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Gnetophyta () is a division of plants (alternatively considered the subclass Gnetidae or order Gnetales), grouped within the gymnosperms (which also includes
conifer Conifers are a group of conifer cone, cone-bearing Spermatophyte, seed plants, a subset of gymnosperms. Scientifically, they make up the phylum, division Pinophyta (), also known as Coniferophyta () or Coniferae. The division contains a single ...
s, cycads, and ginkgos), that consists of some 70 species across the three relict genera: '' Gnetum'' ( family Gnetaceae), '' Welwitschia'' (family Welwitschiaceae), and '' Ephedra'' (family Ephedraceae). Fossilized pollen attributed to a close relative of ''Ephedra'' has been dated as far back as the
Early Cretaceous The Early Cretaceous ( geochronological name) or the Lower Cretaceous (chronostratigraphic name), is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 145  Ma to 100.5 Ma. Geology Pro ...
. Though diverse in the
Early Cretaceous The Early Cretaceous ( geochronological name) or the Lower Cretaceous (chronostratigraphic name), is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 145  Ma to 100.5 Ma. Geology Pro ...
, only three families, each containing a single genus, are still alive today. The primary difference between gnetophytes and other gymnosperms is the presence of vessel elements, a system of conduits that transport water within the plant, similar to those found in flowering plants. Because of this, gnetophytes were once thought to be the closest gymnosperm relatives to flowering plants, but more recent molecular studies have brought this hypothesis into question. Though it is clear they are all related, the exact evolutionary inter-relationships between gnetophytes are unclear. Some classifications hold that all three genera should be placed in a single
order Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of d ...
(Gnetales), while other classifications say they should be distributed among three separate orders, each containing a single family and genus. Most morphological and molecular studies confirm that the genera ''Gnetum'' and ''Welwitschia'' diverged from each other more recently than they did from ''Ephedra''.


Ecology and morphology

Unlike most biological groupings, it is difficult to find many common characteristics between all of the members of the gnetophytes. The two common characteristics most commonly used are the presence of enveloping
bract In botany, a bract is a modified or specialized leaf, especially one associated with a reproductive structure such as a flower, inflorescence axis or cone scale. Bracts are usually different from foliage leaves. They may be smaller, larger, or of ...
s around both the ovules and microsporangia as well as a micropylar projection of the outer membrane of the ovule that produces a pollination droplet,Judd, W.S.; Campbell, C.S.; Kellogg, E.A.; Stevens, P.F.; and Donoghue, M.J. (2008) Plant Systematics: A Phylogenetics Approach. 3rd ed. Sunderland, Massachusetts, USA: Sinauer Associates, Inc. though these are highly specific compared to the similarities between most other plant divisions. L. M. Bowe refers to the gnetophyte genera as a "bizarre and enigmatic" trio because, the gnetophytes' specialization to their respective environments is so complete that they hardly resemble each other at all. ''Gnetum'' species are mostly woody vines in tropical forests, though the best-known member of this group, ''
Gnetum gnemon ''Gnetum gnemon'' is a species of ''Gnetum'' native to southeast Asia and the western Pacific Ocean islands, from Mizoram and Assam in India, south and east through Indonesia and Malaysia to the Philippines, Fiji, and Hawaii in the United States ...
'',Wan T, Liu Z M, Li L F, et al. A genome for gnetophytes and early evolution of seed plants Nature plants, 2018, 4(2): 82. is a tree native to western Malesia. The one remaining species of ''Welwitschia'', '' Welwitschia mirabilis'', native only to the dry deserts of Namibia and Angola, is a ground-hugging species with only two large strap-like leaves that grow continuously from the base throughout the plant's life. ''Ephedra'' species, known as "jointfirs" in the United States, have long slender branches which bear tiny scale-like leaves at their nodes. Infusions from these plants have been traditionally used as a
stimulant Stimulants (also often referred to as psychostimulants or colloquially as uppers) is an overarching term that covers many drugs including those that increase activity of the central nervous system and the body, drugs that are pleasurable and inv ...
, but ephedrine is a controlled substance today in many places because of the risk of harmful or even fatal
overdosing A drug overdose (overdose or OD) is the ingestion or application of a drug or other substance in quantities much greater than are recommended.
.


Fossil Gnetophyta

Knowledge of gnetophyte history through fossil discovery has increased greatly since the 1980s. Gnetophyte fossils have been found that date from the
Permian and the Triassic. Fossils dating back to the Jurassic have been found, though whether or not they belong to the gnetophytes is uncertain. Overall, the fossil record is richest in the early Cretaceous, with fossils of plants, seeds, and
pollen Pollen is a powdery substance produced by seed plants. It consists of pollen grains (highly reduced microgametophytes), which produce male gametes (sperm cells). Pollen grains have a hard coat made of sporopollenin that protects the gametophyt ...
have been found that can clearly be assigned to the gnetophytes.


