Fossils

Outcomes:

Describe the conditions under which fossils form.
Identify different types of fossils.

What Are Fossils?

Fossils are preserved remains or traces of organisms that lived in the past. Most preserved remains are hard parts, such as teeth, bones or shells. Hard parts are less likely to be destroyed before they can become fossils. Even so, a very tiny percentage of living things become fossils. These types of fossils are called body fossils

A group of preserved trilobites, a variety of ancient marine arthropods

The preserved remains of an ammonite

The preserved remains of an Archaeopteryx, a type of dinosaur

The preserved trace of a dinosaur footprint, originally in mud

A preserved trace: burrows made by prehistoric worms

How Fossils Form

The process by which remains or traces of living things become fossils is called fossilisation. Most fossils are preserved in sedimentary rocks

Fossils in Sedimentary Rock

Most fossils form when a dead organism is buried in sediment. Layers of sediment slowly build up. The sediment is buried and turns into sedimentary rock. The remains inside the rock also turn to rock. The remains are replaced by minerals The remains literally turn to stone. This type of fossilization is called permineralization.

Other Ways Fossils Form

Fossils may form in other ways. With complete preservation, the organism doesn't change much. Tree sap may cover an organism and then turn into amber. The original organism is preserved so that scientists might be able to study its DNA . Organisms can also be completely preserved in tar or ice.

Complete Preservation

Molds and casts are another way organisms can be fossilized. A mold is an imprint of an organism left in rock. The organism's remains break down completely. Rock that fills in the mold resembles the original remains. The fossil that forms in the mold is called a cast. Molds and casts usually form in sedimentary rock. With compression, an organism's remains are put under great pressure inside rock layers. This leaves behind a dark stain in the rock.

Molds and Casts. A mold is a hole left in rock after an organism's remains break. A cast forms from the minerals that fill that hole and solidify

Compression. A dark stain is left on a rock that was compressed. These leaves were fossilized by compression

Why Fossilisation Is Rare

It’s very unlikely that any given organism will become a fossil. The remains of many organisms are consumed. Remains also may be broken down by other living things or by the elements. Hard parts, such as bones, are much more likely to become fossils. But even they rarely last long enough to become fossils. Organisms without hard parts are the least likely to be fossilised. Fossils of soft organisms, from to jellyfish, are very rare.

Summary

Fossils are the remains or traces of living organisms.
Body fossils are usually hard parts, such as bones, teeth, and shells.
Trace fossils are the traces of organisms, not parts of them. Trace fossils include footprints, feces, and burrows.
Very few fossils preserve soft parts. Insects may be preserved in amber. Insects and other animals may be preserved in ice.
Some fossils are created when minerals replace the organic material. This is called permineralization.
A fossil may be in the form of a mold. A mold is the depression left in the shape of the material.
A fossil may be preserved as a cast. A cast is the rocky material that fills the mold

Review

What is a fossil and how is it formed?
How do scientists use fossils to learn about past life forms and environments?
What are the different types of fossils and how are they classified?
What are the limitations of using fossils as evidence for evolution and Earth's history?
How can the age of fossils be determined using relative and absolute dating techniques?

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