Tetraxylopteris
Encyclopedia
Tetraxylopteris is a genus of extinct vascular plant
s of the Middle to Upper Devonian
(around ). Fossils were first found in New York State, USA. A second species was later found in Venezuela.
age (around ). T. reposana was found in the Campo Chico Formation, north-west Venezuela, in beds believed to be of Frasnian
age .
The overall shape of Tetraxylopteris consisted of a complex system of branches. The main stem was 'pseudomonopodial', i.e. it divided dichotomously to produce side stems while the main stem maintained its identity. The main and side stems then bore three orders of branches (i.e. the first branches from the stems divided twice more). (There were possibly four orders in T. reposana.) The branches were arranged in opposite pairs with successive pairs at right angles (decussate
). In T. schmidtii, the final order of branches bore appendages arranged oppositely and decussately; in T. reposana, the appendages were opposite and decussate on the penultimate order of branches and spirally or helically arranged on the final order. The appendages branched dichotomously up to three times and were three dimensional, although some appear flattened in the fossilized specimens. They have been called "non-laminate proto-leaves" by Beerling and Fleming, reflecting the belief that such structures were precursors to true leaves, which evolved by first 'planation' – flattening to produce a two dimensional branched structure – and then 'webbing' – tissue growing out between the flattened branches.
One consistent difference between the two species is that T. schmidtii had branches which very distinctly narrowed along their length whereas only the third order branches of T. reposana showed any tapering. Another is that T. reposana had swellings at the bases of first and second order branches.
In both species, the primary xylem
strand was central to stems, branches and appendages. In the main stems and branches it was X-shaped in cross-section, corresponding to the four rows of branches. In the final appendages it became circular in cross-section. The xylem development was 'mesarch', i.e. the first maturing protoxylem had later maturing metaxylem on either side. Protoxylem occurred both at the tips of the lobes of the xylem strand and in the centre. The general anatomy of the woody stem resembles that of seed plants.
The spore-forming organs or sporangia
of Tetraxylopteris were born on a very complex 'fertile branching system'. Firstly the main axis of the system branched twice dichotomously. Then each of the four resulting branches was three times pinnate
. Each ultimate unit had an elongated sporangium at its end which split longitudinally to release the spores which were trilete, ranging from around 70 to 170 µm in diameter.
The complex three-dimensional branching pattern imples that both species would have been open bushy plants. The exact height cannot be determined from the fossils, which consist of broken-off portions. The longest known section of T. schmidtii is 50 cm – rather less for T. reposana, which may nevertheless have been the taller species. An overall height of a few metres has been proposed. Hammond and Berry suggest that T. reposana may have grown in dense thickets so that plants supported one another.
of plants which reproduced in a similar way to fern
s, but had stems whose internal structure resembled gymnosperm
s. The class was divided into two orders
, the more 'primitive' Aneurophytales and the more 'advanced' Archaeopteridales. (The former order had earlier been proposed by Kräusel & Weyland in 1941 for taxa ancestral to both ferns and pteridosperms.) Tetraxylopteris was placed in the Aneurophytales. Another specimen from the same location was initially described as Sphenoxylon, but was later shown to be a poorly preserved stem of Tetraxylopteris.
In 2005 a further species, Tetraxylopteris reposana, was described by Hammond and Berry. The specific epithet reposana is derived from 'El Reposo', the name of the hacienda near the fossil locality. Hammond and Berry suggest that Proteokalon Scheckler & Banks (1971) may be synonymous with Tetraxylopteris.
Hammond and Berry agree with Beck on the systematic taxonomy of the genus.
, traditionally classified as a "trimerophyte" rather than a progymnosperm
like Tetraxylopteris.
Other researchers have produced rather different analyses. Rothwell's analysis separates the "trimerophytes" and progymnosperms, with only the latter being closely related to seed plants.
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 of the Middle to Upper 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...
(around ). Fossils were first found in New York State, USA. A second species was later found in Venezuela.
Description
Fossils of Tetraxylopteris have so far been discovered in two locations. T. schmidtii was named from the Catskill Clastic Wedge, New York State, United States of America, in rocks of Middle to Upper DevonianDevonian
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 (around ). T. reposana was found in the Campo Chico Formation, north-west Venezuela, in beds believed to be of Frasnian
Frasnian
The Frasnian is one of two faunal stages in the Late Devonian epoch. It lasted from 385.3 ± 2.6 million years ago to 374.5 ± 2.6 million years ago. It was preceded by the Givetian stage and followed by the Famennian stage...
age .
The overall shape of Tetraxylopteris consisted of a complex system of branches. The main stem was 'pseudomonopodial', i.e. it divided dichotomously to produce side stems while the main stem maintained its identity. The main and side stems then bore three orders of branches (i.e. the first branches from the stems divided twice more). (There were possibly four orders in T. reposana.) The branches were arranged in opposite pairs with successive pairs at right angles (decussate
Decussation
Decussation is used in biological contexts to describe a crossing.Examples include:* In the brain, where nerve fibers obliquely cross from one lateral part to the other, that is to say they cross at a level other than their origin...
