forms when any kind of rock has changed by heat, pressure or chemical activity
sometimes form without any mineralization. Examples include pollen and forms with hard shells.
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.
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.
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.
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.
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.
The fossil record refers to the record of life on Earth that has been preserved, discovered, and studied by palaeonto logists. But the record is incomplete and often skewed in favour of some organisms over others.
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.