6. See text
The order Polypodiales encompasses the major lineages of polypod ferns, which comprise more than 80% of today's fern species. They are found in many parts of the world including tropical, semitropical and temperate areas. The characteristics of this group include: sporangia with a vertical annulus interrupted by the stalk and stomium; indusia laterally or centrally attached (or lost); gametophytes green, chordate, and surficial.
Historically, the ferns have undergone many different classifications (see review by Christenhusz and Chase, 2014). Smith et al. (2006) carried out the ﬁrst higher-level pteridophyte classiﬁcation published in the molecular phylogenetic era. Smith referred to the ferns as monilophytes, dividing them into four groups. The vast majority of ferns were placed in the Polypodiopsida, and that arrangement has persisted through all subsequent systems, despite some changes in nomenclature. Polypodiopsida is used in the strict sense (sensu stricto) by Smith et al. since it later came to be applied to all ferns (sensu lato), while this large group became known as Polypodiidae. This group is also informally known as the leptosporangiate ferns, while the remaining three groups (subclasses) ar referred to as eusporangiate ferns. The Polypodiidae have been divided into seven orders since that study, the largest of which is Polypodiales. The phylogenetic relationship between the orders of Polypodiidae is shown in this cladogram.
Smith et al. (2006) divided the Polypodiales into fifteen families, a practice continued in their 2008 revision. The Polypodiales were placed in the leptosporangiate ferns, class Polypodiopsida. Fifteen families, including members of the eupolypods, comprising two unranked clades, were recognized: Lindsaeaceae, Saccolomataceae, Dennstaedtiaceae, Pteridaceae, eupolypods I (Aspleniaceae, Thelypteridaceae, Woodsiaceae, Blechnaceae, and Onocleaceae), and eupolypods II (Dryopteridaceae, Lomariopsidaceae, Tectariaceae, Oleandraceae, Davalliaceae, and Polypodiaceae). While many of these families had previously been recognized with similar superscriptions, the authors noted that Dryopteridaceae was more narrowly bounded than in historical circumscriptions, which had included their Tectariaceae, Onocleaceae, and Woodsiaceae. The circumscription of Lomariopsidaceae changed dramatically, most historical genera of that family (except Lomariopsis and Thysanosoria) being moved to Dryopteridaceae, while Cyclopeltis and Nephrolepis were added. Saccolomataceae were removed from the dennstaedtioids. Cystodium was tentatively placed in Lindsaeaceae, away from its historical position with the tree ferns. Woodsiaceae was acknowledged to be of uncertain circumscription and perhaps paraphyletic; the inclusion of Hypodematium, Didymochlaena, and Leucostegia perhaps also rendering Dryopteridaceae paraphyletic. The grammitids were included in Polypodiaceae to render that family monophyletic.
The linear sequence of Christenhusz et al. (2011), intended for compatibility with the classification of Chase and Reveal (2009) which placed all land plants in Equisetopsida, reclassified Smith's Polypodiopsida as subclass Polypodiidae and placed the Polypodiales there. This classification incorporated new phylogenetic evidence to make several changes at the familial level, resulting in an expansion to 21 families. Lonchitis and Cystodium were removed from the Lindsaeaceae and incorporated into new families, Lonchitidaceae and Cystodiaceae respectively. Within eupolypods I, Woodsiaceae proved to be paraphyletic and was reduced to the genera Cheilanthopsis, Hymenocystis, and Woodsia, while the remainder of its genera were removed to Cystopteridaceae, Diplaziopsidaceae, Rhachidosoraceae, Athyriaceae, and Hemidictyaceae. Within eupolypods II, Nephrolepis was placed in a new family, the Nephrolepidaceae, due to uncertainty in its phylogenetic placement, while Hypodematiaceae was split from Dryopteridaceae to contain the three problematic genera mentioned by Smith et al.
