Zosterophyllopsida
Encyclopedia
The 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 plant
s in the fossil record, and had a world-wide distribution. They were probably stem-group lycophytes, forming a sister group to the ancestors of the living lycophytes. By the late Silurian
(late Ludlovian, about ) a diverse assemblage of species existed, examples of which have been found fossilised in what is now Bathurst Island
in Arctic
Canada
.
The stems of zosterophylls were either smooth or covered with small spines known as enation
s, branched dichotomously, and grew at the ends by unrolling, a process known as circinate vernation
. The stems had a central vascular column
in which the protoxylem
was exarch, and the metaxylem developed centripetally. The sporangia
were kidney-shaped (reniform), with conspicuous lateral dehiscence and were borne laterally in a fertile zone towards the tips of the branches.
The zosterophylls were named after the aquatic flowering plant
Zostera from a mistaken belief that the two groups were related. David P. Penhallow
's generic description of the type genus Zosterophyllum refers to "Aquatic plants with creeping stems, from which arise narrow dichotomous branches and narrow linear leaves of the aspect of Zostera." Zosterophyllum rhenanum was reconstructed as aquatic, the lack of stomata on the lower axes giving support to this interpretation. However, current opinion is that the Zosterophylls were terrestrial plants, and Penhallow's "linear leaves" are interpreted as the aerial stems of the plant that had become flattened during fossilization.
Stomata were present, particularly on the upper axes. Their absence on the lower portions of the axes suggests that this part of the plants may have been submerged, and that the plants dwelt in boggy ground or even shallow water. In many fossils these appear to consist of a slit-like opening in the middle of a single elongated guard cell, leading to comparison with the stomata of some mosses. However, this is now thought to result from the loss of the wall separating paired guard cells during fossilisation.
s were placed in the 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, and in 1975 Banks developed his earlier proposal to split it into three groups, which he put at the rank
of subdivision. One of these was the subdivision Zosterophyllophytina, named after the genus Zosterophyllum
. For Banks, zosterophyllophytes or zosterophylls comprised plants with lateral sporangia which released their spores by splitting distally (i.e. away from their attachment), and which had exarch strands of xylem
. Bank's classification produces the hierarchy:
Those who treat most of the extant groups of plants as divisions may raise both the zosterophylls and the Lycophytina sensu Banks to the rank of division:
In their cladistic study published in 1997, Kenrick and Crane provided support for a clade uniting both the zosterophylls and the lycopsids, producing a classification which places the zosterophylls in a class Zosterophyllopsida of the subdivision Lycophytina:
This approach has been widely used alongside previous systems. A consequence is that "lycophyte" and corresponding formal names such as "Lycophyta" and "Lycophytina" are used by different authors in at least two senses: either excluding zosterophylls in the sense of Banks or including them in the sense of Kenrick and Crane.
A further complication is that the cladograms of Kenrick and Crane show that the zosterophylls, broadly defined, are paraphyletic, but contain a 'core' clade of plants with marked bilateral symmetry and circinate tips. The class Zosterophyllopsida sensu Kenrick & Crane may be restricted to this core clade, leaving many genera (e.g. Hicklingia
, Nothia
) with no systematic placement other than Lycophytina sensu Kenrick & Crane, but nevertheless still informally called "zosterophylls".
Under whatever name and rank, the zosterophylls have been divided into orders
and families
, e.g. the Zosterophyllales containing the Zosterophyllaceae and the Sawdoniales containing the Sawdoniaceae. Since the publication of cladograms showing that the group is paraphyletic divisions of the class have been less used, being ignored, for example, in the 2009 paleobotany textbook by Taylor et al.
for the polysporangiophyte
s (plants with branched stems bearing sporangia
), based on cladistic analyses of morphological features. This suggests that the zosterophylls were a paraphyletic stem group, related to the ancestors of modern lycophytes.
Genera which are included at or around the zosterophyll position in the cladogram or have otherwise been included in the group by at least one source, and hence may be considered zosterophylls in the broad sense, are listed below.
B = included by Banks in his 1975 description of Zosterophyllophytina.
Genera may not be assigned to this group by other authors; for example, Adoketophyton was regarded by Hao et al., who named the genus, as having evolved separately from the lycopsids, so that its taxonomic placement was uncertain.
Taxon
|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...
was first established by Banks in 1968 as the subdivision
Phylum
In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....
Zosterophyllophytina; they have since also been treated as the division
Phylum
In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....
Zosterophyllophyta and the 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...
Zosterophyllopsida. They were among the first 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...
s in the fossil record, and had a world-wide distribution. They were probably stem-group lycophytes, forming a sister group to the ancestors of the living lycophytes. By the late 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...
