Stephen P. Broker
The history of plant life on earth is studied through an examination of living forms, as well as of fossilized plant remains from the past. In the next section of this unit the major divisions of plants, both living and extinct, are briefly described. Let us now consider the various ways in which plant fossils have been formed.
Paleobotany is the science that studies fossilized plant material. Fossils the word means ‘extracted from the earth’ or ‘dug up’ are produced through a combination of physical and chemical processes acting on the remains of organisms. Most plants are rapidly decomposed after dying, and in this sense plant fossils are rare. Many plant types grow in areas where preservation is unlikely, such as in uplands; sedimentary deposits are usually required for fossils to form. Soft parts are especially prone to decay. In spite of the odds being very much against any individual plant being preserved, there are many localities throughout the world where fossil plants are abundant.
Plant matter is protected from rapid decomposition by burial in sediment, soon after death. Clay, mud, sand, and suspended particles in the oceans, freshwater lakes and streams, and swamplands are capable of being deposited around plant tissue. Burial in volcanic ash will also protect plant remains. Sedimentary deposits, like clay and sand, undergo compression and solidification in time to yield shales, sandstones, and limestones. It is in this sedimentary rock that one looks for fossils.
Different processes of preservation result in the following types of fossils:
1.
compactions
These are fossils found in peat, brown coals (lignites), and other soft sediments. The plant remains, frequently seeds and fruits of the Tertiary or younger, are entirely of original organic material. They are three-dimensional fossils which have been somewhat flattened and reduced in volume. As with living plant tissue, compactions can be sectioned and examined microscopically for cellular structure.
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2.
compressions
and
impressions
When fine-grained sedimentary deposits containing dead plants are subjected to the extreme weight of overlying strata, so that the air and water are driven out, compressions and impressions may form. The plant matter, usually leaves but also trunks, stems, and roots, is reduced to a “thin carbonaceous film” or is “coalified.” This method of fossilization is also known as carbonization. If a two-dimensional positive image is formed, it is called a compression. In this case the only original organic matter left of the leaf or stem is the thin layer of carbon. Negative images, or imprints, are called impressions. They are also two-dimensional, but they are lacking in any original material. Those compressions which have remnants of waxy leaf cuticle can give thin peels suitable for microscopic examination. Leaf venation is evident. The cellular structure of underlying remains can be detected. Some deformation of the plant does occur. Compressions and impressions are not necessarily found in concert.
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Among the more abundant of sedimentary deposits containing compressions and impressions are the coal deposits of the Carboniferous Period. In the case of coal formation, the coal itself, which we call a fossil fuel, consists of vegetable matter too altered to yield recognizable fossils. The well-known fossils of the Carboniferous swamps come from the clay and silt deposits above and below the coal layer. Hard coal (anthracite) is in fact the end product of a series of physical and chemical processes which convert peat first into lignite, then into soft, or bituminous coal. When considering energy production and use, it should be realized that anthracite burns cleaner and hotter than soft coals, and is in
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3.
petrifactions
In some sedimentary deposits the surrounding water is high in mineral content; silica (SiO2) or Calcium Carbonate (CaCO3) may be present, but also iron and manganese oxides. These salts precipitate out of solution and collect inside any plant matter present. The plant matter may be replaced during mineralization, but usually cell cavities are filled, enclosing the original plant material. Thin sections from throughout these three-dimensional fossils reveal beautifully preserved cell structure under the microscope. Striking colors result from the presence of the oxides. The best-known petrifactions are the “petrified woods”, particularly those of the Petrified Forest in Arizona and of Yellowstone National Park. These are coniferous trees of the Triassic. Silicified or petrified woods are not the only examples of petrifactions. The coal balls of the Carboniferous, which contain well-preserved material from the lowland coal swamps, are also products of this type of fossilization.
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4.
casts
and
molds
These fossils actually consist of no original plant material, and they show no cellular structure. They are, though, very useful and accurate renditions of surface structure or internal layering of wood and bark. When a root or stem trapped in sediment decays away, the surrounding deposits may be solid enough to retain their shape. The mold which results solidifies as a faithful copy of the exterior surface of the solid plant part. If the cavity left behind is then filled with new deposits, a cast is produced, identical to the original plant. Many times molds and casts occur together. Plants commonly preserved in this fashion are the roots and wood of treelike club mosses and horsetails. The bark of these plants occurred in several layers, and casts very different in appearance may actually come from the same plant. The dinosaur tracks common to the red sandstone of the Connecticut Valley are molds and casts left behind from creatures which walked in clay and mud deposits.