Types of Fossilization



Are all fossils so complete and well-preserved?

Very few circumstances lead to fossils that are as beautiful and complete as this baby mammoth that was frozen in ice. An animal falling into a crevasse or a tar pit does not undergo the scattering and degradation that an animal dying at the surface does and so fossils from these types of rare sites are often fantastic.

Types of Fossilization

Most fossils are preserved by one of five processes outlined below (Figure below):

Five types of fossils: (a) insect preserved in amber, (b) petrified wood (permineralization), (c) cast and mold of a clam shell, (d) pyritized ammonite, and (e) compression fossil of a fern.

Preserved Remains

Most uncommon is the preservation of soft-tissue original material. Insects have been preserved perfectly in amber, which is ancient tree sap. Mammoths and a Neanderthal hunter were frozen in glaciers, allowing scientists the rare opportunity to examine their skin, hair, and organs. Scientists collect DNA from these remains and compare the DNA sequences to those of modern counterparts.



The most common method of fossilization is permineralization. After a bone, wood fragment, or shell is buried in sediment, mineral-rich water moves through the sediment. This water deposits minerals into empty spaces and produces a fossil. Fossil dinosaur bones, petrified wood, and many marine fossils were formed by permineralization.

Molds and Casts

When the original bone or shell dissolves and leaves behind an empty space in the shape of the material, the depression is called a mold. The space is later filled with other sediments to form a matching cast within the mold that is the shape of the original organism or part. Many mollusks (clams, snails, octopi, and squid) are found as molds and casts because their shells dissolve easily.


The original shell or bone dissolves and is replaced by a different mineral. For example, calcite shells may be replaced by dolomite, quartz, or pyrite. If a fossil that has been replace by quartz is surrounded by a calcite matrix, mildly acidic water may dissolve the calcite and leave behind an exquisitely preserved quartz fossil.


Some fossils form when their remains are compressed by high pressure, leaving behind a dark imprint. Compression is most common for fossils of leaves and ferns, but can occur with other organisms.


  • amber: Fossilized tree sap.
  • cast: A mold filled with sediment and hardened to create a replica of the original fossil.
  • mold: An impression made in sediments by the hard parts of an organism.
  • permineralization: Fossilization in which minerals in water deposit into empty spaces in an organism.


  • Very few fossils preserve soft parts; some insects are preserved in amber and animals may be preserved in ice.
  • Some fossils are created when minerals replace the organic material.
  • A fossil may be in the form of a mold, which is the depression left in the shape of the material or a cast, which is rocky material that filled the mold.


Practice II

Use this resource to answer the questions that follow.


1. What do paleontologists study?

2. At least how old are most fossils?

3. What are trace fossils?

4. What can be learned from trace fossils?

5. What are mold fossils?

6. What are resin fossils?

7. How are resin fossils formed?

8. What are body fossils?

9. How are body fossils formed?


1. Why are there so few fossils of soft parts?

2. If a snail shell is buried in mud and then infused with mineral rich water what type of fossilization has occurred?

3. What types of fossils are most likely to form by compression and why?


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