Lepidodendrales

(paleobotany) The giant club mosses, an order of extinct lycopods (Lycopodiopsida) consisting primarily of arborescent forms characterized by dichotomous branching, small amounts of secondary vascular tissue, and heterospory.

An extinct order of the class Lycopsida which, together with Isoetales, forms the monophyletic rhizomorphaleans, the most derived and diverse group of clubmosses. The lepidodendraleans are best known as the scale trees that dominate most reconstructions of Upper Carboniferous swamps, where they were the main constituent of many coal-forming peats.

Lepidodendraleans are represented most frequently in the fossil record as characteristic “tire-track” bark fragments, which reflect the regular geometric arrangements of leaves, although taxonomic revisions have focused on the more complete information obtained from fossils preserved three-dimensionally in petrified peats. Reconstruction of the plants from their component organs has revealed that the classic trees possessed a wide range of growth architectures and shared lowland habitats with other smaller-bodied genera, reflecting a major radiation that occurred in the Late Devonian and perhaps Early Carboniferous.

Rhizomorphic lycopsids are distinguished by their centralized rootstock (rhizomorph), which permitted bipolar growth; both aerial and subterranean axes could branch, unlike the unipolar rhizomes of other, more primitive lycopsids. This trait and the newly acquired ability to produce wood allowed many rhizomorphic lycopsids to form large upright trees, with trunks up to 115 ft (35 m) tall and 3 ft (1 m) in diameter.

Unlike most seed plants, branches functioned primarily for reproduction and propagule dispersal rather than light capture; even when mature, they formed open canopies that cast little shade.

The warming and drying of the global climate toward the end of the Late Carboniferous greatly reduced the diversity of the rhizomorphic lycopsids, beginning with the most derived and specialized genera such as Lepidophloios and eventually leaving only the increasingly reduced and ecologically specialized isoetaleans to survive to present day. See also Isoetales; Lycophyta; Lycopsida.