Fossil: Definition, Types, Characteristics & Examples

Fossil: Definition, Types, Characteristics & Examples
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  • 0:00 Definition of Fossils
  • 0:40 Types of Fossils
  • 1:25 Characteristics and…
  • 2:30 How Fossils Form
  • 3:40 Lesson Summary
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Lesson Transcript
Instructor: Julie Zundel

Julie has taught high school Zoology, Biology, Physical Science and Chem Tech. She has a Bachelor of Science in Biology and a Master of Education.

Fossils provide scientists with many clues about Earth's history, offering evidence of dinosaurs and strange plants that existed in the past. This lesson will define a fossil, look at different types and characteristics of fossils, and then describe how fossils are formed.

Definition of Fossils

Have you ever wondered how scientists know so much about the earth's past? For example, how they know Hadrosaurs, a group of duck-billed dinosaurs, lived in Alaska 90 million years ago? Or how they know a relative of today's horsetail plant lived 150 million years ago?

Paleontologists, or scientists who study fossils, help paint a picture of what life used to look like on earth millions of years ago. Fossils are remains or traces of past organisms that have been preserved by nature. And here's a fun fact: the word 'fossil' is Latin and actually means 'obtained by digging,' which makes sense because they are often buried.

Types of Fossils

There are many ways fossils can form, and we'll get to all that in a minute. First, let's discuss the two main ways that fossils can be classified:

  • Body fossils
  • and trace fossils

The remains of a Hadrosaur are an example of a body fossil, or fossils of the actual organism. Typically, hard structures like bones, shells, and teeth fossilize more often than soft-bodied structures like tissues or plant leaves, but as is seen with the horsetail relative, plants can become fossils.

Hadrosaur skull fossils depicted in this photograph are classified as body fossils
hadrosaur

The other type of fossil is called a trace fossil, where evidence of the organism but not the actual organism is preserved, such as a track, a burrow, a nest, or even feces.

Characteristics and Examples of Fossil Formation

The many different ways organisms are preserved gives fossils different characteristics. Let's explore a few ways fossils can form.

Permineralization is when an organism dies and sediment enters the body, filling in the pores of a leaf, or a bone or a shell. Oftentimes, minerals from the sediment will eventually replace the organism's tissues, leaving a replica of the original organism.

A petrified log in Arizona is an example of permineralization
Petrified wood

Mold fossils and cast fossils occur when an organism leaves an imprint that is fossilized. Mold fossils are when an imprint is made and that imprint is fossilized, whereas cast fossils are imprints that are filled in and then fossilized.

This is a mold fossil of a bivalve, like a clam or oyster, from a dig sight in Ohio
Mold fossil of a bivalve organism

For paleontologists, finding an organism that is preserved without any changes to its composition is a huge find. For example, when an organism is preserved through freezing, soft tissue like muscle and skin stays intact; or when an organism falls in tree amber and then is fossilized, the organism is preserved. This is called whole body preservation.

How Fossils Form

You know some of the different types of fossils, but let's look at how fossils form. Fossilization, or fossil formation, is actually pretty rare. The right organism has to be in the right place at the right time. When an organism dies, it usually gets dispersed by organisms eating it or by the elements like wind and water. In addition, many organisms do not fossilize well because of their soft bodies.

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