Ways Of Fossilisation

[RomanK]

There was a question in one of my previous topics about fossilisation or fossils creation and Auspex, thanks him, made shot but clear explanation. I'd like to quote T. Taylor's palaeobatany book just to make his explanation wider.

COAL AND CHARCOAL

Technically, coal comes under the definition of a compression fossil, since it represents a complex, heterogeneous mixture of macromolecular organic compounds derived from plant material that has been compressed over time (Scott, 1987). In general, the lower the rank of the coal (the degree of coalification), the more details of plant structure one can observe. The higher the rank, the more the coal has been metamorphosed and the higher the carbon content.

Ranks from lowest to highest are lignite, subbituminous coal, bituminous coal, and anthracite. Lignite represents an early stage in coal formation, so the plant parts within lignite are not excessively crushed or decayed and are generally easily recognizable. In some instances, SEM has been a useful tool in identifying plant parts preserved in lignites (Alvin and Muir, 1969). In some lignites it is possible to tease apart the plant fragments and make whole mounts of various structures, for example in the Brandon lignite, a famous early Miocene plant locality in Vermont, USA (Haggard and Tiffney, 1997).

Bituminous coal is more metamorphosed and the plant parts are more flattened, but it is still possible to study plant fragments within the coal. Anthracite coal, the most highly metamorphosed type, is altered to such an extent that little of the original plant material is recognizable. Some coals can be thin sectioned for microscopic examination, and pollen grains, spores, and fragments of cuticle can be discerned.

In other instances it is possible to coat the polished surface of coal with epoxy resin and etch it in a low-temperature plasma asher (Winston, 1989). Pieces of coal, or peels of the etched surfaces, can then be examined by light microscopy or SEM to determine the biological composition of the coal. This procedure makes it possible to quantify the plants in various types of coals in instances where coal ball permineralizations (see section “ Cellular Preservation”) are not available (Winston, 1986). Coal can also be macerated using chemicals that break down the solid coal and release the plant fragments. It is possible to recognize cuticle, pieces of bark, bits of wood, solidified resins, and especially spores and pollen grains in this type of preparation. Examination of these components allows one to determine the kinds of plants that were growing in the ancient swamps where the coal was formed. Application of 13 C nuclear magnetic resonance (NMR) and pyrolysis-gas chromatography–mass spectrometry techniques has been used to define stages in the coalification process more accurately (Hatcher et al., 1989). This same technique has also been used for Cenozoic leaf tissues and wood to identify various biomolecules (Yang et al., 2005). The components of coals can also be useful in documenting paleoecology (Poole et al., 2006).

Macerals are defined as the organic constituents that comprise coal as seen in polished thin sections. The system of maceral types was originally proposed by the paleobotanist Marie C. Stopes ( FIG. 1.31 ) in 1919, expanded in 1935, and is constantly kept up to date (ICCP, 2001). Maceral names end in -inite; for example, funginite is made up of fungal spores and various fungal bodies, secretinite is composed secretory deposits formed by medullosan seed ferns (Lyons, 2000), and sporinite consists of the sporopollen in walls of fossil pollen and spores. More detailed studies of coal composition provide valuable information about environmental parameters. For example, G. Taylor et al. (1989) suggested that the association of alginite and inertodetrinite (redeposited small particles of fusinite) in the Permian coals of Australia indicates a paleoenvironment of wet, cool summers and freezing winters. It is possible to distinguish between angiosperm and gymnosperm woods in some coals using macerals (Sэkorovб et al., 2005).

Fossil charcoal or fusain (carbonaceous residue that results from the incomplete combustion of organic material; also called fusinite) is also an important source of paleobotanical data (Cope and Chaloner, 1985; Lupia, 1995), with charcoalified plant remains dating back to the earliest land plants (Glasspool et al., 2004b). There are several techniques used to examine fossil charcoal (Sander and Gee, 1990; Guo and Bustin, 1998; Figueiral et al., 2002) which provide information on taphonomy and paleoecology (Scott et al., 2000), including past atmospheric composition (Scott and Glasspool, 2006) and the presence of fire in paleoecosystems (Uhl et al., 2004, 2007a ; Collinson et al., 2007). The discovery of beautifully preserved charcoalified flowers in Cretaceous (Tiffney, 1977; Friis and Skarby, 1981) rocks from around the world has contributed large amounts of information to our knowledge of early flowering plants (see Chapter 22).

