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Types of Fossils Found in Rock

Fossils are the physical evidence of life that existed during times before recorded human history. The term "fossil" refers to the remains of living organisms or impressions of their physical forms and activities. The earliest fossils date from around 3.5 billion years ago. Fossils are found all over the world, although only a small percentage of life becomes fossilized. Most organisms decay without a trace.

Mold Fossils
- A mold fossil is an imprint or trace of an organism. For example, if a shell makes an imprint in sand and the sand hardens, the shell dissolves and leaves a space with the same shape as the shell in the rock that is formed. This space is called a mold. When sediments harden to become rock a fossil is formed that has the shape of the shell that produced the mold.
Cast Fossils
- A cast fossil is formed when a mold fossil is filled with a mineral of some kind, possibly due to water depositing minerals in the mold. Over time, the mold fills with materials which harden to create a duplicate of the fossil that was originally in the space, thus creating a cast fossil.
Trace Fossils
- Trace fossils are proof of the activities of ancient organisms. These fossils include things like foot prints or burrows. Traces are made all around us, such as our footprints in the snow. Trace fossils tell us about the environment in which the ancient organism lived. A single animal leaves behind thousands of traces. Virtual Museum Canada states "Trace fossils are moments of time that have been captured forever in the rock record. They are a celebration of life."
True Form Fossils
- True form fossils are the actual bodies of organisms which have been replaced by minerals. Common examples include limbs, torsos or heads of organisms. Whereas mold and cast fossils are formed by an impression of the organism, true form fossils are formed by when part of the organism itself is replaced by minerals that harden to become rock.

Rocks & Minerals