The fossil record shows that ferns originated more than 360 million years ago, and between 300 and 100 million years ago they were the most diverse group of plants (see Fig. 33.2). Then, as angiosperms evolved, their diversity markedly decreased. It is not clear why angiosperm evolution should have so affected ferns and horsetails, which had coexisted with a variety of seed plant groups for more than 200 million years.
However, the most diverse group of ferns today evolved after the rise of the angiosperms. These are the polypod ferns (about 9300 species; see Fig. 33.11f). Thus, many of the fern species present today are likely to have evolved to occupy habitats newly created by the development of angiosperm forests. Approximately 40% of living ferns are epiphytes, many of them growing in tropical rain forests dominated by angiosperm trees.
Polypod ferns may owe some of their evolutionary success to an enhanced capacity for spore dispersal. Horsetails, whisk ferns, and a few other fern groups make large sporangia similar to those produced by early vascular plants. Most ferns, however, have a distinctive sporangium (called a leptosporangium) whose wall is only a single cell thick. Polypod ferns have leptosporangia with a line of thick-