Body and trace fossils

A fossil represents part of a once living organism (this is called a body fossil) or has been formed by the action of an organism (a trace fossil).


The process of preserving the organism's body or traces of activity is called fossilisation. It is very rare for the body of an organism to be preserved intact with no alteration and most fossils have undergone significant changes since the death of the organism.

Generally, the older the fossil, the greater the alteration from the original material.

Exercise 1

Give an example of an extinct organism being preserved with no alteration.

Changes to the organic components

An organism is composed of a mixture of different tissues, some are hard and resistant to decay, while others are soft and are broken down relatively easily.

Exercise 2

Using humans as examples, make a list of the different types of tissues in the body.

For each one say if it would decay easily.

Even within the hard parts of organisms, there may be organic material that can be altered by processes after death.

Insects are composed of an organic material called chitin. This hard exoskeleton is almost perfectly preserved when the creature becomes covered by sticky tree resin, which hardens to form amber. But the soft tissues inside the exoskeleton do decay.

Some organisms are found to be mummified. The Egyptians did this with their dead pharaohs, but it can also happen naturally in a dry environment. Basically this involves a complete drying out of the body preventing decay.

In some parts of the world there are pools of tar or peat bogs. If an organism becomes trapped in this material the antiseptic properties and low oxygen conditions prevent decay. Tar pits in America have been excavated to reveal ice-age animals such as mastodons and big cats.


Carbonisation is an important means of fossilisation. Under pressure, the volatile elements in the soft tissues are driven off leaving the remains behind, rich in carbon.

This method is particularly seen in plant fossils, graptolites and arthropods.

Altering the minerals in the hard parts

The hard parts such as shells, bones and teeth are much more likely to be fossilised, we say they have a higher preservation potential.

Many shells are composed of a type of calcium carbonate called aragonite. Over time these shell remains recrystallise to form a more stable form of calcium carbonate called calcite. This recrystallisation process destroys a lot of the finer details but the overall shape and form of the shell remains.

Petrifaction is a word that literally means, "turning to stone". This can happen in two ways. Within the structure of the shell or bone there are spaces left by the decay of organic materials. These spaces can become filled by minerals carried by water, in a process called impregnation. The minerals carried by the water may replace the actual materials of the hard parts. This is called mineral replacement.

Loss of hard parts

In some cases the hard parts of the organism are not preserved directly, they become dissolved away leaving a space in the sediment called a mould. This can then be filled by minerals or sediment forming a cast.

Exercise 3

Draw diagrams to show how moulds and casts form.

Exercise 4

Explain the differences between internal and external moulds and casts.

Transported and derived fossils

Fossils are used to determine the ancient environment. Unfortunately sometimes the organisms may have been transported from the environment they were living in to the site of deposition.


Another common problem with fossils is that they may become free of the rock that contains them, broken out by weathering and erosion. These fossils may then become deposited in new sediment. This is a problem because if you use the fossils to work out the age of the rock, you will get very confused. These are called derived fossils.

Life and death assemblages

Some organisms are preserved in their life positions. Such a collection of fossils is called a life assemblage. If the remains have been deposited after the organisms died, the resulting collection is called a death assemblage.

Exercise 5

What observations will indicate whether you are examining a life or death assemblage?

ESTA GEOTREX The Geology Teachers Resource Exchange Contributor: Owain Thomas Establishment: Amman Valley

School Date: 22:04:05