This page will go through the primary types of fossil preservation that occur throughout the fossil record. Read more about the fossil record and types of preservation on the Digital Encyclopedia of Ancient Life here.
In this case, the fossilized biological material has undergone no changes. The skeletal material is the same composition as it was in life. The example here are teeth of skates and rays – this is relatively common with teeth as CaPO4 (phosphate) is a very stable mineral. Often, species of fossil animals are named only by their teeth. The other bones in the body are very porous and have a more difficult time with erosional processes.
Permineralization is often called ‘petrification‘. This occurs when a fluid that contains chemicals that fill pore spaces. Over time, these chemicals can accumulate in the pores, grow, and turn into minerals. As the minerals become larger, they push out organic matter, completely changing the composition of the fossil. This fluid can be from groundwater, lakes, or even the ocean. Once the fluid fills the pores the chemistry of the organism begins to change.
Eventually the minerals are deposited inside the pore space and the fluid evaporates leaving the excess minerals in the pores, cells, or even tissue of the organism. This type of preservation is common in wood and bone, and other very porous materials.
There is a Petrified Forest in the United States National Park Service.
Recrystallization occurs when the bond arrangements within the mineral’s crystal lattice change. This alters the structure but not the chemical composition, resulting in a polymorph (same chemical formula but different shape/form). This is very common with aragonite to calcite transition in the fossil record. The chemical structure of calcite is more stable than aragonite and therefore preserves better through time.
Read more about aragonite and calcite on the Common Minerals in Biomineralization page.
Replacement occurs when the actual skeletal composition changes completely. The crinoid pictured here, originally had a skeleton of calcite (CaCO3) but the original skeleton was replaced with pyrite (FeS2). In the fossil record the two common minerals that replace original skeletal material: (1) pyrite (or fool’s gold) and (2) silica (SiO2) a form of quartz.
In many cases, the original skeletal material completely dissolves and all that remains is a mold of the organism. There are two main types of moldic preservation: casts and imprints. They often can be preserve together or with other forms of preservation.
Internal mold of a gastropod. These images are both sides of the same specimen, notice the lack of detail. All you see is the spiral shape. Casts can either be internal or external molds. When an organism dies, its shell can fall to the bottom of the ocean. This shell can be infilled with mud or sand once the organic material has been removed by decay and scavengers. The actual shell material dissolves back into the water and what you are left with is a mold of the inside of the shell. A similar process can happen with the exterior of the shell.
Imprints often occur alongside carbonization (see below). This happens when there is no skeletal or organic material remaining but rather an imprint or a pressing of the fossil outline. It’s very similar to when you would color on a piece of paper over a leaf as a child. You get the outline and some details of the anatomy but it is a two dimensional object.
Carbonization of fossil material occurs when there is a lot of heat and pressure. This occurs from many rock layers being piled on top of the layer the fossils are in. These are very often plant remains. This type of preservation was common during a period of time called the Carboniferous, which had a lot of plant life and swamps!