Smith et al. (2006) carried out the first higher-level
pteridophyte classification published in the
molecular phylogenetic era, and considered the ferns (monilophytes), with four
classes. They placed the whisk ferns and related taxa in the class Psilotopsida, with two
orders.
Mark W. Chase and
James L. Reveal (2009) classified them as two separate subclasses, Psilotidae and Ophioglossidae, corresponding to those orders within a much broader grouping, the class
Equisetopsida sensu lato. Christenhusz
et al., 2011, included both the
Ophioglossales and
Psilotales orders in the Ophioglossidae subclass. This was continued by both
Christenhusz and
Chase (2014) and by the Pteridophyte Phylogeny Group (2016). Under the latter the subclass is one of four subclasses of
Polypodiopsida (ferns) and contains two orders, two families, 12 genera, and an estimated 129 species. The relationships between the two orders, Psilotales and Ophioglossales, has long been unclear and was only confirmed by molecular systematic studies. Psilotales have
rhizomes instead of real roots, and the roots of Ophioglossales lack both branching and
root hairs. The
gametophytes of both orders are
heterotrophic and often subterranean, obtaining nutrients from
mycorrhiza instead of light.
Photosynthesis happens exclusively in the
sporophyte. The following cladogram shows a likely phylogenic relationship between subclass Ophioglossidae and the other Polypodiopsida subclasses. The first three small subclasses are sometimes informally referred to as
eusporangiate ferns, in contrast to the largest subclass, Polypodiidae or
leptosporangiate ferns. }}}} The two orders,
Ophioglossales and
Psilotales are
sister groups to each other. ==References==