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10.2: Types of Fossilization

Difficulty Level: Basic Created by: CK-12
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Where could you find the best fossils?

The best fossils would probably be ones that you could get DNA from. DNA holds the genes of an organisms. The genes could tell scientists a lot about what the organisms was really like. Where could you find a fossil with DNA? How about a melting glacier? You might be able to find a creature that had been frozen into the ice for a long time. A frozen animal would be very well preserved!

How Fossils Form

The process by which remains or traces of living things become fossils is called fossilization. Most fossils are preserved in sedimentary rocks.

Fossils in Sedimentary Rock

Most fossils form when a dead organism is buried in sediment. Layers of sediment slowly build up. The sediment is buried and turns into sedimentary rock. The remains inside the rock also turn to rock. The remains are replaced by minerals. The remains literally turn to stone. This type of fossilization is called permineralization. Fossilization is illustrated in Figure below.

Fossilization. This flowchart shows how most fossils form.

Other Ways Fossils Form

Fossils may form in other ways. With complete preservation, the organism doesn't change much. As seen in the Figure below, tree sap may cover an organism and then turn into amber. The original organism is preserved so that scientists might be able to study its DNA. Organisms can also be completely preserved in tar or ice.

Complete Preservation. This dragonfly looks the same as it did the day it died millions of years ago!

Molds and casts are another way organisms can be fossilized. A mold is an imprint of an organism left in rock. The organism's remains break down completely. Rock that fills in the mold resembles the original remains. The fossil that forms in the mold is called a cast. Molds and casts usually form in sedimentary rock. With compression, an organism's remains are put under great pressure inside rock layers. This leaves behind a dark stain in the rock. You can see two examples of these fossils in the Figures below and below.

Molds and Casts. A mold is a hole left in rock after an organim's remains break. A cast forms from the minerals that fill that hole and solidify.

Compression. A dark stain is left on a rock that was compressed. These leaves were fossilized by compression.

Why Fossilization is Rare

It’s very unlikely that any given organism will become a fossil. The remains of many organisms are consumed. Remains also may be broken down by other living things or by the elements. Hard parts, such as bones, are much more likely to become fossils. But even they rarely last long enough to become fossils. Organisms without hard parts are the least likely to be fossilized. Fossils of soft organisms, from bacteria to jellyfish, are very rare.


  • amber: Fossilized tree sap.
  • cast: A mold filled with sediment and hardened to create a replica of the original fossil.
  • compression: Remains are put under great pressure inside a rock. This leaves behind a dark fossil stain.
  • fossilization: The processes that change a living organism or trace of one into a fossil.
  • mold: An impression made in sediments by the hard parts of an organism.
  • permineralization: The replacement of parts of an organisms by minerals.


  • Very few fossils preserve soft parts. Insects may be preserved in amber. Insects and other animals may be preserved in ice.
  • Some fossils are created when minerals replace the organic material. This is called permineralization.
  • A fossil may be in the form of a mold. A mold is the depression left in the shape of the material.
  • A fossil may be preserved as a cast. A cast is the rocky material that fills the mold.


Use this resource to answer the questions that follow.

Fossils at http://www.youtube.com/watch?v=A5i5Qrp6sJU (0:58)

  1. What are fossils?
  2. What type of rocks are fossils found in?
  3. What are sediments?
  4. Explain how a fossil is created.
  5. What factors have exposed sedimentary rock?


  1. Why are there so few fossils of soft parts?
  2. A snail shell is buried in mud. The shell is filled with mineral-rich water. What type of fossilization has occurred?
  3. Compare and contrast molds and casts.
  4. Why is fossilization rare?

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Fossilized tree sap.


Mold filled with sediment and hardened to create a replica of the original fossil.


Remains are put under great pressure inside a rock; this leaves behind a dark fossil stain.


Processes that change a living organism or trace of one into a fossil.


Impression made in sediments by the hard parts of an organism.


Replacement of parts of an organisms by minerals.

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Difficulty Level:
6 , 7
Date Created:
Jan 04, 2013
Last Modified:
Aug 29, 2016
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