Plants can be preserved in the geologic record in various ways: as a mould, as compression/impression fossils, as permineralized fossils and even as unaltered plant remains.
- A compression fossil forms simply by plant remains that became embedded and buried by accumulating sediments. The water is squeezed out and the plant flattened. The original organic can be conserved as thin carbonaceous film forming a silhouette of the original plant or lost completely. In this case the rocks preserve only an impression of the former plant. This impression can be refilled by minerals, forming a "cast" of the original plant. In this kind of fossil the general morphology of the plant can be studied.
Fig.1. Rhacopteris asphlenites from the Carboniferous showing preservation by compression (carbonaceous film) and impression.
Fig.2. Fossil Ficus species from the Eocene lagerstätte of Bolca (Italy), here the original plant tissue was replaced by reddish coloured iron oxides.
Fig.3. Asplenium scolopendrium preserved as imprint in Holocene travertine. If such an imprint is refilled with other minerals a three-dimensional cast of the former plant can be formed.
Fig.4. Cast of a piece of wood composed of silica discovered in a volcanic tuff from the Permian.
Coal is a compression fossil in which the organic substance is enriched in carbon and depleted by other volatile elements like hydrogen or nitrogen in various degrees, when plant remains are still visible it is classified as Lignite, when completely homogenized it is called Anthracite.
Fig.5. Example of coal in thin section, showing collapsed Megaspores, amalgamated plant fragments (Fusinite) and homogenous matrix.
- Permineralization is the classic petrifaction; the organic substance of the tissue is replaced by minerals deposited from percolating fluids. These minerals can be calcium carbonate, iron oxides or hydroxides, iron sulphides, silica, or - in cold environments - also ice. This kind of fossil is preserved undeformed and can deliver information about the three-dimensional structure of the plant.
Fig.6. Polished transverse section of a tree fern of the genus Psaronius, showing the structure of the false trunk, composed of roots growing together.
- Some tissues of plants are exceptional stable, like the cuticle covering the surface of a plant, or the Sporopollenin, organic substance forming the hull of spores and pollen grains. These components can be conserved, under the right conditions, unaltered for million of years.
The epidermis and surface of plants possesses distinctive characters useful for taxonomic classification, like for example the superficial cell pattern, or the shape and distribution of the stomata, papillae and glands, spores and pollen grains also possess very distinctive surface patterns.
If microbial activity is inhibited or oxygen not available also the less resistant plant tissues can be embedded in sediments and conserved unaltered. The most important fossils of this kind were recovered from lake sediments, amber and packrat middens.
In lakes due differences in temperature and density of the water the bottom can became depleted of oxygen, preventing organisms to colonize and feed on organic material, also a rapid sedimentation quickly covers the plant remains, optimal conditions for fossilisation. Some lakes, especially swamps, contain also high concentration of organic substances, washed into the water from the land, that kill microbes, impregnate and preserve organic material.
Amber preserves organic material in various ways, when hardened resin has a limited mechanical protection effect, it protects the organism from scavengers and weather and isolates it from oxygen, which can oxidize the organic material or enable bacteria to destroy the inclusion. Resins of plants possess also an antibiotic effect and therefore kills microorganisms - in Egypt this effect was used to sterilize artificial mummies. The content of "sugar" in the resin probably also draw moisture out of the tissue, preventing further microbial activity.
Fig.7. Example of plant tissue preserved in amber, trichomes of a oak tree florescence.
Despite these cases it is very rare that a plant becomes completely fossilized - especially large plants, like trees, tend to be embedded in sediments only in fragments.
This causes major problems; some plant tissue tends to be overrepresented in the fossil record. For example wood and bark are more resistant than non lignified plant tissue and will became fossilized more easily - this can produce the effect that herbaceous plants are generally underrepresented in the geologic record. In contrast leaves or pollen are produced in great quantities even by a single plant, this can produce a bias of the stratigraphic record to some species of plants.
Fragmented plant remains also make it difficult to reconstruct the overall morphology of a single plant species, and worse, cause a proliferation of artificial plant species. A single plant can produce diverse organs and tissue and therefore fossils - like foliages (which often various shapes and sizes on a single individual), roots, stem, branches, bark, male or female cones in gymnosperms, blossoms and fruits in angiosperms.
Many fossils were found dissociated and attributed to individual species, only subsequent discoveries revealed in some cases which tissues belong together, forming a single plant species. For example the extinct Lepidodendron-tree, related to the modern Lycopsids, was reconstructed by merging together at least seven single "species" (Stigmaria - roots, Knorria - bark, Lepidophloios - bark, Lepidostrobus - cone, Lepidophylloides - leaf etc.) based on the single organs or tissues of the former plant.
Also fragments of plants not necessarily represent a real association of plants - a biocenosis. Plants or parts of them can be transported by wind (for example leaves) or by water (for example wood), therefore plant species of various regions and ecosystems can became deposited and mixed together in a sedimentary basin. This false association of plants is called taphocenosis - a death assemblage.
Fig.8-10. Litter in a modern forest, showing fragmentation of a single or few tree species in various "morphospecies" based on the single plant organs, like cones, needles, branches, wood, intermixing with herbaceous plants and even stranger live forms as fungi are, forming possibly in the geologic record a future taphocenosis.
Many reconstructions of former landscapes show an incredible biodiversity of plants mixed chaotically together, but observing modern landscapes we note that most plant associations are characterized by few plant species, dominating a specific habitat and that there is a continuum of various habitats following in succession. We will, for example, find a specific plant-association on a shore, and a specific plant-association on the dry highland.
Fig.11. "Coal swamp" as imagined by Z. Burian in 1972 (found in SPINAR 1976).
A concrete example of the biased reconstruction of the former vegetation is the classic and in textbooks ubiquitous "coal swamp", that I will discuss soon…
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WILLIS, K.J. & McELWAIN, J.C. (2002): The evolution of plants. Oxford University Press - Oxford: 378
SPINAR, Z.V. (1976): Quando l´uomo non c´era. Fratelli Fabbri Editori, Milano: 228
ARENS, N.C. et al. (1998): Plant Fossils and Their Preservation. (Accessed 18.06.2011)