Origin and evolution of lycopods

Abstract

The lycopods are known from as early as Sieginean Stage of the Lower Devonian. Lower and Middle Devonian lycopods were all herbaceous. Arborescent taxa appeared by Upper Devonian (e.g., Cyclostigma and Lepidosigillaria). The microphyllous foliage of lycopods seem to have originated from enations as well as telomic trusses. The lycopods attained peak of their evolution during the Upper Carboniferous. Towards the close of the Carboniferous and dawn of the Permian, with gradual dwindling and disappearance of swamps, the lepidodendrids suffered drastic decline numerically and phytogeographically. General aridity of the Triassic resulted in acute dwarfing as evidenced by Pleuromeia. This trend continued further resulting in the highly telescoped Nathorstiana during the Cretaceous. The earlier lycopods were homosporous; heterospory appeared by Upper Devonian. Heterospory ran rampant in the Lepidodendrales. The ultimate in heterospory and the approach to seed habit could be witnessed in Lepidocarpon. Four discrete types of strobilus organization could be recognized by the Lower Carboniferous, viz, 1. Lepidostrobus type, 2. Mazocarpon type, 3. Achlamydocarpon type, and 4. Lepidocarpon type. Recent studies point towards the origin of lycopods along two different pathways, with both Zosterophyllopsida and Rhyniopsida representing the progenitors. All available evidence shows that Lycopsida constitutes a ‘Blind Alley’ in the evolution of vascular plants.