Lycopsids

Lycopsids, or Lycopodiophyta, are an ancient lineage of vascular plants that includes the modern clubmosses (Lycopodiaceae), spikemosses (Selaginellaceae), and quillworts (Isoetaceae). They are among the oldest surviving lineages of vascular plants, with a fossil record extending back to the late Silurian (~420 million years ago). In the Carboniferous and Permian periods, large arborescent lycopsids formed extensive forests and contributed to coal deposits.

Morphology and reproduction: Most living lycopsids are small, ground-covering or epiphytic plants with simple, often needle-like leaves (microphylls) arranged along dichotomously branching stems. Many species produce sporangia in strobili (cones) on the tips of fertile shoots. Lycopodiaceae are primarily homosporous, producing a single type of spore; Isoetaceae and Selaginellaceae are heterosporous, producing microspores and megaspores.

Ecology and distribution: Today's lycopsids occur in tropics and temperate regions worldwide. Clubmosses and spikemosses prefer moist, shaded habitats, while quillworts are often aquatic or marshy. Fossil lycopsids included tall, tree-like forms with scale-like leaves, which dominated Carboniferous forests and contributed to peat and coal formation.

Taxonomy and legacy: The group comprises three extant families: Lycopodiaceae (clubmosses), Isoetaceae (quillworts), and Selaginellaceae (spikemosses). They are among the earliest diverging lineages of vascular plants, providing key insights into early land ecosystems and plant evolution.