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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.

Five ways fossils can form

  1. Permineralization occurs when minerals carried by water are deposited around a hard structure. They may also replace the hard structure itself.
  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.
  3. Amber preserved are organisms that become trapped in tree resin that hardens into amber after the tree gets buried underground.
  4. Trace fossils record the activity of an organism. They include nests, burrows, imprints of leaves, footprints and poo.
  5. Preserved remains record the activity of an organism. They include nests, burrows, imprints of leaves, and footprints.Trace fossils form when an entire organism becomes encased in material such as ice or volcanic ash or immersed in bogs.

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
Stage 1 A dinosaur dies and is buried before the remains are completely destroyed. Image: illustration
© Australian Museum

fossilisation step 2
Stage 2 Over time, layers of sediment build up and press down on the buried remains. Image: illustration
© Australian Museum

fossilisation step 3
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. Image: illustration
© Australian Museum

fossilisation step 4
Stage 4 The fossils remain within the rock until uncovered through erosion or excavation. Image: illustration
© Australian Museum