In North America, Carboniferous coals of different ages are typically characterized by the dominance of different types of swamp plants (Cross and Phillips, 1990). Among the Pennsylvanian coal beds for example, lycopsids, tree ferns, calamites, seed ferns, and cordaites constitute the major types of tropical–subtropical arborescent plants that contributed to the peat formation. Although Carboniferous peat swamps have represented the model system in most interpretations of coal-forming ecosystems, the plants lived in atypical terrestrial communities in which the pH and available nutrients were low. It is now becoming apparent that such factors as types of litter accumulation, nature of the biomass, preservational characteristics of certain tissue systems, microbial diversity, biology of the plants, paleoclimate, and paleogeography are but a few of the parameters necessary to understand and properly interpret coal-forming ecosystems through geologic time (Cross and Phillips, 1990; DiMichele et al., 2002, 2007a). An important study by Gastaldo et al. (2004) underscores the fine resolution needed to understand fossil plant community structure, using an in situ three-tiered forest above a Pennsylvanian coal in Alabama, USA, as the data set. Detailed sampling of this fossil forest indicated that the proportion of canopy, understory, and ground-cover plants was variable across the study area, and that wet–dry gradients and/or increasing habitat specialization did not control the distribution of the plants species in this swamp ecosystem.

In rare instances, a coal is formed that consists entirely of cuticular fragments and amorphic organic material (DiMichele et al., 1984). The cuticle is so abundant that it can be peeled off in thin layers. This type of coal is termed a paper coal, alluding to its papery appearance, and is known from relatively few localities, some as early as the Devonian, for example the famous Orestovia paper coal from Siberia (Ergolskaya, 1936a ; Krassilov, 1981a). It is a simple matter to isolate these cuticular fragments by using a chemical base such as potassium hydroxide. The cuticle can then be washed, stained in some cases, and mounted directly on slides for examination (DiMichele et al., 1984; Kerp and Barthel, 1993). Lenses of leaf fragments may sometimes be preserved within coals; these apparently formed in small depressions containing acidic water, which inhibited the normal degradation activities of various microorganisms (Gastaldo and Stub, 1999). In other instances, coals have been found to be made up exclusively of algal remains (see Chapter 4), some as early as the Precambrian (Tyler et al., 1957).

Kerogen is a type of fossilized insoluble organic matter that is widely found in sedimentary rocks, and is a common component of various paleobotanical preparations. It is the most abundant form of organic carbon on Earth—more even than coal deposits. The presence of kerogen in rocks has been used as evidence of some of the earliest life on Earth (Moreau and Sharp, 2004; see Chapter 2). Understanding the chemical composition and source of kerogen, termed “ typing ” the kerogen, is especially important, since these factors help to determine the petroleum-generating potential of source rocks. In the past, kerogen and coal were generally analyzed using just thin sections and light microscopy. Today, both substances are also characterized using standard geochemical methods, such as pyrolysis, gas chromatography–

mass spectrometry (GC–MS), analysis of carbon isotopes for total organic carbon (TOC), and others.

There are a variety of other techniques available today to investigate the nature of the organic matter that remains after fossilization and to compare the carbonaceous residue to determine the chemical-structural characteristics.

Raman spectroscopy has been used to characterize carbonaceous matter in highly metamorphosed rocks for some time (Nestler et al., 2003), but has recently been applied to microscopic fossils preserved in chert of varying ages (Schopf et al., 2005). Unlike most geochemical techniques, Raman spectroscopy is a non-destructive means to analyze ancient organic matter. It provides information on the original biochemistry of the organism, and can help resolve the nature of certain ancient fossil-like organisms (see Chapter 2).

