Rhyniophyta
Encyclopedia
Rhyniopsida is a class
of extinct early vascular plant
s, with one family
, Rhyniaceae, found in the Early Devonian (around ). They are polysporangiophyte
s, since their sporophyte
s consisted of branched stems bearing sporangia
(spore-forming organs). They lacked leaves or true roots but did have simple vascular tissue
. The group was first placed in a subdivision of the division Tracheophyta under the name Rhyniophytina (see Polysporangiophyte: Taxonomy for alternative names). Informally, they are often called rhyniophytes. More recently the name paratracheophytes has been suggested, to distinguish such plants from 'true' tracheophytes or eutracheophytes.
As originally defined, the group was found not to be monophyletic
since some of its members are now known to lack vascular tissue; these have been moved to the class Horneophytopsida
, which is defined as lacking true vascular tissue. Currently, Rhyniopsida includes the genera
Huvenia
, Rhynia
, and Stockmansella
, all from the Devonian
.
One of the most important radiations for land plants occurred in the early Devonian
(Pragian), when the first certain rhyniophytes appear in the fossil record, making this rich fossil discovery of major importance to paleobotany
. It has been suggested that the poorly preserved Eohostimella
, found in deposits of Early Silurian age (Llandovery, around ), may be a rhyniophyte.
for the polysporangiophytes in which the Rhyniaceae are shown as the sister group of all other tracheophytes (vascular plants). The other former "rhyniophytes", such as Horneophyton and Aglaophyton, are placed outside the tracheophyte clade, as they did not possess true vascular tissue (in particular did not have tracheid
s).
s found in the Rhynie chert
Lagerstätte
- rich fossil beds in Aberdeenshire
, Scotland
, and roughly coeval sites with similar flora. Used in this way, these terms refer to a floristic assemblage of more or less related early land plants, not a taxon
. Though the rhyniophytes are well-represented, plants with simpler anatomy
, like Aglaophyton
, are also common; there are also more complex plants, like Asteroxylon
, which has a very early form of leaves.
The Rhynie flora is unusual for the excellent preservation of early vascular plant
s, in addition to plants transitional between vascular and non-vascular. The fossils contain sufficient internal detail to determine vascular organization and to distinguish sporangia and gametangia
. This has led to the recognition of species which apparently had an isomorphic alternation of generations (gametophytes and sporophytes of similar prominence), a condition unknown among land plants today. Because the plants were buried in situ, rather than after transport to a distant location, important morphological details and ecological information can be obtained. The site also preserves other organisms such as arthropods and fungi that lived in the Rhynie ecosystem
.
Class (biology)
In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
of extinct early vascular plant
Vascular plant
Vascular plants are those plants that have lignified tissues for conducting water, minerals, and photosynthetic products through the plant. Vascular plants include the clubmosses, Equisetum, ferns, gymnosperms and angiosperms...
s, with one family
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
, Rhyniaceae, found in the Early Devonian (around ). They are polysporangiophyte
Polysporangiophyte
Polysporangiophytes, also called polysporangiates or more formally Polysporangiophyta, are plants in which the spore-bearing generation has a structure of branching stems terminating in sporangia...
s, since their sporophyte
Sporophyte
All land plants, and some algae, have life cycles in which a haploid gametophyte generation alternates with a diploid sporophyte, the generation of a plant or algae that has a double set of chromosomes. A multicellular sporophyte generation or phase is present in the life cycle of all land plants...
s consisted of branched stems bearing sporangia
Sporangium
A sporangium is an enclosure in which spores are formed. It can be composed of a single cell or can be multicellular. All plants, fungi, and many other lineages form sporangia at some point in their life cycle...
(spore-forming organs). They lacked leaves or true roots but did have simple vascular tissue
Vascular tissue
Vascular tissue is a complex conducting tissue, formed of more than one cell type, found in vascular plants. The primary components of vascular tissue are the xylem and phloem. These two tissues transport fluid and nutrients internally. There are also two meristems associated with vascular tissue:...
. The group was first placed in a subdivision of the division Tracheophyta under the name Rhyniophytina (see Polysporangiophyte: Taxonomy for alternative names). Informally, they are often called rhyniophytes. More recently the name paratracheophytes has been suggested, to distinguish such plants from 'true' tracheophytes or eutracheophytes.
As originally defined, the group was found not to be monophyletic
Monophyly
In common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...
since some of its members are now known to lack vascular tissue; these have been moved to the class Horneophytopsida
Horneophytopsida
Horneophytopsida is a class of extinct plants which consisted of branched stems without leaves, true roots or vascular tissue, found from the Late Silurian to the Early Devonian . They are the simplest known polysporangiophytes, i.e. plants with sporophytes bearing many spore-forming organs on...
, which is defined as lacking true vascular tissue. Currently, Rhyniopsida includes the genera
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
Huvenia
Huvenia
Huvenia is a genus of extinct plants of the Early Devonian , found in slate deposits of the Rhenish Massif. The sporophyte generation consisted of leafless stems , which appear to be flattened, and which branch dichotomously. The strand of conducting tissue contains simple tracheids, making this a...
