Zosterophyllum Sp
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''Zosterophyllum'' was a genus of
Silurian The Silurian ( ) is a geologic period and system spanning 24.6 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 shortest period of the Paleozo ...
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Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...
vascular land plants with naked branching axes on which usually kidney-shaped sporangia were arranged in lateral positions. It is the type genus for the group known as
zosterophyll The zosterophylls are a group of extinct land plants that first appeared in the Silurian period. The taxon was first established by Banks in 1968 as the subdivision Zosterophyllophytina; they have since also been treated as the division Zosteroph ...
s, thought to be part of the lineage from which modern lycophytes evolved. More than 20 species have been described.


Description

The diagnostic features of the genus have changed since its first description in 1892, as the original species (''Zosterophyllum myretonianum'') has become better known, and as other species have been discovered. ''Zosterophyllum'' is a
vascular plant Vascular plants (), also called tracheophytes () or collectively Tracheophyta (), form a large group of land plants ( accepted known species) that have lignified tissues (the xylem) for conducting water and minerals throughout the plant. They al ...
. The axes (stems) are naked, lacking leaves or outgrowths ("enations"). When branching occurs, the branches are either isotomous (equally sized) or pseudomonopodial (one branch is larger than the other but still clearly involves division of the original axis rather a distinct side growth). The
sporangia A sporangium (; from Late Latin, ) is an enclosure in which spores are formed. It can be composed of a single cell or can be multicellular. Virtually all plants, fungi, and many other lineages form sporangia at some point in their life cy ...
are upright on short stalks. In face view, they are flattened, usually kidney-shaped (reniform). They open (dehisce) along the top forming two equally sized valves. Sporangia are grouped into a compact spike in which they are either helically arranged or form distinct rows (e.g. ''Z. llanoveranum''). ''Z. myretonianum'' is thought to have been semiaquatic.


Taxonomy

The genus ''Zosterophyllum'' was erected in 1892 by David P. Penhallow for the type species ''Zosterophyllum myretonianum'', based on fossils found at Myreton quarry near
Dundee Dundee (; sco, Dundee; gd, Dùn Dè or ) is Scotland's fourth-largest city and the 51st-most-populous built-up area in the United Kingdom. The mid-year population estimate for 2016 was , giving Dundee a population density of 2,478/km2 or ...
, Scotland, in
Lower Devonian The Early Devonian is the first of three epochs comprising the Devonian period, corresponding to the Lower Devonian series. It lasted from and began with the Lochkovian Stage , which was followed by the Pragian from and then by the Emsian, w ...
rocks (from about ). Species with radially symmetrical spikes of sporangia have been placed in subgenus ''Zosterophyllum'', those with bilaterally symmetrical spikes in subgenus ''Platyzosterophyllum''. Hao and Xue in 2013 used the absence of terminal
sporangia A sporangium (; from Late Latin, ) is an enclosure in which spores are formed. It can be composed of a single cell or can be multicellular. Virtually all plants, fungi, and many other lineages form sporangia at some point in their life cy ...
to place some species, such as ''Z. llanoveranum'', in the
paraphyletic In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...
order
Gosslingiales Gosslingiales is an order of extinct zosterophylls. The zosterophylls were among the first vascular plants in the fossil record, and share an ancestor with the living lycophytes. The group has been divided up in various ways. Hao and Xue in 20 ...
, a group of zosterophylls considered to have indeterminate growth, with fertile branches generally showing
circinate vernation Vernation (from ''vernal'' meaning ''spring'', since that is when leaves spring forth in temperate regions) is the formation of new leaves or fronds. In plant anatomy, it is the arrangement of leaves in a bud. In pine species, new leaves are short ...
(initially curled up). Other species, such as ''Z. myretonianum'', were not placed in the order, as they did not have terminal sporangia.


