Lesson Objectives

  • Explain why it is rare for an organism to be preserved as a fossil.
  • Distinguish between body fossils and trace fossils.
  • Describe five types of fossilization.
  • Explain the importance of index fossils, and give several examples.


Throughout human history, people have discovered fossils and wondered what they are and what they represent. In ancient times, fossils inspired legends of monsters and other strange creatures. The Chinese writer, Chang Qu, 2,000 years ago reported the discovery of “dragon bones,” which were probably dinosaur fossils (Figure below). Look at the two photos below and try to trace the origin of the creature on the left.
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The griffin, a mythical creature with a lion
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Ammonites (left) and elephant skull (right).
Ancient Greeks named ammonites after the ram god Ammon since they look like the coiled horns of a ram. Legends of the Cyclops may be based on fossilized elephant skulls found in Crete and other Mediterranean islands(Figure above). Can you see why?
Many of the real creatures whose bones became fossilized were no less marvelous than the mythical creatures they inspired (Figure below).
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The giant pterosaur Quetzalcoatlus had a wingspan of up to 12 meters (39 feet).
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Argentinosaurus had an estimated weight of 80,000 kg, equal to the weight of seven elephants!
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Other fossils, such as the trilobite Kolihapeltis sp (left) impress us with their bizarre forms. These suture marks on an ammonite fossil (right) display a delicate beauty.

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) (Figure below). Collections of fossils are known as fossil assemblages.
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Coprolite from a meat-eating dinosaur.
The process of a once-living organism becoming a fossil is called fossilization. Fossilization is very rare: 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).
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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).
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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.
Despite these problems, there is a rich fossil record. How does an organism become fossilized?
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 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).
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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.
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People buried by the extremely hot eruption of ash and gases at Mt. Vesuvius in 79 AD.
Unusual circumstances may lead to the preservation of a variety of fossils, as at the La Brea Tar Pits in Los Angeles, California (Figure below).
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Although the animals trapped in the Ta Brea Tar Pits probably suffered a slow, miserable death, their bones were preserved perfectly by the sticky tar.
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) and (Figure below). The 145 million-year-old Solnhofen Limestone in Germany has fossils of soft body parts that are not normally preserved (Figure below) and (Figure below).
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The Burgess shale contains soft-bodied fossils
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Anomalocaris, meaning
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A brittle star from the Solnhofen Limestone.
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The famous Archeopteryx fossil from the Solnhofen Limestone has distinct feathers and was one of the earliest birds.

Types of Fossilization

Most fossils are preserved by one of five processes outlined below (Figure below):
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Five types of fossils: (A) Insect preserved in amber (B) Petrified wood (permineralization) (C) Cast and mold of a clam shell (D) Pyritized ammonite (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.

Clues from Fossils

Fossils are our best form of evidence about Earth history, including the history of life. Along with other geological evidence from rocks and structures, fossils even give us clues about past climates, the motions of plates, and other major geological events.

History of Life on Earth

That life on Earth has changed over time is well illustrated by the fossil record. Fossils in relatively young rocks resemble animals and plants that are living today. In general, fossils in older rocks are less similar to modern organisms. The history of life will be discussed in the Earth's History chapter.

Environment of Deposition

By knowing something about the type of organism the fossil was, geologists can determine whether the region was terrestrial (on land) or marine (underwater) or even if the water was shallow or deep. The rock may give clues to whether the rate of sedimentation was slow or rapid. The amount of wear and fragmentation of a fossil allows scientists to learn about what happened to the region after the organism died; for example, whether it was exposed to wave action.

Geologic History

The presence of marine organisms in a rock indicates that the region where the rock was deposited was once marine. Sometimes fossils of marine organisms are found on tall mountains indicating that rocks that formed on the seabed were uplifted (Figure below).
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The summit of Mt. Everest, the world


By knowing something about the climate a type of organism lives in now, geologists can use fossils to decipher the climate at the time the fossil was deposited. For example, coal beds form in tropical environments but ancient coal beds are found in Antarctica. Geologists know that at that time the climate on the Antarctic continent was much warmer. Recall from the chapter about plate tectonics that Wegener used the presence of coal beds in Antarctica as one of the lines of evidence for continental drift.

Lesson Summary

  • Fossils are the remains of ancient life. Body fossils are the remains of the organism itself; trace fossils are burrows, tracks, feces, or other evidence of activity.
  • Fossilization is a very rare process. The chances of becoming a fossil are enhanced by quick burial and the presence of hard parts, such as bones or shells.
  • Fossils form in five ways: by preservation of the remains, permineralization, molds and casts, replacement, and compression.
  • Types of organisms that make good index fossils are widespread but only existed for a short period of time. Index fossils help scientists to determine the approximate age of a rock layer and to match that layer up with other rock layers.
  • Fossils give clues about the history of life on Earth, environments, climate, geologic history, and other events of geological importance.

Review Questions

1. What factors make it more likely that an animal will be preserved as a fossil?
2. What are the five main processes of fossilization?
3. A scientist wants to determine the age of a rock. The rock contains an index fossil and an ancient relative of a living organism. Which is more useful for dating the rock, and why?
4. The island of Spitzbergen is in the Arctic Ocean, near the North Pole. Fossils of tropical fruits have been found in coal deposits in Spitzbergen. What does this indicate?

Further Reading / Supplemental Links

This site is all about fossils: http://www.fossils-facts-and-finds.com/index.html
American Museum of Natural History site devoted to the links between mythic creatures and the organisms that inspired them: http://www.amnh.org/exhibitions/mythiccreatures
Fossil myths and legends: http://www.tonmo.com/science/fossils/mythdoc/mythdoc.php
More about the Burgess Shale: http://www.geo.ucalgary.ca/~macrae/Burgess_Shale
More about the Solenhofen Limestone: http://www.ucmp.berkeley.edu/mesozoic/jurassic/solnhofen.html
The story of Otzi the Iceman: http://en.wikipedia.org/wiki/%C3%96tzi_the_Iceman


trace fossil Evidence of the activity of an ancient organism; e.g. tracks, tubes, and bite marks. permineralization Fossilization in which minerals in water deposit into empty spaces in an organism. mold An impression made in sediments by the hard parts of an organism. microfossil A fossil that must be studied with the aid of a microscope. index fossil A fossil indicates the relative age of the rock in which it is found. Index fossils come from species that were widespread but existed for a relatively brief period of time. fossilization The process of becoming a fossil. fossil Any remains or trace of an ancient organism. cast A mold filled with sediment and hardened to create a replica of the original fossil. body fossil The remains of an ancient organism. Examples include shells, bones, teeth, and leaves. amber Fossilized tree sap.

Points to Consider

  • What are some other examples of mythical creatures that may be based on fossils?
  • Why is it so rare for an animal to be preserved as a fossil?
  • Some organisms are more easily preserved than others. Why is this a problem for scientists who are studying ancient ecosystems?
  • Why are examples of amazing fossil preservation so valuable for scientists?
  • Many fossils of marine organisms have been found in the middle of continents, far from any ocean. What conclusion can you draw from this?