Rhyniophytina is a subdivision of extinct early vascular plants that are considered to be similar to the genus Rhynia, found in the Early Devonian (around ). Sources vary in the name and rank used for this group, some treating it as the class Rhyniopsida, others as the division Rhyniophyta. The informal term rhyniophytes is also used. The first definition of the Rhyniophytina was by Banks,:8 since when there have been many redefinitions,:96-97 including by Banks himself. "As a result, the Rhyniophytina have slowly dissolved into a heterogeneous collection of plants ... the group contains only one species on which all authors agree: the type species Rhynia gwynne-vaughanii".:94 When defined very broadly, the group consists of plants with dichotomously branched, naked aerial axes ("stems") with terminal spore-bearing structures (sporangia).:227
|Reconstruction of Rhynia gwynne-vaughanii|
The group was described as a subdivision of the division Tracheophyta by Harlan Parker Banks in 1968 under the name Rhyniophytina. The original definition was: "plants with naked (lacking emergences), dichotomizing axes bearing sporangia that are terminal, usually fusiform and may dehisce longitudinally; they are diminutive plants and, in so far as is known, have a small terete xylem strand with a central protoxylem.":8, With this definition, they are polysporangiophytes, since their sporophytes consisted of branched stems bearing sporangia (spore-forming organs). They lacked leaves or true roots but did have simple vascular tissue. Informally, they are often called rhyniophytes or, as mentioned below, rhyniophytoids.
However, as originally circumscribed, the group was found not to be monophyletic since some of its members are now known to lack vascular tissue. The definition that seems to be used most often now is that of D. Edwards and D.S. Edwards: "plants with smooth axes, lacking well-defined spines or leaves, showing a variety of branching patterns that may be isotomous, anisotomous, pseudomonopodial or adventitious. Elongate to globose sporangia were terminal on main axes or on lateral systems showing limited branching. It seems probable that the xylem, comprising a solid strand of tracheids, was centrarch.":216 However, Edwards and Edwards also decided to include rhyniophytoids, plants which "look like rhyniophytes, but cannot be assigned unequivocally to that group because of inadequate anatomical preservation", but exclude plants like Aglaophyton and Horneophyton which definitely do not possess tracheids.:214-215
In 1966, slightly before Banks created the subdivision, the group was treated as a division under the name Rhyniophyta. Taylor et al. in their book Paleobotany use Rhyniophyta as a formal taxon,:1028 but with a loose definition: plants "characterized by dichotomously branched, naked aerial axes with terminal sporangia".:227 They thus include under "other rhyniophytes" plants apparently without vascular tissue.:246ff
More recently the name paratracheophytes has been suggested, to distinguish such plants from 'true' tracheophytes or eutracheophytes.
Not all authors agree on the hierarchy above or below the Rhyniophytina,:96-97 or even use the lower levels of the hierarchy when writing taxonomic papers. The following are some of the names which may be used:
In 2004, Crane et al. published a cladogram 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 tracheids). However, both Horneophyton and Aglaophyton have been tentatively classified as tracheophytes in at least one recent cladistic analysis of Early Devonian land plants.
Partial cladogram by Crane et al. including the more certain rhyniophytes:
(See the Polysporangiophyte article for the expanded cladogram.)
The general term "rhyniophytes" or "rhyniophytoids" is sometimes used for the assemblage of plants 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.
Aberlemnia is a genus of extinct vascular plants of the Early Devonian (around 420 to 390 million years ago), which consisted of leafless stems with terminal spore-forming organs (sporangia). Fossils found in Scotland were initially described as Cooksonia caledonica. A later review, which included new and more complete fossils from Brazil, showed that the specimens did not fit the circumscription of the genus Cooksonia; accordingly a new genus Aberlemnia was proposed.Aphyllopteris
Aphyllopteris is a poorly known genus of extinct Devonian land plants formerly placed in the Rhyniophytina. It is considered an artificial group for sterile naked axes that branch pseudomonopodially. Early Devonian records of this genus are found in Belgium, France, Norway, Poland, and Russia, and possibly Uzbekistan. Putative Middle to Late Devonian records of this genus include A. delawarensis reported from Frasnian outcrops in New York and Aphyllopteris sp. reported from the Givetian outcrops of the Beckers Butte Member of the Martin Formation in Arizona.Eorhynia
Eorhynia is a genus of extinct plants of the Late Silurian (Přídolí, around 430 to 420 million years ago) which somewhat resemble Rhynia. Fossils were found in Podolia in modern Ukraine.Hedeia
Hedeia is a genus of early land plants of uncertain affinity. It comprises erect axes terminating in corymbose clusters of erect sporangia.The type species, H. corymbosa, was first described by Isabel Cookson from a few specimens in fine sandstone from near Alexandra, Victoria, Australia. She gave no derivation of the generic name. At the time, the locality was regarded as being of Silurian age, but it is now known to be Early Devonian. It is claimed that an undescribed species, also from Victoria, extends from the Early Devonian back to the Late Silurian.H. parvula from Kazakhstan and H. sinica from China are also of Early Devonian age.
