Enter the Coal Swamp Forest: The giant Lycopsid

"The "coal swamp" is one of the most powerful images in palaeontology. Dense, dark, and damp populated by strange trees, giant dragonflies, and sluggish tetrapods resting on rotting logs -a diorama can be found in almost every museum and is short-hand for the Carboniferous tropics. However appealing, this visual representation of the coal-swamp forest, based on analogy with modern tropical rainforests, is largely inaccurate."

W.A. DIMICHELE (2001): "Paleobiology II."

Fig.1. Illustration by Zdenek Burian for the book "Prehistoric Animals" (1956) showing a Carboniferous landscape - note in the background a dense forest of tree like lycopsids of the genus Lepidodendron.

Unlike modern forests, that are dominated by two large plant classes (tropical forests for example by angiosperms and boreal forests by gymnosperms) or by only a few species, the Carboniferous forest was composed of at least four classes and more than 10 orders of plants - with strikingly different morphology and ecology. The "Coal swamp" has therefore no modern analogy, despite the classic iconography is inspired mostly by modern Cypress swamps found for example in the modern Everglades of Florida.
Modern swamps and mires are characterized by a spatially gradient in nutrients and hydrology, these factors change also over time. The groundwater table is the dominant factor affecting the plant assemblage. Various fossil evidences show that the water level of the Coal swamp also strongly varied in time: preserved stumps are signs of an increase in the water table, killing vegetation and preserving it, charcoal layers show a decrease and drying up of the mire. The climate and therefore the environment of the Carboniferous swamp was not so monotonous as depicted in our imagination - there were phases of inundation and phases of drought.

Fig.2. Sketch of the outcrop in Victoria Park (Glasgow) preserving various stumps of Stigmaria (Lepidodendron) by Chris Meadows (1880).

The clubmosses, Class Lycopsida (or Lycopodiopsida) appear today as morphologically simple herbaceous plants, but are a very honourable and old group; fossil specimens reach back to the time when the first organisms colonized the dry land during the Silurian. Today more than 1.100 species are described, very similar in their basic morphology: from a horizontally creeping rhizome vertical branches grow straight upwards; these branches support the sporangia (spore producing organs) and are covered by small scaly leaves.

Fig.3. The modern clubmoss Huperzia selago.

Fig.4. ... and the spikemoss Selaginella helvetica.

However during the Carboniferous several lycopsid groups achieved giant size and developed very different shapes - the dominant "tree" seen in most of the reconstructions of the Carboniferous Coal swamps is a giant lycopsid.
The genus Lepidodendron (or "scale tree"), a 35m high tree like lycopsid, was known for almost 200 years from the imprints of the "bark", showing a typical regular patter with the scars of the single leaves. Despite its common appearance in most of the reconstructed landscapes of the Carboniferous epoch, Lepidodendron was surely limited in his geographical and temporal range. The plant was adapted to wet conditions, water transported also it spores - however such habitats were not characteristic for the entire Carboniferous, during the late Carboniferous the climate became drier and the genus Lepidodendron was replaced soon by smaller lycopsid genera, reaching almost 1 meter in height.

Today various different fossils were merged together to reconstruct the morphology of a Lepidodendron tree: The lower part of the stem, referred in the past as Knorria, was smooth, only the upper part, referred as Lepidophloios, was covered with small, needle shaped leaves (Lepidophylloides) similar to the branches of modern lycopsids. An important difference to modern lycopsids was the position of the sporangia - collocated at the end of dichotomously branching twigs and similar to a cone, known previously as the fossil genus Lepidostrobus.

However this reconstruction similar to a modern tree with stem and branches is valid only for a short phase of reproduction of the plant, when the organism finally produces a terminal sporangium. Until this phase it is more plausible assuming that Lepidodendron resembled much more a simple, unbranched lycopsid, forming very open forests with scattered small and large individuals.

Fig.5. Schematic reconstruction of Lepidodendron as adult, fertile individual and younger individual lacking branches with spore cones, probably the usual habit to be spotted in the Carboniferous forest. Depending from author and reconstruction method, the branches of the mature plant were displayed as standing upright or sag to the ground. The scale tree is named after the typical structure preserved on the bark - the leaf cushion - structure that supported small, needle like leaves covering the upper part of the plant.

To be continued...

Bibliography:

BRIGGS, D.E.G. & CROWTHER, P.R. (2003): Palaeobiology II. Blackwell Publishing: 583
SPINAR, Z.V. (1976): Quando l´uomo non c´era. Fratelli Fabbri Editori, Milano: 228
WILLIS, K.J. & McELWAIN, J.C. (2002): The evolution of plants. Oxford University Press - Oxford: 378