What Is a Lycopod? Giant Trees That Were Really Overgrown Mosses
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
The forests that became most of the world's coal were built from plants whose closest living relatives are small creeping things a few centimetres high. The trees reached forty metres, were hollow, and reproduced in a way no modern tree does.
The group and its survivors
Lycopods are an ancient lineage of vascular plants, separate from the ferns and from the seed plants, distinguished by small leaves each served by a single unbranched vein, which is a different arrangement from the branching venation of nearly every other plant. They reproduce by spores rather than seeds, which ties them to moist conditions. The living representatives are the club mosses, the spike mosses and the quillworts, all small and inconspicuous, occupying damp ground and rock crevices. Those modest plants are the remnant of a lineage that once dominated the land, which is the standard fate of very old groups. Their fossil record extends back over four hundred million years, making them among the earliest vascular plants and placing their origin close to the colonisation of land itself.
The coal forest trees
The Carboniferous forms bore little resemblance to their living relatives:
- •Heights reaching thirty to forty metres, with trunks a metre or more across at the base
- •Trunks that were largely hollow, supported by a thick outer layer rather than by wood in the modern sense
- •Distinctive diamond or hexagonal patterns on the trunk surface, left by the scars of shed leaves, which make the fossils immediately recognisable
- •Little or no branching until near the top, where the crown divided into a compact structure
- •Roots forming an unusual branching organ that was anatomically more like a modified stem than a root
- •A growth strategy in which the plant grew as an unbranched pole for years, reproduced once at the top, and died
How they made the coal
The forests of the Carboniferous accumulated organic matter at a rate unmatched before or since, and the resulting deposits are the source of most of the world's coal. Several factors combined. Extensive low-lying wetlands were flooded repeatedly by shallow seas, burying plant material in waterlogged oxygen-poor conditions where it could not decay fully. The plants themselves produced enormous quantities of resistant material, particularly in the thick outer layer of the trunks. And the growth strategy, producing a large plant quickly and then dying, generated material in pulses. The once-popular explanation that fungi capable of breaking down wood had not yet evolved, allowing material to accumulate unchallenged, has been substantially revised, since the timing does not fit as neatly as was claimed and since the burial conditions are sufficient on their own.
The other coal forest plants
These trees dominated and did not stand alone, and the associated flora is recognisable in the same deposits. Horsetail relatives grew as tall jointed stems in dense stands, reaching over ten metres, and their living descendants are the small horsetails found in damp ground everywhere. Tree ferns formed an understorey and in places a canopy. Seed ferns, an extinct group bearing seeds on fern-like foliage, were abundant and are the source of many of the leaf fossils collected from coal spoil. Early conifer relatives occupied drier ground away from the swamp and became more important as conditions changed. The animals in these forests are known too, including the largest arthropods that ever lived, amphibians of considerable size and the earliest reptiles, and the whole assemblage is preserved in enough detail to reconstruct the ecosystem rather than only its components.
When the forests failed
The coal forests collapsed towards the end of the Carboniferous as the climate dried, in an episode that fragmented the continuous wetland into isolated patches and eliminated most of the tree lycopods. Ferns and seed plants expanded into the space, and the composition of the world's vegetation shifted permanently towards groups less dependent on standing water for reproduction. The event has been studied as one of the clearest cases of a rainforest collapse in the fossil record, with measurable consequences for the animals living in those forests, including amphibians, which declined, and early reptiles, which were less tied to water and did better. The parallel to present concerns is obvious and is drawn explicitly in the literature, with the qualification that the timescales involved were far longer than anything happening now.
The takeaway
Small leaves with a single unbranched vein and reproduction by spores mark the group, whose living members are club mosses a few centimetres high. The Carboniferous trees reached forty metres, were largely hollow, grew as unbranched poles, reproduced once and died. Their remains in waterlogged oxygen-poor swamps became most of the world's coal, and the forests collapsed when the climate dried.