(late Ludlovian, about ) a diverse assemblage of species existed, examples of which have been found fossilised in what is now Bathurst Island
Bathurst Island
A member of the Canadian Arctic Archipelago, Bathurst Island is one of the Queen Elizabeth Islands in Nunavut Territory, Canada. The area of the island is estimated at , making it the 54th largest island in the world and Canada's 13th largest island. It is uninhabited.The island is low-lying with...
in Arctic
Arctic
The Arctic is a region located at the northern-most part of the Earth. The Arctic consists of the Arctic Ocean and parts of Canada, Russia, Greenland, the United States, Norway, Sweden, Finland, and Iceland. The Arctic region consists of a vast, ice-covered ocean, surrounded by treeless permafrost...
Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
.
Morphology
Enation
Enations are scaly leaflike structures, differing from leaves in their lack of vascular tissue. They are created by some leaf diseases. Also found on some early plants such as Rhynia, where they are hypothesized to aid in photosynthesis....
s, branched dichotomously, and grew at the ends by unrolling, a process known as circinate vernation
Vernation
Vernation is the formation of new leaves or fronds. In plant anatomy, it is the arrangement of leaves in a bud....
. The stems had a central vascular column
Stele (biology)
In a vascular plant, the stele is the central part of the root or stem containing the tissues derived from the procambium. These include vascular tissue, in some cases ground tissue and a pericycle, which, if present, defines the outermost boundary of the stele...
in which the protoxylem
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...
was exarch, and the metaxylem developed centripetally. The 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 kidney-shaped (reniform), with conspicuous lateral dehiscence and were borne laterally in a fertile zone towards the tips of the branches.
The zosterophylls were named after the aquatic flowering plant
Flowering plant
The flowering plants , also known as Angiospermae or Magnoliophyta, are the most diverse group of land plants. Angiosperms are seed-producing plants like the gymnosperms and can be distinguished from the gymnosperms by a series of synapomorphies...
Zostera from a mistaken belief that the two groups were related. David P. Penhallow
David P. Penhallow
David Pearce Penhallow was a Canadian-American botanist, paleobotanist and educator.Born in Kittery Point, Maine, Penhallow graduated from Massachusetts Agricultural College in 1873 . When his former professor, William S...
's generic description of the type genus Zosterophyllum refers to "Aquatic plants with creeping stems, from which arise narrow dichotomous branches and narrow linear leaves of the aspect of Zostera." Zosterophyllum rhenanum was reconstructed as aquatic, the lack of stomata on the lower axes giving support to this interpretation. However, current opinion is that the Zosterophylls were terrestrial plants, and Penhallow's "linear leaves" are interpreted as the aerial stems of the plant that had become flattened during fossilization.
Stomata were present, particularly on the upper axes. Their absence on the lower portions of the axes suggests that this part of the plants may have been submerged, and that the plants dwelt in boggy ground or even shallow water. In many fossils these appear to consist of a slit-like opening in the middle of a single elongated guard cell, leading to comparison with the stomata of some mosses. However, this is now thought to result from the loss of the wall separating paired guard cells during fossilisation.
Taxonomy and classification
At first most of the fossilized early land plants other than bryophyteBryophyte
Bryophyte is a traditional name used to refer to all embryophytes that do not have true vascular tissue and are therefore called 'non-vascular plants'. Some bryophytes do have specialized tissues for the transport of water; however since these do not contain lignin, they are not considered to be...
s were placed in the 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, and in 1975 Banks developed his earlier proposal to split it into three groups, which he put at the rank
Taxonomic rank
In biological classification, rank is the level in a taxonomic hierarchy. Examples of taxonomic ranks are species, genus, family, and class. Each rank subsumes under it a number of less general categories...
of subdivision. One of these was the subdivision Zosterophyllophytina, named after the genus Zosterophyllum
Zosterophyllum
Zosterophyllum 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...
. For Banks, zosterophyllophytes or zosterophylls comprised plants with lateral sporangia which released their spores by splitting distally (i.e. away from their attachment), and which had exarch strands 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...
. Bank's classification produces the hierarchy:
- Division Tracheata
- Subdivision †Zosterophyllophytina = zosterophyllophytes, zosterophylls
- Subdivision Lycophytina = lycopods
- + other subdivisions
Those who treat most of the extant groups of plants as divisions may raise both the zosterophylls and the Lycophytina sensu Banks to the rank of division:
- Division Zosterophyllophyta = zosterophylls, zosterophyllophytes
- Division Lycophyta = lycophytes
In their cladistic study published in 1997, Kenrick and Crane provided support for a clade uniting both the zosterophylls and the lycopsids, producing a classification which places the zosterophylls in a class Zosterophyllopsida of the subdivision Lycophytina:
- Division Tracheata
- Subdivision Lycophytina = lycophytes
- Class †Zosterophyllopsida = zosterophylls
- Class Lycopodiopsida = lycopsids
This approach has been widely used alongside previous systems. A consequence is that "lycophyte" and corresponding formal names such as "Lycophyta" and "Lycophytina" are used by different authors in at least two senses: either excluding zosterophylls in the sense of Banks or including them in the sense of Kenrick and Crane.
