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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere
TRIASSIC-JURASSIC MASS EXTINCTION
Let’s finish this chapter with one last bit of evidence pertaining to the kind of respiration present in late Triassic animals. One of the striking new findings of the latest GEOCARBSULF has been the level of Triassic oxygen. Only several years ago the minimum oxygen levels of the past 300 million years were universally pegged at the Permian-Triassic boundary of 250 million years ago. But that time of low oxygen has been substantially moved and now may correspond more closely than previously thought with the Triassic-Jurassic boundary of 200 million years ago. Thus, we are confronted with the possibility that oxygen was lower in the late Triassic than in the early part of the period—perhaps as low as 13 percent of the atmosphere at sea level, or much less than modern-day levels. This time corresponds to one of the major changes of the Triassic: the winnowing out of most land vertebrates, with the exception of the first dinosaurs, the saurischians—creatures with pneumatized bones. This realization comes from new data compiled for this book by one of my grad students, Ken Williford, who very painstakingly went through the literature describing the various Triassic vertebrates and their ranges. The results of this long search are shown in the diagram below.
In the figure below, the Triassic-Jurassic mass extinction, one of the five most deadly mass extinctions of the past 500 million years, is shown as the horizontal black line in the middle of the figure. Here the saurischian (dinosaurs) are considered to have had air sacs.
The data gathered for this figure showed that every group exceptthe saurischian dinosaurs was undergoing reduction (or at best, maintaining roughly equal diversity) in the time intervals leading up to and after the Triassic-Jurassic mass extinction. The groups with the simplest lungs (amphibians and thecodont reptiles) fared the worst and many groups that had been very successful early in the Triassic, such as the phytosaurs, underwent complete extinction. Both amphibians and thecodonts probably had very simple lungs inflated by rib musculature only. Mammals and advanced therapsids of this time, probably both having diaphragm-inflated lungs, did better, but crocodiles, presumably with abdominal pumps, did poorly. The success of the saurischians may have been due to a multitude of factors (food acquisition, tem-