The supercontinent Pangaea showing regions of low oxygen.
A depiction of the supercontinent Pangaea some 250 million years ago shows regions where oxygen was insufficient for animal survival. Because oxygen levels decrease with increasing altitude, the more mountainous "hatched" areas would have prevented intermingling and reproduction between nearby animal populations.
Credit: Nicolle Rager, National Science Foundation, based on Pangaea map data, Paleogeographic Atlas Project, University of Chicago
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