Classification

With just three well-defined genera within an entire division, there still is understandable difficulty in establishing an unambiguous interrelationship among them; in earlier times matters were even more difficult and we find for example Pearson in the early 20th century speaking of the
class Class 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 differentl ...
Gnetales, rather than the order. G.H.M. Lawrence referred to them as an order, but remarked that the three families were distinct enough to deserve recognition as separate orders.Lawrence, George Hill Mathewson. Taxonomy of vascular plants. Macmillan, 1951 Foster & Gifford accepted this principle, and placed the three orders together in a common class for convenience, which they called Gnetopsida. In general the evolutionary relationships among the seed plants still are unresolved, and the Gnetophyta have played an important role in the formation of phylogenetic hypotheses. Molecular phylogenies of extant gymnosperms have conflicted with morphological characters with regard to whether the gymnosperms as a whole (including gnetophytes) comprise a monophyletic group or a
paraphyletic In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...
one that gave rise to angiosperms. At issue is whether the Gnetophyta are the
sister group In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and t ...
of angiosperms, or whether they are sister to, or nested within, other extant gymnosperms. Numerous fossil gymnosperm clades once existed that are morphologically at least as distinctive as the four living gymnosperm groups, such as Bennettitales, ''
Caytonia ''Caytonia'' is an extinct genus of seed ferns. Description ''Caytonia'' has berry like cupules with numerous small seeds arrayed along axes Whole plant reconstructions Different organs attributed to the same original plant can ...
'' and the glossopterids. When these gymnosperm fossils are considered, the question of gnetophyte relationships to other seed plants becomes even more complicated. Several hypotheses, illustrated below, have been presented to explain seed plant evolution. Morphological studies have supported a close relationship between Gnetophyta, Bennettitales and the
Erdtmanithecales Erdtmanithecales is an extinct order of gymnosperm plants known from the Mesozoic era. Known remains include pollen organs, seed cones and seeds associated with '' Eucommiidites'' pollen, which is considered diagnostic for the order. The order was ...
. Recent research by Lee, Cibrian-Jaramillo, ''et al.'' (2011) suggests that the Gnetophyta are a sister group to the rest of the gymnosperms, contradicting the anthophyte hypothesis, which held that gnetophytes were sister to the flowering plants.


Gnetifer hypothesis

In the gnetifer hypothesis, the gnetophytes are sister to the
conifer Conifers are a group of conifer cone, cone-bearing Spermatophyte, seed plants, a subset of gymnosperms. Scientifically, they make up the phylum, division Pinophyta (), also known as Coniferophyta () or Coniferae. The division contains a single ...
s, and the gymnosperms are a
monophyletic In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic gro ...
group, sister to the angiosperms.The gnetifer hypothesis first emerged formally in the mid-twentieth century, when vessel elements in the gnetophytes were interpreted as being derived from tracheids with circular bordered pits, as in conifers. It however only gained strong support with the emergence of molecular data in the late 1990s. Although the most salient morphological evidence still largely supports the anthophyte hypothesis,some more obscure morphological commonalities between the gnetophytes and conifers lend support to the gnetifer hypothesis.These shared traits include: tracheids with scalariform pits with tori interspersed with annular thickenings, absence of scalariform pitting in primary xylem, scale-like and strap-shaped
leaves A leaf (plural, : leaves) is any of the principal appendages of a vascular plant plant stem, stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", wh ...
of '' Ephedra'' and '' Welwitschia''; and reduced sporophylls.


Anthophyte hypothesis

From the early twentieth century, the anthophyte hypothesis was the prevailing explanation for seed plant evolution, based on shared morphological characters between the gnetophytes and angiosperms. In this hypothesis, the gnetophytes, along with the extinct order Bennettitales, are sister to the angiosperms, forming the "anthophytes". Some morphological characters that were suggested to unite the anthophytes include vessels in wood, net-veined leaves (in '' Gnetum'' only),
lignin Lignin is a class of complex organic polymers that form key structural materials in the support tissues of most plants. Lignins are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidity ...
chemistry, the layering of cells in the apical meristem,
pollen Pollen is a powdery substance produced by seed plants. It consists of pollen grains (highly reduced microgametophytes), which produce male gametes (sperm cells). Pollen grains have a hard coat made of sporopollenin that protects the gametophyt ...
and megaspore features (including thin megaspore wall), short cambial initials, and lignin syringal groups. However, most genetic studies, as well as more recent morphological analyses, have rejected the anthophyte hypothesis. Several of these studies have suggested that the gnetophytes and angiosperms have independently derived characters, including flower-like reproductive structures and tracheid vessel elements, that appear shared but are actually the result of parallel evolution.