Another approach that has been used to examine the chemical composition of fossil plant materials involves energydispersive X-ray microanalysis (EDXMA). With this technique the elemental composition and spatial distribution of fossils can be studied without damaging the specimen (Briggs et al., 2000). Other have used EDXMA to map the distribution of elements in fossil cells (Boyce et al., 2001), and cell walls (Boyce et al., 2002).

IMPRESSIONS

When a paleobotanist splits a rock that contains fossil plant fragments along a bedding plane, it is sometimes possible to see the carbonaceous fi lm of a compression along one face, and a negative imprint of the plant part, with little or no carbon adhering, on the other face ( FIG. 1.32 ); these two faces are called part and counterpart in paleobotany. The fossil with little or no carbonaceous material is called an impression The imprint will show all the surface details of the compression, such as leaf shape and venation, but there is no actual plant material, that is no carbon, preserved. If you have ever seen a leaf imprint in a concrete sidewalk, you have seen an impression. The process involved in the formation of an impression is also analogous to these modern “ fossils. ” Such imprints are formed when leaves fall and settle into the wet concrete just after it is poured. As the concrete hardens, it conforms to the contours of the lower side of the leaf that rests on it. Eventually, the leaf disintegrates and the pieces are blown away, but a negative replica of the leaf remains on the hardened concrete. If you have ever put your initials in wet concrete, you have formed an impression.

The impression of dinosaur footprints represents an excellent example of this type of fossilization process. When several footprints are of the same type or a series of trackways are discovered in close proximity, it may be possible to extrapolate the stride of the organism and, from this, infer something of the biomechanics of the animal.

No cellular details can normally be seen on an impression because there is no adhering organic material, but, in some instances, especially where the matrix is exceedingly fine grained, a replica of the impression can be made with latex or similar material. The replica faithfully reproduces whatever surface details were on the original organism when it was impressed into the mud. Examination of part of the replica with the SEM may reveal details with great clarity, such as the pattern of the epidermal cells, hairs, glands, or other surface features. Some impression fossils are covered with mineral encrustations of different composition, for example iron (Spicer, 1977). These deposits may be the result of the activities of microorganisms during the decay process.

Regardless of their origin, however, the mineral crust may provide an excellent replica of the surface of the plant part, and this can be studied using a variety of imaging modes.

MOLDS AND CASTS

In addition to two-dimensional plant parts, such as leaves, three-dimensional structures, such as stems, seeds, or fruits, can also be carried into sites where sediment is accumulating and buried. During fl ood events, massive trunks and tree branches can be moved some distance before they are eventually deposited. If these plant parts became crushed over time, they would be preserved as compression or impression fossils. If, however, the sediment surrounding the three-dimensional plant parts hardens before the plant fragment is crushed, the sediment will form a three-dimensional negative, or mold, of the plant fragment. As the plant material disintegrates, a hollow remains in the sediment, which can subsequently be filled in with sediment, thus forming a cast inside the mold. The surface of the cast and the mold can often faithfully reproduce the surface features of a particular plant part, such as characteristic leaf bases on the surface of a stem or the ornamentation of seeds ( FIG. 1.35 ) and fruits.

The sediment that fills in the cavity of the mold and solidifies becomes a three-dimensional cast of the original plant part In almost all molds and casts no actual plant material remains, but the surface contours are the same as those of the original plant part. The formation of fossil molds and casts parallels the method by which a sculptor creates a bronze statue. The sculptor does not carve directly on a block of bronze, but creates a sculpture with some other medium— wood or wax perhaps. A mold is then constructed around the original sculpture and, when the mold is complete, the original is removed in some fashion (disassembling the mold temporarily or melting the wax). When the mold is reassembled, molten bronze is poured into it, and an exact replica of the original sculpture (but one that involves none of the original material in that sculpture) is formed. Rates of sedimentation in certain areas where molds and casts were formed must have been spectacular. As an example, the sea cliffs at Joggins, Nova Scotia, reveal exposed casts of Pennsylvanian tree trunks 3–8 m tall. The trees must have been buried quite rapidly in place. Sediment hardened and the trees subsequently died, leaving hollows (molds) in the hardened rock that were subsequently filled with other sediments (casts).

