The zosterophylls are a group of extinct

land plants The embryophytes () are a clade of plants, also known as Embryophyta (Plantae ''sensu strictissimo'') () or land plants. They are the most familiar group of photoautotrophs that make up the vegetation on Earth's dry lands and wetlands. Embryophy ...

that first appeared in the

Silurian The Silurian ( ) is a geologic period and system spanning 23.5 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the third and shortest period of t ...

period. The

taxon In biology, a taxon (back-formation from ''taxonomy''; : taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular name and ...

was first established by Banks in 1968 as the subdivision Zosterophyllophytina; they have since also been treated as the division Zosterophyllophyta or Zosterophyta and the

class Class, Classes, or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used d ...

or plesion Zosterophyllopsida or Zosteropsida. They were among the first

vascular plant Vascular plants (), also called tracheophytes (, ) or collectively tracheophyta (; ), are plants that have lignin, lignified tissues (the xylem) for conducting water and minerals throughout the plant. They also have a specialized non-lignified Ti ...

s in the fossil record, and had a world-wide distribution. They were probably

stem-group In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor. ...

lycophyte The lycophytes, when broadly circumscribed, are a group of vascular plants that include the clubmosses. They are sometimes placed in a division Lycopodiophyta or Lycophyta or in a subdivision Lycopodiophytina. They are one of the oldest lineag ...

s, 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 The Arctic (; . ) is the polar regions of Earth, polar region of Earth that surrounds the North Pole, lying within the Arctic Circle. The Arctic region, from the IERS Reference Meridian travelling east, consists of parts of northern Norway ( ...

Canada Canada is a country in North America. Its Provinces and territories of Canada, ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, making it the world's List of coun ...

Morphology

The stems of zosterophylls were either smooth or covered with small spines known as enations, 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 An exarch (; from Ancient Greek ἔξαρχος ''exarchos'') was the holder of any of various historical offices, some of them being political or military and others being ecclesiastical. In the late Roman Empire and early Byzantine Empire, ...

, and the metaxylem developed centripetally. The

sporangia A sporangium (from Late Latin, ; : sporangia) is an enclosure in which spores are formed. It can be composed of a unicellular organism, single cell or can be multicellular organism, multicellular. Virtually all plants, fungus, fungi, and many ot ...

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 plants are plants that bear flowers and fruits, and form the clade Angiospermae (). The term angiosperm is derived from the Ancient Greek, Greek words (; 'container, vessel') and (; 'seed'), meaning that the seeds are enclosed with ...

''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.

Taxonomy and classification

At first most of the fossilized early land plants other than

bryophyte Bryophytes () are a group of embryophyte, land plants (embryophytes), sometimes treated as a taxonomic Division (taxonomy), division referred to as Bryophyta ''Sensu#Common qualifiers, sensu lato'', that contains three groups of non-vascular pla ...

s were placed in the

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''., cited in 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

strands of

xylem Xylem is one of the two types of transport tissue (biology), tissue in vascular plants, the other being phloem; both of these are part of the vascular bundle. The basic function of the xylem is to transport water upward from the roots to parts o ...

. 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:, p. 1028 :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'', '' 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 Order, ORDER or Orders may refer to: * A socio-political or established or existing order, e.g. World order, Ancien Regime, Pax Britannica * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * H ...

and

families Family (from ) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictability, structure, and safety as ...

, 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

In 2004, Crane et al. published a unified

cladogram A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...

for the polysporangiophytes (plants with branched stems bearing

), based on cladistic analyses of morphological features. This suggests that the zosterophylls were a

paraphyletic Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...

stem group, related to the ancestors of modern lycophytes.

Genera

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 broadest sense, are listed below. "B" indicates genera included by Banks in his 1975 description of Zosterophyllophytina. * '' Adoketophyton'' * '' Anisophyton'' * '' Barinophyton'' * '' Bathurstia'' (B) * '' Crenaticaulis'' (B) * '' Danziella'' * '' Deheubarthia'' * '' Demersatheca'' * '' Discalis'' * '' Distichophytum'' (B) * ''

Gosferia Gosferia was a genus of zosterophylls with curved axes and renal sporangia. It is known from Belgium Belgium, officially the Kingdom of Belgium, is a country in Northwestern Europe. Situated in a coastal lowland region known as the Low ...

'' (= ''Forgesia'') * '' Gosslingia'' (B) * '' Guangnania'' * '' Gumuia'' * '' Hicklingia'' * '' Hsua'' * '' Huia'' * '' Jugumella'' * '' Konioria'' * '' Macivera'' * '' Nothia'' * '' Oricilla'' * '' Protobarinophyton'' * '' Ramoferis'' * ''Rebuchia'', see ''Distichophytum'' * '' Sawdonia'' (B) * '' Serrulacaulis'' * '' Tarella'' * '' Thrinkophyton'' * '' Trichopherophyton'' * '' Ventarura'' * '' Wenshania'' * '' Xitunia'' * '' Yunia'' * '' Zosterophyllum'' (B) 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. Barinophytes, like ''Barinophyton'', have been considered to be possible lycopsids, or to fall between the lycopsids and the

euphyllophyte The euphyllophytes are a clade of plants within the tracheophytes (the vascular plants). The group may be treated as an unranked clade, a division under the name Euphyllophyta or a subdivision under the name Euphyllophytina. The euphyllophytes ...

References

External links