[2][3][4] The group has also been treated as the class Pteridopsida or Polypodiopsida,[5] although other classifications assign them a different rank.

[6] Older names for the group include Filicidae and Filicales, although at least the "water ferns" (now the Salviniales) were then treated separately.

The mature sporangia have a wall that is just a single cell thick,[8] and are typically covered with a scale called the indusium, which can cover the whole sorus, forming a ring or cup around the sorus, or can also be strongly reduced to completely absent.

[10] Smith et al. (2006) carried out the first higher-level classification of ferns based on molecular phylogenetics.

[11] Later classifications renamed the group Polypodiidae, initially as a subclass of Equisetopsida sensu lato.

There has been some challenge to recent molecular studies, claiming that these provide a skewed view of the phylogenetic order because they do not take into account fossil representatives.

For example, fossils assigned to the Dicksoniaceae, a member of the Cyatheales, are known from the Lower Jurassic (201 to 175 million years ago).