Preservation of Fossils:
Various types of fossils are observed to have been preserved in sedimentary rocks. The type of fossil preservation depends mostly upon the nature and composition of the original parts and the different conditions for deposition and burial. This web page will cover the basic forms of fossil preservation for both plants and animals. Note that each type of fossil preservation may vary slightly from one case to another due to the random nature and variation in depositional setting, mineralization, groundwater, and burial.
Complex organic compounds that make up plant tissues are likely to undergo chemical changes through a process called carbonization. Such an unique process occurs for dead plant materials when their watery components leave a carbon residue that is much more stable over a long period of time. When a plant is covered by sediment, pressure from the weight above begins squeezing out any liquid and gas that may still be present in the tissue. The plant tissue then becomes flattened and a carbon residue is left behind. In some cases, plant material buried deep within sediment may become heated. If this should happen, the remaining carbon may leave a darker, charcoal appearance.
Petrified wood falls under a special category of replacement called permineralization. In such cases, all of the original plant material has been replaced with minerals, while usually retaining the original structure of the bark. Permineralization typically leaves behind an array of colors. When trees die and fall to the ground, they usually begin the decay process; however, if they are quickly buried by sediment, they may begin the process of permineralization. As moving groundwater begins flushing away the original plant tissue, dissolved minerals, such as silica ions may become supersaturated. If this happens, quartz will completely replace the original material by a process of mineralization.
This ammonite has been replaced with the iron sulfide mineral called pyrite (fool's gold). This replacement process is commonly referred to as pyritization. Such fossilization occurs when chemical changes take place underground as groundwater saturates the sediment. Moving groundwater containing dissolved chemicals commonly fill the original shell material atom by atom, and in rare cases, the original shell material may be fossilized showing great detail of the shell's ornamentation. It should be noted that other types of invertebrate fossils, such as crinoid stems, brachiopods, and bivalves can also undergo pyritization or any other type of replacement process.
Many fossils are preserved as a result of recrystallization (the formation of new crystals). This is especially true when a skeleton or shell is made more of an unstable crystalline mineral, like aragonite, with a closely related but more stable form, such as calcite. Aragonite has a mineral composition of calcium carbonate with a slightly different atomic structure than calcite. It is very common for mollusk shells to originally form as the mineral aragonite; however, once a shell becomes buried, the aragonite chemically alters to calcite. Once this transformation takes place, preservation of the shell is more likely to occur within the rock. The left photo depicts half of an ammonite shell with its many chamber sections exposed. The sections were once composed of aragonite, but later chemically altered to yellow calcite. Through the processes of mineral precipitation, the center whirl, with its chamber sections, was the first to form.
A complete replacement of an organism's structure with another material is called replacement. The left photo is of a fossilized clam shell that has partially been replaced by yellow calcite. After deposition and burial, the original shell material has long since eroded away and filled with a fine-grained sediment. Later, the sediment hardened through a process called lithification to form a chalky variation of limestone. As groundwater passed through and around the fossilized shell, the chalky-limestone partially dissolved away. The groundwater then became supersaturated with calcium carbonate, and as it did so, the mineral-rich water precipitated out to form calcite.
Under exceptional conditions, organisms can be fossilized in an unaltered state. Marine- and lake-dwelling invertebrate animals that have a calcareous skeleton or shell may basically preserve unaltered, even in rocks of great age. The concept of unaltered preservation is a little misleading. It does not mean that the organism is unchanged. As a matter of fact, nucleic acids (DNA and RNA), proteins, pigments, and soft tissues often degrade before preservation begins. Tissues, if present, usually lose their water content; however, organic matter (carbon) will not change into another substance.
Special Forms of Preservation:
When an organism is buried in accumulating sediments and subjected to a great deal of pressure in a relatively short time, all that is left is a thin film of carbonized residue which shows the outline of an organism. You may not think that a black or brownish-red outline of an organism is very useful, but often times compression can preserve soft tissue, such as skin, hair, cellular structure, etc. of an animal or plant. Compressions are often the only source of this information. Such forms of preservation can provide paleontologists with a better understanding of what the organism was, how it lived and behaved, and the environmental conditions present at the time. The compression outline may also reveal very fine details, such as downy feathers shown in the bird fossil to the left.
An impression has undergone the same kind of exposure and pressure as a compression fossil, but there is no carbon film present. An impression is really a two-dimensional "negative" of an organism. Impressions are most commonly found that have large, essentially two-dimensional parts, such as plant leaves, fish scales, and shells. Having a little trouble visualizing how an impression fossil forms? Well, then smooth out some clay, flatten it out, and place a bottle cap or anything relatively flat on top of the clay while pressing somewhat firmly. When you remove the object (i.e. sea shell), an impression of the original shell will be revealed. In some cases, impressions can show great detail of the organism. Note the ribs and growth lines of the scallop shell (left photo) that was preserved in chert. The growth lines are so well-preserved that one can determine past climate conditions.
Preserved in Amber:
Amber, the fossilized resin of a plant or tree, sometimes preserves not only the external, but also the internal structure of an organism. Insects, spiders, frogs, small lizards, and pollen and spores may be preserved in this way. So, how does amber form? First the resin must be resistant to decay. Many trees produce resin, but in most cases resin deposits are broken down by physical and biological processes. Exposure to sunlight, rain fall, and temperature extremes are just a few ways in which resin can be disintegrated. Micro-organisms, such as bacteria and fungi also assist in the breakdown of resin. In order for resin to survive long enough to turn into amber, it must be resistant to such destructive forces. It takes millions of years for resin to harden to amber; therefore, resin must be buried rapidly to ensure its suvival.
Preserved in Tar and Sand:
A mixture of tar and sand has embalmed many insects. Such insects could be stable for thousands but not millions of years. So how do animals and insects get caught in tar and sand? Tar is simply a mixture of viscous, black liquid and minerals that often gives the illusion of water. Unaware of the illusion, a thirsty animal or insect falls into the tar and gets trapped. As the animal or insect sinks into the tar, its flesh or tissue begins to decay, and over a long period of time, the tar hardens to solid rock, thus preserving the external form of the animal or insect. Because of the fast decaying processes that take place in fresh tar, soft tissues are rarely preserved.
Some fossils degrade and disappear in the sediment below leaving a void space called a mold. As time passes, the mold can be filled in with a fine-grained sediment. The fine-grained sediment then hardens through a process of cementation or compaction to form a new structure, but in the likeness of the original material. In such cases, a record of the fossil's internal and external structures may be provided. Steinkerns may reveal internal anatomy of an organism, such as muscle attachment and other details of soft tissue structure. It should be noted that the German name Steinkern has been assigned to all internal fossil structures.