). In T. schmidtii, the final order of branches bore appendages arranged oppositely and decussately; in T. reposana, the appendages were opposite and decussate on the penultimate order of branches and spirally or helically arranged on the final order. The appendages branched dichotomously up to three times and were three dimensional, although some appear flattened in the fossilized specimens. They have been called "non-laminate proto-leaves" by Beerling and Fleming, reflecting the belief that such structures were precursors to true leaves, which evolved by first 'planation' – flattening to produce a two dimensional branched structure – and then 'webbing' – tissue growing out between the flattened branches.
One consistent difference between the two species is that T. schmidtii had branches which very distinctly narrowed along their length whereas only the third order branches of T. reposana showed any tapering. Another is that T. reposana had swellings at the bases of first and second order branches.
In both species, the primary 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...
strand was central to stems, branches and appendages. In the main stems and branches it was X-shaped in cross-section, corresponding to the four rows of branches. In the final appendages it became circular in cross-section. The xylem development was 'mesarch', i.e. the first maturing protoxylem had later maturing metaxylem on either side. Protoxylem occurred both at the tips of the lobes of the xylem strand and in the centre. The general anatomy of the woody stem resembles that of seed plants.
The spore-forming organs or 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...
of Tetraxylopteris were born on a very complex 'fertile branching system'. Firstly the main axis of the system branched twice dichotomously. Then each of the four resulting branches was three times pinnate
Pinnate
Pinnate is a term used to describe feather-like or multi-divided features arising from both sides of a common axis in plant or animal structures, and comes from the Latin word pinna meaning "feather", "wing", or "fin". A similar term is pectinate, which refers to a comb-like arrangement of parts...
. Each ultimate unit had an elongated sporangium at its end which split longitudinally to release the spores which were trilete, ranging from around 70 to 170 µm in diameter.
The complex three-dimensional branching pattern imples that both species would have been open bushy plants. The exact height cannot be determined from the fossils, which consist of broken-off portions. The longest known section of T. schmidtii is 50 cm – rather less for T. reposana, which may nevertheless have been the taller species. An overall height of a few metres has been proposed. Hammond and Berry suggest that T. reposana may have grown in dense thickets so that plants supported one another.
Taxonomy
The genus was created by Beck in 1957 for the species Tetraxylopteris schmidtii. Initially Beck described the genus as possibly a precursor to the seed ferns or pteridosperms. In 1960, Beck created the name Progymnospermopsida for a classClass (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 plants which reproduced in a similar way to fern
Fern
A fern is any one of a group of about 12,000 species of plants belonging to the botanical group known as Pteridophyta. Unlike mosses, they have xylem and phloem . They have stems, leaves, and roots like other vascular plants...
s, but had stems whose internal structure resembled gymnosperm
Gymnosperm
The gymnosperms are a group of seed-bearing plants that includes conifers, cycads, Ginkgo, and Gnetales. The term "gymnosperm" comes from the Greek word gymnospermos , meaning "naked seeds", after the unenclosed condition of their seeds...
s. The class was divided into two orders
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...
, the more 'primitive' Aneurophytales and the more 'advanced' Archaeopteridales. (The former order had earlier been proposed by Kräusel & Weyland in 1941 for taxa ancestral to both ferns and pteridosperms.) Tetraxylopteris was placed in the Aneurophytales. Another specimen from the same location was initially described as Sphenoxylon, but was later shown to be a poorly preserved stem of Tetraxylopteris.
In 2005 a further species, Tetraxylopteris reposana, was described by Hammond and Berry. The specific epithet reposana is derived from 'El Reposo', the name of the hacienda near the fossil locality. Hammond and Berry suggest that Proteokalon Scheckler & Banks (1971) may be synonymous with Tetraxylopteris.
Hammond and Berry agree with Beck on the systematic taxonomy of the genus.
- Class Progymnospermopsida Beck (1960)
- Order Aneurophytales Kräusel & Weyland (1941)
- Genus Tetraxylopteris Beck (1957)
- Order Aneurophytales Kräusel & Weyland (1941)
Phylogeny
A cladogram published in 2004 by Crane et al. places Tetraxylopteris in a paraphyletic stem group, basal to the seed plants (spermatophytes). It is grouped with PerticaPertica
Pertica is a genus of extinct vascular plants of the Early to Middle Devonian . It has been placed in the "trimerophytes", a strongly paraphyletic group of early members of the lineage leading to modern ferns and seed plants.-Description:...
, traditionally classified as a "trimerophyte" rather than a progymnosperm
Progymnosperm
The progymnosperms are an extinct group of woody, spore-bearing plants that is presumed to have evolved from the "trimerophytes", and eventually gave rise to the gymnosperms. They have been treated formally at the rank of division Progymnospermophyta or class Progymnospermopsida...
like Tetraxylopteris.
Other researchers have produced rather different analyses. Rothwell's analysis separates the "trimerophytes" and progymnosperms, with only the latter being closely related to seed plants.
External links
- Cladogram from
