Fossil Evidence: Definition & Overview

Lesson Transcript
Instructor: Derrick Arrington

Derrick has taught biology and chemistry at both the high school and college level. He has a master's degree in science education.

Fossils are critical tools in science because they give us clues to the types of plants and animals that lived on Earth in the past. In this lesson, you will learn about the types of fossils and how they were formed.

Fossil Evidence

Fossils are essentially clues that have been left behind by various forms of living things. They are instrumental tools for understanding the diverse groups of organisms that have inhabited our planet at one time or another. A fossil is any preserved evidence of an organism. Plants, animals and even bacteria can form fossils in some form.

Although there is a rich diversity of fossils, and more are being discovered every day, the fossil history is like a book that is missing many pages. It is estimated that approximately 99% of species that ever existed have become extinct. Only a small percentage of those organisms have been preserved as fossils or discovered.

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Types of Fossil Evidence

Why are there so few fossils relative to the number of species that have existed? It is most likely because organisms decompose before they have a chance to become fossilized. Only those organisms that are quickly buried in sediment after their death become preserved.

There are numerous types of fossil evidence that have been left behind by organisms. Some of the types of fossil evidence commonly used by scientists include trace fossils, molds and casts, replacement fossils, petrified fossils, amber and original material fossils.

A trace fossil is any indirect evidence left by an organisms. Examples of trace fossils include footprints, burrows and fossilized feces that have been left behind by organisms. They simply leave material evidence that the organisms existed rather than evidence of the actual structure of the organism.

This image shows trace fossils of shells that once existed.
An image of trace fossils.

Molds and casts are also important fossil evidence. Molds are impressions of entire organisms that have been left behind. Casts are molds that have been filled with sediment over time.

This image depicts a mold fossil of a Trilobite.
An image of a mold fossil.

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