Diversity of tetrapod genera from the middle part of the Triassic to the middle part of the Jurassic, a time interval of some 60 million years, with hypothesized breathing systems for tetrapod orders. Note that of all the orders shown, only the saurischian dinosaurs increase in diversity leading up to and after the Triassic-Jurassic mass extinction. Their then unique air sac system may have been the prime reason for this. Previously unpublished.

perature tolerance, avoidance of predators, reproductive success), but my conclusion is that this group was unique in possessing a highly septate lung with air sacs that was more efficient than the lungs of any other lineage and that in the very low-oxygen world that occurred both before and after the Triassic-Jurassic mass extinction, this respiratory system conveyed great competitive advantages. Under this scenario, the saurischian dinosaurs took over the planet at the end of the Triassic and kept that dominance well into the Jurassic because of superior activity levels, which was related to superior oxygen acquisitions.

We now know that, alone among the many kinds of reptilian body plans of the middle to late Triassic, the saurischian dinosaurs diversified in the face of either static, or more commonly reducing, numbers



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