What is a fossil?

Fossils are the remains of a once-living organism and/or its behaviour that has been altered by natural earth processes. There are two main types:

• body fossils represent all or part of the organism’s body, like ammonite and plant fossils.
• trace fossils show evidence of the organism’s behaviour, like worm burrows and arthropod trackways.

Figures of fossil examples with scale included.

The study of fossils is called palaeontology, from the Greek meaning ‘ancient (palaeo-) being (onto-) study (-logy)’. Scientists who study fossils are called palaeontologists. Fossils can help determine the age of a particular rock compared to other rocks or identify the environment in which the rock was formed.

Becoming a fossil

Any living thing — animal, plant, bacterium or fungus — has the potential to become a fossil, but because the process of fossilisation is destructive, not everything that dies is preserved. Most dead organisms are eaten, rot away or are destroyed by the environment before they have the chance to become fossils.

Some animal or plant parts preserve more readily than others. For example, bones fossilise better than flesh and organs because fewer predators and scavengers eat bones, and they are more resistant to physical and chemical destruction. Organisms like jellyfish with no hard parts like bones or shells are rare because they are less likely to be preserved.

Normally, only the toughest parts of the toughest animals and plants become fossils. Occasionally the environment is just right to preserve complete soft-bodied organisms that are rarely fossilised, or an entire ecosystem. These special locations are commonly called lagerstätten, a German word meaning ‘storage place’. These sites are important to scientists as they can tell us a lot about how the animals lived and died, and how the different types of organisms interacted with one another and the environment. Special environmental conditions are generally required to form one of these sites — either the animals were quickly killed and buried through anaerobia (lack of oxygen) or volcanic ash, or the water chemistry was particularly good at preserving the bodies.

Western Australia’s Gogo Formation in the west Kimberley is considered a lagerstätten as several beautifully preserved fish (some including evidence of live birth) have been found — including Western Australia’s State fossil.

How behaviour can be preserved

Sometimes organisms leave behind indications of their behaviour such as tracks or burrows, without leaving behind parts of their bodies. These are known as trace fossils. It can include:

• animal footprints,
• bite marks,
• coprolites (fossil dung)
• preserved nests or burrows

Trace fossils might tell us about how an organism moved, where and how it lived, what it ate, how it hunted, and socialised.

Identifying the organism that made a trace fossil can be hard unless a body fossil is nearby. Sometimes, comparisons with modern species provide clues. Even without a clear culprit, trace fossils reveal valuable insights into ancient behaviors and environments. For example, well-defined tracks suggest a damp, exposed surface, while washed-out tracks indicate underwater formation.

What isn’t a fossil

Very recent remains, like beach shells or a fresh skeleton, aren’t considered fossils unless deeply buried and altered over time and opinions vary on the exact criteria.

Human-made objects, like buildings, pottery, or coins, are artefacts, studied by archaeologists, not palaeontologists. Some rocks also contain mineral patterns that can seem like fossils but don’t have biological origins; these are called pseudofossils.

What is a fossil

Confirming the oldest potential fossils is challenging, as they may lack original biological material, have simple forms, or look unlike any known life. Determining biogenicity (whether a feature represents life) is crucial for studying Archean and Proterozoic rocks but can also be difficult for poorly known or preserved Phanerozoic fossils.

Tests applied to determine whether a feature is a fossil include:

• assessing morphology (body shape, generally compared to living organisms)
• preserved structure and texture
• structural organisation and consistency (both within a single individual and between individuals of the same type)
• biochemistry (if available)

For problematic fossils, this list is expanded to seven main criteria:

1. structures should be in rocks of known provenance (similar structures restricted to a similar area and time frame)
2. structures should be indigenous to the rocks, not on top of the surface
3. structures should be formed at the same time as the sediment
4. structures should have formed in an environment suitable for life
5. structures should be of assured biological origin (for example, preserving chemical indicators of biological tissue)
6. structures should show evidence of taphonomy (differences in preservation caused by decay or fossilisation) under different environmental conditions
7. structures should show biofabric evidence (variations in the types of tissue throughout the organism)

Not all criteria must be strictly met for a structure to be considered a fossil, but each should be evaluated logically. For example, criterion five is hard to prove even in recent fossils, as it requires rare fossilisation conditions and minimal rock alteration.

How fossils are formed

There are four main steps to making a fossil:

• An organism dies. Fossilisation is more likely in oxygen-poor environments (like deep oceans or lakes) where decay is slowed.
• The remains must be rapidly buried in soft sediment to protect them from scavengers, wind, and waves.
• Over thousands to millions of years, surrounding rocks alter the remains chemically and physically, forming fossils through processes like mummification, recrystallisation, or compression. Preservation is best when burial conditions remain stable.
• Finally, erosion exposes the fossil, bringing it to the surface for discovery.

This diagram shows the process of a fish decaying and becoming a fossil: Figure A shows the fish dying, Figure B shows the burial process, Figure C shows the geological changes during fossilisation, and Figure D shows the fossil as it would be found.

How fossils help us

The fossil record reveals that different geological periods had distinct plants and animals, with species evolving, migrating, or going extinct over time.

Fossil assemblages (groups of fossils) help determine the relative age of rocks by comparing them locally or globally, a method called biostratigraphy. Unlike radiometric dating, which provides absolute ages, biostratigraphy offers a relative timeline based on fossils representing specific time periods. It is especially useful for dating sedimentary rocks, which are difficult to date radiometrically.

Fossils, including trace fossils, also provide insights into past environments such as climate, altitude, vegetation, and water conditions. This field is known as palaeoenvironmental studies.

This chart shows some of key origins, changes and extinctions seen in fossil organisms through geological time. These changes help us determine the ages of rocks where fossils are preserved.

All this information helps us better understand the environment around us, how it formed and how it has changed through time. It also allows us to best manage our State’s natural resources for everyone’s benefit.

Western Australian fossils in perspective

Western Australia is endowed with both very recent and very old rocks, including

• some of the oldest geological materials known on Earth (from Jack Hills)
• the oldest convincing evidence of life (Pilbara stromatolites)
• rocks that chart the oxygenation of the Earth’s early atmosphere (in the Pilbara and Hamersley Ranges)

Our State has several interesting and important fossil sites extending over more than 3000 million years of geological history.

Do your part to protect Western Australia’s fossils by staying up to date with the restrictions and responsibilities related to fossils and State geoheritage, following all laws and regulations, and spreading the word to others (including tourists).