Psilophyton
Encyclopedia
Psilophyton is a genus of extinct vascular plant
s. Described in 1859, it was one of the first fossil plants to be found which was of Devonian
age (about ). Specimens have been found in northern Maine, USA; Gaspé Bay, Quebec and New Brunswick, Canada; the Czech Republic; and Yunnan, China. Plants lacked leaves or true roots; spore
-forming organs or sporangia
were borne on the ends of branched clusters. It is significantly more complex than some other plants of comparable age (e.g. Rhynia
) and is thought to be part of the group from within which the modern fern
s and seed plants evolved.
Psilophyton dawsonii is the best-known species. Compressed and mineralized specimens have been found in several locations, particularly in calcareous pebbles on the Gaspé Peninsula, Quebec, Canada. Plants consisted of bare stems (axes) ending in blunt tips. Lower down they repeatedly branched dichotomously; higher up they bore sporangium
-bearing 'units' in two rows on opposite sides of the stems. These units branched, also dichotomously, before terminating in sporangia, so that there were clusters of up to 128 paired, downward curved sporangia, oval in shape and about 5 mm long. Spores were released through a longitudinal slit. The first two branching points of the fertile units appear to have consisted of two closely spaced dichotomous branches in which the middle branch did not develop. The trilete spores were between 40 and 75 µm in diameter.
The internal structure of the stems of P. dawsonii was considerably more complex than that of other plants of a similar age, e.g. Rhynia
. A central strand of xylem
occupied up to a third of the diameter of the stem. In the lower parts of main stems it was circular, enlarging before it divided prior to dichotomous branching. Higher up it became elliptical in cross-section, corresponding to the two rows of fertile branching units. Within the branches of these units, the strand was more-or-less rectangular. The conducting elements of the xylem, the tracheid
s, were of the so-called 'P-type' in which the walls were strengthened by ladder-like (scalariform) bars with circular openings between them. The tissue around the central strand was multi-layered with open spaces below the stomata
.
P. princeps, the first-discovered species, differed from P. dawsonii in having spines on the stems and larger sporangia. P. forbesii is the largest known species of Psilophyton. Reconstructions suggest a height of around 60 cm. Its stems were bare of spines but marked by longitudinal striations. Compared to P. dawsonii, both P. princeps and P. forbesii had a greater distinction between main stems and side branches, which may be considered an 'advanced' feature.
P. crenulatum was found in New Brunswick, Canada, also in rocks of Emsian age. Its branches bore spines up to 6 mm long which divided into two or three projections at their tips. It has resemblances to specimens of Aarabia
, found in Morocco.
P. dapsile was found in Maine, USA. It appears to have been considerably smaller than the previous species, perhaps 30 cm tall, with smooth dichotomously branching stems, and sporangia only 2 mm long. Kasper et al. suggested that the smaller species P. dapsile and P. krauselii, which had mainly dichotomous branches, were the more 'primitive' members of the genus.
P. primitivum was found in Yunnan, China, in the Posongchong Formation, which is of Pragian (Siegenian) age (around ). All other species have been found in North America and Europe, which formed part of the continent of Laurussia in the Devonian. The name reflects a belief that the species is 'primitive', with considerable dichotomous branching and a loose, less tightly branched habit. As with all species of Psilophyton, pairs of sporangia were created by dichotomously branching fertile units, although in P. primitivum there were relatively few per unit (4–8) and they were somewhat loosely clustered. Similarities with P. microspinum and P. parvulum were suggested.
from the Devonian of Gaspé Peninsula, Canada, were thought by Dawson in 1859 to resemble the modern whiskfern, Psilotum
. Accordingly, he named his new genus Psilophyton, the type species being P. princeps. Unfortunately, it later turned out that his description and subsequent reconstruction was based on fragments of three different unrelated plants, which caused confusion for many years. The sporangia were from Psilophyton, but some aerial stems were from what is now Sawdonia, and the rhizomes were from Taeniocrada
.
In 1871, Dawson described specimens which had strong spines as P. princeps var. ornatum. He considered that groups of paired terminal sporangia found with these were part of the same plant, although no actual connection was found. Much later, in 1967, it was shown that fossils called "Psilophyton princeps" had two very different patterns of xylem development: from the centre outwards (centrarch) in P. princeps and from the outside inwards (exarch) in P. princeps var. ornatum. Hueber and Banks selected new specimens as the type for the species P. princeps, and Hueber later transferred P. princeps var. ornatum to a new genus as Sawdonia ornata. Only P. princeps had paired terminal sporangia; those of S. ornata were borne on the sides of the stems. Later additions of species to the genus have been based on the description by Hueber and Banks.
