The fossil record around the world has given scientists access to how life developed and how some of it became extinct. Geologists have found the fossils of plants and animals dating back 550 million years. There are certain circumstances that must occur for a plant or animal to leave behind a fossil. This process is called fossilisation.
Before the three steps of fossilisation begin, the organism must die near enough to water for the soil to be moist. The water hastens decomposition but allows the soft tissue to decay naturally as bacteria and smaller animals feed. In order for a fossil to be created, the creature will have to have some form of skeleton or exoskeleton. There are no fossil records of creatures such as earthworms or spineless sea creatures.
After the decomposition process has begun but before it is complete, sedimentation must occur. For instance, if a creature dies in a river delta and the waters recede, the creature's remains will be covered in sediment, thus ending the complete process of the remains being consumed by other animals. The sooner sedimentation happens, the more likely that fossilisation will occur. The type of sediment also plays a contributing role in whether the process begins. The more mineral rich the soil, the more likely fossilisation will occur.
The second step necessary in the process of fossilisation is permineralization. This step occurs when a second layer of sediment piles on top of the first, moist layer in which the remains are trapped. The pressure of the new layer of sediment or rock on the lower layer turns that layer into mineral over time. The more mineral-rich the top layer is, the greater the likelihood of the creation of a fossil. The minerals seeping down with groundwater help guarantee the fossil solidifies.
The final step that preserves the fossil is time. It takes years of pressure on the original layer of sediment for the fossilisation to formalise and the remains of the creature to turn to mineral. There are many examples of creatures that have been preserved without being fossilised, such as animals from the ice age whose remains are frozen and preserved in ancient ice. These remains are preserved, but not fossilised. For formal fossilisation to occur, the animal must have gone through the decay of its soft tissue.