There are two evolutionarily distinct groups of modern ferns, the leptosporangiates and the eusporangiates.
In the leptosporangiates, the sporangium develops from one cell and is usually only one cell thick. In the eusporangiates, the sporangium develops from several cells and is usually several cells thick. Most botanists believe that the leptosporangiate and eusporangiate ferns separated evolutionarily in the lower Carboniferous (about 340 million years ago) or earlier. Modern leptosporangiate ferns are often placed into the Filicales class, and eusporangiate ferns into the Marattiales or Ophioglossales classes.
While there is general agreement about the natural division between the leptosporangiate and eusporangiate ferns, there is considerable uncertainty about other relationships among the modern ferns. Thus, there have been many proposed classification schemes. The widespread occurrence of polyploidy (see above) and hybridization (see below) in ferns has complicated the determination of evolutionary relationships.