Today's humble clubmosses had ancestors taller than a 12-storey building
Living lycophytes are modest plants, easily mistaken for moss. Their Carboniferous relatives, the Lepidodendrales, grew into trees over 40 metres tall and formed vast forests whose remains helped make coal. The lineage reaches back about 425 million years, making it one of the oldest surviving branches of vascular plants.
Lycophytes are vascular plants that spread by spores rather than seeds. As in ferns and flowering plants, the spore-bearing generation is the dominant, visible plant, but lycophytes split off early from the line leading to all other vascular plants, known as euphyllophytes. Some produce a single kind of spore; others make two sizes. Two anatomical signatures define the group: spore cases borne on the sides of the stem, often kidney-shaped, rather than at the tips, and a central strand of water-conducting tissue whose first-formed cells lie on the outside.
Their leaves are distinctive too. Living lycophytes carry microphylls, leaves served by a single vein, unlike the complex, multi-veined leaves of most other plants. Fossils trace how these may have arisen. Zosterophyllum myretonianum, from the Devonian, had bare stems. Sawdonia ornata sprouted flaps called enations with no plumbing. Asteroxylon sent a vein only as far as the base of each flap, and Leclercqia had fully veined microphylls, suggesting a stepwise path.
Where the boundaries lie is contested. Names such as Lycopodiophyta, Lycophyta and plain lycophyte are used by some authors to include the extinct zosterophylls and by others to leave them out. A 1997 study by Kenrick and Crane treated zosterophylls as an untidy, paraphyletic cluster, while a 2013 analysis by Hao and Xue placed them beneath both lycophytes and euphyllophytes, dissolving the broad group altogether. The odd genus Renalia, with tip-borne yet kidney-shaped spore cases, shows why early land plants are so hard to sort.
The Pteridophyte Phylogeny Group's 2016 consensus puts every living lycophyte in the class Lycopodiopsida, which holds roughly 1,290 to 1,340 species, a small remnant of a group that once ruled the landscape.
Source: Lycophyte
