How do Fossils Form

1. Mold and Cast
These fossils form when the organism's original bone or tissue is removed by processes that occur after burial, such as groundwater flow. The impression can be in mold or cast.

Cast occurs when the organism remains decomposed entirely, leaving a space just like the shape of the organism. The process where the
Minerals or sediment fill the hollow depression

But If minerals fill in this space and form a mineralized 3D shape of the organism it is referred to as a mold. Where a duplicate of a plant or animal is preserved in sedimentary rocks. The organism is buried in sediment and then dissolved by underground water leaving a hollow depression.

However, the mold shows only the original shape and marking of the organisms. It does not reveal the internal structure. Minerals or sediment fills the hollow depression and forms a cast.

They are particularly useful to biologists who can use them to discover extinct animal species and history.
2. Permineralization.
The most common method of fossilization is called permineralization, or petrification. Here, each part of the organism is replaced by minerals, leaving a stone copy of the organism.

It occurs when water drips into the remains, and minerals dissolved in the water drip into the spaces within the remains, where they form crystals. These crystallized minerals cause the remains to harden along with the encasing sedimentary rock.

The original shell or bone becomes buried below the ground and water penetrates the surface. The groundwater contains the calcium carbonate that fills the empty spaces in the material, which over time, hardens and fills the pores full of minerals which in turn form a fossil.

After an organism's soft tissues decay in sediment, the hard parts, particularly the bones are left behind. Bones, teeth, and even woody plant materials such as trees usually remain in this manner.

3. Trace
This fossil's forms usually show tracks that animals made while moving across soft sediment. This sediment later hardens to become sedimentary rock. Animal footprints were made in soft sediment that later formed solid sedimentary rock.

There are three main types of trace fossils. Tracks and trails are produced by an organism walking, crawling, foraging, or resting.

The traces can also be in the form of tracks like footprints and movement, animal trails made in soft sediment that later formed solid sedimentary rock.

The structure of the animal or plant remains in a mineral form. The colors of the minerals that replace the form can be dazzling. Sometimes they are made into art and jewelry

Trace fossils are valuable to paleontologists because by studying these footprints, scientists can discover how the animals moved, which in turn gives important information about the structure and even the life of the species.
4. Preservation
Fossils can be intentionally preserved by humans.
It can be found through unaltered fossils, and this type is rare except as captured in amber, trapped in tar, dried out, or frozen as a preserved wooly mammoth.

Both ice and amber protect the organisms from decay and give them an oxygen-free environment. The entire animal has been preserved, even the soft parts which usually decay and disappear.

Fossils can also be replaced with shellfish and wood. Replacement is when the atomic composition of the original living thing is replaced cell by cell by a new chemical structure.

Typically, the chemical that replaces the original is determined by the groundwater the fossil is lying in. A common type of replacement simplification.

This is when the original living remains are replaced with silica as in the case of petrified forests.

5. Carbonization.
Carbonation generally means a chemical reaction where water transforms the organic material of a plant or animal into a thin film of carbon.

Organic materials like Nitrogen, hydrogen, and oxygen are driven off as gases, leaving an outline of the organism. Organisms fossils often by carbonization include fish, leaves, and the woody tissues of plants.

The remains of the plant or animal are crushed beneath the weight of the rock leaving behind a carbon film, an impression of the former living thing.

This fossil formation is the one in which the organism is formed as a residual, thin film of carbon instead of the original organic matter.

The original organism results in thin layers of carbon or coal.

What is left behind is a carbon film, an impression of the former living thing.