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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.

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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
A petrified log in Arizona is an example of permineralization
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.Let’s use a Hadrosaur as an example to explore the world of fossilization.

The Hadrosaur is happily walking along a riverbank when a mudslide occurs, killing and burying him. Under hundreds of pounds of sediment, the Hadrosaur begins to decay. Time passes and the Hadrosaur continues to get deeper in the sediment. The Hadrosaur and sediment surrounding him begin to compress.Many things can happen at this point: the Hadrosaur can dissolve, leaving a mold or cast fossil behind, or minerals can permeate the bone, leaving a permineralization fossil. One day, a lucky paleontologist is digging on the Colville River in Alaska, and the Hadrosaur is discovered!

Lesson Summary

Fossils give us a glimpse into the earth’s past and show scientists how organisms have changed over time. Let’s review some important terms.

There are two main types of fossils:

  • Body fossils, an actual organism
  • and trace fossils, evidence of an organism

Mold and cast fossils are when an organism leaves an imprint. Permineralization is when sediment enters the pores of the organism, replacing tissue and creating a replica organism. Freezing and getting stuck in amber are two examples of whole body preservation, where the whole body is preserved.Becoming a fossil isn’t easy. An organism must be in the right place at the right time, and that’s no guarantee.

For many soft-bodied organisms, like plants or jellyfish, fossilization is especially challenging. I always thought being buried alive would be a terrible way to go, but maybe I can take comfort in knowing that, if I am buried alive, I have a chance of becoming a fossil.

Fossils: Vocabulary & Definitions

  • Fossil: traces of past organisms preserved by nature
  • Paleontologists: scientists who study fossils for clues to the past
  • Body Fossils: remains of actual organisms
  • Trace Fossils: trace evidence that an organism existed
  • Mold Fossils: an imprint is made and fossilized
  • Cast Fossils: an imprint that is filled in, then fossilized
  • Permineralization: process in which an organism dies and sediment enters the body, filling in the pores of a leaf, or a bone or a shell
  • Whole Body Preservation: occurs when the total organism is fossilized; may occur through freezing or falling in tree amber

Learning Outcomes

When this lesson ends, students should be able to:

  • Define fossils
  • Identify the types of fossils
  • Explain how fossils may form

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