Dawson named another specimen Psilophyton robustius. On the grounds that this had side branches which divided into three as well as dividing into two, in 1956 Hopping moved it to a new genus Trimerophyton.
s, moss
es and hornwort
s) were placed in a single class
, Psilophyta, established in 1917 by Kidston and Lang. As additional fossils were discovered and described, it became apparent that the Psilophyta were not a homogeneous group of plants. In 1968, Banks proposed splitting it into three groups, one of which was the subdivision Trimerophytina, informally called the trimerophytes. Psilophyton was a prominent member of this group. The distinction between main stems and strongly branched lateral stems evident in species such as P. forbesii has been considered to be one of the key steps towards the evolution of the leaves of euphyllophytes (modern members of which are the fern
s and seed plants), based on the theory that such leaves evolved through 'webbing' of flattened lateral branching systems.
In 2004, Crane et al. published a simplified cladogram
for the polysporangiophyte
s, based on a number of figures in Kenrick and Crane (1997). Part of their cladogram is reproduced below (with some branches collapsed into 'basal groups' to reduce the size of the diagram). It shows two of the better-known species of Psilophyton as early-diverging members of the euphyllophytes.
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. Described in 1859, it was one of the first fossil plants to be found which was of 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...
age (about ). Specimens have been found in northern Maine, USA; Gaspé Bay, Quebec and New Brunswick, Canada; the Czech Republic; and Yunnan, China. Plants lacked leaves or true roots; spore
Spore
In biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions. Spores form part of the life cycles of many bacteria, plants, algae, fungi and some protozoa. According to scientist Dr...
-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...
were borne on the ends of branched clusters. It is significantly more complex than some other plants of comparable age (e.g. 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 is thought to be part of the group from within which the modern 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 and seed plants evolved.
Description
Almost all the species of Psilophyton have been found in rocks of Emsian age (around ). One exception is P. krauselii, from the Czech Republic, which is younger, being from the upper part of the Middle Devonian (around ).Psilophyton dawsonii is the best-known species. Compressed and mineralized specimens have been found in several locations, particularly in calcareous pebbles on the Gaspé Peninsula, Quebec, Canada. Plants consisted of bare stems (axes) ending in blunt tips. Lower down they repeatedly branched dichotomously; higher up they bore sporangium
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...
-bearing 'units' in two rows on opposite sides of the stems. These units branched, also dichotomously, before terminating in sporangia, so that there were clusters of up to 128 paired, downward curved sporangia, oval in shape and about 5 mm long. Spores were released through a longitudinal slit. The first two branching points of the fertile units appear to have consisted of two closely spaced dichotomous branches in which the middle branch did not develop. The trilete spores were between 40 and 75 µm in diameter.
The internal structure of the stems of P. dawsonii was considerably more complex than that of other plants of a similar age, e.g. 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...
. A central strand of 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...
occupied up to a third of the diameter of the stem. In the lower parts of main stems it was circular, enlarging before it divided prior to dichotomous branching. Higher up it became elliptical in cross-section, corresponding to the two rows of fertile branching units. Within the branches of these units, the strand was more-or-less rectangular. The conducting elements of the xylem, the 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, were of the so-called 'P-type' in which the walls were strengthened by ladder-like (scalariform) bars with circular openings between them. The tissue around the central strand was multi-layered with open spaces below the 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...
.
P. princeps, the first-discovered species, differed from P. dawsonii in having spines on the stems and larger sporangia. P. forbesii is the largest known species of Psilophyton. Reconstructions suggest a height of around 60 cm. Its stems were bare of spines but marked by longitudinal striations. Compared to P. dawsonii, both P. princeps and P. forbesii had a greater distinction between main stems and side branches, which may be considered an 'advanced' feature.
P. crenulatum was found in New Brunswick, Canada, also in rocks of Emsian age. Its branches bore spines up to 6 mm long which divided into two or three projections at their tips. It has resemblances to specimens of Aarabia
Aarabia
Aarabia is a genus of extinct vascular plants found in central Morocco in outcrops of Early Devonian age . The leafless plant has a complex branching system with a main stem and at least three orders of side branches. In addition to these long branches, stems bore very short branches, which...