Compressions, and casts ( FIG. 1.37 ) are important in showing the external form of plant parts in a three-dimensional fashion. Root casts of trees can provide important morphological information useful in determining the type of soil formation and soil drainage conditions when the plants were growing (Retallack, 1990). They also may reveal specialized taphonomic processes and how degradation of organic tissues may have proceeded (Driese et al., 1997). A special form of cast is the calamite pith cast or stein kern, which is a common form of preservation of larger calamite stems and branches. Pith casts are casts ( FIG. 1.38 ) of the hollow pith or medullary region in calamites and preserve an impression of the outside of the pith cavity, which represents the inside of the vascular tissue and cortex (see Chapter 10 for further details).

An unusual example of a mold and cast is represented by fossil wood that was exposed to colloidal silica during the diagenesis; the silica permeated the cell cavities, but somehow did not impregnate the cell walls. After precipitation of the silica within the cell cavities, the cell walls (=molds) disintegrated and all that is left are casts of the cavities of the wood cells ( FIG. 1.39 ). These cell casts have the negative contours of the insides of the cell walls and show counterparts of specialized wall structures, such as bordered pits (Chapter 7).

CELLULAR PRESERVATION

With few exceptions, none of the preceding types of fossil preservation provide the opportunity to examine the internal structure of a plant or plant part. In the case of permineralizations and petrifactions, however, it is possible to study the internal anatomy of ancient plants (Schopf, 1975). In these fossils, one can examine cells and tissue systems within a plant, as well as produce a series of serial sections that can be used to reconstruct the three-dimensional organization of a structure. This type of fossil is called a permineralization or a petrifaction.

In both types, the process begins when a plant part becomes immersed in water containing a high concentration of dissolved minerals, the most common being silica (silicon dioxide, SiO 2 ), which is often readily available in areas of volcanic activity. The plant part, for example a log, gets thoroughly waterlogged, with water and dissolved minerals permeating all the cells and tissue systems. The dissolved minerals may be silica compounds (silicification) ( FIG. 1.40 ), carbonates (e.g., calcium carbonate, CaCO 3 ), oxides, pyrites (iron sulfide, FeS 2 ), or some other type of chemical. At this stage it is unclear what happens, but something triggers precipitation of the dissolved mineral (e.g., a change of pH) so that it hardens around and within the plant fragment. The cell walls of the plant itself may serve as nucleation sites for the growth of the mineral crystals. When the mineral is completely solidified, the plant fragment, in effect, is entombed within solid rock. The fossil can now be sectioned by various means and examined under the microscope to see internal details of the plant. Although several authors have attempted experimental silicification of wood in the laboratory (Leo and Barghoorn, 1976; Laroche et al., 1989), the preservation seen in fossils is often much better. Preservation of plants as petrifactions or permineralizations probably involves several stages of mineral growth, with different sizes of crystals involved. Some more recent work (Channing and Edwards, 2004) suggests that colloidal or gel phases of some minerals may be involved in the apparently rapid preservation of minute details.

Permineralizations and petrifactions can both be studied by means of thin sections, sometimes called petrographic thin sections ( FIG. 1.41 ) (Hass and Rowe, 1999). The piece of rock containing the fossil is cut and ground thin enough to transmit light through the section, essentially, the same technique that geologists use to make petrographic rock sections. Rock saws are available that can cut through most types of rock matrix; most have steel blades with diamond particles embedded in the cutting edges. Oil or some other coolant is used to keep the blade from getting too hot as it slices through the rock. Saw blades covered with or made of particles of silicon carbide or some other abrasive can be used to cut through softer material. The fossil to be studied is cut out with a saw, and the surface of the fossil is polished with an appropriate abrasive (e.g., silicon carbide of various grades) until it is smooth. The surface showing the fossil is then attached to a piece of glass with some type of adhesive.

After the adhesive has solidified, the glass slide with the piece of permineralized material is placed back in a saw, now a specialized thin-sectioning saw, and the remainder of the rock is cut away to leave as thin a slice as possible. At this point, the rock is still opaque. The next step involves grinding the surface, either by hand on a lapidary wheel or plate, or on a thin-sectioning machine, so that more and more light can pass through the specimen. Eventually, the sliver of rock is thin enough to be examined with a microscope.

