Lepidodendron hickii and Generic Delimitation in Carboniferous Lepidodendrid Lycopods

Lepidodendron hickii is tlie 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 plesiomorphic (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. The ordering of fossil plants into taxonomic hierarchies reflective of phylogeny,, or even of shared morphological characters, has been a problematic part of paleobotany. Difficulties in approximating natural relationships of extinct plants center on frequent lack of organic connection between vegetative and reproductive parts, limits in assessing the developmental significance of morphology from static and nonconnected specimens, and the diversity of names applied to different forms of preservation. These difficulties are reflected by the Carboniferous lepidodendrid lycopods. Years of study have yielded a wealth of morphological information but also produced a tangled web of nomenclature, which inhibits recognition of discrete forms and the evolutionary relationships among them. The reasons for this are complex, rooted in the traditional treatment of certain forms and in the truly untested belief that anatomy is not a diagnostic taxonomic tool and does not correlate with distinctive leafcushion form, the character upon which most "genera" are based. Central to this problem has been an extreme overextension of the name Lepidodendron both to compression and petrifaction fossils. The type species of Lepidodendron (fide Thomas 1970) is L. aculeatum Sternb., a species based on compression specimens with distinctive, vertically elongate leaf cushions that have infrafoliar parichnos. The only species of anatomically-preserved lycopod stems that has leaf cushions with infrafoliar parichnos and general leaf-cushion form of the L. aculeatum-type is L. hickii Watson (Watson 1907). Other anatomically-preserved forms that historically have been placed in Lepidodendron include, totally or in part, Paralycopodites brevifolius (DiMichele 1980), Lepidophloios (DiMichele 1979a), Sublepidophloios, and a group of lepidodendrids including "Lepidodendron" vasculare (DiMichele 1981). These distinctive vegetative forms, differing from each other in leaf-cushion architecture and anatomy, are correlated with different reproductive structures, further emphasizing their distinctiveness. The application of names to them, although not trivial, is secondary in importance to the recognition that they are distinct, that there are not morphological clines among them, and that not all of them can be Lepidodendron, despite historical treatments, if the name is ever to have any biological significance. Lepidodendron hickii, the focus of this report, is the best representative of Lepidodendron anatomy in light of uniquely shared similarities between L. hickii leaf cushions and those of the type species, L. aculeatum. The distinctiveness of L. hickii anatomy when compared to other lepidodendrids with leaf-cushion height; width ratios > 1 suggests that the height: width ratio of cushions is not a sufficient basis for generic circumscription; greater leaf-cushion height