How are fossils created?

Question

Full disclosure: I am a creationist, but I have always wondered about this one. I assume there is a theory and/or repeatable method, but I've never heard it. I'm fairly certain that you can't just leave a carcass on the ground any ol' place and get a fossil, so what are the conditions that must be present and what is the likelihood of those conditions existing in sufficient quantity to generate the fossil record?

Please, no flames, no "Well, yeah, how do you explain..." replies. This is an honest question and I promise to read any links provided. (A reasonable number of them anyway...)

Answer 1

Fossils are the remains of creatures which existed long ago. Fossils range from thousands of years to many millions of years in age. The earliest fossils date from around 600 million years ago, however recent reports suggest bacteria may have existed up to 3 billion years earlier. To put this in context, the dinosaurs became extinct just 65 million years ago.
Not all former life was preserved as fossils, in fact the vast majority simply vanished without trace. The most likely materials to survive fossilisation are the hard parts such as shells and objects which in life were constructed from resistant materials, such as Coral. In order for softer materials to survive, the conditions must be extremely favoura
Fossils come in a variety of sizes, from minute traces to large skeletons. Trace fossils are clues to former life, they result from the activities or presence of creatures and plants. Examples of these traces include footprints, burrows and root tunnels. At the larger end of the scale, fossils also include bones, the largest of which belong to the dinosaurs, which existed between the Triassic and Cretaceous periods.
The following diagrams illustrate the process of fossilisation, starting with the moment of death. The example used is an ammonite, a shelled creature that lived in the seas around 150 million years ago. (to learn more about Ammonites click here)
Having died, the ammonite slowly sinks to the sea floor. Scavengers feed on the fleshy body of the creature, and after only several weeks all that remains is the shell
Several months after death the shell gradually becomes covered with silt and sand. These layers continue to build, providing a shield around the shell and protecting it from damage. Time continues to pass and more and more layers are deposited. After a few hundred years the shell is several feet beneath the surface.
Time continues to pass, 1,000 years, 10,000 years and more...
Gradually the chemicals in the shell undergo a series of changes. As the shell slowly decays, water infused with minerals passes through it, replacing the chemicals in the shell with rock-like minerals (Calcite, Iron or Silica). This process is scientifically known as 'permineralization'
Over millions of years the original shell is completely replaced by the minerals and what remains is a rock-like copy of the original shell. The fossil has the same shape as the original object, but is actually rock. This process also results in loss of original color.

Answer 2

The previous answer is correct. Remains need to be submerged to become a fossil. The presence of oxygen allows the animal to decay therefore, if the animal or plant is buried in mud for example it slows the decay process and allows the chemical process of fossilization to occur. The composition of the animal material is altered and replaced by harder minerals like the previous answer explained. It is very uncommon for a fossil to form. Its something like 1% of remains turn into fossils. I respect the fact that you have your own views on this subject but are still interested in the geologic side of things.

Answer 3

Permineralization
A permineralized trilobite, Asaphus kowalewskiiPermineralization occurs after burial, as the empty spaces within an organism (spaces filled with liquid or gas during life) become filled with mineral-rich groundwater and the minerals precipitate from the groundwater, thus occupying the empty spaces. This process can occur in very small spaces, such as within the cell wall of a plant cell. Small scale permineralization can produce very detailed fossils. For permineralization to occur, the organism must become covered by sediment soon after death or soon after the initial decaying process. The degree to which the remains are decayed when covered determines the later details of the fossil. Some fossils consist only of skeletal remains or teeth; other fossils contain traces of skin, feathers or even soft tissues. This is a form of diagenesis.


Replacement and compression fossils
In some cases the original remains of the organism have been completely dissolved or otherwise destroyed. When all that is left is an organism-shaped hole in the rock, it is called a mould fossil or typolite. If this hole is later filled with other minerals, it is called a cast fossil and is considered a replacement fossil since the original materials have been completely replaced by new, unrelated ones. In some cases replacement occurs so gradually and at such fine scales that no "hole" in the rock can ever be discerned and microstructural features are preserved despite the total loss of original material.

Compression fossils such as those of fossil ferns are the result of chemical reduction of the complex organic molecules composing the organism's tissues. In this case the fossil consists of original material, albeit in a geochemically altered state. This chemical change is an expression of diagenesis.

To sum up, fossilization processes proceed differently for different kinds of tissues and under different kinds of conditions.


Trace fossils
Trace fossils are the remains of trackways, burrows, footprints, eggs and eggshells, nests, droppings and other types of impressions. Fossilized droppings, called coprolites, can give insight into the feeding behavior of animals and can therefore be of great importance.


Microfossils
'Microfossil' is a descriptive term applied to fossilized plants and animals whose size is just at or below the level at which the fossil can be analyzed by the naked eye. A commonly applied cut-off point between "micro" and "macro" fossils is 1 mm, although this is only an approximate guide. Microfossils may either be complete (or near-complete) organisms in themselves (such as the marine plankters foraminifera and coccolithophores) or component parts (such as small teeth or spores) of larger animals or plants. Microfossils are of critical importance as a reservoir of paleoclimate information, and are also commonly used by biostratigraphers to assist in the correlation of rock units.


Resin fossils
Fossil resin (colloquially called amber) is a natural polymer found in many types of strata throughout the world, even the Arctic. The oldest fossil resin dates to the Triassic, though most dates to the Tertiary. The excretion of the resin by certain plants is thought to be an evolutionary adaptation for protection from insects and to seal wounds caused by damage elements. Fossil resin often contains other fossils called inclusions that were captured by the sticky resin. These include bacteria, fungi, other plants, and animals. Animal inclusions are usually small invertebrates, predominantly arthropods such as insects and spiders, and only extremely rarely a vertebrate such as a small lizard. Preservation of inclusions can be exquisite, including small fragments of DNA.

Pseudofossils
Example of a pseudofossil: this dendrite looks much like a plantPseudofossils are visual patterns in rocks that are produced by naturally occurring geologic processes rather than biologic processes. They can easily be mistaken for real fossils. Some pseudofossils, such as dendrites, are formed by naturally occurring fissures in the rock that get filled up by percolating minerals. Other types of pseudofossils are kidney ore (round shapes in iron ore) and moss agates, which look like moss or plant leaves. Concretions, round or oval-shaped nodules found in some sedimentary strata, were once thought to be dinosaur eggs, and are often mistaken for fossils as well.

Answer 4

Calcium and its compounds are soluble in water. As the bones decay, calcium is removed. To become fossils, the bones have to be exposed to water containing harder, less soluble dissolved minerals. As the calcium gradually dissolves, the more durable minerals are deposited in the space left behind. Eventually, all the calcium is replaced by minerals.