The classification of Christenhusz and Chase (2014) dramatically reduced the number of families recognized in this order to eight by "lumping", reducing many families to subfamilies and expanding the circumscription of Aspleniaceae and Dryopteridaceae to encompass all of eupolypods I and eupolypods II, respectively. In the new Aspleniaceae, the former Cystopteridaceae, Rhachidosoraceae, Diplaziopsidaceae, Aspleniaceae, Thelypteridaceae, Woodsiaceae, Athyriaceae, and Blechnaceae became subfamilies Cystopteridoideae, Rhachidosoroideae, Diplaziopsidoideae, Asplenioideae, Thelypteridoideae, Woodsioideae, Athyrioideae, and Blechnoideae, respectively. The former Hemidictyaceae were included in the Asplenioideae, and the Onocleaceae in the Blechnoideae.
In the new Polypodiaceae, the former Hypodematiaceae, Dryopteridaceae, Lomariopsidaceae, Tectariaceae, Oleandraceae, Davalliaceae, and Polypodiaceae became subfamilies Hypodematioideae, Dryopteridoideae, Lomariopsidoideae, Tectarioideae, Oleandroideae, Davallioideae, and Polypodioideae respectively. Didymochlaena was placed in its own subfamily, Didymochlaenoideae.
Their family phylogeny is shown in this cladogram:
The PPG I classification (2016) used a process intermediate between the to two previous approaches, by introducing a new rank, that of suborder, and organising 26 families within six suborders, largely returning to the families set out by Christenhusz et al. in 2011. In lieu of the expansion of Aspleniaceae and Polypodiaceae, the six suborders allowed taxonomic recognition of the eupolypod clades:
- Saccolomatineae includes the single family Saccolomataceae.
- Lindsaeinae corresponds to the Lindseaceae of Smith et al., and includes the Cystodiaceae, Lindsaeaceae, and Lonchitidaceae. It is probably not monophyletic.
- Pteridineae includes the single family Pteridaceae.
- Dennstaedtiineae includes the single family Dennstaedtiaceae.
- Aspleniinae or Aspleniaceae (informally called eupolypods I) includes the (sub)families Cystopteridaceae, Rhachidosoraceae, Diplaziopsidaceae, Aspleniaceae, Woodsiaceae, Blechnaceae, Athyriaceae, Thelypteridaceae,. Desmophlebiaceae, containing the single genus Desmophlebium, separated from Diplazium in 2016, but can be easily accommodated in Asplenioideae of Christenhusz & Chase
- Polypodiineae or Polypodiaceae (eupolypods II) includes the (sub)families Didymochlaenoideae (containing only Didymochlaena), Hypodematiaceae, Dryopteridaceae, Lomariopsidaceae, Tectariaceae, Oleandraceae, Davalliaceae, and Polypodiaceae.
These suborders are phylogenetically related as shown in this cladogram:
The classification of ferns is certainly not yet stable due to the narrow family concept presented by the PPG I classification. The larger family concepts of Christenhusz & Chase are likely to be more stable. Family classification is a matter of taste, preference and applicability, and only time will tell which classification will be followed by the general user community.
The following diagram shows a likely phylogenetic relationship between the families of the Polypodiales
Now-obsolete families of Polypodiales include:
- Desmophlebiaceae - now in Aspleniaceae
- Drynariaceae - now in Polypodiaceae
- Grammitidaceae - now in Polypodiaceae
- Gymnogrammitidaceae - now in Polypodiaceae
- Hemidictyaceae - now in Aspleniaceae
- Loxogrammaceae - now in Polypodiaceae
- Nephrolepidaceae - now in Lomariopsidaceae or Polypodiaceae subfam. Lomariopsidoideae
- Onocleaceae - now in Blechnaceae or Aspleniaceae subfam. Blechnoideae
- Platyceriaceae - now in Polypodiaceae
- Pleursoriopsidaceae - now in Polypodiaceae
- Vittariaceae - now in Pteridaceae
Polypodiales may be regarded as one of the most evolutionarily advanced orders of monilophytes (ferns), based on recent genetic analysis. They arose and diversified about 100 million years ago, probably subsequent to the diversification of the angiosperms.
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