Sometimes the piece of glass to which the fossil material is attached is the actual slide used for study. In that case, a permanent adhesive, such as epoxy resin, may be used. Some prefer to transfer the ground specimen to a clean microscope slide. In those instances, a cement that can later be remelted is used, such as Lakeside thermoplastic resin. Before the thin section is transferred, it is coated with a transparent plastic like material to keep the section intact. This thin slice is then placed on a clean slide with a natural or synthetic mounting medium and covered with a cover glass. Once the medium has hardened, the slide can be examined with a compound microscope. Some paleobotanists use no cover glass, but rather examine the rock surface directly using oil immersion microscope objectives; this method has been applied very successfully with the Early Devonian Rhynie chert (see Chapter 8).

PERMINERALIZATION

In a permineralization, minerals fill the cell lumina and the intercellular spaces, but do not completely replace the cell walls. The cell walls still consist of organic matter, although they may be chemically altered to various degrees.

Chemically, the various layers of the cell wall may still be distinct (Boyce et al., 2002), and the permineralization may faithfully reproduce the microstructure of the wall, for example the position of cellulose microfibrils (Smoot and Taylor, 1984). Cellular contents have even been described from permineralizations! The processes involved in the formation of certain types of permineralization in silica are being studied in modern hot springs ecosystems like Yellowstone National Park, USA (Channing and Edwards, 2004), and in filamentous microbes from similar ecosystems in New Zealand (Renaut and Jones, 2003; Jones et al., 2004; Phoenix et al., 2005). These studies underscore the complexity of the preservation process. In general it involves the formation of opaline silica (opal-A) films that coat structures and colloidal silica that permeates cells and tissue systems. More recent work has shown that bacteria may be involved in or even necessary for many mineralization processes, and the fi eld of geomicrobiology is a rapidly growing area of study. In biomineralization, the bacteria may serve as catalysts for chemical reactions and also as nucleation sites for mineralization (see Konhauser, 2007 for additional information on this topic).

An analogy of the process of permineralization is the technique used to embed and section living biological material.

For example, a piece of plant is killed and fixed in an appropriate chemical. It is then passed through a series of alcohols to dehydrate the tissue, and finally transferred to molten paraffin or plastic. When the paraffin is cooled, the plant part is completely embedded in it—paraffin is present within the tissues as well as around them. The entombed specimens can then be serially sectioned to reveal details of the cells and tissue systems.

Edited November 10, 2010 by RomanK

[pleecan]

Great article!

PL

[JimB88]

Thanks Roman, that's really helpful. Ive found wood impressions in coal - do you ever find anything in the coal?

[RomanK]

Thanks Roman, that's really helpful. Ive found wood impressions in coal - do you ever find anything in the coal?

Hi Jim, no I didn't. There aren't the big pieces of the coal at the spoils, but using microscope you can find at least pollen.

[BobWill]

Since I didn't see the original post there may be a good reason why you have limited the list to exclude the other ways of fossilization but here are a few more just for grins: frozen tundra, peat and tar pits, resin inclusion, dessication, trace, and bioimmuration.

[RomanK]

Since I didn't see the original post there may be a good reason why you have limited the list to exclude the other ways of fossilization but here are a few more just for grins: frozen tundra, peat and tar pits, resin inclusion, dessication, trace, and bioimmuration.

Question was from cowsharks:

"A question from a vertebrate/shark-tooth hunter/collector.....are these fern leaves, branches, bark, actually fossils or are they impressions made in the material, and the material is then fossilized or turned into stone? Just curious because I've often wondered about this because I know wood can become fossilized, but I've never seen something like a leaf be a fossil. I've often wondered also about trilobites. Are some of the trilobites that folks find the actual bug that became a fossil, like the kind they find in Morocco, or are some just "impressions" in stone? Just curious since I don't collect this kind of stuff.

Either way, the stuff you found looks like really nice specimens, even to someone who primarily collects shark teeth

Daryl. "

You can easely add Bob any other ways of fossilisation here. Thanks, Roman