Leptosporangiate fern

The leptosporangiate ferns were first recognized as a group, the "Leptosporangiateen", by Karl Ritter von Goebel in 1881, who placed the eusporangiate ferns with seed plants and vascular plants into a coeval "Eusporangiateen". As this classification artificially split the ferns, Christian Luerssen subdivided the homosporous ferns only into Eusporangiatae and Leptosporangiatae in 1884–9. The latter group was treated at a variety of ranks in subsequent systems of classification. The subclass "Polypodiidae" was first published and used for the homosporous leptosporangiate ferns by Cronquist, Takhtajan and Zimmermann in 1966, typified on Polypodium L.. Other contemporary classifications used the name "Filicidae" for this subclass.[10]

Smith et al. (2006) carried out the first higher-level classification of ferns based on molecular phylogenetics. They included heterosporous water ferns (Salviniales) (placed in a separate subclass by Cronquist et al. due to their highly modified morphology) within the leptosporangiate ferns, which they elevated to the rank of class as the Polypodiopsida (published by Cronquist et al. to include all ferns).[5]

The common ancestor of Salviniales, Cyatheales and Polypodiales went through a whole genome duplication.[11]

Later classifications renamed the group Polypodiidae, initially as a subclass of Equisetopsida sensu lato.[6] This subclass comprises leptosporangiate ferns as opposed to the remaining three subclasses which are informally referred to as eusporangiate ferns. The following diagram shows a likely phylogenic relationship between subclass Polypodiidae and the other Equisetopsida subclasses in that system[4]

In 2014, Christenhusz and Chase grouped all the fern subclasses together as Polypodiophyta[3] and in 2016 the Pteridophyte Phylogeny Group (PPG) adopted the class Polypodiopsida sensu lato for the four fern subclasses. The following cladogram shows the phylogenic relationship between the subclasses according to the PPG. The first three small subclasses being informally grouped as eusporangiate ferns, in contrast to the Polypodiidae or leptosporangiate ferns. Polypodiidae is shown as a sister group of Marattiidae.[1]

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.[17] However, the molecular studies have clarified relations among families that had already been thought to be polyphyletic before the advent of molecular information but that were left in their polyphyletic ranks because there was not enough information to do otherwise.[18] The classification of ferns using these molecular studies, which have generally supported one another, reflects the best information available at present, because traditional morphological characters are not always informative in elucidating evolutionary relationships among ferns.[3]

The leptosporangiate ferns have a substantial fossil record. For example, fossils assigned to the Dicksoniaceae, a member of the Cyatheales, are known from the Lower Jurassic (201 to 175 million years ago).[19] A number of other extinct families have been described. They are not included in the classification systems used for extant ferns, and so most cannot be assigned to orders used in these systems. Taylor et al. (2009) use the order "Filicales", which corresponds to four Polypodiidae orders in more modern systems: Hymenophyllales, Gleicheniales, Schizaeales and Cyatheales. The unplaced families include:[20]

• Anachoropteridaceae
• Botryopteridaceae
• Kaplanopteridaceae
• Psalixochlaenaceae
• Sermayaceae
• Skaaripteridaceae
• Tedeleaceae
• Tempskyaceae