Gnepine hypothesis

The gnepine hypothesis is a modification of the gnetifer hypothesis, and suggests that the gnetophytes belong within the conifers as a sister group to the Pinaceae. According to this hypothesis, the conifers as currently defined are not a monophyletic group, in contrast with molecular findings that support its monophyly. All existing evidence for this hypothesis comes from molecular studies since 1999. A 2018 phylogenomic study estimated the divergence between Gnetales and Pinaceae at around 241 millions of years ago, in the early Triassic while a 2021 study placed it earlier, in the
Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonifero ...
. However, the morphological evidence remains difficult to reconcile with the gnepine hypothesis. If the gnetophytes are nested within conifers, they must have lost several shared derived characters of the conifers (or these characters must have evolved in parallel in the other two conifer lineages): narrowly triangular leaves (gnetophytes have diverse leaf shapes), resin canals, a tiered
proembryo Plant embryonic development, also plant embryogenesis is a process that occurs after the fertilization of an ovule to produce a fully developed plant embryo. This is a pertinent stage in the plant life cycle that is followed by dormancy and germina ...
, and flat woody ovuliferous cone scales. These kinds of major morphological changes are not without precedent in the Pinales, however: the Taxaceae, for example, have lost the classical cone of the conifers in favor of a single-terminal ovule, surrounded by a fleshy aril.


Gnetophyte-sister hypothesis

Some partitions of the genetic data suggest that the gnetophytes are sister to all of the other extant seed plant groups. However, there is no morphological evidence nor examples from the fossil record to support the gnetophyte-sister hypotheses.


