[Laboratory III -- Plant Fossils and Their Preservation]

Conditions Required for Plant Fossil Preservation

Three conditions are required for the preservation of plant fossils:

  1. Removing the material from oxygen-rich environment of aerobic decay;
  2. "Fixing" the organic material to retard anaerobic decay;
  3. Introducing the fossil to the sedimentary rock record (a.k.a., burial).

Consequently, plant fossils are generally preserved in environments very low in oxygen (e.g., anaerobic sediment) because most decomposers (e.g., fungi, most decomposing bacteria and invertebrates) require oxygen for metabolism. Such sediments are commonly gray, green or black rather than red, a sedimentary signal of oxygen-rich conditions. The "fixing" requirements means that plant material must fall into an environment rich in humic acids or clay minerals, which can retard decay by blocking the chemical sites onto which decomposers fasten their degrading enzymes. Plant material can also be "fixed" by removing degradable organic compounds during the process of charring by wildfire. This is a common and spectacular mode of preservation for flowers. Plant material can then be incorporated into the rock record in areas where sediment is being deposited, which usually, but not always, requires the presence of water. Consequently, streams, flood plains, lakes, swamps, and the ocean are good candidates for fossil-forming systems. Plant fossils are commonly preserved in fine-grained sediment such as sand, silt, or clay, or in association with organic deposits such as peat (coal).

As you look at the various modes of preservation in lab, note the characteristics of the rock matrix in which plant material is preserved. Note color, grain size (i.e., sand, silt, clay), mineral composition (quartz, clay, mica, organic-rich, organic-poor), and any other unusual features. If you aren't familiar with the basic features of sedimentary rocks, they may start out all looking the same…don't worry. Take some extra time to make systematic observations of each specimen: What color is it? Can you see the sediment grains with your naked eye? With a hand lens? How would you classify them (round or angular)? Is the rock shiny or dull? Does it have uniform or mixed composition? Do you notice sedimentary layers or other features (ripple marks, animal tracks)? Just be patient; you'll train your eye to recognize rock types in no time.

Types of Plant Fossils

Six broad categories of plant fossils are commonly recognized. Although these categories seem well-defined, a given fossil may fall into several categories or may elude them all. Consequently, these categories should be thought of as broad modes of preservation rather than shoe boxes into which all fossils must go. When thinking about types of fossils and modes of preservation, it is more important to consider what types of biologically interesting information is or is not present than to fret over strict classifications. With that caveat, the basic types of plant fossils include:

Each type of plant fossil carries different types of anatomical and biological information. Consequently, to piece together the most complete picture of an ancient plant, paleobotanists hope for the same type of plant or plant part will be preserved in several different styles.

Compressions

Compressions are plant parts that have suffered physical deformation such that the three-dimensional plant part is compressed to more-or-less two-dimensions. Compressions retain organic matter, usually more or less coalified. Compressions of leaves (VG 1:1) , for example, differ from impressions in that some organic substance, often cuticle, is preserved. Peat, lignite, and coal are essentially compressions of thick accumulations of plant debris relatively free of encasing mineral sediment.

Compressions are excellent records of external form, especially for planar structures like leaves. They often preserve cuticle that can be recovered by dissolving the mineral matter in hydrofluoric acid (HF) or disaggregating in mild peroxide (VG 1:2). The cuticle retains the imprint of epidermal cells, but other than this, cellular information can seldom be recovered from compressions. Consequently, compressions generally preserve plants at the organ, organism, and/or environment level. In addition, because compressions preserve organic material, carbon isotopic studies can be performed on compressions. From these studies, paleobotanists can sometimes recognize the biochemical signature of C3, C4, and CAM photosynthetic physiology in extinct species.

Study the assortment of compressions available. Note color, texture, and type of deformation (usually flattening) experienced by each specimen. Notice distortion in original morphology is introduced by flattening.

Examine samples of coal, noting their weight, texture and surface features (dull vs. shiny). Identify discrete plant parts in the different ranks (essentially grades of metamorphism) of coal. Coal geologists recognize a continuum of degree of metamorphism in coal: peat - lignite - bituminous coal (A-C) - anthracite. Peat is an accumulation of virtually unaltered plant material, while anthracite is nearly pure carbon with little trace of the original plant material. All materials can be burned for fuel, but the energy content per weight increases with degree of metamorphism and the proportion of impurities generally decreases. Study thin sections of coal at the microscope. Can you identify specific plant parts in the thin sections? Study samples of fusain (fossil charcoal).

