Lepidodendron hickii and Generic Delimitation in Carboniferous Lepidodendrid Lycopods

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Abstract
Lepidodendron hickii is the only anatomically preserved Carboniferous lepidodendrid with leaf cushions referable to the compression-based L. aculeatum, the type of Lepidodendron. Historically Lepidodendron has come to encompass several kinds of arborescent lycopods with distinctive vegetative and reproductive morphologies, among which no truly intermediate forms are known. These include totally or in part Lepidophloios (and Sublepidophloios, a possible congener), Paralycopodites, "Lepidodendron" sensu L. vasculare and others, and a large number of fragmentary specimens of varying affinities. The discrete nature of the character states defining these genera suggests that anatomy is sufficient for delimitation of lepidodendrid genera. Simple leaf-cushion shape is of dubious value in the delimitation of Lepidodendron because the "Lepidodendron"-shape appears to be pleiomorphic (ancestral). Lepidodendron hickii is associated with Achlamydocarpon takhtajanii megasporangium-sporophyll units, which suggests close alliance with the herbaceous L. serratum, also a producer of A. takhtajanii. At the generic level, Lepidodendron (sensu L. hickii and L. serratum) is most similar phenetically to Lepidophloios.