Fossil lineages

Ephedraceae *'' Leongathia'' V.A. Krassilov, D.L. Dilcher & J.G. Douglas 1998 Koonwarra fossil bed, Australia, Early Cretaceous (
Aptian The Aptian is an age in the geologic timescale or a stage in the stratigraphic column. It is a subdivision of the Early or Lower Cretaceous Epoch or Series and encompasses the time from 121.4 ± 1.0 Ma to 113.0 ± 1.0 Ma (million years ago), a ...
) *'' Jianchangia'' Yang, Wang and Ferguson, 2020 Jiufotang Formation, China, Early Cretaceous (Aptian) *'' Eamesia'' Yang, Lin and Ferguson, 2018 Yixian Formation, China, Early Cretaceous (Aptian) *'' Prognetella'' Krassilov et Bugdaeva, 1999 Yixian Formation, China, Early Cretaceous (Aptian) (initially interpreted as an angiosperm) *'' Chengia'' Yang, Lin & Wang, 2013, Yixian Formation, China, Early Cretaceous (Aptian) *'' Chaoyangia'' Duan, 1998 Yixian Formation, China, Early Cretaceous (Aptian) *'' Eragrosites'' Yixian Formation, China, Early Cretaceous (Aptian) *'' Gurvanella'' China, Mongolia, Early Cretaceous *'' Alloephedra'' China, Early Cretaceous *'' Amphiephedra'' China, Early Cretaceous *'' Beipiaoa'' China, Early Cretaceous *'' Ephedrispermum'' Portugal, Early Cretaceous (Aptian-Albian) *'' Ephedrites'' China, Early Cretaceous *'' Erenia'' China, Mongolia, Early Cretaceous *'' Liaoxia'' China, Early Cretaceous *'' Dichoephedra'' China, Early Cretaceous Gnetaceae *'' Khitania'' Guo et al. 2009 Yixian Formation, China, Early Cretaceous (Aptian) Welwitschiaceae *'' Priscowelwitschia'' Dilcher et al., 2005 Crato Formation, Brazil, Early Cretaceous (Aptian) *'' Cratonia'' Rydin et al., 2003 Crato Formation, Brazil, Early Cretaceous (Aptian) *'' Welwitschiostrobus'' Dilcher et al., 2005 Crato Formation, Brazil, Early Cretaceous (Aptian) ''Incertae sedis'': *''
Drewria ''Drewria potomacensis'' is a Cretaceous megafossil member of the Gnetales, from the Potomac Group, hence its name. It was possibly a shrub.Crane, Peter R., and Garland R. Upchurch Jr. "Drewria potomacensis gen. et sp. nov., an Early Cretaceous ...
'' Crane & Upchurch, 1987
Potomac Group The Potomac Group is a geologic group in Delaware, Maryland, New Jersey, and Virginia. It preserves fossils dating back to the Cretaceous period. An indeterminate tyrannosauroid and '' Priconodon crassus'', a nodosaurid, are known from indetermi ...
, USA, Albian (possible affinities to Welwitschiaceae) *'' Bicatia'' Friis, Pedersen and Crane, 2014
Figueira da Foz Formation Figueira may refer to: Places Brazil * Figueira, Paraná Cape Verde * Figueira Pavão, Fogo Island * Figueira (Maio), Maio Island * Figueira da Naus, Santiago Island Portugal * Figueira da Foz, a city and municipality in the district of Coimbr ...
, Portugal, Early Cretaceous (late Aptian early Albian),
Potomac Group The Potomac Group is a geologic group in Delaware, Maryland, New Jersey, and Virginia. It preserves fossils dating back to the Cretaceous period. An indeterminate tyrannosauroid and '' Priconodon crassus'', a nodosaurid, are known from indetermi ...
, USA, Albian (possible affinities to Welwitschiaceae) *'' Liaoningia'' Yang et al, 2017 Yixian Formation, China, Early Cretaceous (Aptian) *'' Protognetum'' Y. Yang, L. Xie et D.K. Ferguson, 2017
Daohugou The Haifanggou Formation (), also known as the Jiulongshan Formation (), is a fossil-bearing rock deposit located near Daohugou () village of Ningcheng County, in Inner Mongolia, northeastern China. The formation consists of coarse conglomerates ...
, China, Middle Jurassic ( Callovian) *'' Itajuba'' Ricardi-Branco et al, 2013, Crato Formation, Brazil, Early Cretaceous (Aptian) *'' Protoephedrites'' Rothwell et Stockey, 2013 Canada, Valanginian (possible ephedroid affinities) *'' Siphonospermum'' Rydin et Friis, 2010 Yixian Formation, China, Early Cretaceous (Aptian) *'' Welwitschiophyllum'' Dilcher et al., 2005 Crato Formation, Brazil, Early Cretaceous (Aptian), Akrabou Formation, Morocco, Late Cretaceous (
Cenomanian The Cenomanian is, in the ICS' geological timescale, the oldest or earliest age of the Late Cretaceous Epoch or the lowest stage of the Upper Cretaceous Series. An age is a unit of geochronology; it is a unit of time; the stage is a unit in the s ...
- Turonian) (Initially interpreted as a member of Welwitschiaceae, later considered uncertain). *'' Dayvaultia'' Manchester et al. 2021
Morrison Formation The Morrison Formation is a distinctive sequence of Late Jurassic, Upper Jurassic sedimentary rock found in the western United States which has been the most fertile source of dinosaur fossils in North America. It is composed of mudstone, sandsto ...
, USA, Late Jurassic (
Tithonian In the geological timescale, the Tithonian is the latest age of the Late Jurassic Epoch and the uppermost stage of the Upper Jurassic Series. It spans the time between 152.1 ± 4 Ma and 145.0 ± 4 Ma (million years ago). It is preceded by the K ...
) Possible gnetophytes (not confirmed as members of the group) *'' Archaestrobilus''
Trujillo Formation The Trujillo Formation is a geologic formation in Texas and New Mexico. It preserves fossils dating back to the upper Triassic period. It is also known as the Trujillo Sandstone. Description The formation consists mostly of gray to brown or re ...
, Texas, United States, Upper Triassic *'' Dechellyia''-'' Masculostrobus'' Mongolia, Early Cretaceous (Aptian-Albian) *'' Dinophyton''
Chinle Formation The Chinle Formation is an Upper Triassic continental geological formation of fluvial, lacustrine, and palustrine to eolian deposits spread across the U.S. states of Nevada, Utah, northern Arizona, western New Mexico, and western Colorado. In Ne ...
, United States, Upper Triassic *'' Nataligma''
Molteno Formation The Molteno Formation is a geological formation found in several localities in Lesotho and South Africa. It lies mainly south of Maseru, near Burgersdorp, Aliwal North, Dordrecht, Molteno, and Elliot. It extends as far north as Matatiele in the ...
, South Africa, Upper Triassic (Carnian) *'' Palaeognetaleana'' Wang, 2004, China, Upper Permian *''
Sanmiguelia ''Sanmiguelia lewisi'' is an extinct plant genus, possibly of flowering plants. The fossil was first described from Late Triassic Chinle Formation in Colorado, and later in Lower Jurassic Moenave Formation in Utah Utah ( , ) is a state ...
'' United States, Late Triassic-Early Jurassic *'' Eoantha'' Russia, Early Cretaceous *'' Bassitheca'' Morrison Formation, USA, Late Jurassic (Tithonian)


References

Other Sources: * * {{Taxonbar, from=Q309147 Plant divisions Extant Permian first appearances