Fossils are the preserved remains of ancient lifeforms that remain in the geological record, as a result of the fossilisation process.
This process allows the prehistoic organisms to retain their original forms even after millions of years. For instance, these organisms be immacultely preserved when they are encased in amber, ice, or tar. Very occasionally, they may be mummified if caught in a enclosed environment, like an airtight cave. Most often, the hard parts of a specimen’s anatomy are preserved as an imprint within mineral deposits.
Specimen parts, especially their configuration, are best preserved when they are rapidly buried. Over time, more layers of sediment build up and the weight places immense pressure on a specimen. In a resulting lithification process, water is pushed out of the soft layers of sediment and the mud and sand is compressed into hard rock. The surrounding minerals replace the specimen’s hard parts which dissolves away in a process known as permineralisation. The resulting remineralised, three-dimensional image of the original skeleton is a fossil. Over time, plate tectonics in the earth’s crust shift the sediment layer back up to the surface, and weather erosion exposes the fossil within.