Types of Fossils

Fossils are the preserved remains or impressions of ancient organisms that lived millions or even billions of years ago. They provide scientists with a window into the past, offering clues about the evolution of life on Earth, past environments, and the creatures that inhabited them. Here are the major types of fossils:

Body Fossils: These are the most common type of fossil and consist of the actual remains of plants and animals that have been preserved over time. Examples include bones, teeth, shells, leaves, and petrified wood.

Fossilization processes proceed differently according to tissue type and external conditions.

Types of Fossils


Permineralization is a process of fossilization that occurs when an organism is buried. The empty spaces within an organism (spaces filled with liquid or gas during life) become filled with mineral-rich groundwater. Minerals precipitate from the groundwater, occupying the empty spaces. This process can occur in very small spaces, such as within the cell wall of a plant cell.

Types of Fossils According to Their Formation
A pyritised Echioceras ammonite from Charmouth.

Casts and Molds

In some cases the original remains of the organism completely dissolve or are otherwise destroyed. The remaining organism-shaped hole in the rock is called an external mold. If this hole is later filled with other minerals, it is a cast. An endocast or internal mold is formed when sediments or minerals fill the internal cavity of an organism, such as the inside of a bivalve or snail or the hollow of a skull.

fossils mold
External mold of a bivalve from the Logan Formation, Lower Carboniferous, Ohio

Replacement and Recrystallization

Replacement occurs when the shell, bone or other tissue is replaced with another mineral. In some cases mineral replacement of the original shell occurs so gradually and at such fine scales that microstructural features are preserved despite the total loss of original material. A shell is said to be recrystallized when the original skeletal compounds are still present but in a different crystal form, as from aragonite to calcite.

Carbon films

Carbon films are thin film coatings which consist predominantly of the chemical element carbon. The soft tissues of organisms are made largely of organic carbon compounds, leaving a thin film of carbon residue is left, forming a silhouette of the original organism called a carbon film.

Molecular Fossils

These are microscopic chemical traces of biological molecules like lipids (fats) and proteins preserved in fossils. They can be used to identify the organisms that were present and information about their environment.

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