Thoughts on the early lycopsids and zosterophylls

Morphological, anatomical, and reproductive characteristics of the modern lycopsids are of great value in tracing the ancient lineage of the group. The sporangium is the principal character: lack or misinterpretation of it in fossil specimens places an investigator in a predicament. An informed choice of a group of characters must be made from all those that remain. There are, of course, pitfalls and the defining characters of fossil species of the lycopsids have become confused. Differences in ontogeny of sporangia in early land plants are suggested as a way of separating the early rhyniophytoids with terminal, fusiform, indehiscent sporangia from the cooksonioid plants with terminal, globular or reniform, dehiscent sporangia. The rhyniophytoids are the source of later plant groups other than the lycopsids. Among the cooksonioids, change in the ontogeny of the sporangium in relation to the apical meristem is suggested as the point of differentiation and separation of the zosterophyll and lycopsid lineages. A reinterpretation of Kaulangiophyton suggests that its sporangia are borne adaxially on the leaves rather than terminally on stalks. The apical meristem of Asteroxylon is illustrated and described for the first time and compared with the apical meristem seen in species of Lycopodium. The genus Baragwanathia, questionably the earliest in the lycopsid lineage, is discussed in light of specimens ostensibly dated as of Gorstian age (lower Late Silurian) in Australia. Drepanophycus is perhaps the earliest recognizable member of the lycopsid lineage as opposed to Baragwanathia. The variation in the morphology of lycopsid leaves is of taxonomic value only in differentiating genera, particularly among those in the Devonian. Lineages are suggested diagrammatically.