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zosterophyllophytes (zosterophylls) A group of extinct vascular plants that lived in the Devonian period. They were small herbs, similar in many ways to the rhyniophytes; for example, members of the principal genus, Zosterophyllum, grew about 15 cm tall in swampy areas and had naked stems that branched dichotomously (equally). However, other members evolved lateral branches and scalelike outgrowths. Moreover, the arrangement of their spore-forming organs and xylem indicates that they represent a line of evolution, separate from the rhyniophytes, that led to the lycophytes (e.g. clubmosses) but not to the seed plants.