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 has become better known, and as other species have been discovered. Zosterophyllum is a vascular plant. The axes are naked, lacking leaves or outgrowths. When branching occurs, the branches are either isotomous or pseudomonopodial. The sporangia are upright on short stalks. In face view, they are flattened, usually kidney-shaped. They open 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. Z. myretonianum is thought to have been semiaquatic.
Taxonomy
The genus Zosterophyllum was erected in 1892 by David P. Penhallow for the type speciesZosterophyllum myretonianum, based on fossils found at Myreton quarry near Dundee, Scotland, in Lower Devonian rocks. 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. 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 Craneet al. places the species of Zosterophyllum in a paraphyletic stem group of broadly defined "zosterophylls", basal to the lycopsids. 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.
Species
Species that have been described include:
†Z. australianum Lang & Cookson
†Z. bifurcatum Li & Cai – fragmentary
†Z. deciduum Gerrienne
†Z. divaricatum
†Z. dushanense
†Z. fertile Leclercq
†Z. llanoveranum Croft & Lang
†Z. longhuashanense – fragmentary
†Z. longum Høeg
†Z. minifertillum Hao & Xue
†Z. minor Ananiev
†Z. minorstachyum
†Z. minutum
†Z. myretonianum Penh.
†Z. ovatum Edwards & Li
†Z. qujingense
†Z. ramosum Hao & Wang
†Z. rhenanum Kräusel & Weyland
†Z. shengfengense
†Z. sichuanense – correct genus?
†Z. sinense
†Z. spathulatum – fragmentary
†Z. spectabile
†Z. tenerum Hao & Xue
†Z. xishanense
†Z. yunnanicum Hsü
Some species have been transferred to other genera:
