Psilophytopsida
Encyclopedia
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 root
s 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).
. Three genera were initially included, Rhynia
, Horneophyton
and Psilophyton
. All lacked leaves and true roots, consisting only of branched stems; however they were considered to contain vascular tissue
.
Additional fossil genera were added later. As described by Sporne in 1966, Psilophytopsida consisted of four families:
By 1975, it had become clear that the class had become increasingly unnatural, containing unrelated early vascular plants. It was split up by Banks into three subdivisions: Rhyniophytina, Zosterophyllophytina, and Trimerophytina. Later cladistic analyses of early land plants suggested that at least the rhyniophytes and the trimerophytes were not monophyletic. Separating out 'basal groups', such as the earliest land plants, is intrinsically difficult, since at this stage they contain many shared characters (plesiomorphies) which are not sufficient to distinguish them.
The current classification of former members of the class is largely due to Kenrick and Crane in 1997.
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...
containing one order
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
, 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
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
plants from the Upper Silurian
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
and 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...
periods which lack true root
Root
In vascular plants, the root is the organ of a plant that typically lies below the surface of the soil. This is not always the case, however, since a root can also be aerial or aerating . Furthermore, a stem normally occurring below ground is not exceptional either...
s and leaves
Leaf
A leaf is an organ of a vascular plant, as defined in botanical terms, and in particular in plant morphology. Foliage is a mass noun that refers to leaves as a feature of plants....
, 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
Psilotopsida
Psilotopsida is a class of fern-like plants. It should not be confused with the obsolete class Psilophytopsida. As circumscribed by Smith et al. Psilotopsida contains two families, Psilotaceae and Ophioglossaceae, placed in orders Psilotales and Ophioglossales, respectively...
, which refers to a class of living ferns, containing only Psilotaceae (whisk-ferns) and Ophioglossaceae (moon-worts and adder's-tongue ferns).
Description
The class was created in 1917 by Kidston and Lang for fossils found in the Rhynie Chert BedRhynie 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...
. Three genera were initially included, 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...
, Horneophyton
Horneophyton
Horneophyton, a member of the Horneophytopsida, was an 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.-Description:...
and Psilophyton
Psilophyton
Psilophyton is a genus of extinct vascular plants. Described in 1859, it was one of the first fossil plants to be found which was of Devonian age . Specimens have been found in northern Maine, USA; Gaspé Bay, Quebec and New Brunswick, Canada; the Czech Republic; and Yunnan, China...
. All lacked leaves and true roots, consisting only of branched stems; however they were considered to contain 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:...
.
Additional fossil genera were added later. As described by Sporne in 1966, Psilophytopsida consisted of four families:
- Rhyniaceae
- Rhynia – now placed in the class Rhyniopsida
- Horneophyton – now placed in the class HorneophytopsidaHorneophytopsidaHorneophytopsida 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...
- CooksoniaCooksoniaCooksonia is an extinct grouping of primitive land plants. The earliest Cooksonia date from the middle of the Silurian ; the group continues to be an important component of the flora until the Early Devonian, a total time span of...
– now considered to be paraphyletic basal trachaeophyteVascular plantVascular 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 - YarraviaYarraviaYarravia 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...
– uncertain position
- Zosterophyllaceae – now placed in the class ZosterophyllopsidaZosterophyllopsidaThe Zosterophylls are a group of extinct plants. The taxon was first established by Banks in 1968 as the subdivision Zosterophyllophytina; they have since also been treated as the division Zosterophyllophyta and the class Zosterophyllopsida. They were among the first vascular plants in the fossil...
- ZosterophyllumZosterophyllumZosterophyllum was a genus of Silurian-Devonian land plant with branching axes.Some species have been transferred to other genera:* Z. artesianum to Danziella artesiana* Z. contiguum to Demersatheca contigua...
- Zosterophyllum
- Psilophytaceae
- PsilophytonPsilophytonPsilophyton is a genus of extinct vascular plants. Described in 1859, it was one of the first fossil plants to be found which was of Devonian age . Specimens have been found in northern Maine, USA; Gaspé Bay, Quebec and New Brunswick, Canada; the Czech Republic; and Yunnan, China...
– now considered to be a basal euphyllophyte
- Psilophyton
- Asteroxylaceae – now placed in the order DrepanophycalesDrepanophycalesDrepanophycales is an order of extinct plants of the Division Lycopodiophyta of ?Late Silurian to Late Devonian age , found in North America, China, Russia, Europe, and Australia. Sometimes known as the Asteroxylales or Baragwanathiales....
- AsteroxylonAsteroxylonAsteroxylon 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...
- Asteroxylon
By 1975, it had become clear that the class had become increasingly unnatural, containing unrelated early vascular plants. It was split up by Banks into three subdivisions: Rhyniophytina, Zosterophyllophytina, and Trimerophytina. Later cladistic analyses of early land plants suggested that at least the rhyniophytes and the trimerophytes were not monophyletic. Separating out 'basal groups', such as the earliest land plants, is intrinsically difficult, since at this stage they contain many shared characters (plesiomorphies) which are not sufficient to distinguish them.
The current classification of former members of the class is largely due to Kenrick and Crane in 1997.