Understanding Fossils: Formation, Dating, and Key Insights

What Are Fossils?

Fossils are the mineralized remains of living things, including imprints or traces of their activity. Most commonly, the hard and less alterable parts of animals and plants fossilize, such as bones, shells, and teeth. In many cases, the body or its remains have disappeared completely, but its shape is imprinted on a rock as a mold, as seen with fossil shells.

Impressions or casts left by soft-bodied organisms like worms and squid are also considered fossils. Other examples include footprints, tracks, tubes or galleries built as housing, and coprolites (fossilized feces). An entire animal or plant does not usually fossilize unless a series of special conditions allows it to be encased in materials that preserve it from decomposition.

The Process of Fossilization

The remains of living organisms left in the open decompose rapidly due to the combined action of external geological agents, fungi, and bacteria, often disappearing completely. However, if these organic remains are covered by a material that insulates them from atmospheric contact, they can fossilize. Fossilization is a series of chemical transformations in which organic matter is replaced by inorganic minerals, allowing the exact structure and shape of the original organism to be retained.

Using Fossils to Date Rocks

Fossils are commonly used to determine the age of the rocks in which they are embedded. However, not all fossils can be used for dating; only those that lived for a limited time on Earth are useful for this purpose. For example, ammonites lived during the Mesozoic Era and trilobites during the Paleozoic Era. Therefore, their presence can determine the age of the rocks containing them. These are known as index fossils or guide fossils.

Information Provided by Fossils

Besides their use in geochronology, fossils provide other valuable information:

• Paleogeological Indicators: They help determine the position of continents in the past. They have been used to demonstrate the theory of continental drift, as identical plant and animal fossils have been found on continents now separated by oceans, showing they were once united.
• Environmental and Climatic Indicators: A layer in a sedimentary series often contains various associated fossil genera and species. Just like living organisms today, these fossils were linked to specific habitats. By studying these associations, we can learn about the environmental and climatic conditions in which they lived.
• Evolutionary Process Indicators: Fossils allow us to learn about extinct species and, more importantly, understand their succession through time. This enables us to trace different evolutionary lineages of plants and animals and see their relationship with modern forms.