Phylogeny

A cladogram published in 2004 by Crane et al. places the species of ''Zosterophyllum'' in a
paraphyletic In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...
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. ...
of broadly defined "
zosterophyll The zosterophylls are a group of extinct land plants that first appeared in the Silurian period. The taxon was first established by Banks in 1968 as the subdivision Zosterophyllophytina; they have since also been treated as the division Zosteroph ...
s", basal to the lycopsids (living and extinct clubmosses and relatives). On this view, the genus is not
monophyletic In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic gro ...
. A cladistic analysis by Hao and Xue in 2013 agreed that ''Zosterophyllum'' is not monophyletic, with the three species of ''Zosterophyllum'' they included falling into different clades, some being closer to the
Gosslingiales Gosslingiales is an order of extinct zosterophylls. The zosterophylls were among the first vascular plants in the fossil record, and share an ancestor with the living lycophytes. The group has been divided up in various ways. Hao and Xue in 20 ...
than others. Their analysis differed in producing a monophyletic clade of zosterophylls (their Zosterophyllopsida).


Species

Species that have been described include: * †''Z. arcticum'' Kotyk (1998) nom. inv. * †''Z. australianum'' Lang & Cookson (1931) * †''Z. bifurcatum'' Li & Cai (1977) – fragmentary * †''Z. bohemicum'' (Kräusel & Weyland 1933) Obrhel (1959) * †''Z. bokkeveldense'' Plumstead (1978) * †''Z. confertum'' Gossmann et al (2021) * †''Z. deciduum'' Gerrienne (1988) * †''Z. devriesii'' Plumstead (1978) * †''Z. dispersum'' Zakharova (1984) * †''Z. divaricatum'' Gensel (1982) * †''Z. dushanense'' Li & Cai (1977) * †''Z. fertile'' Leclercq (1942) * †''Z. llanoveranum'' Croft & Lang (1942) * †''Z. longhuashanense'' Li & Cai (1977) – fragmentary * †''Z. longum'' (Høeg 1942) Høeg (1967) * †''Z. minifertillum'' Hao & Xue (2013) * †''Z. minor'' Ananiev (1960) * †''Z. minorstachyum'' Xue (2009) * †''Z. myretonianum'' Penhallow (1892) * †''Z. ovatum'' Edwards & Li (2018) * †''Z. qujingense'' Hao, Xue & Wang (2007) * †''Z. ramosum'' Hao & Wang (2000) * †''Z. reflexum'' Kotyk (1998) nom. inv. * †''Z. rhenanum'' Kräusel & Weyland (1935) * †''Z. shengfengense'' Hao et al. (2010) * †''Z. sinense'' Li & Cai (1977) * †''Z. spathulatum'' Li & Cai (1977) – fragmentary * †''Z. spectabile'' Schweitzer (1979) * †''Z. tenerum'' Hao & Xue (2013) * †''Z. xishanense'' Hao, Xue & Wang (2007) * †''Z. uimenense'' Udodov et al. (1971) * †''Z. yunnanicum'' Hsu (1966) Some species have been transferred to other genera: * ''Z. artesianum'' Danzé-Corsin (1956) to ''
Danziella ''Danziella'' is a genus of extinct vascular plants of the Early Devonian (around ). 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 ...
artesiana'' * ''Z. contiguum'' Li & Cai (1977) to ''
Demersatheca ''Demersatheca'' is a genus of extinct vascular plants of the Early Devonian (Pragian, around ). Fossils were first found in the Posongchong Formation of eastern Yunnan, China. The plant had smooth leafless stems at least 1 mm in diameter, ...
contigua'' * ''Z. longa'' Wang (2007) to '' Yanmenia longa'' * ''Z. subverticillatum'' Li & Cai (1977) to ''
Adoketophyton ''Adoketophyton'' is a genus of extinct vascular plants of the Early Devonian (Pragian stage, around ). The plant was first described in 1977 based on fossil specimens from the Posongchong Formation, Wenshan district, Yunnan, China. These were ...
subverticillatum'' * ''Z. sichuanense'' Geng (1992) to '' Ornicephalum sichuanense'' * ''Z. minutum'' Tims (1980) to '' Gippslandites minutus''


References


Bibliography

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External links


Cladogram
from {{Taxonbar, from=Q4024838 Silurian plants Devonian plants Silurian first appearances Devonian extinctions Paleozoic life of New Brunswick Paleozoic life of Nunavut Zosterophylls Prehistoric lycophyte genera