producing a host of new kinds of vertebrate body plans—and respiration systems as well. The most famous of the new Triassic body plans was a bipedal form that we call dinosaurs.
Because of its general interest and rather sensational aspects, perhaps the most commonly asked question about dinosaurs is the manner of their extinction. The 1980 hypotheses by the Alvarez group that Earth was hit 65 million years ago by an asteroid and that the environmental effects of that asteroid rather suddenly caused the Cretaceous-Tertiary mass extinction in which the dinosaurs were the most prominent victims, keeps this question paramount in people’s minds. The fact that this controversy is rekindled every several years by some new finding brings it to the surface once again. Thus its preeminence even supercedes the question of whether or not the dinosaurs were warm-blooded. Way down on the list of questions about dinosaurs is the inverse of the extinction question—not why they died out, but why they evolved in the first place.
We know when they first appeared, in the second third of the Triassic Period (some 235 million years ago), and we know what these earliest dinosaurs looked like: most were like smaller versions of the later and iconic Tyrannosaurus rex and Allosaurus. They were bipedal forms that quickly became large. What has not been largely known or even considered is the new understanding that 230 million years ago was the time when oxygen may have been nearing its lowest level since the Cambrian Period.
So here is a new view here: dinosaurs evolved during, or immediately before, the Triassic oxygen low, a time when oxygen was at its lowest value of the last 500 million years—and their body plan is a result of adaptation to low oxygen.