A further complication is that the cladograms of Kenrick and Crane show that the zosterophylls, broadly defined, are paraphyletic, but contain a 'core' clade of plants with marked bilateral symmetry and circinate tips. The class Zosterophyllopsida sensu Kenrick & Crane may be restricted to this core clade, leaving many genera (e.g. Hicklingia
Hicklingia
Hicklingia is a genus of extinct plants of the Middle Devonian . Compressed specimens were first described in 1923 from the Old Red Sandstone of Scotland...
, Nothia
Nothia
Nothia was a genus of Early Devonian vascular plants whose fossils were found in the Rhynie chert in Scotland. It had branching horizontal underground stems and leafless aerial stems bearing lateral and terminal spore-forming organs . Its aerial stems were covered with small 'bumps' , each...
) with no systematic placement other than Lycophytina sensu Kenrick & Crane, but nevertheless still informally called "zosterophylls".
Under whatever name and rank, the zosterophylls have been divided into 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...
and families
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
, e.g. the Zosterophyllales containing the Zosterophyllaceae and the Sawdoniales containing the Sawdoniaceae. Since the publication of cladograms showing that the group is paraphyletic divisions of the class have been less used, being ignored, for example, in the 2009 paleobotany textbook by Taylor et al.
Phylogeny and genera
In 2004, Crane et al. published a unified cladogramCladogram
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 (plants with branched stems bearing 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...
), based on cladistic analyses of morphological features. This suggests that the zosterophylls were a paraphyletic stem group, related to the ancestors of modern lycophytes.
Genera which are included at or around the zosterophyll position in the cladogram or have otherwise been included in the group by at least one source, and hence may be considered zosterophylls in the broad sense, are listed below.
- AdoketophytonAdoketophytonAdoketophyton is a genus of extinct vascular plants of the Early Devonian . The plant was first described in 1977 based on fossil specimens from the Posongchong Formation, Wenshan district, Yunnan, China. These were originally named Zosterophyllum subverticillatum; later the species was...
- AnisophytonAnisophytonAnisophyton was a genus of Silu-Devonian land plant with branching axes.A cladogram published in 2004 by Crane et al. places Anisophyton in the core of a paraphyletic stem group of broadly defined "zosterophylls", basal to the lycopsids .-External links:* from...
- BarinophytonBarinophytonBarinophyton was a genus of Silu-Devonian land plant with branching axes.A cladogram published in 2004 by Crane et al. places Barinophyton in the core of a paraphyletic stem group of broadly defined "zosterophylls", basal to the lycopsids .-External links:* from...
- BathurstiaBathurstiaBathurstia was a genus of scrambling Silu-Devonian land plant with isotomously-branching axes that grew to heights of 30 cm. It is aligned with the Zosterophylls, and produced Calamospora-type spores....
(B) - CrenaticaulisCrenaticaulisCrenaticaulis was an early genus of slender, dichotomously branching, leafless land plants, known from the Devonian period and first described in 1969. They were probably allied to the zosterophylls, and are assigned to subdivision Zosterophyllophytina, or class Zosterophyllopsida. They bore...
(B) - DanziellaDanziellaDanziella is a genus of extinct vascular plants of the Early Devonian . Fossils found in the Artois region of northern France were first described as Zosterophyllum artesianum, but a later review by Edwards showed that they did not fit the circumscription of that genus.-Description:Compressed...
- DeheubarthiaDeheubarthiaDeheubarthia was a genus of Silu-Devonian land plant with branching axes.A cladogram published in 2004 by Crane et al. places Thrinkophyton in the core of a paraphyletic stem group of broadly defined "zosterophylls", basal to the lycopsids .-External links:* from...
- DemersathecaDemersathecaDemersatheca is a genus of extinct vascular plants of the Early Devonian . Fossils were first found in the Posongchong Formation of eastern Yunnan, China. The plant had smooth leafless stems at least 1 mm in diameter, but only regions which bore spore-forming organs or sporangia are well-known...
- DiscalisDiscalisDiscalis is a genus of extinct vascular plants of the Early Devonian . The name is derived from the Greek , referring to the disc-shaped sporangia...
