the hypothesis of long-term atmospheric oxygen reduction across the Permian-Triassic time interval.

HABITABLE LAND AREA AT THE END OF THE PERMIAN

A last aspect of the Permian extinction relates to altitude and oxygen. Just as oxygen content diminishes with increasing altitude in our world, so too would a change in altitude during any time in the past act in analogous fashion. But with the rise or drop in oxygen levels, paleoaltitude and its effects on the distribution of organisms would greatly change. Mountain ranges in our world often are barriers to gene exchange, producing different biota on either side of the range. At the end of the Permian just living at sea level would have been equivalent today to breathing at 15,000 feet, a height greater than that found atop Mount Rainer in Washington State. Thus even low altitudes during the Permian would have exacerbated this, so that even a modest set of hills would have isolated all but the most low-oxygen-tolerant animals. The result would be a world composed of numerous endemic centers hugging the sea-level coastlines. The high plateaus of many continents may have been without animal life save for the most altitude tolerant. This goes against expectation based on continental position.

Because the continents 250 million years ago were all merged into one gigantic supercontinent (named Pangea), we would expect a world where there were very few terrestrial biotic provinces, since animals would be able to walk from one side of the continent to the other without an Atlantic Ocean in the way. But altitude became the new barrier to migration, and new studies of various vertebrate faunas appear to show a world of many separate biotic provinces, at least on land. The work of Roger Smith and myself in the Karoo desert, of Mike Benton in Russia, of Christian Sidor in Niger, and of Roger Smith in Madagascar showed that each of these separate localities had distinct and largely nonoverlapping faunas, as predicted by the Huey and Ward model of altitude. This can be formalized as follows:

Hypothesis 7.3: During times of low oxygen, altitude creates barriers to migration and gene flow. Low-oxygen times there-



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