How are fossils formed?

Fossils are formed in different ways, but most are formed when a plant or animal dies in a watery environment and is buried in mud and silt. Soft tissues quickly decompose leaving the hard bones or shells behind. Over time sediment builds over the top and hardens into rock. It is when the processes of erosion occur that these secrets in stone are revealed to us.

What does the word fossil mean?

The term fossil refers to any trace of past life. A fossil may be an an organisms remains, such as plant, shells, teeth or bones. A fossil record can also be of the activity of an organism such as footprints, burrows and faeces.

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

Five ways fossils can form

  1. Permineralization occurs when dissolved minerals carried by ground water fill the cellular spaces of plants and animals. The dissolved minerals crystalise and produce 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.
  2. Natural casts form when flowing water removes all of the original bone or tissue, leaving just an impression in sediment. Minerals fill in the mold, recreating the original shape of the organism. these are commonly marine invertebrates like shells.
  3. Amber preserved are organisms that become trapped in tree resin that hardens into amber after the tree gets buried underground. Examples include insects, pollen, lizards and frogs.
  4. Trace fossils record the activity of an organism. They include nests, burrows, imprints of leaves, footprints and poo.
  5. Preserved remains record intact remains of animals, often including preserved skin, muscle, bone, hair and internal organs. Fossils form when an entire organism becomes encased in material such as ice or volcanic ash or buried in peat bogs. This is a much rarer form of preservation than the other forms above. Examples are mammoths,

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

From dinosaur to fossil

The remains of a dinosaur have to be buried before they completely decompose or are eaten by scavengers. The conditions of burial must then be suitable for the remains to leave an impression or have their organic material replaced by minerals. Finally, the fossils must survive millions of years of pressure, uplift and erosion if they are to come back to the surface.

So what are the chances of any dead animal turning into a fossil? Many millions to one – so we certainly appreciate the fossils we find.

By far the most common fossil remains are those of shelled invertebrate aquatic creatures such as snails, corals, and clams.

Fossils of terrestrial (land) animals are scarcer than those of plants. In order to become fossilised, animals must die in a watery environment and become buried in the mud and silt. Because of this, most terrestrial animals never get the chance to become fossilised unless they die next to a water source. There may be whole genus' of terrestrial animals for which no fossil record has been discovered. We may never know how diverse these animals were.

Dinosaur fossil

Replacement of organic material in bone with minerals – dinosaur femur (thigh bone), Camarasaurus supremus. F 17743. USA, Late Jurassic, 156–146 million years ago.

Image: Stuart Humphreys
© Australian Museum

Four stages of fossilisation

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

Stage 1: A dinosaur dies and is buried before the remains are completely destroyed.

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

Stage 2: Over time, layers of sediment build up and press down on the buried remains.

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

Stage 3: Dissolved minerals, transported by ground-waters in the sediment, fill tiny spaces in the bones. The combination of pressure, chemical reactions and time eventually turns the sediments into rock and the bones into mineralised fossils.

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

Stage 4: The fossils remain within the rock until uncovered through erosion or excavation.