"History of Geology" will be dedicated until the end of the world year to two topics - the evolution of paleoart and - appropriately - the supposed age and end of the earth. A first glimpse on paleoart introduced the early soft-tissue reconstructions of animals, however also other organisms are worth to be studied, reconstructed and displayed.

"There are so many plants on the earth, that there is a danger to thinking them trivial...[]

"Carl Sagan in "Cosmos, ep. 2 - One Voice in the Cosmic Fugue"

Plants are possibly the first fossils recognized as the remains of once living organisms - as many fossil species still resemble extant species. Similar to other fossil groups, also paleoart involving plants experienced various phases. Before the 19th century pictorial representation of single fossils dominated following the tradition of early "collections and classifications of empirical knowledge".

Already in 1699 the English naturalist Edward Lhwyd (1660-1709) depicted various fossil plants in his catalogue "Lithophilacii Britannica ichnographia,...[]". Also the "father of modern binomial nomenclature" Carl von Linné (1707-1778) studied and published some notes about plant fossils hosted in the cabinets of curiosities of Swedish noblemen. The oldest published work dealing almost exclusively with fossil plants is the "Herbarium diluvianum" by the Swiss scholar Johann Jakob Scheuchzer (1672-1733), published in various edition between 1709 and 1723.

Fig.1. Plate from J.J. Scheuchzer´s opus magnus "Herbarium diluvianum" (1709-1723) (image in public domain).

In 1804 the German Geologist Ernst Freiherr von Schlotheim (1764-1832) published a description of Permian plants with the title "Beschreibung merkwürdiger Kräuter-Abdrücke und Pflanzen-Versteinerungen/Description of curious imprints of herbs and plant-petrifactions", where he classified various fossil plants comparing their supposed ecology with modern plants - the first real book on paleobotany.

Fig.2. Plate from Caspar von Sternberg´s "Versuch einer geognostisch-botanischen Darstellung der Flora der Vorwelt/Attempt of a geognostic-botanical presentation of the flora of the ancient world" (1820-1838), showing the "bark" of an extinct Lepidodendron-"tree" related to the modern clubmosses (image in public domain).

Thereafter the interest of naturalists shifted soon from single specimens to entire plant communities. The increased mining and quarrying activity in Victorian Britain and during the Industrial Revolution exposed especially remains of Paleozoic (541 to 252,2 million years ago) plants, preserved in coal layers.

Fig.3. & 4. "Vegetation of the Devonian Period, restored", a scenery published in the book "The Geological History of Plants" (1888) by Canadian geologist John William Dawson (1820-1899), like the Carboniferous "Lepidodendron tree" shown below (images in public domain).Similar to paleoart involving dinosaurs also this iconic scenery persisted for decades in popular media. Still today "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." (cited from W.A. DIMICHELE (2001): "Paleobiology II").

Unlike modern forests, 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 (359,2 to 299,0 million years ago) forest was composed of at least four classes and more than ten orders of plants - with different morphology and ecology. The plant community found in a "coal swamp" has therefore no modern analogy. Despite this fundamental problem, also most illustrations tend to oversimplify or generalize ancient ecosystems.

For example modern swamps and mires are characterized by a spatial gradient of various ecological factors, like nutrients and hydrology, and to complicate this situation these factors can change also over time. Stumps found in fossil coal layers are signs of a high water level, killing and preserving parts of plants. Charcoal layers show a decrease of the water level and drying up of the mire. The climate and therefore the environment of the Carboniferous swamp was not so monotonous as we like to think - there were phases of inundation followed by phases of drought.

Also focusing on a single species can be problematic. Today clubmosses (Lycopsida) are small, herbaceous plants, however during the Carboniferous several varieties of giant clubmosses evolved - the dominant "tree" shown in most reconstructions of coal swamps.

Fig.5. A modern clubmoss-species (Clavatum sp.) with sterile and fertile "branches", characterized by the "cone-like" terminal sporangia. The organs of reproduction mark the end of growth of single "branches" of the plant, in most modern clubmoss-species therefore the sporangia are positioned laterally.

The genus Lepidodendron (or "scale tree"), a 35m high ancient clubmoss-species, was known for almost 200 years from the fossil imprints of the "bark", characterized by a typical regular patter of scars left by needlelike leaves (lik shown in fig.2. and fig.4.). Despite being common in most reconstructed Carboniferous landscapes, Lepidodendron was a plant limited in its geographical and temporal range. Adapted to a wet environment, it became extinct during the late Carboniferous when the climate became drier, replaced by other, smaller clubmoss-species.

Also the "classic" anatomy of Lepidodendron shown in many drawings is probably not entirely correct. An important difference to modern clubmosses is the position of the organs of reproduction - the sporangia (reseambling a sort of a cone, like shown in fig.4.-C) - in the fossil species. The sporangia are found at the end of twig-like structures. Many artists like to show Lepidodendron resembling a modern tree - characterized by a stem with various branches supporting the cones. However a terminal sporangium could only be produced by a full-grown plant. Younger and immature specimens resembled probably modern clubmosses, with simple unbranched "trunk".

It is therefore more plausible to assume that Lepidodendron formed very open forests, with single small, unbranched and large, branched specimens, accompanied by various other plant species.

Bibliography:

BOWDEN, A.J.; BUREK, C.V. & WILDING, R. (eds.) (2005): History of Palaeobotany - Selected essays. Geological Society Special Publication 241: 304

DAVIDSON, J.P. (2008): A History of Paleontology Illustrations. Indiana University Press, Bloomington: 217