Impressions

Impressions are two-dimensional imprints of plants or their parts found, most commonly, in fine-grained sediment such as silt or clay. Impressions are essentially compressions sans organic material. If the sediment is very fine-grained, impressions may faithfully replicate remarkable details of original external form, regardless of subsequent consolidation of the sediment. Study the specimens of impressions, noting the several categories of plant parts represented. Because of their shape, texture, and abundance, leaves are among the most common organ preserved in impression (VG 1:1). Impressions may also occur if, when layers of rock are split apart, the organic material adheres to only one side of the rock. In this case, the side with organic material is the compression, known as the "part", while the corresponding impression known as the "counterpart".

One particularly interesting type of impression forms in "dirty" sand (VG 1:3) . In this type of sediment, relatively coarse sand grains are mixed with silt and clay. This type of sediment is common in river and flood plain environments so is important for terrestrial plant preservation. When a leaf falls into this type of sediment and begins to decay, the first organic bonds to break leave charged molecular tails hanging off the leaf surface. This charged tail attracts clay particles with opposite charge that linger within the sediment. The clay migrates to the leaf surface, coating the organic structure. This has two remarkable consequences: First, further decay is retarded because clay is occupying sites of organic reaction. Second, the fine clay allows remarkable detail to be preserved. Because most of the sediment is relatively coarse (sand), the organic material is lost later, but an exquisitely detailed impression is retained in the clay film. This mode of preservation is important in the Dakota Sandstone flora of Cretaceous age. It is also important in the preservation of remarkable animal fossils such as the Jurassic bird Archaeopteryx and the strange Cambrian invertebrates of the Burgess Shale.

Impressions, like compressions, record information about external shape and morphology of plant organs. However, because they lack organic material, cuticle and organic carbon cannot be recovered from them. In cases of impressions in very fine-grained sediment, some cellular detail can be recovered by making a latex of silicone rubber cast of the impression.

Casts and Molds

When sediment is deposited into cavities left by the decay of plant parts, a cast results (VG 1:4). A mold is essentially a cavity left in the sediment by the decayed plant tissue. Molds are generally unfilled, or may be partially filled with sediment. Casts and molds commonly lack organic matter, but a resistant structure like periderm may be preserved as a compression on the outside of the cast or the inside of a mold. Casts and molds may be found together with the cast filling the mold.

Molds are formed when soft sediment surrounding the structure lithifies or hardens before the structure decays. When the mold fills in with sediment that subsequently hardens, a cast is formed. Casts of an internal hollow structure like a pith cavity are also common. Pith casts can be confusing because you are looking at the inside of the fossil-what in life would have been empty space. Like compressions and impressions, casts and molds record external (or sometimes internal) organ features well, but provide no cellular or tissue information. Unlike compressions/impressions, molds and casts often are truer records of the original three-dimensional shape of the structure. Casts of ancient trees are among the most impressive plant fossils (VG 1:5).

Permineralizations

Permineralization occurs when the plant tissues are infiltrated with mineral-rich fluid. Minerals (commonly silica, carbonate, phosphate or pyrite or rarely other minerals) precipitate in cell lumens and intercellular spaces, thus preserving internal structures of plant parts in three dimensions. This type of preservation is known as "structural preservation". Because organic material (commonly cell walls but in some cases finer detail) is preserved, permineralizations can yield detailed information about the internal structure of the once-living plant (VG 1:6). When mineral matter actually replaces the cell-wall and other internal structures, the preservation may be called petrifaction. In petrified specimens, cellular details are lost with the organic material of the cell wall. Please note that we are using these words in a precise, scientific sense. "Petrified" also has a colloquial meaning that might encompass what we distinguish as "permineralized". Therefore, for the purpose of this course, permineralized wood preserves the cellular detail of wood anatomy and the lignin of cell walls that has been "fixed" by a mineral in-filling. This is much like bioplastic in-filling cellular structures when one makes a histological thin section. Petrified wood, on the other hand, lacks such cellular preservation.