It is sometimes suggested that Hedeia and Yarravia are merely different preservations of the same type of plant.Horneophyton
Horneophyton, a member of the Horneophytopsida, is an extinct early plant which may form a "missing link" between the hornworts and the Rhyniopsida. It is among the most abundant organisms found in the Rhynie chert, a Devonian Lagerstätte in Scotland. A single species, Horneophyton lignieri, is known. Its probable female gametophyte is the form taxon Langiophyton mackiei.Huvenia
Huvenia is a genus of extinct plants of the Early Devonian (Pragian or Siegenian stage, around 411 to 408 million years ago), found in slate deposits of the Rhenish Massif. The sporophyte generation consisted of leafless stems (axes), which appear to be flattened, and which branch dichotomously. The strand of conducting tissue contains simple tracheids, making this a vascular plant (tracheophyte). The sporangia (spore-forming organs) are borne on the ends of short branching stems (sporangiophores) rather than terminating main stems as in some other early land plants. Sporangia appear to be twisted, but it is not clear whether this feature was present in life or developed after death.In 2004, Crane et al. published a cladogram for the polysporangiophytes, in which Huvenia is placed in the Rhyniaceae. (See that article for the cladogram.)Lycopodolica
Lycopodolica is a genus of extinct plants of the Late Silurian (Přídolí, around 430 to 420 million years ago). Fossils were found in the Rashkov Beds in Podolia in modern Ukraine. Plants there are preserved as compressions without internal detail. Lycopodolica had stems (axes) which appear to have branched and which are covered with lax, hair- or thread-like outgrowths. Considered to be a lycophyte, Lycopodolica differs from Baragwanathia in the nature of its outgrowths or enations.Polysporangiophyte
Polysporangiophytes, also called polysporangiates or formally Polysporangiophyta, are plants in which the spore-bearing generation (sporophyte) has branching stems (axes) that terminate in sporangia. The name literally means many sporangia plant. The clade includes all land plants (embryophytes) except for the bryophytes (liverworts, mosses and hornworts) whose sporophytes are normally unbranched, even if a few exceptional cases occur. While the definition is independent of the presence of vascular tissue, all living polysporangiophytes also have vascular tissue, i.e., are vascular plants or tracheophytes. Fossil polysporangiophytes are known that have no vascular tissue, and so are not tracheophytes.Psilophytopsida
Psilophytopsida is a now obsolete class containing one order, Psilophytales, which was previously used to classify a number of extinct plants which are now placed elsewhere. The class was established in 1917, under the name Psilophyta, with only three genera (Rhynia, Horneophyton and Psilophyton) for a group of fossil plants from the Upper Silurian and Devonian periods which lack true roots and leaves, but have a vascular system within a branching cylindrical stem. The living Psilotaceae, the whisk-ferns, were sometimes added to the class, which was then usually called Psilopsida. This classification is no longer in use.The class should not be confused with the current use of the name Psilotopsida, which refers to a class of living ferns, containing only Psilotaceae (whisk-ferns) and Ophioglossaceae (moon-worts and adder's-tongue ferns).Rhynia
Rhynia is a single-species genus of Devonian vascular plants. 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. Rhynia gwynne-vaughanii was a member of a sister group to all other eutracheophytes, including modern vascular plants.Sciadophyton
Sciadophyton is a morphotaxon of lower Devonian plants known only from compression fossils. It is interpreted as the monoicous gametophyte of a vascular land plant, because its vascularised branches end in a cup-shaped structure bearing gametangia, both antheridia and archegonia, but little structural information is preserved at the cellular level. It formed rosettes of stems, which may have radiated from a basal gametophytic corm-like thallus or from a central 'stem' or even from a root system, although there is not enough evidence to discriminate between these possibilities.Stockmansella
Stockmansella is a genus of extinct plants of the Middle Devonian (Eifelian stage, around 393 to 388 million years ago), fossils of which have been found in north-west Germany. The sporophyte generation consists of prostrate dichotomizing stems (axes) up to 10 cm long and around 3mm wide, which at intervals produce narrower smooth upright stems. These bear sporangia (spore-forming organs) on short lateral branches (sporangiophores). The prostrate stems have bulges from which rhizoids form. Both prostrate and upright stems have a central strand of conducting tissue which contains simple tracheids, so that Stockmansella is a vascular plant.The genus was created by Fairon-Demaret for fossil forms previously assigned to Taeniocrada but which differ in having single lateral sporangia. (She initially gave the genus the name Stockmansia, but this had already been used for a genus of ferns.) The form genus Sciadophyton is thought to be the gametophyte stage of several early land plants, including Stockmansella, although as these forms have only been found as compressed fossils, their morphology is not entirely clear.In 2004, Crane et al. published a cladogram for the polysporangiophytes, in which Stockmansella is placed in the Rhyniaceae, sister to all other tracheophytes (vascular plants).Syllabus der Pflanzenfamilien
Syllabus der Pflanzenfamilien (1892–) by Adolf Engler (1844–1930) is a complete revision of plant families down to generic level and often even further. As such it forms part of the Engler system of plant taxonomy.
Engler's starting point was that of Eichler who had been the first to use phylogenetic principles, and reflected the new post-Darwinian perspective, although Engler himself did not think that his was. His modified Eichler schema first appeared in 1886 in his Guide to Breslau Botanic Garden (of which he was the director) and was expanded in his Syllabus in 1892. This reflected the new post-Darwinian perspective. Engler's Syllabus der Pflanzenfamilien first appeared in 1892 with the title Syllabus der Vorlesungen über specielle und medicinisch-pharmaceutische Botanik. Many subsequent editions have appeared since, and it was continued after Engler's death in 1930. The most recent edition was the 13th in 2009. A number of references to the Engler system actually refer to later revisions ('modified Engler system') undertaken by Melchior and colleagues, the 12th edition of the Syllabus, also referred to as the Melchior system.Yarravia
Yarravia is a genus of extinct vascular plants mainly known from fossils found in Victoria, Australia. Originally the rocks in which they were found were considered to be late Silurian in age; more recently they have been found to be Early Devonian (Pragian, around 410 million years ago). Specimens consist only of incomplete leafless stems, some of which bore groups of spore-forming organs or sporangia which were fused, at least at the base.