, found in Morocco.
P. dapsile was found in Maine, USA. It appears to have been considerably smaller than the previous species, perhaps 30 cm tall, with smooth dichotomously branching stems, and sporangia only 2 mm long. Kasper et al. suggested that the smaller species P. dapsile and P. krauselii, which had mainly dichotomous branches, were the more 'primitive' members of the genus.
P. primitivum was found in Yunnan, China, in the Posongchong Formation, which is of Pragian (Siegenian) age (around ). All other species have been found in North America and Europe, which formed part of the continent of Laurussia in the Devonian. The name reflects a belief that the species is 'primitive', with considerable dichotomous branching and a loose, less tightly branched habit. As with all species of Psilophyton, pairs of sporangia were created by dichotomously branching fertile units, although in P. primitivum there were relatively few per unit (4–8) and they were somewhat loosely clustered. Similarities with P. microspinum and P. parvulum were suggested.
Taxonomy
Leafless, dichotomously branching fossils bearing spines and possessing vascular tissueVascular 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:...
from the Devonian of Gaspé Peninsula, Canada, were thought by Dawson in 1859 to resemble the modern whiskfern, Psilotum
Psilotum
Psilotum is a genus of fern-like vascular plants, one of two genera in the family Psilotaceae, order Psilotales, and class Psilotopsida...
. Accordingly, he named his new genus Psilophyton, the type species being P. princeps. Unfortunately, it later turned out that his description and subsequent reconstruction was based on fragments of three different unrelated plants, which caused confusion for many years. The sporangia were from Psilophyton, but some aerial stems were from what is now Sawdonia, and the rhizomes were from Taeniocrada
Taeniocrada
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 tissues...
.
In 1871, Dawson described specimens which had strong spines as P. princeps var. ornatum. He considered that groups of paired terminal sporangia found with these were part of the same plant, although no actual connection was found. Much later, in 1967, it was shown that fossils called "Psilophyton princeps" had two very different patterns of xylem development: from the centre outwards (centrarch) in P. princeps and from the outside inwards (exarch) in P. princeps var. ornatum. Hueber and Banks selected new specimens as the type for the species P. princeps, and Hueber later transferred P. princeps var. ornatum to a new genus as Sawdonia ornata. Only P. princeps had paired terminal sporangia; those of S. ornata were borne on the sides of the stems. Later additions of species to the genus have been based on the description by Hueber and Banks.
Dawson named another specimen Psilophyton robustius. On the grounds that this had side branches which divided into three as well as dividing into two, in 1956 Hopping moved it to a new genus Trimerophyton.
Phylogeny
At first most of the early polysporangiophytes (land plants other than liverwortLiverwort
Liverwort may refer to either*Marchantiophyta, a division of non-vascular plants*Hepatica, a genus of spring flowersliverworts are part of the bryophytes group and the bryophytes of the PlantaeIn the bryophytres group their are mosses too....
s, moss
Moss
Mosses are small, soft plants that are typically 1–10 cm tall, though some species are much larger. They commonly grow close together in clumps or mats in damp or shady locations. They do not have flowers or seeds, and their simple leaves cover the thin wiry stems...
es and hornwort
Hornwort
Hornworts are a group of bryophytes, or non-vascular plants, comprising the division Anthocerotophyta. The common name refers to the elongated horn-like structure, which is the sporophyte. The flattened, green plant body of a hornwort is the gametophyte plant.Hornworts may be found worldwide,...
s) were placed in a single class
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...
, Psilophyta, established in 1917 by Kidston and Lang. As additional fossils were discovered and described, it became apparent that the Psilophyta were not a homogeneous group of plants. In 1968, Banks proposed splitting it into three groups, one of which was the subdivision Trimerophytina, informally called the trimerophytes. Psilophyton was a prominent member of this group. The distinction between main stems and strongly branched lateral stems evident in species such as P. forbesii has been considered to be one of the key steps towards the evolution of the leaves of euphyllophytes (modern members of which are the 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 and seed plants), based on the theory that such leaves evolved through 'webbing' of flattened lateral branching systems.
In 2004, Crane et al. published a simplified cladogram
Cladogram
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 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, based on a number of figures in Kenrick and Crane (1997). Part of their cladogram is reproduced below (with some branches collapsed into 'basal groups' to reduce the size of the diagram). It shows two of the better-known species of Psilophyton as early-diverging members of the euphyllophytes.