as a subclass of Equisetopsida sensu lato. 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 In 2014, Christenhusz and Chase grouped all the fern subclasses together as Polypodiophyta 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. Subdivision In both the Christenhusz and Chase, and the PPG classification, the extant Polypodiidae are divided into seven orders, 44 families, 300 genera, and an estimated 10,323 species. Phylogenetic relationships The following phylogram shows a likely relationship between the other vascular plant classes and the leptosporangiate ferns. It was formerly unclear about the relationship between Equisetopsida, Psilotopsida, and Marattiopsida, but recent studies have shown that Equisetopsida is most likely sister to Psilotopsida. Discussion of molecular classification 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. 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. 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. Extinct families 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 (). 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: Anachoropteridaceae Botryopteridaceae Kaplanopteridaceae Psalixochlaenaceae Sermayaceae Skaaripteridaceae Tedeleaceae Tempskyaceae References Bibliography </ref>