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.
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== thumb|Zosterophyllum fossils from North Rhine-Westphalia; left: showing curled (circinnate) branch tip; right: with sporangium 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).
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