Edaphic Aridity as a Factor in Angiosperm Evolution

Ancient terranes of granitic and metamorphic rocks regularly form steep-sided mountains (bornhardts), domes, ridges, and plains. They provide dry edaphic sites in climatic regions ranging from wet tropical to desert, enabling edaphic deserts to occur in proximity to tropical rain forest, savanna and other types of vegetation, including those in temperate climates. Similar dry edaphic sites have been present throughout angiosperm history. Their scattered distribution and steep gradients of decreasing moisture have provided environmental opportunities that have encouraged the origin of taxa adapted to varying grades of drought. These arid sites may also have served as loci from which some taxa readapted to nearby moister sites. During periods of climatic amelioration, as in Cenomanian and later times, dry edaphic sites appear to have been local refugia for unique taxa adapted to drought whose decendants have persisted to the present. Inasmuch as evolutionary rate would tend to accelerate in local dry sites, the preceding relations may account for the rarity of "missing links" in the fossil plant record, for the "sudden appearance" in the record of taxa that are already fully evolved, and for the occurrence today of unique adaptive types that have no fossil record because they developed in dry edaphic sites remote from areas where most of the fossil record accumulated.