Gleicheniales is an order of

fern The ferns (Polypodiopsida or Polypodiophyta) are a group of vascular plants (plants with xylem and phloem) that reproduce via spores and have neither seeds nor flowers. They differ from mosses by being vascular, i.e., having specialized tissue ...

s in the subclass Polypodiidae (the

leptosporangiate fern The Polypodiidae, commonly called leptosporangiate ferns, formerly Leptosporangiatae, are one of four subclasses of ferns, the largest of these being the largest group of living ferns, including some 11,000 species worldwide. The group has also ...

s). The Gleicheniales has spore records potentially as early as the

Early Carboniferous Early may refer to: Places in the United States * Early, Iowa, a city * Early, Texas, a city * Early Branch, a stream in Missouri * Early County, Georgia * Fort Early, Georgia, an early 19th century fort Music * Early B, stage name of Jamaican d ...

, but the oldest unambiguous macrofossil records date to the

Early Permian 01 or 01 may refer to: * The year 2001, or any year ending with 01 * The month of January * 1 (number) Music * '01 (Richard Müller album), ''01'' (Richard Müller album), 2001 * 01 (Urban Zakapa album), ''01'' (Urban Zakapa album), 2011 * ''01011 ...

Description

These ferns are characterized by root

stele A stele ( ) or stela ( )The plural in English is sometimes stelai ( ) based on direct transliteration of the Greek, sometimes stelae or stelæ ( ) based on the inflection of Greek nouns in Latin, and sometimes anglicized to steles ( ) or stela ...

s having 3–5 protoxylem poles and antheridia with 6–12 narrow, twisted or curved cells in their walls, Gleichenia-type spore wall ultrastructure. Otherwise, their habitus is highly diverse, including plants with the typical fern

frond A frond is a large, divided leaf. In both common usage and botanical nomenclature, the leaves of ferns are referred to as fronds and some botanists restrict the term to this group. Other botanists allow the term frond to also apply to the lar ...

s, others whose leaves resemble those of

palm tree The Arecaceae () is a family of perennial, flowering plants in the monocot order Arecales. Their growth form can be climbers, shrubs, tree-like and stemless plants, all commonly known as palms. Those having a tree-like form are colloquially c ...

s, and yet others again which have undivided leaves. They are

tropical The tropics are the regions of Earth surrounding the equator, where the sun may shine directly overhead. This contrasts with the temperate or polar regions of Earth, where the Sun can never be directly overhead. This is because of Earth's ax ...

ferns, most diverse in

Asia Asia ( , ) is the largest continent in the world by both land area and population. It covers an area of more than 44 million square kilometres, about 30% of Earth's total land area and 8% of Earth's total surface area. The continent, which ...

and the

Pacific The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean, or, depending on the definition, to Antarctica in the south, and is bounded by the cont ...

region.

Classification

In the molecular phylogenetic classification of Smith et al. in 2006, the Gleicheniales were placed in class

Polypodiopsida The ferns (Polypodiopsida or Polypodiophyta) are a group of vascular plants (plants with xylem and phloem) that reproduce via spores and have neither seeds nor flowers. They differ from mosses by being vascular, i.e., having specialized tissue ...

(the

s). Three families, Dipteridaceae, Gleicheniaceae, and Matoniaceae were recognized. 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 Gleicheniales there. The circumscription of the order and its families was not changed, and that circumscription and placement in Polypodiidae has subsequently been followed in the classifications of Christenhusz and Chase (2014) and PPG I (2016). The fossil taxon '' Microphyllopteris'' is used for some

Mesozoic The Mesozoic Era is the Era (geology), era of Earth's Geologic time scale, geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceous Period (geology), Periods. It is characterized by the dominance of archosaurian r ...

Gleicheniales that cannot be reliably assigned to the present-day orders. The

Triassic The Triassic ( ; sometimes symbolized 🝈) is a geologic period and system which spans 50.5 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.4 Mya. The Triassic is t ...

'' Antarctipteris'' and '' Gleichenipteris'' are sometimes ascribed to the Gleicheniaceae, but are probably better considered Gleicheniales ''

incertae sedis or is a term used for a taxonomy (biology), taxonomic group where its broader relationships are unknown or undefined. Alternatively, such groups are frequently referred to as "enigmatic taxa". In the system of open nomenclature, uncertainty ...

''. The oldest possible macrofossils of the Gleicheniales stem group are members of the genus '' Oligocarpia'' (fertile fronds) from the Carboniferous. However, these have alternatively been considered members of fossil family Sermayaceae. The oldest evidence of stem (basal) group of Gleicheniales was discovered in the wall ultrastructure of miospores '' Maiaspora'' from the Visean (Middle Mississippian) of Russia and United Kingdom. The oldest unambiguous macrofossil records, belonging to the stem member of family Gleicheniaceae, are known from the Wuda Tuff flora of China, dating to the Asselian, approximately 298.34 million years ago. In historical treatments, the order has sometimes been treated as a subclass Gleicheniatae of the Pteridopsida, with the

taxa In biology, a taxon (back-formation from ''taxonomy''; : taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular name and ...

treated as families here upranked to orders, so that a distinct subclass can be established for the

s. In other treatments, they were expanded to include the

filmy fern The Hymenophyllaceae, the filmy ferns and bristle ferns, are a family of two to nine genera (depending on classification system) and about 650 known species of ferns, with a cosmopolitan distribution, subcosmopolitan distribution, but generally r ...

s (order Hymenophyllales), as well as the similar-looking

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

'' Hymenophyllopsis'' (as order Hymenophyllopsidales). The resultant group was treated as

class Class, Classes, or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used d ...

Gleichenopsida alongside the Pteridopsida, which would then be limited to the leptosporangiate ferns. However, this class is not

monophyletic In biological cladistics for the classification of organisms, monophyly is the condition of a taxonomic grouping being a clade – that is, a grouping of organisms which meets these criteria: # the grouping contains its own most recent co ...

but rather a basal grade, retaining ancient traits among the living ferns. Irrespective of their modern taxonomic treatment, the Gleicheniales were formerly included in the order

Polypodiales The Order (biology), 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 Tropics, tropical, semitropical and Temperate clima ...

. But the ferns in the loose sense are much too diverse a group to be shoehorned into one

taxon In biology, a taxon (back-formation from ''taxonomy''; : taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular name and ...

at such a low rank.

Footnotes

References

* (2006): A classification for extant ferns. ''Taxon'' 55(3): 705–731
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{{Taxonbar, from=Q1209668 Fern orders