- DistichophytumDistichophytumDistichophytum is a genus of extinct vascular plants of the Late Silurian to Early Devonian , around . The genus has a tangled taxonomic history, also being known as Bucheria and Rebuchia ....
(B) - GosslingiaGosslingiaGosslingia was a genus of Silu-Devonian land plant with branching axes.A cladogram published in 2004 by Crane et al. places Gosslingia in the core of a paraphyletic stem group of broadly defined "zosterophylls", basal to the lycopsids .-External links:* from...
(B) - GumuiaGumuiaGumuia is a genus of extinct vascular plants of the Early Devonian . The genus was first described in 1989 based on fossil specimens from the Posongchong Formation, Wenshan district, Yunnan, China....
- HicklingiaHicklingiaHicklingia is a genus of extinct plants of the Middle Devonian . Compressed specimens were first described in 1923 from the Old Red Sandstone of Scotland...
- HsuaHsuaHsua is a genus of extinct vascular plants, known from the Devonian. its name of the genus honours the Chinese palaeobotanist Jen Hsu.- Features :...
- HuiaHuia (plant)Huia is a genus of extinct vascular plants of the Early Devonian . The genus was first described in 1985 based on fossil specimens from the Posongchong Formation, Wenshan district, Yunnan, China.-Description:...
- KonioriaKonioriaKonioria was a genus of Silu-Devonian land plant with branching axes.A cladogram published in 2004 by Crane et al. places Konioria in the core of a paraphyletic stem group of broadly defined "zosterophylls", basal to the lycopsids .-External links:* from...
- MaciveraMaciveraMacivera is a genus of extinct vascular plants. Fossils were found in sediments in Bathust Island, Nunavut, Canada, from the upper Silurian . The leafless stems branched dichotomously and were relatively thin, being between 0.7 and 1.0 mm wide...
- NothiaNothiaNothia was a genus of Early Devonian vascular plants whose fossils were found in the Rhynie chert in Scotland. It had branching horizontal underground stems and leafless aerial stems bearing lateral and terminal spore-forming organs . Its aerial stems were covered with small 'bumps' , each...
- OricillaOricillaOricilla was a genus of Silu-Devonian land plant with branching axes.A cladogram published in 2004 by Crane et al. places Oricilla in the core of a paraphyletic stem group of broadly defined "zosterophylls", basal to the lycopsids .-External links:* from...
- ProtobarinophytonProtobarinophytonProtobarinophyton was a genus of Silu-Devonian land plant with branching axes.A cladogram published in 2004 by Crane et al. places Protobarinophyton in the core of a paraphyletic stem group of broadly defined "zosterophylls", basal to the lycopsids .-External links:* from...
- Rebuchia, see Distichophytum
- Sawdonia (B)
- SerrulacaulisSerrulacaulisSerrulacaulis was a genus of Silu-Devonian land plant with branching axes.A cladogram published in 2004 by Crane et al. places Serrulacaulis in the core of a paraphyletic stem group of broadly defined "zosterophylls", basal to the lycopsids .-External links:* from...
- TarellaTarellaTarella was a genus of Silu-Devonian land plant with branching axes.A cladogram published in 2004 by Crane et al. places Tarella in the core of a paraphyletic stem group of broadly defined "zosterophylls", basal to the lycopsids .-External links:* from...
- ThrinkophytonThrinkophytonThrinkophyton was a genus of Silu-Devonian land plant with branching axes.A cladogram published in 2004 by Crane et al. places Thrinkophyton in the core of a paraphyletic stem group of broadly defined "zosterophylls", basal to the lycopsids .-External links:* from...
- TrichopherophytonTrichopherophytonTrichopherophyton is a genus of extinct vascular plants of the Early Devonian . Fossils were found in the Rhynie chert, Scotland...
- VentaruraVentaruraVentarura is a genus of extinct vascular plants of the Early Devonian . Fossils were found in the Windyfield chert, Rhynie, Scotland. Some features, such as bivalved sporangia borne laterally and the anatomy of the xylem, relate this genus to the zosterophylls...
- WenshaniaWenshaniaWenshania is a genus of extinct vascular plants found in the Posongchong Formation, Yunnan, China, which is of Early Devonian age . Plants consisted of leafless stems with simple dichotomous branching, and bore spore-forming organs or sporangia all around the sides of stems...
- 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...
(B)
B = included by Banks in his 1975 description of Zosterophyllophytina.
Genera may not be assigned to this group by other authors; for example, Adoketophyton was regarded by Hao et al., who named the genus, as having evolved separately from the lycopsids, so that its taxonomic placement was uncertain.