How do fossils form?

Fossils are formed in many different ways, but most are formed when a living organism (such as a plant or animal) dies and is quickly buried by sediment (such as mud, sand or volcanic ash). Soft tissues often decompose, leaving only the hard bones or shells behind (but in special circumstances the soft tissues of organisms can be preserved). After the organism has been buried, more sediment, volcanic ash or lava can build up over the top of the buried organism and eventually all the layers harden into rock (they become ‘lithified’). It is only when the process of erosion - when the rocks are worn back down and washed away - occurs that these once living organisms are revealed to us from within the stones.

Pliosaur fossil maintainace
Sheldon Teare observing and working on Eric the Pliosaur in the Material Conservation laboratory. March 2018. Image: Nick Langley
© Australian Museum

What does the word fossil mean?

The term fossil literally means ‘dug up’, which comes from the Latin word fossilis. A fossil refers to any remains or traces of past life that are preserved in the rock record. Fossils include the organisms remains, such as plant or animal tissues, shells, teeth or bones and even bacteria (!), but can also include traces of life such as foot prints or worm burrows. Fossils tell us about the history of life on Earth, which we refer to as the fossil record.

Five common ways that fossils form

  1. Permineralisation occurs when dissolved minerals carried by ground water fill up space inside the cells of plants and animals. The dissolved minerals crystalise in these cellular spaces and eventually form rocks in the shape of the animal or plant. This is the most common type of fossil preservation and examples include teeth, bones, shells and wood. We may refer to these fossils as having been petrified.
  2. Impression fossils form when the organisms original bone or tissue is removed by processes that occur after burial, such as ground water flow. If the organisms remains are decomposed entirely, leaving an empty space in the shape of the organism, it is referred to as a cast. If minerals fill in this empty space and form a mineralised 3D shape of the organism it is referred to as a mould. Many marine invertebrates like shells form fossils in this way.
  3. Amber can preserve organisms if they become trapped in tree resin. The resin will eventually harden to form a golden amber that has been shown to preserve fossils up to 100 million years old.
  4. Trace fossils record the activity of an organism. They include nests, burrows, footprints and coprolites (better known as fossilised poo!).
  5. Soft tissues can also be preserved. These include the intact remains of organisms and often include preserved skin, muscle, bone, hair and internal organs. Soft tissue fossils form in special circumstances that often need rapid burial and low oxygen environments that stop the organism from decomposing or being scavenged by other organisms. These circumstances can occur when the entire organism becomes rapidly encased in material such as ice or volcanic ash or buried in peat bogs or trapped in amber! Although this is a rare form of preservation, when it does happen the fossils are exceptionally well-preserved and provide a lot of information about these past forms of life. When fossil sites are found with this kind of exceptional soft tissue preservation they are often referred to as Lagerstätte (which means ‘storage place’ in German). An example of this kind of soft tissue preservation comes from the worlds best-preserved woolly mammoth (a baby mammoth named Lyuba) discovered from within ice that formed 40,000 years ago.

Fish and plant fossil
Fish and plant fossil. An Australian 20 cent coin can be seen for scale. Image: Yong Yi Zhen
© Australian Museum

How does a dinosaur become a fossil?

fossilisation step 1
Diagram of stage 1 of 4 of fossilisation process. Image: illustration
© Australian Museum

Stage 1: A dinosaur dies while swimming in a lake. The fleshy parts of the dinosaur decompose, but the hard bones remain intact at the bottom of the lake.

fossilisation step 2
Diagram of stage 2 of 4 of fossilisation process. Image: illustration
© Australian Museum

Stage 2: Sediment builds up over the dinosaur’s remains, and over time a thick sediment bed accumulates on top, protecting the dinosaur bones from scavenging animals.

fossilisation step 3
Diagram of stage 3 of 4 of fossilisation process. Image: illustration
© Australian Museum

Stage 3: Gradually, the bones are replaced by minerals transported in the ground-water and they turn the bones to stone (this process is known as permineralisation and is described above). As more sediment accumulates over time the pressure and compaction builds up and eventually the bones and sediment layers become the bed rock.

fossilisation step 4
Diagram of stage 4 of 4 of fossilisation process. Image: illustration
© Australian Museum

Stage 4: The dinosaur bones are preserved within the rocks until they are uncovered through erosion or excavated by palaeontologists.

What is the fossil record?

The fossil record refers to the record of life on Earth that has been preserved, discovered, and studied by palaeontologists. But the record is incomplete and often skewed in favour of some organisms over others.

For example, by far the most common fossil remains are those of shelled invertebrate creatures such as snails, corals, and clams that live in aquatic environments (lakes, river and the sea).

Fossils of terrestrial (land-based) organisms are much scarcer than those that live in aquatic environments. In order for these terrestrial organisms to become fossilised, they must either become buried in an aqueous environment like a lake or a river, or become buried by ash which would occur during a volcanic eruption. Because of this, most terrestrial organisms never get the chance to become fossilised. There may be whole groups of terrestrial organisms for which no fossil record has been discovered. But this means there are many more discoveries to made about the history of life on Earth! So, we continue to investigate, explore, and uncover the puzzle that is Earth’s fossil record.