Taeniocrada
Encyclopedia
Taeniocrada is a genus of extinct plants of Devonian
age . It is used as a form genus
for fossil plants with leafless flattened stems which divided dichotomously and had prominent midribs regarded as containing vascular tissue
s. It has been suggested that some species assigned to this genus were aquatic.
Taeniocrada decheniana, from the Lower Devonian, had separate fertile stems which repeatedly branched in a dichotomous fashion ending in sporangia between 3 and 7 mm long. A few sporangia were borne on the sides of stems. The species was found in dense stands. It has been suggested that it was aquatic or semi-aquatic because it apparently did not have stomata
on the flattened stems.
Taeniocrada dubia, from the Lower Devonian, was originally considered to be a rhyniophyte, i.e. a very early vascular plant
, but this has been questioned. The central strand appears to have been composed of tubes of differing diameters with helical thickenings which were part of the original cell wall, rather than being produced as the cell matured as would be the case in the xylem
of vascular plants.
Taeniocrada stilesvillensis, from the Upper Devonian of New York, had stems which divided either into two equal branches (i.e. dichotomously) or so that one branch was more of a 'main stem' than the other (i.e. pseudomonopodially). Ridges along the stem bore hair-like structures. True vascular tissue
was present.
A further nine species still considered to be part of the genus are listed and in some cases briefly described by Taylor (1986).
, used for fossil plants with flattened membrane-like stems, which were leafless with a prominent central thickened strand and which showed dichotomous branching. In 1986 Taylor noted that as more species had been added to the genus, the characters it possessed became wider, so that some species had sporangia
which were at the ends of stems (terminal), others had sporangia borne on the sides of stems (lateral). Some species had smooth stems, other had stems with 'emergences' (e.g. spines or hairs). Some were known to have vascular tissue, others not. He concluded that the genus no longer matched any existing description and that its species probably belonged to more than one genus. He also suggested that the flattening of the stems might be artefacts of preservation.
In 1985, Fairon-Demaret created a new genus for fossils previously assigned to Taeniocrada but which had single lateral sporangia. She transferred T. langii (named by Stockmans in 1939) to Stockmansella
langii, leaving Taeniocrada for fossil plants with terminal sporangia borne on branched structures.
genus (i.e. a mixture of species with no close common ancestor). Some species may belong to the Rhyniopsida as defined by Kenrick and Crane.
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...
age . It is used as a form genus
Form taxon
Form classification is the classification of organisms based on their morphology, which does not necessarily reflect their biological relationships...
for fossil plants with leafless flattened stems which divided dichotomously and had prominent midribs regarded as containing 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:...
s. It has been suggested that some species assigned to this genus were aquatic.
Description
Key features of the original definition of the genus Taeniocrada were that it possessed leafless flattened stems with prominent midribs which appeared to contain vascular tissues. As more species were added to the genus, its definition became less clear. Three of the better-known species are T. decheniana, T. dubia and T. stilesvillensis.Taeniocrada decheniana, from the Lower Devonian, had separate fertile stems which repeatedly branched in a dichotomous fashion ending in sporangia between 3 and 7 mm long. A few sporangia were borne on the sides of stems. The species was found in dense stands. It has been suggested that it was aquatic or semi-aquatic because it apparently did not have stomata
Stoma
In botany, a stoma is a pore, found in the leaf and stem epidermis that is used forgas exchange. The pore is bordered by a pair of specialized parenchyma cells known as guard cells that are responsible for regulating the size of the opening...
on the flattened stems.
Taeniocrada dubia, from the Lower Devonian, was originally considered to be a rhyniophyte, i.e. a very 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...
, but this has been questioned. The central strand appears to have been composed of tubes of differing diameters with helical thickenings which were part of the original cell wall, rather than being produced as the cell matured as would be the case in the xylem
Xylem
Xylem is one of the two types of transport tissue in vascular plants. . The word xylem is derived from the Classical Greek word ξυλον , meaning "wood"; the best-known xylem tissue is wood, though it is found throughout the plant...
of vascular plants.
Taeniocrada stilesvillensis, from the Upper Devonian of New York, had stems which divided either into two equal branches (i.e. dichotomously) or so that one branch was more of a 'main stem' than the other (i.e. pseudomonopodially). Ridges along the stem bore hair-like structures. True 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:...
was present.
A further nine species still considered to be part of the genus are listed and in some cases briefly described by Taylor (1986).
Taxonomy
The genus Taeniocrada has a somewhat complex taxonomic history. It was created by White with the species T. lesquereuxii for fossils previously regarded as algae but which proved to have vascular tissue. (The date of creation is variously given as 1902, 1903 and 1913.) It was basically a form genusForm taxon
Form classification is the classification of organisms based on their morphology, which does not necessarily reflect their biological relationships...
, used for fossil plants with flattened membrane-like stems, which were leafless with a prominent central thickened strand and which showed dichotomous branching. In 1986 Taylor noted that as more species had been added to the genus, the characters it possessed became wider, so that some species had 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...
which were at the ends of stems (terminal), others had sporangia borne on the sides of stems (lateral). Some species had smooth stems, other had stems with 'emergences' (e.g. spines or hairs). Some were known to have vascular tissue, others not. He concluded that the genus no longer matched any existing description and that its species probably belonged to more than one genus. He also suggested that the flattening of the stems might be artefacts of preservation.
In 1985, Fairon-Demaret created a new genus for fossils previously assigned to Taeniocrada but which had single lateral sporangia. She transferred T. langii (named by Stockmans in 1939) to 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...
langii, leaving Taeniocrada for fossil plants with terminal sporangia borne on branched structures.
Phylogeny
Following Taylor (1986), Crane et al. (2004) regard Taeniocrada as a polyphyleticPolyphyly
A polyphyletic group is one whose members' last common ancestor is not a member of the group.For example, the group consisting of warm-blooded animals is polyphyletic, because it contains both mammals and birds, but the most recent common ancestor of mammals and birds was cold-blooded...
genus (i.e. a mixture of species with no close common ancestor). Some species may belong to the Rhyniopsida as defined by Kenrick and Crane.