Silica permineralization (silification (VG 1:7)) commonly occurs in areas where silica-rich volcaniclastic sediments are weathering, for example the famous upright trees in Yellowstone National Park or nearby Calistoga, California. Silification is also an important preservational mode for Precambrian microbial remains deposited in near-shore marine environments.

Permineralization with calcium carbonate (calcite or dolomite) is particularly common in Carboniferous coal seams (VG 1:9), where whole regions of peat were permineralized. Called coal balls (because of their sometimes round or ellipsoidal shape (VG 1:10)) or widow makers (because of their tendency to drop out of mine roofs onto the heads of unsuspecting miners), these fossils commonly preserve a hodge-podge of plants and plant organs.

Permineralizations in pyrite (an iron-sulfur mineral) are particularly important in Devonian rocks where coal balls and well-preserved compactions are rare or unknown. These pyritized fossils often occur in the presence of sea water (a source of sulfur), and are characteristic of plant tissues washed into marine basins. Pyrite permineralizations offer a challenge to the museum curator because iron in pyrite exists in a reduced state and tends to oxidize when exposed to air. Upon oxidization, most of the structures are lost. This is called "pyrite disease" in fossils and is characterized by a mold-like appearance on the cut surface of the coal ball. To prevent destruction, the surface can be coated with a sealant. Coal balls can also be stored in an low-oxygen medium like glycerin or antifreeze.

Permineralization with phosphate is uncommon for land plants, but can be important in some types of marine settings.

Compactions

In peat, brown coals (lignite), middens and soft sediments, plant remains may retain their external form with only slight volume reduction due to compaction. Such tissues are not mineralized, retain resistant organic material, and may show unidirectional compression (flattening). Internal structure, especially of thick-walled, hard fruits is sometimes well preserved. These fossils may be sectioned by microtome or embedded and treated much like living tissues. Compactions are most common in the youngest plant fossils. Examine specimens of Tertiary fruits and seeds, and thin sections made from them. Pollen and spores are also preserved as compactions (VG 1:8). The material making up their outer shells (sporopollenin) is extremely resistant to decay and can remain for hundreds of millions of year practically unaltered in the rock record. However, the pollen and spore shells, once spherical, are flattened by the compressive forces of lithification.

Molecular Fossils

As more becomes known about the chemistry of modern plants, paleobotanists have begun to examine the fossil record for corresponding chemical data. For example, characteristic breakdown products of chlorophylls and lignins have been found in well-preserved fossil leaves. Lipids and their derivatives have also been recovered from sediments. Some carbohydrate break-down products may also survive in sediment. A special class of these, oleananes, are formed by flowering plants, some ferns and lichens. An increase in abundance of these molecules in sediments of mid to Late Cretaceous age is used to document the increasing abundance of flowering plants (Moldowan et al, 1994). In another stunning example, genetic material was recovered from Tertiary leaves, and the age of material from which DNA and RNA is recovered seems to be greater with every issue of Nature. As testament to the anoxic requirement for preservation of most molecular fossils, the RNA recovered from fossil leaves degrades within a few seconds when exposed to air, so special preparation techniques were developed to harvest and transport the material to the lab for processing.

Molecular fossils are recovered and studied using chromatographic techniques, mass spectrometry, and spectrophotometry. The preservation of these chemical products is highly variable, and depends on oxygen levels during deposition, temperatures experienced by the rocks since preservation, and many other physical and chemical factors. In a similar vein, geochemists have investigated the chemistry of petroleum and its precursors in an attempt to understand its formation.

Fossil DNA and RNA have also been making headlines in the scientific press. In some exceptional cases, genetic material or proteins have been sufficiently well-preserved to permit their use in the reconstruction of evolutionary relationships, in much the same way as one might sequence living organisms. However, much of this work is controversial due to the difficulty of preserving and isolating these fragile molecules. Also, contamination by other materials is a common and difficult to recognize problem.

A more mainstream application of organic chemistry to the study of ancient plants is that of stable carbon isotopes. During photosynthesis, plants reduce carbon from carbon dioxide to form organic molecules. This ratio of carbon-12 to carbon-13 in the resulting compounds gives information on the proportion of these isotopes in the atmosphere (interesting for geological questions relating to global carbon cycling) and about the physiology of the plant itself. In lecture we will discuss several applications of these techniques.




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