Once an organism dies, it often ends up in a place where it is covered in sediment such as sand, mud, or volcanic ash. The type of sediment an organism is buried in can significantly affect the type of fossil that results. Aquatic organisms usually end up buried in fine mud and silt, which are more likely to preserve delicate structures like shells or bones, while organisms buried under volcanic ash, can result in very fossils. detailed. However, tar, or asphalt, can trap and preserve organisms.

The sticky nature of the tar prevents scavengers from removing the remains and the environment allows for exceptional preservation. Over time, layers of sediment build up on the buried remains, increasing pressure on them, causing compaction and drawing out remaining water in the tissues.

Most often, fossils of the harder parts of an organism, such as bones, teeth, shells, and wood, end up being preserved because these materials are more durable than soft tissues, such as muscles, skin and organs, which decompose much more quickly. In rare cases, under exceptional conditions such as extreme freezing, mummification, or entrapment in amber, soft tissues may also be preserved.

During burial, minerals in the surrounding sediment begin to replace the organic material present in the hard parts of the body. This process, called permineralization, occurs when mineral-rich water seeps into the pores and cavities of bones, teeth, or shells. Over time, minerals crystallize within these spaces, preserving the original structure.

This gradual mineralization transforms the remains into a rock-like substance, such as calcium carbonate or silica that can replace the original material, transforming bones or shells into fossilized forms. Fossils can remain buried for millions of years and gradually become part of the geological record.