Pteridophyte

Of the pteridophytes, ferns account for nearly 90% of the extant diversity.[2] Smith et al. (2006), the first higher-level pteridophyte classification published in the molecular phylogenetic era, considered the ferns as monilophytes, as follows:[3]

• Division Tracheophyta (tracheophytes) - vascular plants
  • Subdivision Lycopodiophyta (lycophytes) - less than 1% of extant vascular plants
  • Sub division Euphyllophytina (euphyllophytes)
    • Infradivision Moniliformopses (monilophytes)
    • Infradivision Spermatophyta - seed plants, ~260,000 species

where the monilophytes comprise about 9,000 species, including horsetails (Equisetaceae), whisk ferns (Psilotaceae), and all eusporangiate and all leptosporangiate ferns. Historically both lycophytes and monilophytes were grouped together as pteridophytes (ferns and fern allies) on the basis of being spore-bearing ("seed-free"). In Smith's molecular phylogenetic study the ferns are characterised by lateral root origin in the endodermis, usually mesarch protoxylem in shoots, a pseudoendospore, plasmodial tapetum, and sperm cells with 30-1000 flagella.[3] The term "moniliform" as in Moniliformopses and monilophytes means "bead-shaped" and was introduced by Kenrick and Crane (1997)[4] as a scientific replacement for "fern" (including Equisetaceae) and became established by Pryer et al. (2004).[5] Christenhusz and Chase (2014) in their review of classification schemes provide a critique of this usage, which they discouraged as irrational. In fact the alternative name Filicopsida was already in use.[6] By comparison "lycopod" or lycophyte (club moss) means wolf-plant. The term "fern ally" included under Pteridophyta generally refers to vascular spore-bearing plants that are not ferns, including lycopods, horsetails, whisk ferns and water ferns (Marsileaceae, Salviniaceae and Ceratopteris), and even a much wider range of taxa. This is not a natural grouping but rather a convenient term for non-fern, and is also discouraged, as is eusporangiate for non-leptosporangiate ferns.[7]

However both Infradivision and Moniliformopses are also invalid names under the International Code of Botanical Nomenclature. Ferns, despite forming a monophyletic clade, are formally only considered as four classes (Psilotopsida; Equisetopsida; Marattiopsida; Polypodiopsida), 11 orders and 37 families, without assigning a higher taxonomic rank.[3]

Furthermore, within the Polypodiopsida, the largest grouping, a number of informal clades were recognised, including leptosporangiates, core leptosporangiates, polypods (Polypodiales), and eupolypods (including Eupolypods I and Eupolypods II).[3]

In 2014 Christenhusz and Chase, summarising the known knowledge at that time, treated this group as two separate unrelated taxa in a consensus classification;[7]

• Lycopodiophyta (lycopods) 1 subclass, 3 orders, each with one family, 5 genera, approx. 1,300 species
• Polypodiophyta (ferns) 4 sublasses, 11 orders, 21 families, approx. 212 genera, approx. 10,535 species
  • Subclass Equisetidae Warm.
  • Subclass Ophioglossidae Klinge
  • Subclass Marattiidae Klinge
  • Subclass Polypodiidae Cronquist, Takht. & Zimmerm.

These subclasses correspond to Smith's four classes, with Ophioglossidae corresponding to Psilotopsida.

The two major groups previously included in Pteridophyta are phylogenetically related as follows:[7][8][9]

Pteridophytes consist of two separate but related classes, whose nomenclature has varied.[3][10] The system put forward by the Pteridophyte Phylogeny Group in 2016, PPG I, is:[2]

• Class Lycopodiopsida Bartl. – lycophytes: clubmosses, quillworts and spikemosses; 3 extant orders

• Order Lycopodiales DC. ex Bercht. & J.Presl – clubmosses; 1 extant family
• Order Isoetales Prantl – quillworts; 1 extant family
• Order Selaginellales Prantl – spikemosses; 1 extant family

• Class Polypodiopsida Cronquist, Takht. & W.Zimm. – horsetails and ferns; 11 extant orders
  • Subclass Equisetidae Warm. – horsetails; 1 extant order, family and genus (Equisetum)
    • Order Equisetales DC. ex Bercht. & J.Presl – 1 extant family
  • Subclass Ophioglossidae Klinge – 2 extant orders
    • Order Psilotales Prant – whisk ferns; 1 extant family
    • Order Ophioglossales Link – grape ferns; 1 extant family
  • Subclass Marattiidae Klinge – marattioid ferns; 1 extant order
    • Order Marattiales Link – 1 extant family
  • Subclass Polypodiidae Cronquist, Takht. & W.Zimm. – leptosporangiate ferns; 7 extant orders
    • Order Osmundales Link – 1 extant family
    • Order Hymenophyllales A.B.Frank – 1 extant family
    • Order Gleicheniales Schimp – 3 extant families
    • Order Schizaeales Schimp. – 3 extant families
    • Order Salviniales Link – 2 extant families
    • Order Cyatheales A.B.Frank – 8 extant families
    • Order Polypodiales Link – 26 extant families

In addition to these living groups, several groups of pteridophytes are now extinct and known only from fossils. These groups include the Rhyniopsida, Zosterophyllopsida, Trimerophytopsida, the Lepidodendrales and the Progymnospermopsida.

Modern studies of the land plants agree that all pteridophytes share a common ancestor with seed plants. Therefore, pteridophytes do not form a clade but constitute a paraphyletic group.