Many other animals changed body plans in response to extremes in oxygen and so too did the dinosaurs, in my view. The dinosaur body plan is radically different from earlier reptilian body plans and appears in virtually a dead heat (and in great global heat) with the oxygen minimum. Perhaps this is a coincidence. But because many of the aspects of “dinosaurness” can be explained in terms of adaptations for life in low oxygen, that seems unlikely.

To formalize this: the initial dinosaur body plan (evolved first by saurischian dinosaurs such as Staurikosaurus and the somewhat younger Herrerasaurus) was in some part in response to the low-oxygen conditions of the time:

Hypothesis 8.1: The initial dinosaur body plan of bipedalism evolved as a response to low oxygen in the middle Triassic. With a bipedal stance the first dinosaurs overcame the respiratory limitations imposed by Carrier’s Constraint. The Triassic oxygen low thus triggered the origin of dinosaurs through the formation of this new body plan.

The fossil record shows that the earliest true dinosaurs were bipedal and came from more primitive bipedal thecodonts slightly earlier in the Triassic. These thecodonts (diapsids) were the ancestors of the lineage giving rise to the crocodiles as well and may have been either warm-blooded or heading that way. Bipedalism was a recurring body plan in this group, and there were even bipedal crocodiles early on. Why bipedalism, and how could it have been an adaptation to low oxygen?

Earlier we saw how even most modern-day lizards cannot breath while they run, and this is due to their sprawling gait. Modern-day mammals show a distinct rhythm by synchronizing breathtaking with limb movement. Horses, jackrabbits, and cheetahs (among many other mammals) take one breath per stride. Their limbs are located directly beneath the mass of the body and to allow this the backbone in these quadruped mammals has been enormously stiffened compared to the backbones of the sprawling reptiles. The mammalian backbone bows slightly downward and then straightens out with running, and this slight up-and-down bowing is coordinated with air inspiration and



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