Lepidodendron sp.

Lepidodendron is an extinct genus of arborescent (tree-like) lycopsid plants that thrived during the Carboniferous period, approximately 359 to 299 million years ago. Often referred to as “scale trees” due to their distinctive bark pattern, Lepidodendron and its close relatives formed vast, swampy forests across the equatorial regions of the ancient supercontinent Euramerica. These plants are among the most iconic representatives of the Palaeozoic coal-forming flora, contributing extensively to the coal seams of Europe and North America.

Lepidodendron belonged to the class Lycopodiopsida, a lineage of vascular plants within the division Lycopodiophyta—one of the most ancient groups of land plants, and the most ancient lineage of vascular plant that still has extant representitives. Lycopsids are characterized by the presence of microphylls (small leaves with a single unbranched vein), dichotomous branching, and reproduction via spores rather than seeds. While modern lycopsids, such as Lycopodium and Selaginella, are modest in size and typically low-growing, their Carboniferous ancestors reached tree-like proportions.

Lepidodendron trees could grow up to 30 meters in height, with trunks exceeding 1 meter in diameter. These giants were among the tallest plants of their time and formed dense forests in lowland peat-forming environments. Unlike modern woody trees, Lepidodendron did not form true wood through secondary growth. Instead, their support structure was largely provided by a thick, photosynthetic outer cortex and a relatively narrow band of vascular tissue. The central pith of the stem was hollow or spongy, and the entire trunk was covered in a quilted pattern of diamond-shaped leaf scars—remnants of fallen microphylls that once covered the surface densely.

Reproduction in Lepidodendron was spore-based and highly specialised. The trees produced large, cone-like strobili at the tips of branches, bearing megaspores and microspores in separate structures (a condition known as heterospory). These spores were dispersed into the humid air of the Carboniferous swamps, where gametophytes would develop independently of the parent plant. Lepidodendron’s life cycle, like other lycopsids, alternated between a dominant sporophyte (the tree) and a short-lived gametophyte stage.

The internal anatomy of Lepidodendron reveals its unique evolutionary strategy. Without the extensive secondary xylem seen in gymnosperms or angiosperms, Lepidodendron relied on rapid vertical growth and short lifespans to dominate the swamp forests. Many specimens show limited evidence of age rings or long-lived tissues, suggesting that individual trees may have lived only a few decades before releasing spores and dying, similar to a monocarpic growth strategy.

The bark of Lepidodendron, known as Lepidodendroid bark, is often fossilized in exquisite detail. It consists of overlapping rhomboidal leaf cushions with a central vascular bundle scar and lateral parichnos grooves, which aided gas exchange. These bark impressions are some of the most recognizable plant fossils from the Carboniferous and are commonly mistaken for reptilian skin impressions by non-specialists.

Lepidodendron’s root system, classified under the name Stigmaria, consisted of horizontal rhizomes that bore rootlets in a spiral arrangement. These structures anchored the plant in the waterlogged soils of coal swamp environments. Like the aerial parts of the plant, the roots lacked true wood but were capable of significant girth and spread, often forming dense mats that stabilised the swamp substrate. The rootlets leave similar trace marks on the surface of the rhizomes as the microphylls do on the main stem, though they tend to be rounder and more spaced out.

The ecological dominance of Lepidodendron and its relatives during the Carboniferous coincided with a high-oxygen atmosphere and warm, humid climate. These conditions were ideal for spore dispersal and rapid growth in peat-forming wetlands. However, the genus declined during the late Carboniferous and early Permian, as the global climate became cooler and drier. This shift in climate, known as the Carboniferous rainforest collapse, fragmented the widespread tropical wetlands and led to the replacement of lycopsid-dominated forests by more drought-tolerant seed plants, such as early conifers and seed ferns.

Lepidodendron is often preserved in situ within coal measures, with upright stumps rooted in paleosols (fossil soils) and associated with other swamp flora like Sigillaria, Calamites, and Cordaites.