Earth's Systems: What are fossils?By Encyclopaedia Britannica, adapted by Newsela staff

Grade Level 404/21/2017

Word Count 679

The fossil of a fish called Priscacara. This fish lived more than 33 million years ago. It is now extinct. The fossil was found in Wyoming. Photo from: Wikimedia Commons.

Fossils are the remains of ancient animals and other life forms. They are preserved in the earth. Most people think of fossils as preserved bones or shells of ancient animals. However, there are many forms of fossils. Scientists have even found fossils of early forms of bacteria, or germs.

Not all remains of living things become fossils. An organism must be preserved after it dies to become a fossil. If it is not preserved, it will quickly decay. The best places for fossils to form are wet areas such as riverbeds. The most common fossils come from hard tissues. These are tissues such as shells, bones and tree trunks. Soft tissues can also become fossils if they are preserved quickly. However, this is uncommon.

Scientists have found fossils of thousands of organisms that lived in the past. Together these fossils make up the fossil record. This record tells scientists about the history of life on Earth.

How Fossils Form

There are several ways that fossils can form. One way is called permineralization. This begins when an organism dies in an area like a wetland. After it dies, its body is buried under sediment. Sediment is material like silt, sand and gravel. 

Fossil molds and casts preserve an impression of remains buried in sediment. The shape of the organism left in the sediment is called a mold. The sediment that fills the mold recreates the shape of the remains. This is called a cast. Image from: Encyclopaedia Britannica.

Over time, more and more sediment presses down on the organism. The organism is gradually replaced with minerals from the water that leaks through the sediment. Eventually the remains become hard as stone. This

can take thousands to millions of years. Bones, teeth, shells and tree trunks are usually preserved through permineralization.

Remains buried in sediment leave a 3-D print of their shape. Over time, water moves through the sediment. It gradually destroys the original organism. But minerals fill in the print, creating a mold. The sediment that fills in the mold also becomes mineralized. In this way the sediment recreates the shape of the organism. The result is called a cast. Ancient sea creatures were commonly preserved as molds and casts.

Trace fossils provide evidence of an organism's activity. They include footprints, trails and nests.

Another way a fossil can form is through carbonization. This is how soft tissues become fossils. Soft tissues include leaves, stems, seeds, insects and feathers. During carbonization, the tissues are squeezed between two rocks. Over time, this creates a 3-D print of the tissues.

A fossil can also form when an object is trapped inside a material. This is called an inclusion fossil. Insects trapped in amber are one example. They form when insects become trapped in the sticky resin of a tree. Over time, the resin hardens into amber.

The Fossil Record

The fossil record shows us what organisms looked like a long time ago. Many of these organisms died out long ago. The fossil record shows how the animals changed over time.

The fossil record goes back more than 3 billion years. The oldest known fossils are very tiny creatures. They look like bacteria, or germs. Other ancient fossils look like jellyfish, sea anemones and worms.

What Fossils Teach

Scientists use fossils to learn about ancient organisms. They also use them to learn about the ancient world. Fossils provide evidence of ancient climates and ecosystems. They also show how the land has changed over time. For example, there are fossils of sea creatures high up in the Rocky Mountains. This shows that these great mountains were once under the sea.

A fossil from the Burgess Shale, an area of the Rocky Mountains in Canada.

These fossils taught us that parts of the mountain range used to be under water millions of years ago. Photo from: Wikimedia Commons.