, Rhynia
Rhynia
Rhynia gwynne-vaughanii was the sporophyte generation of a vascular, axial, free-sporing diplohaplontic embryophytic land plant of the Lower Devonian that had anatomical features more advanced than those of the bryophytes, and was basal to modern vascular plants or eutracheophytes.-Description:R...
, and Stockmansella
Stockmansella
Stockmansella is a genus of extinct plants of the Middle Devonian , fossils of which have been found in north-west Germany. The sporophyte generation consists of prostrate dichotomizing stems up to 10cm long and around 3mm wide, which at intervals produce narrower smooth upright stems...
, all from the Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
.
One of the most important radiations for land plants occurred in the early Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
(Pragian), when the first certain rhyniophytes appear in the fossil record, making this rich fossil discovery of major importance to paleobotany
Paleobotany
Paleobotany, also spelled as palaeobotany , is the branch of paleontology or paleobiology dealing with the recovery and identification of plant remains from geological contexts, and their use for the biological reconstruction of past environments , and both the evolutionary history of plants, with a...
. It has been suggested that the poorly preserved Eohostimella
Eohostimella
Eohostimella heathana is an early, probably terrestrial, "plant" known from compression fossils of Early Silurian age . The chemistry of its fossils is similar to that of fossilised vascular plants, rather than algae...
, found in deposits of Early Silurian age (Llandovery, around ), may be a rhyniophyte.
Phylogeny
In 2004, Crane et al. published a cladogramCladogram
A cladogram is a diagram used in cladistics which shows ancestral relations between organisms, to represent the evolutionary tree of life. Although traditionally such cladograms were generated largely on the basis of morphological characters, DNA and RNA sequencing data and computational...
for the polysporangiophytes in which the Rhyniaceae are shown as the sister group of all other tracheophytes (vascular plants). The other former "rhyniophytes", such as Horneophyton and Aglaophyton, are placed outside the tracheophyte clade, as they did not possess true vascular tissue (in particular did not have tracheid
Tracheid
Tracheids are elongated cells in the xylem of vascular plants that serve in the transport of water and mineral salts. Tracheids are one of two types of tracheary elements, vessel elements being the other. All tracheary elements develop a thick lignified cell wall, and at maturity the protoplast...
s).
Rhynie flora
The general term "rhyniophytes" or "rhyniophytoids" is sometimes used for the assemblage of plantPlant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...
s found in the Rhynie chert
Rhynie chert
The Rhynie chert is an Early Devonian sedimentary deposit exhibiting extraordinary fossil detail or completeness . It is exposed near the village of Rhynie, Aberdeenshire, Scotland; a second unit, the Windyfield chert, is located some 700 m away...
Lagerstätte
Lagerstätte
A Lagerstätte is a sedimentary deposit that exhibits extraordinary fossil richness or completeness.Palaeontologists distinguish two kinds....
- rich fossil beds in Aberdeenshire
Aberdeenshire
Aberdeenshire is one of the 32 unitary council areas in Scotland and a lieutenancy area.The present day Aberdeenshire council area does not include the City of Aberdeen, now a separate council area, from which its name derives. Together, the modern council area and the city formed historic...
, Scotland
Scotland
Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...
, and roughly coeval sites with similar flora. Used in this way, these terms refer to a floristic assemblage of more or less related early land plants, not a taxon
Taxon
|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...
. Though the rhyniophytes are well-represented, plants with simpler anatomy
Anatomy
Anatomy is a branch of biology and medicine that is the consideration of the structure of living things. It is a general term that includes human anatomy, animal anatomy , and plant anatomy...
, like Aglaophyton
Aglaophyton
Aglaophyton major was the sporophyte generation of a diplohaplontic, pre-vascular, axial, free-sporing land plant of the Lower Devonian . It had anatomical features intermediate between those of the bryophytes and vascular plants or tracheophytes.A. major was first described by Kidston and Lang in...
, are also common; there are also more complex plants, like Asteroxylon
Asteroxylon
Asteroxylon is an extinct genus of plants of the Division Lycopodiophyta known from anatomically preserved specimens in an Early Devonian deposit of chert at Rhynie, Aberdeenshire in North-East Scotland that has been dated at...
, which has a very early form of leaves.
The Rhynie flora is unusual for the excellent preservation of early vascular plant
Vascular plant
Vascular plants are those plants that have lignified tissues for conducting water, minerals, and photosynthetic products through the plant. Vascular plants include the clubmosses, Equisetum, ferns, gymnosperms and angiosperms...
s, in addition to plants transitional between vascular and non-vascular. The fossils contain sufficient internal detail to determine vascular organization and to distinguish sporangia and gametangia
Gametangia
A gametangium is an organ or cell in which gametes are produced that is found in many multicellular protists, algae, fungi, and the gametophytes of plants...
. This has led to the recognition of species which apparently had an isomorphic alternation of generations (gametophytes and sporophytes of similar prominence), a condition unknown among land plants today. Because the plants were buried in situ, rather than after transport to a distant location, important morphological details and ecological information can be obtained. The site also preserves other organisms such as arthropods and fungi that lived in the Rhynie ecosystem
Ecosystem
An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....
.
External links
- http://www.palaeos.com/Plants/Rhyniophytes/index.html
- http://www2.ncsu.edu/unity/lockers/project/dendrology/index/plantae/vascular/vascular.html
- Cladogram from
