Zosterophyllum

''Zosterophyllum'' was a genus of Silurian-Devonian 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 zosterophylls, 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. 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 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, Scotland, in Lower Devonian 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 to place some species, such as ''Z. llanoveranum'', in the paraphyletic order Gosslingiales, a group of zosterophylls considered to have indeterminate growth, with fertile branches generally showing circinate vernation (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 stem group of broadly defined "zosterophylls", basal to the lycopsids (living and extinct clubmosses and relatives). On this view, the genus is not monophyletic.

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 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 artesiana''
* ''Z. contiguum'' Li & Cai (1977) to ''Demersatheca contigua''
* ''Z. longa'' Wang (2007) to '' Yanmenia longa''
* ''Z. subverticillatum'' Li & Cai (1977) to ''Adoketophyton 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