How are fossils formed?

Ammonoid fossil from the Lower Jurassic of England. Source: Museum Victoria.

For about 3000 million years, life was present only in the oceans. The oldest fossils are therefore of marine creatures. When marine animals or plants died, their remains accumulated on the sea floor where they were buried by mud, sand or silt. Over very long periods, these sediments became sedimentary rock, and the animal or plant remains became encased in the rock.

By 430 million years ago, animals and plants had colonised the land. When land animals or plants died, the soft parts usually decomposed or were eaten by scavengers. However, if the hard parts (bones, shells, wood) are quickly covered by water, sand, or even volcanic ash, they might be preserved. Teeth are the hardest parts of an animal and are most likely to be preserved.

For more information go to Fossils and the Scientific Process in Dinosaurs & Fossils.

Stages in the formation of a fossil.

Click to enlarge images.

Cockle shell replaced by opal, Cretaceous, Coober Pedy, South Australia. Source: Museum Victoria.

Bone, wood and shell, although hard, have minute air spaces. When buried, water containing dissolved minerals may seep into these spaces and deposit minerals. Often, over millions of years, all of the original bone or shell dissolves away leaving a complete mineral replacement embedded in the surrounding rock. The bones, wood and shell are then said to be petrified; i.e. turned to stone.

Fossil skeletons are sometimes found relatively intact, although usually they have been scattered, and only parts of the skeleton are able to be recovered. Sometimes, the entire animal has decayed away, leaving a 'mould' that has then been filled by sediments or minerals, making a natural 'cast'. Similarly, footprints made in soft ground have created moulds that were later filled, making casts. And in some locations, impressions of skin have been discovered.

Insect preserved in amber, Oligocene, Baltic region. Source: Museum Victoria.

Fossils are not only found in rock:

• Animals have been trapped in natural tar pits and been beautifully preserved.
• Mammoths and other animals that lived during the ice ages have been trapped in ice, or frozen ground, so that flesh, hair and even stomach contents have been preserved.
• Extinct insects have been found in fossil tree sap (amber).