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Gnetophyta () is a division of plants (alternatively considered the subclass Gnetidae or order Gnetales), grouped within the
gymnosperm The gymnosperms ( lit. revealed seeds) are a group of seed-producing plants that includes conifers, cycads, '' Ginkgo'', and gnetophytes, forming the clade Gymnospermae. The term ''gymnosperm'' comes from the composite word in el, γυμν ...
s (which also includes conifers, cycads, and ginkgos), that consists of some 70 species across the three relict genera: ''
Gnetum ''Gnetum'' is a genus of gymnosperms, the sole genus in the family Gnetaceae within the Gnetophyta. They are tropical evergreen trees, shrubs and lianas. Unlike other gymnosperms, they possess vessel elements in the xylem. Some species have been ...
'' ( 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. Though diverse in the Early Cretaceous, only three families, each containing a single
genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...
, 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 plant Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek words ('container, vessel') and ('seed'), and refers to those plants t ...
s. 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 ...
(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 bracts around both the ovules and microsporangia as well as a micropylar projection of the outer membrane of the ovule that produces a
pollination Pollination is the transfer of pollen from an Stamen, anther of a plant to the stigma (botany), stigma of a plant, later enabling fertilisation and the production of seeds, most often by an animal or by Anemophily, wind. Pollinating agents can ...
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'',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 Namibia (, ), officially the Republic of Namibia, is a country in Southern Africa. Its western border is the Atlantic Ocean. It shares land borders with Zambia and Angola to the north, Botswana to the east and South Africa to the south and ea ...
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, 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 The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last period of the Paleo ...
and the
Triassic The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest per ...
. Fossils dating back to the
Jurassic The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...
have been found, though whether or not they belong to the gnetophytes is uncertain. Overall, the fossil record is richest in the early
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...
, with fossils of plants,
seed A seed is an embryonic plant enclosed in a protective outer covering, along with a food reserve. The formation of the seed is a part of the process of reproduction in seed plants, the spermatophytes, including the gymnosperm and angiosper ...
s, and pollen 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 differently ...
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 In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups ...
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 one that gave rise to angiosperms. At issue is whether the Gnetophyta are the sister group 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 The gymnosperms ( lit. revealed seeds) are a group of seed-producing plants that includes conifers, cycads, '' Ginkgo'', and gnetophytes, forming the clade Gymnospermae. The term ''gymnosperm'' comes from the composite word in el, γυμν ...
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. 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 conifers, and the
gymnosperm The gymnosperms ( lit. revealed seeds) are a group of seed-producing plants that includes conifers, cycads, '' Ginkgo'', and gnetophytes, forming the clade Gymnospermae. The term ''gymnosperm'' comes from the composite word in el, γυμν ...
s are a monophyletic 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 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 ''Gnetum'' is a genus of gymnosperms, the sole genus in the family Gnetaceae within the Gnetophyta. They are tropical evergreen trees, shrubs and lianas. Unlike other gymnosperms, they possess vessel elements in the xylem. Some species have been ...
'' only), lignin chemistry, the layering of cells in the apical meristem, pollen 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 The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest per ...
while a 2021 study placed it earlier, in the Carboniferous. 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 In polymer chemistry and materials science, resin is a solid or highly viscous substance of plant or synthetic origin that is typically convertible into polymers. Resins are usually mixtures of organic compounds. This article focuses on nat ...
canals, a tiered proembryo, 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) *'' 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'' 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 indeterm ...
, USA, Albian (possible affinities to Welwitschiaceae) *'' Bicatia'' Friis, Pedersen and Crane, 2014 Figueira da Foz Formation, 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 indeterm ...
, 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 In the geologic timescale, the Valanginian is an age or stage of the Early or Lower Cretaceous. It spans between 139.8 ± 3.0 Ma and 132.9 ± 2.0 Ma (million years ago). The Valanginian Stage succeeds the Berriasian Stage of the Lower Cretace ...
(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- Turonian) (Initially interpreted as a member of Welwitschiaceae, later considered uncertain). *'' Dayvaultia'' Manchester et al. 2021 Morrison Formation, USA, Late Jurassic ( Tithonian) 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 N ...
, 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 Ea ...
, 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 The Moenave Formation is a Mesozoic ...
'' 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