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How Fossilization Creates Fossils

The different types of fossils include body and trace fossils; all are part of the fossil record.

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Fossils I: Fossilization

What kind of fossil is this?

As a paleontologist it would be great to find a new species of dinosaur or the best preserved specimen of a species like Tyrannosaurus rex. But lots of important information can be gained from less....um...glamorous finds. One example is this fossil coprolite from a meat-eating dinosaur. Fortunately, fossil poo doesn't stink!

Fossils were Parts of Living Organisms

It wasn't always known that fossils were parts of living organisms. In 1666, a young doctor named Nicholas Steno dissected the head of an enormous great white shark that had been caught by fisherman near Florence, Italy. Steno was struck by the resemblance of the shark’s teeth to fossils found in inland mountains and hills (Figure below).

Fossil Shark Tooth (left) and Modern Shark Tooth (right).

Most people at the time did not believe that fossils were once part of living creatures. Authors in that day thought that the fossils of marine animals found in tall mountains, miles from any ocean could be explained in one of two ways:

  • The shells were washed up during the Biblical flood. (This explanation could not account for the fact that fossils were not only found on mountains, but also within mountains, in rocks that had been quarried from deep below Earth’s surface.)
  • The fossils formed within the rocks as a result of mysterious forces.

But for Steno, the close resemblance between fossils and modern organisms was impossible to ignore. Instead of invoking supernatural forces, Steno concluded that fossils were once parts of living creatures.

How Fossils Form

A fossil is any remains or traces of an ancient organism. Fossils include body fossils, left behind when the soft parts have decayed away, and trace fossils, such as burrows, tracks, or fossilized coprolites (feces) as seen above. Collections of fossils are known as fossil assemblages.

Fossilization is Rare

Becoming a fossil isn't easy. Only a tiny percentage of the organisms that have ever lived become fossils.

Why do you think only a tiny percentage of living organisms become fossils after death? Think about an antelope that dies on the African plain (Figure below).

Hyenas eating an antelope. Will the antelope in this photo become a fossil?

Most of its body is eaten by hyenas and other scavengers and the remaining flesh is devoured by insects and bacteria. Only bones are left behind. As the years go by, the bones are scattered and fragmented into small pieces, eventually turning into dust. The remaining nutrients return to the soil. This antelope will not be preserved as a fossil.

Is it more likely that a marine organism will become a fossil? When clams, oysters, and other shellfish die, the soft parts quickly decay, and the shells are scattered. In shallow water, wave action grinds them into sand-sized pieces. The shells are also attacked by worms, sponges, and other animals (Figure below).

Fossil shell that has been attacked by a boring sponge.

How about a soft bodied organism? Will a creature without hard shells or bones become a fossil? There is virtually no fossil record of soft bodied organisms such as jellyfish, worms, or slugs. Insects, which are by far the most common land animals, are only rarely found as fossils (Figure below).

A rare insect fossil.

Conditions that Create Fossils

Despite these problems, there is a rich fossil record. How does an organism become fossilized?

Hard Parts

Usually it’s only the hard parts that are fossilized. The fossil record consists almost entirely of the shells, bones, or other hard parts of animals. Mammal teeth are much more resistant than other bones, so a large portion of the mammal fossil record consists of teeth. The shells of marine creatures are common also.

Quick Burial

Quick burial is essential because most decay and fragmentation occurs at the surface. Marine animals that die near a river delta may be rapidly buried by river sediments. A storm at sea may shift sediment on the ocean floor, covering a body and helping to preserve its skeletal remains (Figure below).

This fish was quickly buried in sediment to become a fossil.

Quick burial is rare on land, so fossils of land animals and plants are less common than marine fossils. Land organisms can be buried by mudslides, volcanic ash, or covered by sand in a sandstorm (Figure below). Skeletons can be covered by mud in lakes, swamps, or bogs.

People buried by the extremely hot eruption of ash and gases at Mt. Vesuvius in 79 AD.

Unusual Circumstances

Unusual circumstances may lead to the preservation of a variety of fossils, as at the La Brea Tar Pits in Los Angeles, California. Although the animals trapped in the La Brea Tar Pits probably suffered a slow, miserable death, their bones were preserved perfectly by the sticky tar. (Figure below).

Artists concept of animals surrounding the La Brea Tar Pits.

In spite of the difficulties of preservation, billions of fossils have been discovered, examined, and identified by thousands of scientists. The fossil record is our best clue to the history of life on Earth, and an important indicator of past climates and geological conditions as well.

Exceptional Preservation

Some rock beds contain exceptional fossils or fossil assemblages. Two of the most famous examples of soft organism preservation are from the 505 million-year-old Burgess Shale in Canada (Figure below). The 145 million-year-old Solnhofen Limestone in Germany has fossils of soft body parts that are not normally preserved (Figure below).

(a) The Burgess shale contains soft-bodied fossils. (b) Anomalocaris, meaning “abnormal shrimp” is now extinct. The image is of a fossil. (c) A brittle star from the Solnhofen Limestone. (d) The famous Archeopteryx fossil from the Solnhofen Limestone has distinct feathers and was one of the earliest birds.


  • body fossil: The remains of an ancient organism. Examples include shells, bones, teeth, and leaves.
  • trace fossil: Evidence of the activity of an ancient organism; e.g. tracks, tubes, and bite marks.


  • Fossils are the remains or traces of living organisms: body fossils are the remains and trace fossils are the traces.
  • Fossils are mostly made of the hard parts of organisms; there are few soft-bodied fossils.
  • Some of the best preserved fossils form in extremely unusual circumstances like the La Brea tar pits.


Use this resource to answer the questions that follow.


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. Give three examples of body fossils and trace fossils.

2. Under what conditions do fossils form?

3. Why are more fossils of marine organisms than of land organisms?

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