How Fossils Form

A paleontologist working to document fossil sites for the park's paleontological resources inventory. Theodore Roosevelt National Park, North Dakota.

NPS photo by Patrick Wilson.

Introduction

Fossils are the evidence of life preserved in a geologic context. How this evidence is preserved in the geologic record can be quite variable. Sometimes all or part of an ancient organism is preserved as a fossil. Other times, all that remains is a trace of that organism, like a fossil footprint left in wet sand.

Fossilization Processes

A variety of changes to organic remains during fossilization. Fossilization processes include:

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  Permineralization and Replacement
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  Recrystallization
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  Molds, Casts, & Steinkerns
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  Impressions and Compressions

  (including Carbonization)

• 

  Preserved Remains

  Drying, Freezing, Amber, Natural Asphalt

The Odds of Becoming a Fossil

The odds of any individual organism becoming fossilized is very small, but overall the fossil record provides a rich history of life on Earth. Yet it is incomplete. Fossils are much more likely when the organisms lived in areas that were undergoing active sedimentation. Fossils of organism that lived in marine environments or on rivers and their floodplains are much more common than those that lived in mountainous terrain or other uplands. Fossils of winged and terrestrial insects and birds are fairly rare, except for those of some water birds.

Two factors greatly increase the odds of the remains of an organism being fossilized:

• Rapid burial: Burial by sediment soon after an organism dies prevents the remains from decomposition, weathering, and scavenging.

• Hard parts: The hard parts of organisms, such as shells, bones, teeth, and wood, are more durable than soft tissues and are much more likely to preserved as fossils.

Taphonomy is the study of what happens to an organism between its death and when it is a fossil.

Sometimes rare circumstances can come together to produce exceptionally rich fossil deposits called Lagerstätten. Lagerstätten can contain either especially high numbers of fossils, exquisite preservation so that fine details and/or soft tissues are retained, or both.

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Fossil Types

Overall, there are two main categories of fossils: body fossils and trace fossils.

• Body Fossils: Parts or all of the body of past animals, plants, and microorganisms.

• Trace Fossils (ichnofossils): The evidence of biological activity (like a footprint) of an organization made by the activity of an organism while it is alive.

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