Climatically driven biogeographic provinces of Late Triassic tropical Pangea. 2011. J. Whiteside, et al. PNAS, Published online before print May 13.
More than 200 million years ago, mammals and reptiles lived in their own separate worlds on the supercontinent Pangaea, despite little geographical incentive to do so. Mammals lived in areas of twice-yearly seasonal rainfall; reptiles stayed in areas where rains came just once a year. Mammals lose more water when they excrete, and thus need water-rich environments to survive. link
